Download Diagnostic reference levels as a quality assurance tool

Australian Institute of Radiography
Literature review
The Radiographer 2009; 56 (3): 32–37
Diagnostic reference levels as a quality assurance tool
KD Edmonds
Medical Physics Section, Medical Radiation Branch, Australian Radiation Protection and Nuclear Safety Agency,
Yallambie, Victoria 3085, Australia.
Correspondence [email protected]
Abstract The objective of diagnostic reference levels (DRL) in radiology is to assist in the optimisation of radiation dose
to patients, while maintaining diagnostic image quality, and to detect unusually high doses that do not contribute significantly to the clinical outcome of a medical imaging examination. DRL have been in existence overseas for more than a
decade and its influence has contributed to a steady decline in dose for general radiography and fluoroscopic procedures.
High dose modalities such as CT and interventional procedures are increasing dramatically both locally and internationally resulting in the unwanted outcome of a significant increase in population cumulative effective dose. This calls for
urgent dose reduction and dose constraint measures. Utilising DRL is one method of optimising patient dose. Some local
and international DRL dose levels for some common radiographic, interventional and CT examinations are presented as
a suggestion for the application of this methodology in Australian radiology practice.
Keywords: diagnostic reference levels, guidance levels, radiation dose, reference values, quality assurance.
Introduction
The objective of establishing diagnostic reference levels (DRL)
in diagnostic imaging (also previously known as Guidance Levels
or Reference Values1,2) is to provide radiology and other departments that use x-ray imaging with a convenient DRL dose comparison to ensure that radiation doses to patients are kept within
reasonable limits.
The main task of radiation protection is not only to minimise the stochastic risks but also to avoid deterministic injuries.
Stochastic refers to effects whose probability increases with
increasing dose and for which there is no threshold dose. Any
dose, no matter how small, has the potential to cause harm and
this becomes apparent years after the exposure. Examples are
leukaemia and hereditary effects. Deterministic effects are those
in which the severity of the effect, rather than the probability,
increase with increasing dose and for which there is a threshold
dose. Examples are epilation, erythema and hematologic damage
and are known as early effects.3
A DRL, as defined by the International Commission on
Radiological Protection (ICRP), is “a form of investigation level,
applied to an easily measured quantity, usually the absorbed dose
in air, or tissue-equivalent material at the surface of a simple
phantom or a representative patient.”4
The ICRP recommends the establishment of reference levels
as a method of optimising the radiation exposure to patients. This
is accomplished by comparison between the numerical value of
the diagnostic reference level (derived from relevant national,
regional or local data) and the mean or other appropriate value
observed in practice for a suitable reference group of patients or a
suitable reference phantom.
Another definition by the Council of the European Union in its
Council Directive 97/43 defines DRL as “dose levels in medical
radiodiagnostic practices or, in the case of radio-pharmaceuticals,
levels of activity, for typical examinations for groups of standardsized patients or standard phantoms for broadly defined types
of equipment. These levels are expected not to be exceeded for
standard procedures when good and normal practice regarding
diagnostic and technical performance is applied.”36 It reinforces
the concept of references doses applying only to “standard” or
“representative” patients. DRL therefore are not dose limits but
a guide of good practice. It is not a dose constraint and the DRL
values are not used for regulatory or commercial purposes. DRL
act as an investigation trigger if the numerical values are consistently exceeded.
Background
The need for DRL
Patient exposures in diagnostic radiology are increasing at a
disquieting rate for certain radiographic, fluoroscopic and CT
examinations. Regulla and Elder5 pointed out that data obtained
from the United Nations Scientific Committee on the Effects of
Atomic Radiation (UNCEAR) show that there are significant
differences in national radiation exposures and a very uneven
distribution of patient doses among world population for the same
or similar procedures. Mean annual x-ray effective dose of the
population can vary by up to a factor of 60. In the United States,
studies such as the Nationwide Exposure X-ray Trends (NEXT)
surveys also showed that patient doses in radiology vary considerably from one facility to the next.6 Gray, et al.1 posed the question,
“why one radiology facility should use an exposure that is 10,
20 or 126 times greater than another facility to produce a radiographic image?” Johnston and Brennan7 and Carroll and Brennan8
also reported wide variations in patient doses for the same radiographic examinations among hospitals in the UK and Europe.
