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INTERNATIONAL ATOMIC ENERGY AGENCY
Distance Assisted Training Programme
for
Nuclear Medicine Technologists
Edited by: Heather E. Patterson, Brian F. Hutton
PET / CT Radiation Safety Author: Jocelyn Towson Module:
Unit:
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(version 4)
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PET / CT Radiation Safety Contents
Page
Subject flowchart
d
Outline
1
Radiation from PET radionuclides
4
Radiation from a CT scanner
7
Radionuclide sources in a PET center
- Vials and injection systems
- How much activity for patients?
- How accurately do you need to dispense FDG?
- The patient as a source of radiation
- Other PET sources
10
Radiation controls
- Containment of radioactive material
- Radiation shields
- Distance and Time
21
How the workload and the workspace affect your exposure
- Clinical procedures
- Layout of the PET center
- Uptake areas and the scanner room
35
12
17
23
30
37
How to measure radiation in PET/CT environments
42
Radiation safety for patients
- The radiation dose from FDG and CT
- Pregnant patients
- Lactating patients
- Pediatric patients
- Advice for the patient, their family and carers
47
Conclusion
56
49
52
53
54
- Radiation Safety Guidelines for operating a PET/CT scanner
Glossary
58
c
PET / CT Radiation Safety Outline Radiation from PET radionuclides Radiation from a CT scanner Radionuclide sources in a PET centre Vials Activity and for Injections patients OL Accuracy for FDG OL OL
Patient as a source Other PET sources Radiation Controls Containment Shielding Distance and Time Exercise Estimating dose rates
How workload and workspace affect exposure Clinical Procedures Layout of PET centre Uptake areas Scanner rooms Measuring radiation in PET/CT environments Exercise Recording dose rates
OL
Radiation Safety for Patients Dose from FDG & CT carers Pregnant Lactating patients patients Pediatric Advice for patients patient, family & OL
Radiation Safety Guidelines for operating PET/CT scanner OL On‐line exercises d
OL
PET/CT Radiation Safety
Technical Writer:
Production Editor:
Jocelyn Towson
Heather Patterson
Outline:
This Unit covers the radiation safety aspects of PET/CT scanning which the
nuclear medicine technologist should know. It aims to build on the knowledge
of radiation protection in general nuclear medicine which an experienced
technologist is assumed to have. PET has a significant impact on the radiation
safety of both staff and patients. The occupational exposure of technologists
usually increases when PET imaging is introduced. The clinical exposure of
patients from a PET/CT scan is generally higher than most other nuclear
medicine imaging procedures. The technologist therefore needs to understand
the radiation sources with which he or she works, how to minimize
occupational exposure to themselves and other health care workers, and how
to give appropriate radiation safety advice for the patient, their family and
other carers. The technologist also has a significant role in the optimization of
the patient’s radiation exposure, i.e. the radiation dose is kept to the minimum
required without compromising the image quality required for interpretation
by the physician.
The range of PET services may vary widely from one center to another,
depending on local arrangements such as supply of PET radiopharmaceuticals,
involvement of nursing and medical staff and support from scientific staff
including physicists and radiochemists or radiopharmacists. For the purpose of
this Unit, a straightforward service is assumed:
- the center has a modern PET scanner incorporating a multi-slice CT scanner
- the center uses mainly 18F-fluorodeoxyglucose (FDG)
- FDG is supplied either in a multi-dose vial or in unit doses
- FDG is administered by intravenous injection, and
- oncology scans account for the bulk of the clinical workload.
It should be evident by the end of the subject that an effective radiation
protection program in PET depends on investment in suitable equipment and
teamwork between all staff members.
Introduction
The use of Positron Emission Tomography (PET) is growing rapidly because a
form of glucose (FDG) labeled with fluorine-18 is such a valuable tracer for
imaging changes in the uptake of glucose in body tissues, especially in cancer,
and because the supply of F18 is expanding. Modern PET scanners incorporate
a multi-slice CT scanner for anatomical information. The addition of the CT
scan improves the physician’s ability to localize suspicious areas of FDG
uptake on the PET scan with more confidence. The CT scan is also used to
correct the PET image for attenuation of F18 photons (lost by absorption or
scatter) in the patient’s body. This is a much faster method than the
radionuclide transmission sources of older PET scanners.
1
Nuclear medicine technologists get a higher radiation dose from their work
than people in most other occupations. However their doses are usually much
less than the legal limit of 20 millisieverts per year. For example, the following
graph shows the dose to workers in various industries who wore TLD
dosimeters provided by the Australian Radiation and Nuclear Safety Authority
in 2004. People working in nuclear medicine practices (none of which in this
survey operated a PET service) received the most, with 75% of doses being
within 3.5 mSv per year.
Australian TLD data for occupational exposures
3rd quartile annual photon doses
NUMed = General nuclear medicine practices
ARPANSA PMS report TR139 [2004]
μSv/y 4000
3500
3000
2500
2000
1500
1000
500
R
ad
io
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nc
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uc
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in
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0
Figure 1:
Australia TLD data for occupational exposure
It was recognized many years ago that when nuclear medicine facilities start to do PET scanning, the radiation dose of the nuclear medicine technologists increased. Figure 2: More recently, CT scanners have been combined with PET (and SPECT) in a dual imaging system. The CT scan exposes PET technologists to a negligible amount of radiation, although it significantly increases the radiation dose to the patient. However CT has a large impact on radiation safety for staff because it is a fast imaging modality, allowing many patients to be scanned per day. 2
Objectives On completion of this Unit, you will understand..
• Radiation from the decay of clinical PET radionuclides
• Primary and secondary radiation from a modern CT scanner
• Sources of F18 in a PET center
• Containment and shielding for PET sources
• Maximizing distance from FDG sources
• Minimizing close contact time with FDG patients
• How the clinical workload and workspace affect your exposure
• How to measure radiation in a PET/CT center
• How PET/CT clinical protocols determine the patient’s radiation dose
• Radiation advice for the patient, their family and carers
You will be able to..
• Check if shields for dispensing, calibrating and injecting FDG are adequate
• Assist in preparing the patient for a PET/CT scan
• Safely dispense and inject the correct amount of FDG
• Move and position the patient quickly and efficiently
• Observe radiation safety rules for operation of a CT scanner
• Deal with FDG spills including urinary incontinence
• Evaluate the results of personnel dosimeter reports
• Identify and report any situations which could lead to unplanned exposure
or where exposure could be reduced.
Technical terms are introduced in Bold Italic font and listed in a Glossary. Time Check: Allow 15 hours to study of this subject and complete the exercises in your
Workbook and on-line.
Pre-requisites:
You should have a good understanding of radiation safety within a general
nuclear medicine service.
You should revise Module 1, Unit 2a and Module 2, Unit 2b and understand
the following:
i.
What is Effective Dose?
ii.
What is Equivalent dose?
iii.
What is Extremity Dose?
iv.
What are the Dose Limits for occupational exposure in your country?
3
Time Check: Allow 1.5 hrs to study this section and complete exercises in your
Workbook.
Radiation from PET radionuclides
Emissions from radioactive decay can be considered in two groups:
a) electrically charged particles including beta particles and positrons, which
travel a very short distance from where the radioactive atom disintegrated
b) electromagnetic photons - either x-rays or gamma rays - which can travel
much further than charged particles, and possibly penetrate through the
source and its container.
For example, consider a patient injected with FDG. The positrons are emitted at
high speed from the nuclei of F18 atoms in tissues where FDG had
accumulated. They rapidly lose energy as they slow down, contributing most
of the internal dose to the patient. When the positrons are nearly stationary,
each one combines with an electron and is annihilated (vanishes!) less than a
millimeter from the FDG molecule where it came from. The mass of the
electron and positron that disappeared is turned into energy, creating two 511
keV gamma rays (annihilation photons) which are emitted in opposite
directions. Some of the energy of these photons is absorbed in the patient’s
body, adding to the patient’s internal dose. Of course many photons will
escape from the patient’s body and some of them can be used for imaging, as
described in other subjects e.g. Basic Physics, Radiation Safety, Module 1. Any
photons coming out of the body can cause an external dose to the technologist
or anyone else nearby.
Here is a comparison of the main photon emissions of some PET radionuclides
compared to technetium-99m and iodine-131. The dose rate at a distance of one
meter from a small glass vial containing the same amount of activity - 1
gigabecquerel (1 GBq) - is shown in the last column:
Table 1
Half-life
Photon energy (keV) and photon Dose rate in tissue at
1 metre from 1 GBq
yield (number of photons per
disintegration)
in 10mL glass vial
18F
110 mins 511 keV (1.92)
158 μSv/h
15O
511 keV (2)
2 mins
167 μSv/h
13N
511 keV (2)
10 mins
163 μSv/h
11C
511 keV (2)
20 mins
163 μSv/h
99mTc 6 hrs
140 keV (0.89)
22 μSv/h
131I
8 days
364 keV (0.82), 662 keV (0.07)
64 μSv/h
[This information was obtained from the Radionuclide and Radiation Protection Data
Handbook (2002) published in the journal Radiation Protection Dosimetry, Vol 98 No.1,
2002].
The difference in dose rate from F18 compared to other radionuclides used in
nuclear medicine is striking.
4
Dose rate microsieverts/hour @ 1m, 1GBq in vial
18
Tl-201
22
Tc-99m
25
Ga-67
64
I-131
158
F-18
46
Mo-99
0
20
40
60
80
100
120
140
160
Figure 3: Dose rates in microsieverts / hour at 1 meter from 1 GBq of various radionuclides in a vial. NOTE - The dose rate at 1 metre from an FDG patient will be less than the dose
rate from the same amount of activity in a small source like a vial or syringe,
because of attenuation of some of the photons in the patient.
Another obvious difference between F18 and other radionuclides used in
general nuclear medicine is the much shorter half life. What effect does this
have on radiation safety? For the patient, radionuclide half life is one of several
factors which determine the radiation dose from a radiopharmaceutical – short
half life means lower dose. In fact, because of the positrons, the internal
radiation dose from an FDG PET procedure using a few hundred MBq is
similar to the dose from many 99mTc procedures, despite the short half life.
The short half life of F18 does not have much benefit for the radiation safety of
staff.
As you will find out later in this Unit, most of the technologist’s whole body
dose is likely to come from contact with injected patients. FDG scans are
usually completed within the first half life when dose rates are highest, and
many patients can be scanned each day. From the technologist’s point of view,
the dose rate is quite high all the time regardless of the F18 half life.
Most of the technologist’s extremity dose to the hands comes from dispensing
and calibrating FDG syringes. If the FDG is supplied as a bulk solution once or
maybe twice a day, then the vial must contain much more activity than will be
injected, to allow for decay. The radioactive concentration in the vial may be
very high, requiring very small volumes to be withdrawn for injection. This
will increase the radiation dose to the technologist’s hands during dispensing.
