Download Radiation Dosimetry of the Patient – Chapter 24, Bushberg

Radiation Dosimetry of the Patient
Robert L. Metzger, Ph.D.
1. Dosimetry

Radiation dosimetry is primarily of interest because radiation
dose quantities serve as indicators of the risk of biologic
damage to the patient

The biologic effects of radiation can be classified as either
deterministic (non-stochastic) or stochastic

Deterministic or non-stochastic effects are believed to be
caused by cell killing
 if a sufficient number of cells in an organ or tissue are killed,
its function can be impaired
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1. Dosimetry

Deterministic or non-stochastic effects
 effects include terratogenic effects to the embryo or fetus,
skin damage and cataracts
 a threshold can be defined below which the effect will not
occur
 for doses greater than the threshold dose, the severity of the
effect increases with the dose

to assess the likelihood of a deterministic effect on an organ
from an imaging procedure, the dose to that organ is
estimated
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1. Dosimetry


A stochastic effect is caused by damage to a cell that produces
genetically transformed but reproductively viable descendants
 cancer and hereditary effects of radiation
 probability of a stochastic effect, instead of its severity increases
with dose
 No dose thresholds below which the effects cannot occur
The NRC’s radiation dose limits described in Chapter 23 are intended
to limit the risks of stochastic effects and to prevent the non-stochastic
effects
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1. Dosimetry

Entrance Skin Exposure
 The radiation exposure incident on a patient is the entrance skin
exposure
 Skin doses are easy to measure but they are poor indicators of
patient risk
 They do not take into account the exposed area, penetrating
power of the x-ray beam, or the radiosensitivity of the exposed
region

At diagnostic energies, the f-factor (roentgen-to-rad) conversion is
close to 1.0 so that dose is numerically equal to exposure
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1. Dosimetry

Dose-Area Product (DAP)
 Product of patient entrance skin exposure and cross-sectional
area of the x-ray beam (exposed area)
 Units are in mGy-cm2 or mrad-cm2
 Used in fluoroscopy
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1. Dosimetry

Radiation Dose
 Radiation dose is defined as the absorbed energy per unit mass but
this says nothing about the total mass of tissue exposed and the
distribution of the absorbed energy
 Would you prefer to receive a dose of 10 mGy to the whole body or
20 mGy to the finger?
 The 10 mGy whole body dose represents about 1,000 times the
ionizing energy absorbed for a 70-kg person with a 35 g finger
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1. Dosimetry

Imparted energy
 the total amount of energy deposited in matter is called the
imparted energy (Joules), is the product of the dose (Gray)
and the mass (Kg) over which the energy is imparted
 assume each 1-cm slice of a head CT scan delivers a 30 mGy
dose to the tissue in the slice
 If the scan covers 15 cm, the dose is still the same, however
the imparted energy is approx. 15 times that of a single slice
(you also have to consider scatter from adjacent slices, about
10-25%)
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1. Dosimetry

The disadvantage of imparted energy is that it does not account
for the different sensitivities of the exposed tissue to biologic
damage

Effective dose is used for comparing risk of stochastic effects
 E (Sv) = wT x HT
 has shortcomings, wT were developed from epidemiologic
data and incorporate significant uncertainties
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1. Dosimetry

Organ Doses
 It is possible to estimate organ doses from a given entrance
skin exposure (ESE)
 Organ doses are substantially lower than skin dose
 For AP projections, the embryo dose will be between 1/3rd and
1/4th the ESE (in the direct beam)
 For PA projections, the embryo dose will be about 1/6th of the
ESE (in the direct beam)
 For LAT projection, the embryo dose will be about 1/20th of the
ESE (in the direct beam)
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c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 59.
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1. Dosimetry

Comparing ESE is useful for assessment of equipment performance and
calibration, when a comprehensive analysis of effective dose is unnecessary
c.f. Bushberg, et al.
The Essential
Physics of Medical
Imaging, 2nd ed., p.
797.
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1. Risk

The International Commission on Radiological Protection (ICRP)
estimates the risk of fatal cancer for exposures to adults of
working age to be 0.004 deaths per Sv or 0.0004 per rem

this translates to 1 cancer death per 2,500 people receiving
an effective dose of 10 mSv (1 rem)