These patient doses are attributed to a wide range of factors such
as type of image receptor, exposure factors, fluoroscopic times,
number of images, type of anti-scatter grid and level of quality
control as reviewed by Seeram,9 Bushong10 and Parry, et al.11
In Australia, the Australian Radiation Protection and Nuclear
Safety Agency (ARPANSA) Code of Practice, Section 3.1.8
Diagnostic reference levels as a quality assurance tool
(Radiation Protection Series No.14) states that “the Responsible
Person must establish a program to ensure that radiation doses
administered to a patient for diagnostic purposes are:
1 Periodically compared with DRL for diagnostic procedures for
which DRL have been established in Australia
2 If DRL are consistently exceeded, reviewed to determine
whether radiation has been optimised.”12
In addition, the ARPANSA Safety Guide, Section 7.8
(Radiation Protection Series No.14.1), suggests that “as part of
the QA program, patient dose surveys are undertaken periodically
to establish that the doses are acceptable when compared with
published DRL.” It also recommends that accrediting bodies such
as RANZCR and the Australian Council on Healthcare Standards
consider including compliance with DRL for a core set of examinations. If the radiology department observes dose values consistently exceeding the DRL, then this warrants further investigation
however some flexibility should be allowed if higher doses are
indicated by sound clinical judgement.13,14
However, at this point in time, there are no DRL published in
the Code of Practice or the Safety Guides. It would seem logical
therefore to use published values from the literature from extensive surveys in countries with similar healthcare settings e.g.
similar levels of education and training for imaging technologists,
radiologists and similar provision of imaging equipment.15
History of DRL
National surveys of patient doses from x-ray examinations in
Europe and the USA since the 1950s have demonstrated wide
variations in doses between radiology departments and illustrated
the need for quantitative guidance on patient exposure. It was only
at this stage that dose measurements to patients began in earnest.
National surveys in the USA and UK concentrated on measuring
entrance surface doses with or without backscatter for common
radiographic projections. The Nationwide Evaluation of X-ray
Trends in the USA in the 1970s measured entrance skin exposure free-in-air for average exposure technique factors or used
a standard phantom. The NRPB national patient dose survey in
the UK in the 1980s measured entrance surface dose directly on
the surface of the patient (including backscatter) using thermoluminescence dosemeters. A European trial supporting the Quality
Criteria for Diagnostic Radiographic Images in 1991 used the
same technique.37
Dose guidelines began to appear in late 1980s. First was the
USA, promoted by the Centre for Devices and Radiological Health
(CDRH) in conjunction with the Conference of Radiation Control
Program Directors Inc. Then in the UK it was conducted by the
National Radiation Protection Board in collaboration with relevant
professional bodies. Europe then followed with reference doses
incorporated into Working Documents by EC Study Groups.38
International recommendations then appeared on how to
measure and set reference dose levels based on the initiatives led
by the USA and the UK. The ICRP Publication 60 first made
mention of the concept of “investigation levels” in 1990 followed
by the current definition of DRL in ICRP Publication 73 in 19964
and the EC Medical Exposure Directive in 1997.36
The United Kingdom introduced DRL in 1990 for common
diagnostic examinations based on a national patient dose survey
in the mid-1980s conducted by the NRPB, now known as the
Health Protection Agency (HPA). They are now based on the
five-yearly reviews of the National Patient Dose Database and are
currently in their third review.16
The International Atomic Energy Agency (IAEA) in 1996 and
The Radiographer
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2002 also issued advice on the use of DRL (or guidance levels)
in their safety standard series and included guidance levels for
typical adult patient doses for general radiography, CT, fluoroscopy and mammography.2,14,
Many countries worldwide have now incorporated the European
Community Directive in national legislative documents.1Several
organisations currently providing guideline documents for establishing DRL include the ICRP,4,20 the Health Protection Agency in
the UK,16,21 the Commission of European Communities22 and the
American College of Radiology.23
Aim
The overall aim of DRL are to better manage patient dose in
diagnostic radiology using the principle of optimisation which is
defined as exposure to radiation from justified activities should
be kept as low as reasonably achievable, social and economic
factors being taken into account. The European Commission and
the ICRP provide a range of tools to achieve this.15,24
Data obtained from patient dose surveys show that typical
patient doses for the same type of x-ray examinations can vary
considerably from one radiology practice to another. The establishment of DRL therefore is to give an indication of unusually
high values. The DRL are usually set at the third quartile value of
the distribution of typical doses derived from dose surveys both
nationally and internationally. Using the third quartile or 75th
percentile is a compromise between being overly stringent and
overly complacent.3
Essentially, if mean doses exceed a reference level dose an investigation should take place to establish the cause and take corrective
action, unless the dose was clinically justified. Reference doses were
also used to provide a trigger for practices in need of investigation
and hopefully lead to dose optimisation. ICRP 734 recommended
that DRL values be selected by professional bodies, be reviewed at
regular intervals and be specific to a country or region.