5
Figure 4:
Examples of dose rates from ‘single dose’ small sources from
various radionuclides.
The short half life of F18 is useful for the management of spills and
contaminated items such as bed linen, used vials and syringes, because
overnight storage is usually sufficient to reduce the radiation to background
levels before disposal.
Key points: &
•
The high dose rate from F18 is due to high photon energy and high photon
yield
•
The short half life of F18
- does not have any benefit for reducing the radiation dose of the technologist
- is useful for limiting the radiation dose to the patient.
- is useful for avoiding long term contamination or radioactive waste.
6
Time Check: Allow 1 hr to study the following section.
Radiation from a CT scanner
The primary radiation from a CT scanner is the beam of x-rays emitted from
the rotating x-ray tube towards the x-ray detectors on the opposite side of the
gantry. The x-ray beam comes out of the thick metal housing surrounding the
tube, through a thin metal window in the housing. The CT beam is shaped like
a fan across the gantry aperture, wide enough to cover the body width of most
patients. In the z-direction along the scanner axis, the CT beam is narrow about 40 mm or so in current multi-slice machines - because it is tightly
collimated to the ring of x-ray detectors. The beam is very intense. A modern
CT tube can produce a high x-ray output for a few minutes without
over-heating.
The energy spectrum of the photons in the primary CT beam is a combination
of intense narrow peaks of characteristic x-rays for the particular anode
material (e.g. tungsten) and a continuous spread of energies (bremsstrahlung
radiation) up to the maximum, or ‘peak’, operating voltage applied to the x-ray
tube. The spectrum of the beam is cut off at about half the maximum energy by
filters are placed in the beam to block low energy photons. (These photons
would otherwise increase the patient’s skin dose and could not reach the
detectors on the other side so are of no use.) The average CT beam energy is
around 70 keV, which is quite high compared to standard radiography and
fluoroscopy but much lower than 511 keV PET gamma rays. A diagram of a CT
spectrum is shown below in Figure 5:
CT PRIMARY BEAM
Tungsten anode, fixed tube current, increasing kVp
No inherent
filtration
[adapted from a PET/CT teaching article in the Journal of Nuclear Medicine Technology] Figure 5: Diagram of a CT spectrum 7
There are also two types of secondary radiation from a CT scanner:
Leakage radiation is x-rays which penetrate the metal housing of the x-ray
tube in any direction, apart from where the useful beam exits. Leakage
radiation is heavily filtered: the metal housing absorbs the low energy x-rays
and only a small proportion of the x-rays at the maximum energy are
transmitted. The amount of leakage is controlled by regulatory standards to a
very low level: the dose rate at one meter from the tube must be less than 1
mSv per hour, with the tube operating continuously at the maximum operating
voltage and current. For the patient, leakage radiation is insignificant
compared to the primary beam. For occupied areas close to the CT scanner,
leakage radiation may influence the design of structural shielding because of its
penetrating ability.
Scatter radiation occurs when x-rays in the primary beam are scattered by any
object in their path, mainly the patient and the scanner bed. Scattered radiation
is of lower energy than the primary beam. The amount of scatter radiation is
quite significant. , If there are no scattering objects in the beam, for example
during a tube warm-up run, there is not much scatter from the detectors into
the room because the photons are mostly absorbed in the gantry. In fact, with a
patient in place the scatter is not uniform in all directions because of absorption
in the gantry. The highest scatter levels are close to the gantry opening on
either side.
Manufacturers provide a scatter distribution map of radiation dose levels
around the scanner, measured with a cylindrical plastic phantom in the middle
of the gantry to represent either the head or body of a patient.
Scatter distribution map
[Biographs 2- and 64-slice]
Figure 5: Scatter distribution map of radiation dose levels around a scanner
8
Figure 6: An isodose contour map of scatter distribution gives a better visual impression of how the dose rate varies significantly around the scanner gantry, in both horizontal and vertical planes. Adding CT to radionuclide dose
Wall at head end of Biograph
μSv/h
1000
100
10
1
0.1
7:00
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
Figure 7: An illustration of the variation in dose rate inside a PET/CT room throughout a day. It was measured with a radiation survey meter located at the wall near the head end of the scanner, in line with the scanner bed. The graph clearly shows that the intensity and the duration of the PET and CT radiation components are very different. At the beginning of the day, you can see spikes of scattered CT radiation and low level PET radiation during warm‐up and calibration runs with a gallium‐68/germanium‐68 phantom. This is followed through the day by a series of a high dose rate spike during a CT scan and low dose rate from F18, for one patient after another. 9
Scattered CT radiation is the reason why staff should not be in the room during
a CT scan, unless they are wearing lead protection. PET radiation, although at a
low level, is very penetrating and is the reason why the scanner room needs
more shielding than a conventional radiology CT room.
Key points:
•
•
•
)Go To
&
The x-rays from a CT scanner are lower energy than PET gamma rays and
much higher intensity.
The primary radiation from the CT scanner is the beam of x-rays from the
rotating x-ray tube through the patient to the detectors on the opposite side.
The secondary radiation from the CT scanner is due to x-ray leakage from the
x-ray tube and x-ray scatter from objects in the primary beam.
your Workbook RadSafety section and complete questions 1 - 2
10
Time Check: Allow 3 hrs to study this section and complete exercises in your Workbook
and on-line.
Radionuclide sources in a PET center
Vials and injection systems
A few PET facilities will have a cyclotron on site but most will have to rely on a
remote cyclotron which is not too far away for delivery of FDG within 2 to 3
hours of production. If FDG supply is not restricted, a busy center can scan
about 20 patients per day per PET/CT scanner, depending on the number of
work shifts.
FDG is usually delivered to the PET center in either unit dose or multi-dose
vials, in suitable packaging in an approved transport container as shown below.
An on-site supply may use a pneumatic tube system as shown below.
Figure 8: Transport containers Figure 9: Pneumatic carrier Note the heavy lead shielding to protect the technologist taking the carrier out of the terminus and transferring the vial from the carrier to a lead pot. 11
Check for any signs of damage to the packaging before you unpack a vial.
If there is any sign of damage, tell your supervisor immediately and do not
touch it unless wearing gloves.
The vial will be labeled with the activity when measured by the supplier or at
some specified time - for example 08:00h. The vial may contain enough FDG for
one or more patients – generally from 0.5 GBq to 20 GBq. The label will also
state the volume of the FDG solution, so that you can calculate how much you
need for a patient’s injection at any time.
Vials should be kept in a small, dedicated storage and dispensing room (which
we will call a hot lab).You should be able to check the activity in the vial, using
a dose calibrator in the hot lab. Of course, the time when you measure the vial
will probably differ from the supplier’s calibration time and you will have to
apply a decay correction before you can confirm the amount. If the supplier’s
calibration and your corrected calibration do not agree within 10%, ask your
supervisor to check the true activity!
The PET technologist is usually the person who dispenses FDG solution from
the vial into a syringe for each patient. In some facilities a nurse, doctor or
radiochemist may dispense the FDG syringes. A few PET facilities may be
supplied with syringes which have already been filled with FDG and labeled
with the activity and time of calibration.
Reference:
Unit X, Module X (PET/CT Scheduling and Workflow Protocols)
The following information on dispensing is intended to reinforce the
description in PET/CT Scheduling and Workflow Protocols, so that you are
fully aware of how to give the correct patient the correct amount of PET
radiopharmaceutical, while keeping your own radiation exposure at a
reasonable level. The focus is on FDG, which accounts for the great majority
of scans in most PET centers.
How much activity?
There are differences between PET centers in how much FDG to use,
depending on factors such as the availability of FDG, the clinical workload
(numbers and types of scan requests) and the PET camera.
In all diagnostic imaging, the amount of radiation used should be ‘optimized’.
This means that the patient’s radiation dose should be kept as low as possible
while ensuring the image quality is adequate for diagnostic purposes.
•
•
Too little FDG activity means poor quality, ‘noisy’ images and possibly longer
scan acquisition times with a greater risk of patient movement.
Too much activity would expose the patient to too much radiation and could
also result in reduced image quality. ‘Too much’ activity may be unlikely, if
FDG is in short supply.
12
The latest scanners can handle higher count rates, allowing more injected
activity and shorter scan times. Rapid imaging reduces the chance of patient
movement during a scan and more patients can be scanned per day.
A PET/CT scan will expose the patient to a significant amount of
from x-rays as well as FDG. Be aware that the patient’s weight will
radiation dose from both components of a PET/CT scan. PET scan
using FDG should be specified for body scans and brain scans
patients and paediatric patients (if applicable).
radiation
affect the
protocols
for adult
Sometimes the supply of FDG is inadequate, or delays during the day have
resulted in loss of FDG by decay for the last patients on the day’s schedule. If
there is not quite enough FDG activity for a particular patient, you could ask
the PET physician if a lower amount than usual is acceptable.
Adults:
Remember that the optimum amount of radionuclide will lie in a range which
depends on your particular PET system: from about 250 to 400 MBq for BGO
detectors in 3D mode, or 300 to 550 MBq for the faster LSO and GSO detectors.
FDG activity for a whole-body scan. Two alternatives are commonly used:
-
fixed amount in the optimum range, or
-
variable amount depending on the patient’s weight, in a range from 4
MBq/kg up to 6 MBq/kg with fast PET systems.
What about very overweight patients?
Most centers have a PET/CT protocol specifically for obese patients. Images
(both PET and CT) could be too noisy if the protocol for a patient of standard
weight (70 kg) is used. It may be the technologist’s responsibility to assess
whether to use the ‘obese’ protocol for a particular patient, say a tall patient
who weighs more than 150 kg or a short patient who weighs more than 120 kg.
The amount of FDG injected can be increased for obese patients but should not
exceed the optimum range for the scanner’s PET detectors. Above the optimum
range it is better to increase the usual acquisition time to compensate for the
loss of counts by attenuation in the body.
What about a brain scan with FDG?
Most centers use a fixed amount, typically in the range of 200 to 370 MBq
regardless of body weight.
Children:
It is very important to use as little activity as you need for a good diagnostic
image. For nuclear medicine scans in general, various methods have been used
for calculating activity depending on weight, age, body surface area or other
factors. In addition, for small infants a Minimum Activity was set much higher
than the amount calculated by body weight. (The intention was to ensure good
quality images from older gamma cameras.)
13
However PET image quality is actually better for very small patients and
should require simply scaling in direct proportion to body weight, with no
need for a Minimum Activity higher than the weight-based estimate.
The calculation of dosage for children should be guided by experience at
specialist paediatric centers, where activity for both whole-body and brain PET
studies is calculated according to the child’s body weight, such as
- Whole-body scans from 4 to 6 MBq/kg
- Brain scans from 3 to 5 MBq/kg.
For example, calculate the amount of FDG required for a child who weighs 30
kg. Suppose the PET center uses 5 MBq/kg for a whole-body scan and 3
MBq/kg for a brain scan.