Because of the linear, no-threshold assumption used in risk
estimates, risk is presumed to be proportional to the effective
dose
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1. Risk

Risk is proportional to the effective dose
 there would be a 1 in 25,000 chance that a fatal cancer
would result from an effective dose of 1 mSv (0.1 rem),
or
 a 1 in 500 chance of a fatal cancer from an effective dose
of 50 mSv (5 rem)

The ICRP estimates the risk to be two or three times higher
for infants and children and substantially lower for adults
older than 50 years of age
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1. Typical Absorbed and Effective doses
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c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 798.
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1. Risks
Effective
Dose
(mSv)
Risk of Fatal
Cancer
(per
million)
Equivalent to
Number of
Cigarettes
Smoked
Chest Radiograph
0.04
1.6
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Skull Exam
0.1
4.0
29
71
Mammography
0.1
4.0
29
71
Thoracic Spine
1.0
40.0
292
714
Pelvis
1.1
44.0
321
786
Abdomen
1.2
48.0
350
857
CT Head
1.8
72.0
526
1286
Lumbar Spine
2.1
84.0
613
1500
Intravenous
Urography
4.2
168.0
1226
3000
CT Pelvis
7.1
284.0
2073
5071
CT Abdomen
7.6
304.0
2219
5429
CT Chest
7.8
312.0
2277
5571
Barium Enema (with
fluoro)
8.7
348.0
2540
6214
Procedure
Equivalent to
Number of
Highway
Miles Driven
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1. Interventional Radiologic Procedures
c.f. Bushberg, et al.
The Essential
Physics of Medical
Imaging, 2nd ed., p.
799.
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2. Radiographic Procedures
Geometry for measuring the output free-in-air of a radiographic system
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 801.
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2. Radiographic Procedures
c.f. Bushberg, et al. The Essential Physics of Medical
Imaging, 2nd ed., p. 802.
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2. Radiographic Procedures
Geometry for measuring the output free-in-air of a radiographic system when
phototiming is used
c.f. Bushberg, et al.
The Essential
Physics of Medical
Imaging, 2nd ed., p.
804.
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3. Effective Dose Comparison with Chest PA Exam
Approx. period
of background
radiation
Procedures
Eff. Dose (mSv)
Equivalent no.
of chest x-rays
Chest PA
0.02
1
3 days
Pelvis
0.7
35
4 months
Abdomen
1.0
50
6 months
CT Chest
8
400
3.6 years
CT Abdomen or
Pelvis
10-20
500
4.5 years
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Question

Assuming the skin entrance dose from a single slice CT study is 5
rad, the dose for a 10 slice examination would be approximately
_____ rad and the imparted energy would be ____ rad (ignore
scatter).
A. 5, 15
B. 15, 5
D. 50, 5
E. 5, 50
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Question

The skin entrance exposure from a CT slice is 2.0 R. Ten
contiguous slices are taken, then dye is injected and 10 slices are
repeated. The total entrance skin exposure is about _____ R.
A. 2.0
B. 2.2
D. 5.0
E. 20.0
You have to consider scatter. 25% of 2 R = 0.5. So 2.5 per scan is
the rad exp. For two scans, 2.5*2 = 5.0
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Question

The national average ESE for a normal 23 cm thick A/P abdomen
film with a 400 speed screen-film system is:
A. 13 mR
B. 150 mR
C. 300 mR
D. 850 mR
E. 3000 mR
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Question

Match the exposure or dose with the appropriate item:
A. 15 mR
B. 40 mR
C. 5 R
D. 10 R
E. 50 mrem
1. CT head scan ESE
2. Lateral chest ESE
3. 10 min fluoro (thin patient)
4. Monthly limit for a pregnant technologist
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Question

Match the exposure or dose with the appropriate item:
A. 15 mR
B. 40 mR
C. 5 R
D. 10 R
E. 50 mrem
1. CT head scan ESE – 4 to 6 R typical
2. Lateral chest ESE – 10-15 mR for PA. 2 to 3 times for Lateral
3. 10 min fluoro (thin patient) – 1-2 R/min for thin patient
4. Monthly limit for a pregnant technologist – 0.5 mSv or 50 mrem
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