Wide variations in patient doses are to be expected and it
is only sensible to compare mean or median values, which is
less influenced by extreme outliers, on representative groups of
patients to monitor trends with time, equipment or technique.
Method
From a practical perspective, the DRL should be expressed
as a readily measurable patient-related quantity for the specified
procedure. For example,
1 General radiographic examinations – either entrance skin dose
(ESD) or the dose area product (DAP)
2 Fluoroscopic examinations – dose area product (DAP)
3 CT examinations – computed tomography dose index ( CTDIw
or CTDIvol ) and the dose length product (DLP).
New CT scanners in accordance with Australian Standards,
AS/NZS 32002.4,25 should display the volume CTDIvol and/or the
DLP on the operator’s console after the selection of technique
factors and prior to the initiation of x-rays.13
DRL used for film-screen technology should not necessarily be
used for new digital radiography without prior adjustment.26
Dosimetry methods
International guidance on patient dosimetry techniques for
x-rays used in medical imaging is published by the International
Commission on Radiation Units and Measurements in ICRU
Report 74.27 This report contains advice on the relevant dosimetric quantities and how to measure or calculate them in a clinical
setting which is directly applicable to the patient dose surveys
needed to estimate population exposure.
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KD Edmonds
The Radiographer
There are various methods of recording dose levels.
Thermoluminescent dosimeters (TLD) are often used for plain
film examinations and include dose contribution from backscatter if placed on the patient or phantom surface. The small TLD
sachets are usually placed in the centre of the irradiated field on
the entrance surface of the patient or phantom. The TLD can be
stuck directly and unobtrusively to the patient’s skin with very
little interference in patient mobility or comfort. They do not
interfere with the examination or obscure important diagnostic
information on the radiographic image. They need to be calibrated
with respect to radiation qualities used in diagnostic radiology.
TLD are also prone to some inaccuracy due to signal fade, nonlinear response and dependency on beam energy.3 Ionisation
chambers are bulky and more difficult to attach to patients. The
parallel plate ionisation chambers measure back scatter but the
lead backed solid state detectors do not. They are not recommended for direct measurement of entrance surface dose on the
skin of the patient. They can, however, be used to make measurements of the absorbed dose to air, in free air, without a patient
or phantom present. The measurements can then be corrected
using appropriate backscatter factors and the inverse square law
to estimate the entrance surface dose. Newer technology such as
optically stimulated luminescence dosimeters (OSLs) and radiochromic film may replace TLD. Radiochromic film is currently
being evaluated by ARPANSA.
Alternatively, the entrance skin dose may also be calculated
from x-ray tube output measurements (mGy/mAs) and the exposure parameters, kVp, filtration and mAs. The incident air kerma
is calculated from the tube output using the inverse square law and
then multiplied by the backscatter factor to obtain the entrance
skin dose.
A dose area product (DAP) meter consists of an ionisation
meter that is usually attached to the x-ray tube collimator and
measures the dose in Gy. square centimetre (Gy cm2) which is
proportional to the beam area and incident air kerma. The unit
unfortunately does not measure backscatter which is important
in higher dose examinations such as cardiac and vascular interventional procedures. However DAP meters can be used for
radiographic and fluoroscopic procedures such as barium meals,
angiography and on mobile image intensifiers.2,3,28
For CT machines, the CTDIw and /or CTDIvol (mGy) and the
DLP values(mGy cm) are conveniently provided at the operator console before or after the examination. The CTDIw is the
weighted sum of the CT dose (or air kerma ) index measured
in the centre and periphery (1 cm under the surface) of a 16 cm
diameter (head) or a 32 cm diameter (trunk) standard polymethylmethacrylate (PMMA) CT dosimetry phantom. The CT dose
index is measured with a 100mm long pencil ionization chamber
inside a standard PMMA CT dosimetry phantom.
1
2
CTDIw = 3 CTDIc + 3 CTDIp where c is the centre position
and p is the peripheral position of the phantom. Units: mGy
CTDIw corrected for pitch is the CTDIvol.
CTDIvol =
CTDIw
pitch
Units: mGy
The DLP is the product of CTDIvol and the scan length of the
examination.