For a body scan, Activity required = 30 x 5 = 150 MBq of FDG
For a brain scan, Activity required = 30 x 3 = 90 MBq of FDG.
An alternative method of calculation is to scale the activity for a standard 70kg
adult in direct proportion to the child’s weight.
For example, suppose the usual activity for adults is 300 MBq for a brain scan
and 400 MBq for a whole-body, and the child weighs 30 kg:
For a body scan, Activity required = (30/70) x 400 = 170 MBq of FDG
For a brain scan, Activity required = (30/70) x 300 = 130 MBq of FDG.
What about newborn or very small infants?
In a survey (2007) of paediatric nuclear medicine practices in the USA, the
Minimum Activity for FDG body scans ranged from 18 to 74 MBq (median 37
MBq). The reviewer commented that “Nowhere is a customized approach to
dosimetry more urgently needed than in neonates and infants whose extremely
small size demands the utmost caution in optimizing minimum dosing activity
without sacrificing image quality” (Treves et al, J Nucl Med 2008).
After some years of debate, the European Association of Nuclear Medicine
(EANM) now recommends 3D acquisition, with a minimum activity of 14 MBq
FDG for whole-body scans and 10 MBq FDG for brain scans.
) Go to
your Workbook RadSafety section and complete questions 3 and then the
following exercise
EANM has a paediatric ‘Dosage Card’ on its website for various
radiopharmaceuticals, where a Baseline Activity is multiplied by a Multiplier
factor specified for different weights.
)Go to the website http://www.eanm.org/scientific_info/guidelines.
Download the pdf version of the Dosage Card and answer the following for an
FDG whole-body scan using 3D: Record the answers in your Workbook
- What is the Baseline Activity of FDG?
- What is the Multiplier for a 30 kg child?
- How much FDG is required?
- How much is the Minimum Activity?
14
•
Then follow the link to the Dosage Card Online. Using this website Calculator,
select 30 kg weight and F18-FDG (3D) Recommended for Children.
How does the answer compare with what you have just worked out?
Key Points: & 1. Calculated amounts should be rounded to the nearest 10 MBq if over 100 MBq or to nearest 5 MBq if less than 100 MBq. It is not practical or necessary to be any more precise when working out how much to inject. 2. With any calculation, the answer depends on the assumptions you start with. Therefore in the examples above there are different answers for the same 30 kg child. 3. For brain scans, the activity to be injected is varied according to the child’s weight, which is not the case with adult patients. The reduction of activity for small children is important for minimizing the radiation dose. The quality of the brain image is not compromised, because the child’s brain is a bigger fraction of the total body weight while young. Comment: As well as radiation exposure, PET scanning of children has many other special issues such as patient preparation, sedation, injection and handling which are best managed within specialist centers. Established paediatric centers should be able to provide advice and support to •
•
new centers. It is important to use less FDG for children than for adults, to reduce their radiation dose. No more than the optimum activity for the PET detectors should be used for adults and obese patients. Exceeding the optimum activity will not improve image quality but it will increase the radiation dose to the patient and staff. How accurately do you need to dispense FDG? In general, radiopharmaceuticals should be dispensed and administered to the patient within +/‐ 10% of the prescribed activity. With FDG this requires careful timing because a 10% change in activity occurs in just 16 minutes of radioactive decay. Technical Challenges: In order to draw up the right amount of activity for the scheduled time of injection, you will need to estimate the volume required at that time, just as you would for a 99mTc radiopharmaceutical. Remember that F18 decays much faster than 99mTc, so the volume should be recalculated if there are any delays or changes of more than 15 minutes to the scheduled times of injection. It is best to fill the syringe and measure the activity close to the time of injection. 15
If the vial contains enough activity for many patients in a day, the radioactive
concentration will be very high at the beginning of the day and the volume
required for the first few patients may be quite small, possibly a small fraction
of 1 mL. It takes skill to draw up such small volumes by hand and not have to
adjust the volume several times after checking the activity in the dose calibrator.
(Especially when the vial and/or syringe are heavily shielded – see later.)
Repeated attempts to draw up the right amount can increase the radiation dose
to the hands. Therefore it may be better to prescribe with a larger tolerance, for
example 350 +/- 50 MBq.
Another alternative for accurate dispensing is to use an automated system for
filling the syringe, but this may not make much difference to your hand dose if
you still have to calibrate and shield the syringe. A few commercial syringe
filling systems are available.
Special note: Dispensing small amounts for small patients:
The activity of FDG used for very small infants should not be less than about 20
MBq, depending on experience and the camera. It is important not to reduce
the FDG activity so much that you can’t acquire a good image in a reasonable
time. It is also important not to overdose the infant by inaccurate dispensing of
small amounts.
For example, suppose an adult dosage is 500 MBq, then 10 MBq more or less
won’t increase the patient’s radiation dose by more than 2% or affect the image
quality. Compare this to a situation where an infant requires 25 MBq. An extra
10 MBq would increase the effective dose from 5.7 mSv to 8 mSv, i.e. by 40% !
So accurate dispensing of very small amounts is important for image quality
and control of radiation dose to children. It can be very difficult to dispense a
small volume from a solution of high radioactive concentration.
- You may be able to obtain FDG in a more dilute form than you would
normally use when dispensing doses for adults.
- You should make sure that the measurement of the FDG syringe in the dose
calibrator is not affected by any contamination of the ion chamber. A
removable Perspex liner is useful protection against contamination.
- You should make sure that the measurement of the FDG syringe in the dose
calibrator is not affected by background from the FDG vial or other sources
such as patients or QC phantoms. Thick, well designed lead shielding
around the ion chamber will improve the accuracy and reliability of dose
measurements, as well as protecting yourself.
A technologist who is new to PET work should first practice dispensing small
volumes using inactive saline and the PET shielding system.
Spreadsheets are a convenient tool for calculating how much activity and
volume to dispense. A spreadsheet for FDG is available on the DAT training
website for you to download and experiment with. It shows how the volume to
be injected will change during the day, assuming a standard amount of FDG
activity for either an oncology whole body scan, or a neurology scan.
16
DISCLAIMER:
This spreadsheet is to be used for teaching purposes only. It is not approved
or authorized by the IAEA for the dispensing of PET radiopharmaceuticals
for real patients in clinical practice. A PET center must establish its own
methods of calculating and verifying radiopharmaceutical activity. These
methods must be part of the quality assurance program and be formally
approved by the Manager.
)Go To DAT website (RADSAF1) which has a link to the example spreadsheet for
FDG activity and doses. Then complete the answers in your Workbook
RadSafety Section for question X.
Key points: &
•
•
The activity of FDG when injected should be within a defined range of the
prescribed amount say +/- 10%, or possibly +/- 50MBq for adults.
Take special care when preparing injections for children. Confirming the correct Patient, Procedure and Injection The FDG syringe is taken from the hot lab in a suitable syringe shield to the patient who is resting in an isolation room or area set aside for injected patients. It is essential to follow correct procedure for confirming the patient’s identity, the purpose of the scan, the radiopharmaceutical and activity immediately before giving the injection. Take ‘time out’ from working in a hurry, to ask the patient to volunteer their name and another item of personal information such as date of birth or home address. If the patient has an identity wrist band and medical records, check those too. Check what type of procedure has been requested and if there are any special requirements – there have been cases where a request has been made for the wrong patient or wrong procedure. Check the activity and other information on the syringe label, particularly if someone else prepared the syringe for you. Only then should you proceed with the injection! By observing such precautions, the ‘wrong patient/wrong injection’ error rate of PET services should be extremely low. The Patient as a source of F18 radiation During and after the injection of FDG, the next source of F18 radiation to consider is the patient. It is a very good idea to prepare venous access in advance by insertion of an intravenous cannula. Review your center’s procedures for hand washing before and after every contact with a patient. The FDG is injected and the intravenous line is flushed with saline. The FDG circulates through the body and gradually accumulates in various organs and tissues. The brain, heart and renal system accumulate the highest concentrations. During this uptake phase, the patient rests to avoid excess uptake of FDG in normal muscle and brain tissues. By one hour after the injection, about 10‐20% of the injected dose will be excreted in urine. There is very little excretion after an hour because FDG stays in the 17
other tissues where it has accumulated. After the uptake period, which may range from 45 to 90 minutes at different PET centers, the patient is sent to the toilet to empty their bladder. If a patient has a urinary catheter (which is rare) the bag should be emptied of urine immediately before the PET/CT scan. The patient is then taken to the scanner room and positioned on the scanner bed. At the end of the scan, the patient is helped off the scanner bed and escorted back to the uptake room or a change room and helped to get ready to leave the PET center. Sometimes the patient may wait until the PET physician has reviewed the images, and may have to have another scan (‘delayed image’) before leaving. There is obviously a need for frequent close contact between staff and the
patient. Some or all of the contact tasks may be done by the PET technologist,
or some may be shared with others including doctors and nurses. It is possible
that the technologist may have to come in close contact with each ‘hot’ patient
at least four times, and more often if there are technical or other issues to sort
out. Dose per task has been measured at several PET facilities.
The following chart is an example of task doses measured at a PET/CT center
where adult patients only are seen, 350 MBq was the standard dose for body
and brain scans, injections were given from a shielded syringe with no
additional body shielding and the uptake period was 1 hour. Some quantitative
brain studies required two arterialized venous blood samples to be taken from
the arm not used for the FDG injection.
direct patient to
toilet
dispense/
calibrate FDG
help patient off
scan bed
unpack FDG vial
set up patient
for scan
inject FDG,
infusion pum p
40m in blood
10m in blood
inject FDG,
m anual flush
0.0
0.5
1.0
1.5
2.0
AVERAGE DOSE PER TASK (μ Sv)
Figure 8: Example of average dose per task when working with the patient. These task doses are very low, because of careful attention to minimizing dose rates and contact times wherever possible. 18
Figure 9: Example doses after administration of 350MBq The dose rate at a set distance – say 1 meter ‐ from a patient just after injection of FDG is less than it was from the same activity in the syringe before injection. This is partly because the patient’s body is a large extended source, instead of a small syringe, and partly due to attenuation of the some of the gamma photons in the patient’s body. However, the dose rates are still quite high. If you look at the localization of FDG in a whole body PET scan you can see that the dose rate around a patient on a scanner bed won’t be the same in all directions. It can be up to 300 μSv/h to someone standing beside a patient injected with 500 MBq: Dose rates @ 0.5m and 2m,
p.i. 500 MBq 18F-FDG
50
300
40
Figure 10: 180
40
10
Dose rates at post injection Dose rates near FDG patients have been measured just after injection (p.i.),
when the activity in the body was known. The values in the table below are
rather high, covering most (95th percentile) of the measurements on a series of
patients.
Table 2
Patient position;
μSv/h per MBq
where dose rate measured
at 0.5m from patient
Standing, at anterior chest
0.60
Supine, at side
0.85
Supine, at head
0.36
Supine, at feet
0.078
Benetar et al, Eur J Nucl Med 2000
19
To estimate dose to staff and others moving around at about a meter from a PET patient, it is more realistic to assume an average dose rate of 0.092 μSv/h per MBq at 1 meter in any direction from the patient. These dose rates are ‘normalised’ for activity, so you can use them to estimate dose rates from any patient by multiplying by the activity in MBq Other radioactive sources in PET centers
Various PET radiopharmaceuticals apart from FDG are used or have been used
in the past, including
- Fluorine 18 fluoride
- Nitrogen 13 ammonia
- Carbon 11 ligands and gases
- Oxygen 15 water and gases
- Rubidium 82 from a generator system containing the parent nuclide
strontium 82.
In addition, solid ‘sealed sources’ are used for transmission scanning, PET
system calibration and dose calibrator quality control. These include sodium 22,
caesium 137 and the parent/daughter pair Germanium 68/Gallium 68.
) Go To DAT website (RADSAF2) and link to XL worksheet to prepare your own
dispensing chart.
) Go To
your Workbook RadSafety Section and complete questions 5 - 6
Key points:
&
•
Most of the injected activity is still in the patient at the end of the uptake period
when the technologist (or nurse) comes in contact with the patient.
•
When the patient is lying on a bed or trolley, the dose rate is highest beside the
bed and lowest at the foot of the bed.
20
Time Check: Allow 3 hrs to study this section and complete exercises in your Workbook and
on-line.
Radiation Controls for PET sources
Containment
Prevention of contamination from liquid radioactive sources is just as
important in PET as in general nuclear medicine. In fact the positrons emitted
from F18 are similar in energy to the beta particles emitted by I131. They have a
maximum range in tissue of nearly 2 mm. A droplet of F18 solution with a
concentration of 100 MBq per mL on the skin is enough to give a dose of 500
mSv (the annual dose limit for a small area of skin) in just 6 minutes.
•
•
FDG must be kept in containers which, as far as possible, will not leak or spill.
Additional layers of containment are recommended. The usual precautions for
nuclear medicine apply to PET:
cover the work surface with a disposable absorbent sheet before dispensing or
injecting
syringes, tubing, 3-way taps etc. should have non-slip connections (e.g. Luer
lock). the luer lock connections on the disposable syringes, tubing, 3-way tap,
intravenous cannula and injection plug used to inject FDG at one PET center.
Figure 11: The luer lock connections on the disposable syringes, tubing, 3‐way tap, intravenous cannula and injection plug used to inject FDG at one PET center. Spills in PET are uncommon and are usually the result of a problem with a urinary catheter or injection line. It is important to check that an iv cannula is working correctly before you inject FDG. 21
Monitoring for surface contamination in the Hot Lab can be difficult if there is a high dose rate from sources such as the FDG vial. In that situation you can do a wipe test: wipe the surface with an alcohol swab held in tongs, and check the swab with a contamination meter in a low‐background area. A small Decontamination Kit or Trolley should be stocked with the usual decontamination supplies including disposable wipe cloths, absorbent plastic‐backed sheets for covering contaminated surfaces, tongs, swabs, plastic rubbish bags, disposable gloves, gown and overshoes, eye protection, detergent dispenser, Caution Radiation tape and labels for waste, a marker pen, scissors and a small contamination survey meter and spare batteries. Figure 12: Example kit / trolley ready for emergency decontamination Positrons from an open source of F18, for example a contaminated surface, can travel a few meters in air. The superficial dose rate (to skin or eyes) one meter away from an open 1 GBq source of F18 is nearly 11 mSv/h, something to remember when you have to clean up a spill. Key points: & •
•
•
•
•
FDG sources should be fully contained. F18 positrons are easily absorbed in
glass and plastic containers but can give a high dose rate to skin if FDG is spilt.
Check patency of iv cannula and tight connections on iv lines before giving
FDG injection
Wear disposable gloves every time you handle FDG sources and check them
for contamination before taking them off.
Keep a fully stocked decontamination kit somewhere central, for example the
Hot Lab
If cleaning up a spill, wear disposable gloves, gown and overshoes, and also
plastic goggles to protect your eyes.
22
Shielding:
Shields for PET sources are essential. The question is: how much? Lead is
commonly used to shield small sources like vials and syringes in general
nuclear medicine. But the shields used for Tc99m are inadequate for PET
radionuclides. This is because the attenuation of photons in different materials
depends very much on their energy.
Revision:
The most useful photon interaction for shielding purposes is the photoelectric
(PE) effect, where all of a photon’s energy is absorbed in a single interaction
with an atom (‘hit’). The PE effect is highly likely at low energies and in
material of high atomic number (Z). It is therefore very important in diagnostic
radiology, where the average energy of x-ray beams is less than 100 keV. It is
also quite useful in nuclear medicine for protection against 99mTc which emits
140 keV gamma rays. For example, leaded aprons with 0.5mm of lead (Z = 82)
are useful in radiology. In nuclear medicine, 2mm of lead is very effective in
shielding the 140keV photons from 99mTc in a syringe. Even a 0.5mm lead apron
can give some protection to anyone working all day close to patients having
cardiac scans with [99mTc]-mIBI.
What about lead aprons for protection of PET staff and escorts?
The heaviest aprons used in radiology are only equivalent to 0.5mm lead at low
energies. They are no use when working with PET sources (which includes
injected patients) where the photon energies are moderate, up to 511 keV.
However a lead apron (or skirt and vest, or coat) should be available in a
PET/CT center for the rare situations when a patient escort needs to stay with a
patient during the CT component of the scan.
At moderate photon energies of a few hundred keV, PE absorption is not likely.
The main process is Compton scattering, interactions with atoms in which the
photon loses some of its energy and changes direction. To be really effective, a
shield must be thick enough for most of the photons to be scattered multiple
times until the energy is low and PE absorption occurs. A simple indication of
the shielding properties of a material is the Half Value Layer (HVL). This is the
thickness which will reduce the radiation intensity to one half. The Tenth
Value Layer (TVL) is the thickness which will reduce the radiation intensity to
one tenth.
Figure 13:
Lead aprons hanging close to scanner ready for use
23
Tungsten is a very good alternative to lead for shielding PET radionuclides in
small sources like vials and syringes, because it can provide equivalent
protection with less thickness and less weight. This is because the main
interaction of the 511 keV photons in a shield is by Compton scattering, which
depends on density as well as atomic number. Tungsten shields are more
difficult to make and more expensive than lead shields.
In the following table 3, you can see the importance of photon energy by
comparing the HVL and TVL values in lead for 99mTc and F18. Also the
importance of density when comparing HVL and TVL values for lead and
tungsten.
Table 3
99mTc
F18
F18
140 keV
511 keV
511 keV
Lead
Lead
Tungsten
Atomic
number
Z = 82
Z = 82
Z = 74
Density
g/cm3
11.35
11.35
19.3
HVL
TVL
<< 1mm
5.6 mm
3.2 mm
0.9 mm
20 mm
11 mm
Two millimeters of lead will reduce the dose rate from a 99mTc source to less
than 1%, i.e. by two TVLs = 1/10th x 1/10th. For F18, the same level of shielding
would need 40 mm of lead. In fact, F18 requires more shielding than I131, and
nearly as much as a Molybdenum-99 generator. In the photos below you
should compare the shielding required for a standard nuclear medicine hot lab
bench with a PET hot lab. For example, look at the vial shields, body shield and
dose calibrator shield. Clearly the nuclear medicine hot lab would not provide
enough protection for PET.
(a) (b) Figure 14: Hot lab dispensing area and shielding for (a) nuclear medicine (b) PET. 24
There are two types of shield:
A. Localized, around a physically small source such as a syringe or vial. The
shield may be fixed in place, for example a body shield on a dispensing
bench. It can also be mobile – for example a sliding bench shield, or a
transport container, or a body shield on wheels.
B. Structural i.e. built into walls, doors and other barriers, for extended
sources such as x-ray machines or PET patients, where localized shielding is
not an option.
Localised shields for PET are clearly going to be very heavy. Their design must
take account of manual handling risks if they have to be moved or
manipulated.
•
•
•
Is there guidance on working with heavy or bulky items at your own place of
employment?
Is there a weight limit on objects which have to be picked up or carried?
Do you have to do a manual handling risk assessment before working with
new equipment?
Vial shields are available in lead or tungsten. Lead pots designed to hold vials
of FDG are typically 30 mm thick. They are too heavy to pick up and hold
upside down while filling a syringe. One option is to support the vial shield on
a strong frame which allows the vial to be inverted. Another option is to use a
long needle and tubing to draw FDG from the vial, but this method might
expose the person to more radiation. In this case, a vent needle is needed to
equalize pressure in the vial. Automated dispensing systems are also available
but can be expensive.
There are times when the vial or syringe has to be handled without its own
shield. A body shield for the unpacking/dispensing bench will provide the
necessary protection. The shield may be L-shaped, with its horizontal base on
the bench top protecting the lower half of the body, and the vertical section
protecting the upper body. Alternatively, the body shield may be a single
vertical slab of lead mounted at the edge of the bench and extending from thigh
to shoulder level. With this type of shield as shown in the example below, the
underside of the bench is also shielded. Body shields are generally made of
lead, about 60 to 70 mm thick. A window of lead glass, equivalent to about 50
mm of lead, is set at an angle in the upper section of the body shield to give a
good view of the bench area on the other side. For best visibility, the lead glass
should be a single thick piece, not a stack of thinner lead glass sheets. The area
behind the body shield must be very well lit so that the syringe markings and
any small bubbles can easily be seen. A light can be fixed above the bench or on
the top rear of the body shield, taking care not to cause distracting reflections in
the glass.
The bench, vial holder, body shield and lead glass window should be of a
suitable height so that all staff can prepare doses quickly and comfortably.
25
Here are examples of vial holders and body shields for dispensing FDG.
Figure 15: Examples of body shielding used in PET Figure 16: Examples of vial shield (solid arrows), syringe shield (dotted arrow) 26
Many different injection systems are available from commercial suppliers. Below is a summary with photos (Figure 17) of the dispensing and injecting procedure used at one PET center, where two syringe shields are used. The first is an inner shield of clear plastic to absorb any positrons and to increase the distance between the fingers and the FDG solution. There is a good view of the contents and the activity can be measured in the dose calibrator with the plastic shield on the syringe. Summary for 18F – FDG Injections Dispensing: ¾ Withdraw predetermined volume of 18F‐FDG for 350 MBq from lead shielded multidose vial into a 5 mL syringe fitted with a plastic shield. ¾ Measure 18F‐FDG activity in dose calibrator; if not within ± 50 MBq adjust volume and recalibrate. ¾ Make up to 5 mL with saline and place in mobile lead syringe shield Injecting: ¾ Wheel 18F‐FDG syringe in shield to patient ¾ Remove needle, connect 18F‐FDG syringe and a saline syringe to i.v. cannula via a 3‐way tap and extension tube. ¾ Inject 18F‐FDG; flush syringe and line with saline ¾ Measure residual 18F‐FDG in dose calibrator Adapted from: Nuclear Fields vial and syringe shields a b c d e f Figure 17: Sequence of events for dispensing and injecting After calibration, the syringe is put in an outer shield of lead or tungsten about 15 to 20 mm thick for transport to the patient. The outer shield is heavy and is usually mounted on a trolley or wheeled support. 27
A similar system is shown below, made of tungsten. The syringe shield is vertical while being moved and then turned to the horizontal for injecting. At the end of the injection, the syringe is discarded in a sharps bin inside a shielded cupboard under the hot lab bench. PET syringe transporter
shield (red arrow), syringe (white arrows)
Figure 18: Syringe is discarded in a sharps bin inside a shielded cupboard under the hot lab bench.