Thus DLP = CTDIvol x length irradiated. Units: mGy cm15
As a starting point it is suggested that the UK 2000 survey
review of DRL for general radiography for adults (Table 1) and
fluoroscopy for adults (Table 2), Paediatric procedures (Table
3), the UK 2003 CT survey (Table 4), and mammography (Table
5), be adopted and /or adapted in the Australian context as there
are currently no established national DRL.29 Some state regulators though have provided local DRL guidance on radiography,
fluoroscopy and CT.34 Dose values should be reviewed as computed/digital radiography becomes more widespread in order to
minimise the detrimental influence of ‘exposure creep’.30 This
phenomenon occurs after the change over from film-screen
radiography to digital radiography where exposure factors may
actually increase in order to reduce image noise. Uncoupling of
display from acquisition in digital radiography introduces the
potential for systematic overexposure without necessarily compromising image quality.31 The wide exposure latitude and linear
response to x-ray energy provides an image appearance that
remains consistent throughout the exposure range and this in turn
provides little feedback to the technologist. Underexposed images
typically have a grainy, mottled appearance that causes radiologists to reject images. Over-exposed images, on the other hand,
have a crisp, sharp appearance. In order to prevent repeating the
image, the technologist may increase exposure factors especially
for manual and mobile radiography. Exposure indices or exposure indicators provided by the various CR/DR manufacturers
also have a wide range of acceptable values and are currently not
standardised throughout the industry.
In the case of CT examinations, care should be taken when
following overseas DRL because of the wide variety of CT scanners and local examination protocols employed. In addition, the
rapid advances in CT technology have also resulted in constantly
changing scanning protocols. Nevertheless, the DRL provided in
Tables 1–5, serve as a rough guide until new DRL emerge from
current surveys in Australia.
In future, a web based interactive dose survey software program will be provided by ARPANSA where each radiology
department can access it to calculate their dose levels and compare them with DRL.
Discussion
The development of DRL practice in diagnostic radiology within
Australia is still at an early stage as no national surveys have been
carried out for any radiological examinations for the express purpose of establishing national DRL. At a local level, various organisations, regulatory authorities and individual practices have carried
out limited general radiography, fluoroscopy and CT surveys.34
There is a clear need to manage (optimise) the radiation doses
from diagnostic radiology in order to minimise the risks from
radiation induced cancers. The establishment and use of DRL is
recommended by international radiation protection organisations
as an important component of the management of these doses
and many countries have incorporated them into their radiation
protection regulations12,36
Data from European countries shows a wide variation in common DRL which may be due to differences in socio-economic
conditions, regulatory regime, activeness of professional bodies and health care implementation (private/public mix etc).7,32
International radiation protection bodies such as the IAEA and
ICRP therefore recommend that each country carry out its own
national wide scale DRL survey. It is for this reason that Australia
must develop its own set of common national DRL.
The introduction of computed and digital radiography in recent
years has had a significant impact on the potential for higher dose
Diagnostic reference levels as a quality assurance tool
The Radiographer
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Table 1: Recommended diagnostic reference doses for individual radiographs on adult patients.
Radiograph
ESD per radiograph (mGy)
DAP per radiograph (Gy cm2)
Skull AP/PA
3
-
Skull LAT
1.5
-
Chest PA
0.2
0.12
Chest LAT
1
-
Thoracic spine AP
3.5
-
Thoracic spine LAT
10
-
Lumbar spine AP
6
1.6
Lumbar spine LAT
14
3
Lumbar spine LSJ
26
3
Abdomen AP
6
3
Pelvis AP
4
3
Adopted from the UK 2000 DRL survey review. 29
Note: Adult is defined as a person of average size (70–80 kg) ESD = Entrance Skin Dose, DAP = Dose Area Product.
Table 2: Recommended diagnostic reference doses for fluoroscopic/interventional examinations on adult patients.
Examination
DAP per exam (Gy cm2)
Fluoroscopy time per exam (mins)
Barium (or water soluble) swallow
11
2.3
Barium meal
13
2.3
Barium follow through
14
2.2
Barium (or water soluble) enema
31
2.7
Small bowel enema
50
10.7
Biliary drainage/intervention
54
17
Femoral angiogram
33
5
Hickman line
4
2.2
Hysterosalpingogram
4
1
IVU
16
-
MCU
17
2.7
Nephrostogram
13
4.6
Nephrostomy
19
8.8
Retrograde pyelogram
13
3
Sialogram
1.6
1.6
T-tube cholangiogram
10
2
Venogram (leg)
5
2.3
Coronary angiogram
36
5.6
Oesophageal dilation
16
5.5
Pacemaker implant
27
10.7
Adopted from the UK 2000 survey review.29 DAP = Dose Area Product.
delivery.30,31 In addition, the exponential increase in CT examinations has lead to the unwanted outcome of a significant increase
in population cumulative effective dose.35 Other causal agents that
are linked to high doses include type of image receptor, exposure
factors, fluoroscopic time, number of images, type of antiscatter
grid and level of quality control.9,10,11
DRL depend significantly on local practice and equipment.