A shielded space, sometimes called a ‘cave’ or ‘castle’, can be constructed for
storage on a bench or floor. A waste cave on the Hot Lab bench can be used to
store ‘Sharps’ containers holding used syringes and needles. Similar shielding
may be needed for storing other sources including transmission sources and
QC phantoms for the PET scanner and a constancy check source of caesium-139
for the dose calibrator. Interlocking lead bricks 50 mm thick are useful for
building caves as they are much easier to handle than large slabs and usually
cost less. Like all lead material in the workplace, the cave should be covered to
prevent contact with skin. Stainless steel or high grade paint finishes are used
and can easily be cleaned with a soft cloth dampened with methylated spirits.
Figure 19:
(a)
Waste cave
(b)
Shielded ‘Sharps’ container
28
With all this shielding, the work bench needs to be reinforced with a metal
frame to carry the weight. A good solution for storing PET phantoms is a
dedicated shield on wheels which can be kept in the scanner room. This
minimizes manual handling and radiation exposure when doing daily QC
measurements.
(a) (b) Figure 20: Shielded phantom on wheels kept in scanner room An ion chamber shield is needed for the dose calibrator – this is done with a
stack of lead rings 50 mm thick supported on a very strong steel frame. The ion
chamber can be recessed into the bench top so that the opening is easy to reach
when putting a syringe in for measurement.
Figure 21:
An ion chamber shield is needed for the dose calibrator
Key points: &
•
•
•
•
•
Custom-made shields are required for all PET sources in the Hot Lab.
Shielding is heavy and strong mechanical support is essential
There may be a significant manual handling risk for portable sources like vials
and syringes unless properly supported on a mechanical support.
Lead should be covered so that the surface does not come in contact with skin
or become worn with use
The vial shield and syringe shields are essential for radiation protection. 29
Spares or spare parts for the dispensing and injecting systems and the dose
calibrator liner should be available at short notice in case of breakage or
contamination.
Shielding for FDG patients
The patient becomes radioactive at the moment of the injection. The person
doing the injection can minimize their dose by using both distance and time.
The alternative is to work behind a body shield, which may be part of the
mobile injection system. However this may be awkward and take longer to
give injections.
The space where the patient waits during the uptake phase is generally
enclosed in structural shielding. It may also be fitted with an internal shield,
either mobile or mounted on a wall, for the protection of carers (such as a
parent of a pediatric patient) or staff who need to attend to the patient (for
example to take a blood sample).
Body Shields
mobile (red arrow), wall mounted (blue arrow)
Figure 22: Staff body shields in injecting room and scanning room Distance and Time
Maximizing distance from FDG sources
The importance of distance in radiation protection can be seen from the
following exercises. Dose rate is proportional to the inverse square of the distance
from the source. For example, if you double the distance from a source, say from
1 meter to 2 meters, you reduce the dose rate by ½ x ½ = ¼.
30
Revision:
The Inverse Square Law (ISL) describes the relation between dose rate (R) and
distance (d) for a small source.
R is proportional to 1/d2
It can be used to calculate the dose rate at location (b) from the known dose
rate at location (a):
Rb = Ra × da2 / db2
For example, suppose you want to estimate the dose rate at a distance of 3
meters from a vial of FDG. We know the dose rate at 1 meter from a vial
containing 1 GBq of FDG is 158 μSv/h, so:
Rb = 158 × 12 / 32
= 158/9 = 18 μSv/h per GBq in the vial.
Strictly speaking, the ISL applies to small sources. However beyond a distance
of 2 meters or more, the ISL is a reasonable method for estimating dose rates
around an FDG patient. You may assume that the average dose rate from FDG
in a patient is 92 μSv/h per GBq at 1 m in any direction from the patient.
Use these standard dose rates per GBq of F18 at 1 meter from small sources and
patients in the following exercises on the Inverse Square Law.
Note that because the standard dose rates are given at 1 meter, you should also use
meters for the distance of interest in your ISL calculations.
Distance can be used for reducing exposure from injected patients in the uptake
phase. The photo below shows a large dedicated open area, well away from
occupied areas. Patients are instructed by intercom to go to the injection
window on the right, then to wait on a comfortable chair and finally to go to
the toilet on the left before walking up the room to the scanner. With this
arrangement the technologists have very little contact with ambulant patients.
It may not be suitable where patients require nursing care. It would be
important to ensure that patients are resting and quiet before and after their
FDG injection, to minimize muscle uptake and brain stimulation.
Injection widow Figure 23: 31
Key points: &
•
•
•
•
Increased distance from a source is an effective method of radiation protection.
By doubling your distance from a small source, the dose rate is reduced to one
quarter of what it was.
Vials containing FDG should never be touched or picked up by hand. Always
use tongs.
Syringes containing FDG should only be held at the plunger end. Never touch
the barrel near the FDG liquid.
When you have to talk to an FDG patient or escort them from one place to
another, try to keep at least one meter away from the patient if possible.
Minimizing contact time with FDG patients
Assuming that your PET center is equipped with suitable shielding for your
protection when dispensing, calibrating and injecting FDG, most of your
radiation dose will come from injected patients. Knowing this is the first step
towards keeping your dose “As Low As Reasonably Achievable” (ALARA).
How much is a reasonable dose?
The dose limit for occupational exposure is 20 mSv a year in most countries.
The dose limit represents a level that must not be exceeded but also should not
be reached in routine work. A reasonable aim is to keep the technologist’s
dose below 5 or 6 mSv per year. This is achieved at many busy PET centers
injecting up to 40 GBq of FDG per week, where the staff are rotated between
work rosters in PET and general nuclear medicine.
Range of technologist doses
whole
body
hands
gamma
camera
mSv/y
2-4
PET
camera
mSv/y
4-6
% of dose
limit
10-30%
20-40
40-80
4-16%
Figure 24: Example range of technologist doses If you are a female technologist, doctor, nurse, receptionist or other staff member and you become pregnant, inform your supervisor or the Radiation Safety Officer (if there is one). They will be able to review your radiation dose history or potential exposure, and tell you what precautions you should take. Your dose while you are pregnant should not exceed 1 mSv. In a PET center, your duties may have to be modified if you wish to stay at work through the pregnancy. 32
Careful planning will minimize the time that staff need to be in close contact
with patients, i.e. within 1 meter. At the time of scanning, the dose rates from
PET patients are generally higher than from diagnostic nuclear medicine
patients. Typical dose rates at 1 meter from the patient at the time of scanning
are shown below for some nuclear medicine and PET procedures.
Nuclear Medicine patients Table 4 Injection Scanned @ hrs μSv/h @ 1m post injection from patient Bone 800 MBq 99mTc 2 3 Lung V/Q 240 MBq 99mTc 0 2 mIBI 1‐day stress/rest 1500 MBq 99mTc 1 8 Lymphoma 370 MBq 67 Ga 24 5 PET 550 MBq 18F 1 30 based on data in Janssen et al. ASLM Bulletin Aug 2000 and Task Group 108 Figure 25: Typical dose rate at 1 meter from the patient Here is a list of suggested methods to eliminate, shorten or postpone close contact times. 1. Make sure that the patient has been fully prepared for the scan before the FDG is injected. This includes checking previous scans, medications, blood sugar level, whether the patient suffers from claustrophobia and may need sedation. Patients should wear loose comfortable clothing with no metal fasteners, like a track suit or hospital gown. The patient should not wear or carry anything with metal in it, including jewelry or keys, and may need to remove dentures. 2. Make sure the patient understands what they will have to do, including resting during the uptake phase, going to the toilet, getting on to and off the scanner bed, what happens during the scan, changing clothes and collecting valuables and scans before leaving. Point out where toilets, lockers, change rooms and the scanner are, and how to use the nurse call button if there is one. 3. With the patient is settled comfortably in the uptake room, set up venous access with an intravenous cannula. Attach either a saline flush syringe on an extension tube with a 3‐way tap or a saline intravenous infusion set. This will allow the FDG to be injected and the line flushed with saline as quickly as possible. It will also reduce the risk of ‘tissuing’ the FDG injection outside the vein. 4. Immediately after the FDG injection is completed, disconnect the syringe and tape the tubing to the patient’s arm. Leave the cannula where it is until much later. Removal of the cannula can be postponed until the scan is completed and the patient is preparing to leave, by which time the dose rate from the patient will be much lower. 33
5. After the uptake phase is complete, escort the patient to the toilet and then to the scanner room but without rushing the patient. 6. Get the scanner ready with the bed in position and a bedside step alongside if necessary before the patient arrives. Supports and restraints for the patient on the scanner bed should be in easy reach. Key points: &
•
Be aware of when and why you have to be in close contact with injected FDG patients. •
Do as much preparation as possible before the patient is injected. •
Speak clearly to patients, reassure them about what is going to happen and make sure they understand what they have to do. •
Discuss with others in the PET center how close contact times with injected patients can be reduced or shared or avoided. )Go to your Workbook RadSafety section and complete questions 7 – 11 which includes an exercise to estimate dose rates around your department. 34
Time Check: Allow 2 hrs to study this section and complete exercises in your Workbook.
How the clinical workload and workspace affect your exposure Radiation exposure to staff depends on the number of patients per day, the amount of FDG injected, the type of procedure and scanner. It also depends on the layout of the center. There will be big differences between PET facilities and you would expect big differences in the radiation exposure to staff. If your personal dosimeter readings are low, your radiation protection is adequate. But if the workload increases significantly, everyone should think carefully about how to minimize dose. •
If possible, Managers should use duty rosters to rotate staff between various areas including scan processing, or to general nuclear medicine as well as PET. Rosters can be effective in sharing the exposure so no one receives a much higher dose than anyone else in their group (e.g. nursing, technology, radiopharmacy). Clinical procedures: If you have plenty of FDG and a new PET/CT scanner, you could be scanning up to twenty patients a day, about twice as many compared to older scanners which used radionuclide transmission sources. •
A high inpatient workload will increase dose because of the greater amount of close contact care required. Most PET examinations are extended region scans for oncology patients. The scan can be straightforward, or there may be variations on the scan protocol which will require you to have more contact with the patient. •
What regions of the body are you asked to scan, for example are the legs and brain included? •
Can the scan be done in a single pass on your scanner, or do you need to reposition the patient? PET scans of the brain are often required. • Do you do EEGs on epilepsy patients? • Do you do quantitative brain scans with FDG, for example where arterialized venous blood samples are collected from the opposite arm to the FDG during the uptake phase? • Do you do paediatric patients? • Or scans for the planning of radiation therapy, where a flat bed is used and the patient’s skin has to be marked up for treatment fields? These procedures are likely to cause longer contact time with patients.
35
NOTE:
Some patients and scan procedures will require more close contact between the PET technologist and the patient. Other people can sometimes assist, for example a nurse who has come to the PET center with the patient. A mobile body shield in the uptake room or scanner room can be used by persons who need to closely supervise paediatric patients, for example a nurse, anaesthetist or parent. Each PET/CT scanner will have different features for handling patients. This will affect how long it takes to set up the patient for the scan. Here are a few points to think about. You may have some ideas for doing tasks efficiently. Can the scanner bed be lowered to about chair height so that the patient can easily get on and off? Some patients will not be able to get on the scanner bed without assistance. A bedside step with a handrail is essential. For fast and safe transfers of inpatients between trolley or bed and the scanner, use a slide board and several staff to assist. When you have to scan a patient who cannot walk; •
Is there good access for the trolley to be wheeled in alongside the scanner bed? •
And can you organize a quick team effort with other staff to transfer the patient to and from the scanner? •
When you set up a patient, •
Where will you position the arms? •
What support is there for the head? •
How will the patient be restrained from moving? •
Where do you have to stand to operate the bed control? You might be able to use the gantry as a shield between yourself and the patient. Or you might be able to use a mobile body shield. When the scan has finished and the patient has to be taken off the bed, remember that patients can feel dizzy when they first sit up, especially if they are elderly, frail or unwell. You must be near enough to provide support and prevent any falls.
Key points: &
•
•
•
•
Technologists and nurses generally get more radiation exposure from
inpatients and frail elderly patients than from outpatients who don’t need
physical support
Be well prepared for close contact tasks, especially nursing care, blood
sampling, EEG monitoring.
Nurse escorts and other carers who accompany the patient may be asked to
help with some tasks.
Staff rosters between PET and general nuclear medicine are useful to share
doses evenly.
36
Workspace: The layout of the department is important. If there are 20 patients per scanner per day, there could be three or four individual uptake rooms or one large uptake area, a toilet and possibly lockers and a change room. These rooms should be grouped near the scanner so that injected patients do not have to move very far. A central location for the Control Room and the PET Hot Lab is convenient for technologists. Here is a workflow diagram showing the movements of PET patients (solid lines) and technologists (dotted lines) in a PET center where the technologists wait in the Control Room between doing injections and scanning patients. Figure 26: Diagram of PET department design The layout of the center should also minimize the number of times that any staff member is near a patient, for example when using internal corridors. Lockers and change cubicles should be well separated from areas used by staff. The dose rate in a corridor used by patients walking between the uptake room, toilet and scanner room is shown below. Contacts are brief but numerous. 37
Figure 27:
The ‘spikes’ indicate increased recordings of dose rates from
injected patients using a PET department corridor during the day.
Aids for patients to move around the center without assistance should be considered. Are doors well signposted? Are there handrails in the corridors? Do chairs have armrests so that patients can get up unaided? Is an intercom system used to instruct patients? Is CCTV used to monitor patients? How long until patients leave the PET center after the end of their scan? If they have to wait for their scan results, is there a separate waiting area for ‘hot’ patients? ) Go To your Workbook RadSafety section and complete question 12 which requires you to draw a floor plan of your PET department, describe the shielding provided and other activities. Shielding of patient areas and the scanner room are required. During the uptake phase, patients rest quietly in individual rooms at most PET centers. The walls of the uptake rooms may need as much as 20 mm of lead or 20 cm of concrete if there are occupied areas adjacent to the patient at a close distance. Shielding design is complex and will not be considered here. It is generally better to use distance to isolate patients, to reduce the need for expensive shielding. 38
Figure 28: Comfortable seating in an uptake room If patients are in a shared area during the uptake phase, they should be separated by shields so that staff preparing and injecting one patient are not exposed to other injected patients at the same time. After the scan patients may return to a change room for removal of the intravenous cannula and assistance with dressing. Before leaving the PET center, they may also be given a light snack, wait for their scan to be reviewed and a copy to be put on CD for them to take to their own doctor. Although each patient may spend only a few minutes in the change room compared to an hour in the uptake room, the frequent use of the change room by many patients may require shielding. The figure below shows the dose rate in the room adjacent to a change room, on a day where it was used by 13 FDG patients. Time of day
Figure 29: Dose rate in room adjacent to change room 39
Shielding for the scanner room protects against both PET and CT radiation. The scanner room doors (wide doors to corridor or waiting area for patients and trolleys, standard door to control room for technologists) should be shielded with lead just like any diagnostic CT room in a radiology department. The ‘Caution X‐Ray On’ warning light should be clearly visible to anyone approaching the doors. The main doors to the scanner room from the corridor should be lockable from the inside, to prevent interruptions. The doors should not be interlocked to the operation of the CT tube. Otherwise, if someone opens the door by mistake the scan would be aborted and the patient would then have to have another scan and additional radiation dose. Figure 30: Door of CT room with warning sign and warning lights Technologists will spend a lot of time in the Control Room next to the scanner, where the dose rate should always be very low. The door between the control room and the scanner should be lead‐lined as in a Radiology CT scan room, and should be kept closed while scanning. An example of a potential unnecessary dose to radiographers if they stand in the open doorway to a CT room is shown below. Figure 31: Example of dose to radiographer standing in open CT doorway 40
The radiation regulatory authority may permit the use of closed circuit TV
cameras for observing the patient and the interior of the scanner room from the
control room. CTV is useful for monitoring patients in multiple uptake rooms,
as shown on this display in a Control Room.
Figure 32: CCTV monitoring of several rooms Otherwise, a leaded glass window is suitable if there is a large distance
between the control room and the PET gantry and the lead equivalence of the
glass is specified at 511 keV as well as for scattered x-rays at 100 keV or less.
(Shielding design for walls, floor and ceiling is a specialized topic which is not
covered in this Unit.)
In many countries, the regulatory authority for radiation will classify areas
where radioactive materials are used as ‘controlled radiation areas’. In these
areas, access by members of the public is restricted unless they are
accompanied by an authorized person and are there on a short visit only. The
PET Hot Lab, the Scanner room and any laboratory area for counting blood
samples should be regarded as controlled radiation areas with restricted
public access. These areas will have a Caution Radiation sign on the door.
Key points: &
•
•
•
•
Uptake rooms, toilets, change cubicles and post-scan waiting areas should be
separated from staff areas by shielding and/or distance.
Radiation dose within the Control Room should be less than 1 mSv per year so
that staff exposure is mainly due to necessary contacts with patients.
Rooms and corridors within the PET center should be arranged to minimize
the distances moved by patients and contact between patients and staff.
Signs, handrails, chairs etc should be designed to allow ambulant patients to
move easily from one room to the next without close contact with staff.
41
Time Check: Allow 3 hrs to study this section and complete exercises in your Workbook and
on-line.
How to measure radiation in PET/CT environments
Dose to PET center staff must be measured with a personal dosimeter which
can detect radiation using film, TLD (thermoluminescent dosimeter) or OSL
(optically stimulated luminescence) dosimeter. ‘Whole body’ dose should be
measured by a dosimeter worn midline at chest or waist level. ‘Extremity’ dose
for anyone who dispenses or injects FDG should be measured by a dosimeter in
a ring worn on the finger, or possibly on a wrist band. Ring badges are shown
below, they should be worn under gloves on the dominant hand.
(a)
Figure 33:
(b)
a) finger-ring TLD badges
b) badges worn under gloves
Many nuclear medicine facilities change dosimeters every three months. In a
busy PET center, it might be better to change them every month or two. It is
important for everyone to change their dosimeters promptly so that the
manager can return them immediately for processing and follow up with an
investigation of any high results. So what is a high result? It will vary between
facilities and countries, but a good guide is
- less than 0.5 mSv per month whole body dose, and
- less than 15 mSv per month extremity dose.
These levels can be used as ‘investigation levels’ for following up the work
practices or other possible causes for a high personal dosimeter result.
)Go to
DAT website, (RADSAF3) you can look at two studies of staff exposure at a
PET center where increasing workload after installation of PET/CT was
associated with a rise in the number of dose reports above investigation levels.
You will see from this guidance that a staff member could receive a dose of 1
mSv from two months’ work in a busy PET center. This is significant because
women who become pregnant are then subject to a lower dose limit, to ensure
that the dose to the fetus is within the limit for a member of the public, i.e. It
does not exceed one millisievert.
42
An electronic personal dosimeter (EPD) is useful for keeping track of radiation
dose over a shorter time frame, for example microsieverts each day. It is
educational for new staff to be able to see how much dose they get when
working on various rosters or as they gain experience and improve their
technique. An EPD is also useful for monitoring other health care workers such
as anaesthetists or nurse escorts who come to the PET center occasionally. The
displayed dose will allow you to reassure them immediately by comparing
their exposure to the legal limit for a member of the public (1 mSv/year) and
‘background’ radiation from natural sources (about 2 mSv/year). Take care to
note whether a dose is displayed in millisieverts or microsieverts when you
record the results!
Dose and dose rate information can be downloaded to a computer from the
more sophisticated EPDs and survey meters, which can give valuable
information about task doses. Below are the dose rate records for one
technologist doing FDG injections all day, and another technologist directing
patients around the center and setting them up on the scanner. Dose is received
in numerous short exposures from contact with patients. At other times, when
the technologists were in the Scanner Control Room, their exposure was
negligible.
Figure 34a. Dose rates for technologist administering injections 43
Figure 34b: Dose rates for technologist attending patients and set up. Find out the annual dose limits in your country for effective dose (measured with a body dosimeter) and extremity dose (measured with a ring or wrist band dosimeter). What is the dose limit for a pregnant woman working with radiation? ) Go to your Workbook Radsafety section question 13, record your dose limits and complete the questions and calculations.