They may also change with time as optimisation strategies become
successful. The UK experience over the past 20 years has shown
that the implementation of DRL together with a dose optimisation
program has resulted in a gradual reduction of doses.29,32
For DRL to succeed, acceptance and application of the concept is
required across as many radiology departments as possible. Complex
calculations will only discourage participation. Furthermore, setting
DRL is a resource intensive activity and requires a national response.
Priority should be given to procedures with greatest dose implications, i.e. CT and Interventional procedures. DRL should be owned
by the professions such as the Australian Institute of Radiography and
the Royal Australian New Zealand College of Radiology. ARPANSA
will assist in facilitating their development.
Conclusion
Overseas experience has shown that the use of DRL have proven be a useful quality assurance tool in optimising patient dose in
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KD Edmonds
The Radiographer
Table 3: Recommended diagnostic reference doses for complete examinations on paediatric patients.
Examination
Standard age (y)
DAP per exam (Gy cm 2)
MCU
0
0.4
1
1.0
5
1.0
10
2.1
15
4.7
Barium meal
0
0.7
1
2.0
5
2.0
10
4.5
15
7.2
Barium swallow
0
0.8
1
1.5
5
1.5
10
2.7
15
4.6
Adopted from the UK 2000 survey review.
29
Table 4: Recommended diagnostic reference levels for CT examinations (CTDIvol and DLP).
Patient group
Scan region
CTDIvol (mGy)
single slice/multi slice
Adults
Post fossa
Cerebrum
Whole exam
65/100
55/65
Abdomen (liver metastases )
Whole exam
13/14
Abdomen and pelvis
(abscess) Whole exam
13/14
Chest, abdomen and pelvis
(lymphoma staging or follow up).
Whole exam
22/26
Chest (lung cancer)
10/13
Chest Hi-res
Whole exam
3/7
Children
0–1 year-old
Head (post fossa)
Head (cerebrum)
Thorax
35
30
12
270 (whole exam)
Head (post fossa)
Head (cerebrum)
Thorax
50
45
13
470 (whole exam)
5-year-old
Head (post fossa)
Head (cerebrum)
Thorax
65
50
20
620 (whole exam)
10-year-old
DLP (mGy cm)
Single slice/multi slice
760/930
460/470
510/560
760/940
430/580
80/170
200
230
370
Adopted from UK 2003 CT dose survey.
Dose values for adults relate to the 16 cm diameter CT dosimetry phantom for examinations of the head and the 32 cm diameter CT dosimetry phantom for
examinations of the trunk.All dose values for children relate to the 16 cm diameter CT dosimetry phantom.
CTDI vol = Computed Tomography Dose Index Volume, DLP = Dose Length Product.
33
Diagnostic reference levels as a quality assurance tool
Table 5: Recommended diagnostic reference level for mammography for a
typical adult patient.
• For film screen examinations using a grid, the mean glandular dose (MGD)
is 2 mGy based on the 4.2 cm acrylic American College of Radiologists phantom.17,18,19
• For a 50% adipose, 50% glandular 5 cm thick phantom the MGD is 3 mGy17
Note: For digital mammography, the values quoted above represent an upper
limit
diagnostic radiology. It is recommended that local dose surveys be
performed annually while national surveys every five years.32,33
The imaging technologist is the main person who decides on
the exposure factors and the visual image quality for the radiologist to make a diagnosis. The technologist should therefore be
aware of the exposure options that minimise radiation doses while
still maintaining good image quality and monitoring dose levels.
DRL would therefore serve as an important means of minimising
radiation doses as well as dose variations at minimal cost to radiology
departments. They also increase staff awareness and imaging technologists will be better equipped to deal with patient enquiries.3
ARPANSA is responsible for carrying out national DRL
surveys in consultation with relevant stakeholders such as the
Royal Australian & New Zealand College of Radiology, The
Australian Institute of Radiography, Australasian College of
Physical Scientists & Engineers in Medicine, Australian & New
Zealand Society of Nuclear Medicine, Department of Health and
Aging and the various State/Territory Regulators.
Acknowledgements
The author thanks Anthony Wallace for his advice in preparing
this article.
The author
KD Edmonds DCR BHA Grad Dip Pub Health
The Radiographer
­37
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