Every PET center should have a radiation survey meter to measure the dose rate from F18 photons and to check for x‐ray leakage from the CT room. After a CT machine is installed, a careful radiation survey should be done in the surrounding rooms while the scanner is operating at high mAs and with a body phantom in the gantry. The survey should pick up any unsuspected penetrations or gaps in the walls and shielding, for example from air conditioning, water pipes or electrical/communications cabling. 44
A contamination meter is needed to check gloves and clothing and any suspected spills of FDG. A good place to install it is at the entrance to the PET dispensing area. Figure 35: Contamination monitor at entrance to hot lab For surveys of radiation dose rate, a pressurized ion chamber detector will give accurate readings of low level radiation at any energy, but it does not respond quickly to changes. It is not suitable for measuring surface contamination. A suitable alternative is a Geiger‐Muller (GM) counter with an energy‐compensated detector. Most GM instruments are calibrated with cesium‐137 (662 keV gamma rays) so the accuracy is good for 511 keV photons. When surveying around a CT room for scattered x‐ray photons in the range from 50 – 100 keV, the accuracy of the GM counter is not as good. However the sensitivity should be sufficient to check for gaps in shielding within the walls and doors of the scanner room, which is more important. If the GM survey meter has a large area detector with a thin window, it is sensitive to positrons as well as photons so it can be used both as a survey meter and a contamination meter. A survey meter with a proportional counter is also suitable for surveys of dose rate and contamination, but is much more expensive than a GM survey meter. A meter with an audible clicking sound is useful for doing surveys, but it should be possible to turn the noise off when there are patients nearby. Other instruments using scintillation counters (such as sodium iodide or plastic scintillators) for contamination surveys are not suitable for dose rate surveys. ) Go to
your Workbook Radsafety section questions 14 and complete the exercise. You are required to record dose rates whilst performing duties around your
department and to record and comment on activity from patients, time, status
of patient etc.
45
Check with your supervisor if your center has a survey meter (with either a Geiger‐Muller detector or an ion chamber detector) which you may use for this exercise to measure dose rates in your department. (if you work in a PET center) Some survey meters are able to log the dose rate in small time intervals and the information can be downloaded to a computer for viewing. This type of instrument is very useful for detailed personnel monitoring, for example to investigate the exposure to PET technologists performing different tasks. Here is an example of the record for a technologist doing FDG injections and other tasks, which were logged on a worksheet during the day. The dose rate was measured in intervals of 32 seconds. From a large number of such records, it was possible to measure the average dose per task. Time of day Figure 36: Log of dose rates and specific activities of a technologist administrating FDG injections. A data‐logging survey meter is also useful for detailed area monitoring. You have already seen several graphed examples of area surveys in a PET center. A very good long‐term monitor for staff areas like the Hot Lab, Control Room, Reporting Room and nurses’ desk is a personal dosimeter fixed at occupied locations, and processed after 3 months. The results will show how effective the shielding is and reveal any long term trends. 46
The dose calibrator is a stable, accurate ion chamber used for measuring radionuclide activity. The ion chamber should have a clear Perspex liner to protect it from contamination. A Quality Assurance program is recommended: •
The constancy of the dose calibrator on all radionuclide settings should be checked daily by its response to a long‐lived sealed source such as caesium‐137. The results should be logged and the checked each day for inconsistencies. •
The accuracy of all nuclear medicine dose calibrators, including PET, should be checked every year or two years with a reference source or set of sources. It is unlikely that a reference source of F18 approved by the national authority would be available. Other approved reference sources such as 99mTc and 131I should be used. •
The linearity of the dose calibrator over the full activity range (say from 20 GBq down to 1 MBq) should be tested every year or two years by the decay of a short‐lived radionuclide (preferably a PET radionuclide, otherwise 99mTc). Note: The linearity method which uses a set of ‘attenuation sleeves’ designed for 99mTc is not suitable with higher energy radionuclides as in PET. •
With 511keV photons, no corrections are required for the source container (either vial or syringe) or syringe volume. •
If the calibrator has to be repaired, the accuracy should be confirmed by the manufacturer or approved agent before it is used again. Key Points: &
•
•
•
•
All staff should wear a personal radiation monitor on the body, changed every
one to three months. They should be informed of the results and know how
their exposure compares to legal dose limits.
Staff who prepare and inject PET radiopharmaceuticals should also wear a
dosimeter on the hand or finger.
Every PET center should have a portable survey/contamination meter to check
dose rates in occupied areas and contamination on surfaces especially the
hands.
The PET dose calibrator should have a clear Perspex liner in the ion chamber to
protect it from contamination, and heavy lead shielding around the ion
chamber to protect staff while measuring PET vials and syringes.
47
Time Check: Allow 2 hrs to study this section and complete exercises in your Workbook.
Radiation safety for the patient The radiation dose from PET/CT (and SPECT/CT) procedures is greater than that for most routine nuclear medicine procedures, but is still considered to be justified because of the value of these scans for the patient’s clinical management. Another Unit in your course will cover how radiation dose is calculated for PET/CT procedures. Here we will look at how the technologist ensures the patient does not get more dose than intended for an adequate study. •
•
•
•
What unit of Absorbed Dose for organs and tissues is used in your country? What does Effective Dose tell you? What is the unit of Effective Dose? Approximately how much radiation do people get each year from natural ‘background’ radiation? ) Go to your Workbook Radsafety section question 15 and answer the set of questions on General Issues, Image Quality and Quality Control and Misadministrations. ) Go to DAT website (RADSAF4) and then use the link to the International Atomic Energy Agency (IAEA) website. Refer to your Workbook question 16 and follow the guide to review all links. There is much useful information on the IAEA website on Radiological Protection of Patients. Note: Now that you are familiar with the basic radiation issues for nuclear medicine patients, you can apply it to PET/CT imaging. ¾ FDG component: The radiation dose depends on the amount of activity injected. The effective dose is about 10 mSv from 500 MBq FDG for a ‘standard adult’ weighing 70 kg. That’s a similar dose to various Tc99m scans using about 1 GBq of activity. Occasionally a radiopharmaceutical may be injected by mistake into local tissue instead of a vein, which is sometimes called ‘extravasation’. For example if a patient has difficult veins for injections, or if the tip of a cannula in the vein is dislodged, some or all of the injection may be injected into surrounding tissues. If an entire FDG dose is extravasated (‘tissued’) at the time of injection, FDG may be taken up in local tissue and will not spread to the rest of the body. Suppose all of a 5 mL injection of 200 MBq FDG was distributed in 5 g of subcutaneous tissue, the absorbed dose of radiation would be over 5 Gy and could cause temporary reddening of the skin (erythema). ¾ CT component: The radiation dose will depend on the purpose of the CT scan. If it is to be used as a diagnostic CT for reporting by a radiologist, then the scan factors which are used in Radiology practices should be used to give high quality images. 48
However, if the CT scan is to be used only for attenuation correction and localizing FDG uptake, customized low‐dose protocols should be used. It is possible to reduce the CT dose significantly (up to 50% or more) with such protocols. (i)
(ii)
PET/CT Scheduling and Workflow Protocols Module X Unit X, the section on Radiation Safety Issues Specific to PET/CT describes how CT dose depends on many factors and how it can be estimated. The dose to the internal organs and tissues of the patient is from a combination of exposure from: the primary beam (only about 3% of the beam exits the patient and strikes the detectors, the rest is absorbed) and scattered radiation both within the primary beam and up to about 10 cm on either side of the beam. Because the radiation beam is directed from all angles around the body, tissue dose is higher around the edge of the body (and head) than in the centre. You should be aware that the breasts, lens of the eye, thyroid and testes are radiosensitive. They will get a relatively high dose from a CT scan because they are superficial organs. In diagnostic radiology, it is possible to angle the head or gantry to avoid exposing the eyes but this may not be possible with PET/CT scanners. It is also possible to use shields to protect superficial organs like the eyes, thyroid and breasts in diagnostic radiology, but such shields would interfere with attenuation correction in PET/CT. In recent years a lot of effort has gone into reducing CT exposure, especially for children. Age‐ and weight‐based protocols are now widely available. Manufacturers have developed features such as Automatic Exposure Control (AEC), which varies the radiation exposure by continuously adjusting the x‐ray tube output according to the size and shape of the scanned region. AEC is valuable because the overall aim is dose reduction while maintaining good image quality. However in some regions of the body AEC can increase dose compared to a fixed mA protocol. The scanner must display dose information on the screen at the conclusion of each CT scan – usually the CTDIvol and the Dose Length Product (DLP), as explained in the CT Unit of this course. For general guidance on dose, professional organizations have recommended Diagnostic Reference Levels (DRLs) for common procedures in radiology and nuclear medicine. However there are no DRLs yet for combined PET/CT procedures. Pregnancy Caution is necessary when deciding if a woman who is or may be pregnant should be exposed to radiation for medical purposes. It is important to know if a female patient is pregnant when she arrives for a PET/CT scan. Signs should be prominently displayed in the PET center. Multiple languages may be needed. 49
IF IT IS POSSIBLE THAT YOU MIGHT BE PREGNANT, PLEASE INFORM OUR STAFF BEFORE YOU HAVE YOUR INJECTION FOR YOUR PET SCAN Figure 37: Displaying a pregnancy caution sign in multiple languages It is also important for women of child-bearing age to be interviewed to
confirm whether they could be pregnant or not.
PET/CT is not a ‘low dose’ procedure. The radiation dose to an embryo/fetus
from a PET/CT scan depends on many factors. FDG crosses the placenta, so the
fetus will be exposed from its own uptake of FDG as well as FDG in the
mother’s body. The dose to an embryo/fetus from a PET/CT scan could be
anywhere in a range from 5mSv to 40 mSv, depending on the stage of the
pregnancy, the injected activity of FDG, the CT scan protocol and the scanned
region. The fetal dose will almost always exceed 1 mSv, the limit for a ‘member
of the public’.
However in some cases a PET/CT scan may provide essential information for
the care of the mother and baby so it should not be ruled out automatically in
pregnancy.
Pregnancy is not always recognized before a woman has a nuclear medicine
scan or an x-ray of the abdomen. ‘Child-bearing age’ is debatable; it is usually
assumed to be from 12 to 50 years of age. Pregnancy is very unlikely or
impossible if a woman has
- had a hysterectomy;
- had a menstrual period in the last 10 days and normally
has regular periods;
- had a tubal ligation more than three months previously
50
and has used other means of contraception for that period;
- not had a sexual relationship for several months; or
- been taking contraception measures without fail.
In all other cases, pregnancy should be regarded as uncertain.
1. Patients not known to be pregnant: The PET doctor should interview the patient. If there is doubt about a pregnancy, the PET doctor should decide whether to perform a pregnancy test (urinary or serum β‐HCG) to confirm the patient is not pregnant, or to postpone the PET study till after the next menstrual period, or to proceed with the study. If a β‐HCG test is equivocal, the scan should be postponed for a few days and the test should be repeated. 2. Pregnant patients: If the PET/CT scan is thought to be justified on medical grounds, minimize the fetal dose by using a Low‐dose CT protocol, especially if the uterus is in or near the region of the body to be scanned. It may also be possible to reduce the amount of FDG injected. By careful choice of FDG activity and scan factors, an acceptable quality body scan should be achieved with a fetal dose of less than 20 mSv. For brain PET/CT studies, fetal dose is virtually all due to FDG and could be as low as 5 or 6 mSv. If there are no useful alternative tests such as MRI or ultrasound which don’t use radiation, the question to ask is: Could the result of the proposed PET/CT scan influence how this patient will be managed? For example, the scan below is of a patient with a large mediastinal mass confirmed as Hodgkins lymphoma when she was 5 months pregnant. The patient had 350 MBq of FDG with a low‐dose CT scan, but no diagnostic CT procedure in Radiology. The PET study was used by her physician to recommend conservative management until the baby could be delivered. The radiation dose to the fetus was 12 mSv, which is below the threshold for neurological or other damage, and has minimal risk of subsequent childhood cancer. 51
Anterior and posterior coronal
sections through uterus
Image showing large mediastinal mass confirmed as Hodgkins lymphoma when the patient was 5 months pregnant Figure 38: There is no need to consider termination of a pregnancy after a PET/CT scan because the radiation dose is too low to harm the development of a fetus. Radiation exposure of a fetus may slightly increase the risk of cancer later in life, as it does with children and adults. However below dose levels of about 50 mSv, the risk is thought to be too small to detect. Lactating patients: Occasionally a scan is requested for a woman who is lactating. It is important to know if a patient is breast‐feeding an infant or small child when she arrives for a PET/CT scan. Signs should be prominently displayed in the PET center: IF
YOU ARE BREAST-FEEDING, PLEASE INFORM
OUR STAFF BEFORE YOU HAVE YOUR INJECTION
FOR YOUR PET SCAN
52
Figure 39: Displaying a breast feeding caution sign in multiple languages Scans show that there is avid uptake of FDG in lactating breast tissue. The dose rate measured on the mother’s chest after the scan will be quite high. However if the milk is expressed and the radioactivity is measured in a gamma counter, almost no radioactivity is found in the milk. The mother should arrange for someone else to bottle‐feed the baby for about six hours after her injection, so that the baby will get very little radiation exposure. After her scan, the mother can express and discard the breast milk or save breast milk for later if she wishes. •
•
•
•
Points to remember regarding lactating patients: FDG accumulates in breast tissue during lactation Small amounts of 18F is secreted in milk Breast feeding should be suspended for 6 hours after injection Close contact dose to an infant on the chest for 30 minutes measured 150 μSv at 3 hours after 480 MBq injection. Pediatric patients: If your PET center does scans of children, you should know its procedure for calculating the amount of FDG to use for children according to their weight. It is also important to use weight‐based low‐dose CT protocols for children, as you will find in the CT Unit of this course. PET/CT protocols for children should already be set up on your scanner. Although many pediatric patients sent for PET/CT scans already have cancer, they may need many follow‐up scans over time so minimizing radiation dose is still very important. For these patients the FDG PET/CT scan is better than other nuclear medicine procedures, giving the best information and with a lower dose than scans using gallium‐67, thallium‐201 or conventional CT. 53
Key points: &
•
•
•
•
•
Double check the patient’s identity and the reason for the scan before you start. Use the correct amount of FDG for the patient’s age and do not exceed the optimum for the scanner. Select the appropriate CT scan protocol for the patient’s age, weight and scanned region Set the scan limits carefully Let your supervisor know if there is a problem with the CT scan which might require a repeat scan. Advice for the patient, their family and carers. Patients will ask about the radiation dose from their PET/CT scan: the answer is that it is similar to having a couple of routine diagnostic procedures in radiology and nuclear medicine. They may understand that PET/CT is a very expensive technology which is not widely available, so it is unlikely to be used without very good justification. The PET center doctor should be able to reassure them that the small risk is balanced by the benefit of the information provided by the scan. Patients may also want to know how their scan might affect their family. This is where the short half life of F18 is useful: the radiation level falls so quickly that there is no need for special precautions when the patient leaves the PET center. If a pregnant woman – say a nurse or family member ‐ arrives to escort the patient home, there will be a small (not dangerous) dose to the fetus which can be minimized by keeping at least a meter distance from the patient for an hour or two. But the PET center is not a suitable place for infants, children and pregnant women to spend several hours waiting for a patient. Key points: & •
•
•
•
At the time of booking a scan, the patient or their carer should be told that infants, children and pregnant women should not accompany them to or from the PET center. There is no need for special precautions for nurses, family and other carers after the patient leaves the PET center, because of the short half life of FDG Lactating patients should not breast feed or bottle feed the baby for about six hours post injection. Patients may go home on public transport. Pregnant nurses should not routinely escort patients to and from PET center, but there is no harm done if they arrive without knowing that PET patients are radioactive. ) Go to your Workbook RadSafety section question 17 where you are required to prepare short written answers to the following: ‐ Patient protection ‐ Protection of children, pregnant woman, breast feeding mothers and accompanying persons ‐ Protection of the public ‐ Staff protection 54
Some of the questions have not been fully covered in this teaching Unit. Then use the link to the IAEA website again (follow the links to Other Modalities) to fill in the gaps in your knowledge.
55
Conclusion
Safety guidelines for technologists operating a PET/CT scanner:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Wear the personal radiation dosimeters issued to you and return them on time
for processing.
Do not leave your radiation dosimeter in the scanner room, hot lab, uptake
room or any other area used by PET patients.
If you are a female technologist and you become pregnant, inform your
supervisor or the Radiation Safety Officer (if there is one). They will review
your radiation dose history and tell you what precautions you should take.
Identify each patient correctly before giving an FDG injection or CT scan. Ask
the patient to volunteer two items of information such as name, age, address. If
a patient is wearing an identity wrist band, check the information on the band.
For every female patient of child-bearing age, confirm that the patient is not
pregnant before you give the FDG injection or do the CT scan. This can be done
by checking the information recorded by the PET doctor who interviews the
patient. If there is any doubt, inform the doctor and wait for instructions.
Check that all metal items have been removed from the patient.
Explain the whole PET/CT procedure. Answer any questions or concerns from
the patient.
Give the injection using shielding, while maximizing distance and minimizing
time to reduce your exposure
At end of uptake phase, direct the patient to the toilet and then to scanner room
while maximizing distance and minimizing time to reduce your exposure
When positioning the patient on the scanner bed, make sure there are no
trailing intravenous lines, bedding, clothing or anything else which could be
caught when the bed moves through the gantry
Before starting the CT scan, check that there is no one in the room except the
patient and that the scan room doors are closed.
Keep a lead apron (0.5mm lead equivalence) on a hanger in the scanner room.
On the rare occasion that a medical escort has to stay with a patient during the
CT scan, that person should wear the apron and stand where the gantry will
shield them from scattered radiation.
Use only defined CT and PET protocols as instructed. The protocols should be
set up with the manufacturer’s advice and medical physics support, to
minimize the patient’s radiation dose while maintaining good image quality.
Select the appropriate protocol for the scan region and the patient’s body size.
Take particular care with handling and protocol when scanning paediatric
patients.
Use the intercom to communicate with the patient, speak clearly to reduce the
risk of the patient moving during the scan.
Stop the scan immediately if a medical emergency happens.
Inform the PET physician if you have to repeat a CT scan on a patient because
of operator error, patient movement, machine or computer malfunction or any
other reason. Record the repeated CT exposure in the patient's file.
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•
Notify the medical physicist and/or manufacturer immediately about any
problem or failure of the scanner or operating system.
If the ‘Caution X-Rays’ warning light outside the scan room door is not
working when a CT scan is in progress, tell your supervisor so it can be fixed
immediately.
If the patient is incontinent while on the scanner bed, get help from the
physicist, radiation safety advisor or other PET staff member while you clean
up the contamination.
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Glossary of words and expressions
Absorbed dose Activity Effective Dose
Equivalent dose
Geiger‐Mueller counter Half life, T1/2 : radioactive or physical Positron Radiation weighting factor Radioactivity Rem Energy imparted by ionising radiations to unit mass of matter. The SI unit of absorbed dose is the gray (Gy), 1 Gy = 1 J/kg. The non‐SI unit was the rad. 1 Gy ≡ 100 rad
The number of nuclear transformations or disintegrations occurring in a quantity of radioactive material per unit time. The SI unit is the becquerel (Bq) which is one disintegration per second. The non‐SI unit was the curie (Ci). e.g. 37 MBq = 1 mCi; 1 MBq = 27 μCi.
The sum of the equivalent doses in all tissues of the body from a particular exposure, weighted by a tissue weighting factor, according to tissue radiosensitivity. Gives an indication of the overall risk of an exposure, independent of the part of the body exposed. The unit is the sievert, 1 Sv = 1 J/kg. The non‐SI unit was the rem. 1 Sv ≡ 100 rem. A modified version of absorbed dose, weighted by a radiation weighting factor, to take into account the biological impact of the type of radiation concerned. The unit is the sievert (Sv). The non‐SI unit was the rem. 1 Sv ≡ 100 rem. For the x‐, gamma and beta radiations used in medicine, equivalent dose in sieverts and absorbed dose in grays are numerically equal and either can be used for tissue doses. A radiation instrument. It consists of a gas‐filled tube containing electrodes, between which there is a high voltage but no current flowing. When ionizing radiation passes through the tube, ions are created in the gas and travel to the electrodes causing a short, intense pulse of current. The number of pulses per second is a measure of the intensity of the radiation. Sometimes called a Geiger counter, or a GM counter. The thickness of a material which reduces the intensity of an x‐ray or gamma ray beam to one half its original value. HVL = 0.693 x µ
approximately, where μ is the linear attenuation coefficient.
A positively charged electron emitted by certain radionuclides. It is unstable, and in combination with an electron it undergoes annihilation to form annihilation radiation. A multiplying factor used to weight the absorbed dose from a given type of radiation to obtain the equivalent dose. Radiation weighting factors range from 1 for electrons and photons to 20 for alpha particles and some neutron energies. The spontaneous disintegration of the nucleus of an unstable radionuclide, generally emitting alpha or beta particles and often accompanied by x‐ or gamma rays. The old unit for equivalent dose. 1 rem = 100 ergs/g 1 rem ≡ 10‐2 Sv = 10 mSv 58
SI units
Sievert (Sv) Specific activity Thermo‐luminescent dosimeter (TLD) The International System of Units. The SI units of activity and absorbed dose, the becquerel and the gray, were adopted in 1975. The old units, the curie and the rad, together with the roentgen and the rem, should no longer be used. The SI unit of equivalent dose and effective dose. It has the same dimensions as the gray, i.e. 1 Sv = 1 J/kg. 1 Sv ≡ 100 rem. Total radioactivity of a given nuclide per gram of a compound, element or radioactive nuclide. A semi‐conductor radiation detector which is capable of storing a fraction of the absorbed ionising radiation and releasing this energy in the form of light when heated at a later time. The amount of light released is a measure of radiation exposure. For example, CaSO4:Dy for whole body TLD badges, and LiF:Mg,Ti for extremity badges
.
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