Diagnostic Reference Levels (DRLs): The concept and use

Vassileva, ICDA-3 2019, Lisbon
Diagnostic reference levels (DRLs):
the concept and use
Jenia Vassileva, Ph.D.
Radiation Protection Specialist
Radiation Protection of Patients Unit
International Atomic Energy Agency, Vienna, Austria

• The national DRL for the bone imaging of adult patient with Tc-99m
phosphates is 700 MBq.
• The typical activity administered to an adult patient in my department
is 750 MBq.
• Is this a legal basis for regulatory inspectors to close the department
because of the dose limits to patients systematically exceeded?

• The national DRL for the bone imaging of adult patient with Tc-
99m phosphates is 700 MBq, and the local DRL is 750 MBq.
• The nuclear medicine physician prescribed for a particular
patient an activity of 800 MBq.
• Is this a break in regulation that triggers legal actions?

Issues with the current use DRLs
• Misuse of DRL values for individual patients
• Misuse of DRL values as a limit for individual patients or
individual examinations
• Use of phantoms or inappropriate measures of radiation output
to set DRL value
• Establishment of DRL values when technology and image quality
requirements are different
• Use of effective dose to set DRLs

Issues with the current use DRLs
• Lack of sufficient patient dose data in the collected sample to set
DRLs
• Lack of specification of imaging procedures and indication based
protocols
• Image quality not confirmed
• Lack of paediatric DRLs
• DRLs adopted from other countries
• DRLs not updated regularly
• Tendency to assume that being below DRLs means adequately
good practice

Learning objectives
• To understand the concept of DRLs
• To introduce dose quantities used for setting DRLs
• To clarify the DRL nomenclature
• To describe steps needed to establish DRLs
• To understand how DRLs should be used

The advent of DRLs
• Large variations in patient doses for the same exam have been
long documented
• The need for improvement long recognized
• Various approaches advocated in 70s, 80s
– Nationwide Evaluation of X-ray Trends (NEXT) survey program initiated by
FDA in the US in 1973, and since then, periodic surveys performed using
phantoms
– First national dose survey using patient samples for radiographic
procedures organized in UK in the mid 1980s
– National reference doses first suggested for the UK in 1989

The advent of DRLs
• National survey in UK in the mid 1980s showed wide variation in
practices between hospitals for similar radiographic procedures
UK Survey of Patient Dose 1984: 20 Hospitals
HPA-RPD-029, Health Protection Agency, UK, 2007

Trends in DRLs with time
• UK has > 20 years of experience with DRLs
– First survey in 1985; Reviews in 1995, 2000, 2005 and 2010
Trend due to better
optimization, including regular
monitoring of patient doses:
The average percentage dose
reduction between surveys
was between 10 and 20%
HPA-RPD-029, Health Protection Agency, UK, 2007
Hart, D., Hillier, M.C., Shrimpton, P.C., 2012. Doses to Patients
from Radiographic and Fluoroscopic X-ray Imaging Procedures in
the UK – 2010 Review Report. HPA-CRCE-034.

CT surveys in UK
• 1st in 1991
• 2nd in 2003
• 3rd in 2011
• 4th launched March 2019
Max/min = 38 48
Adult chest CT (lung cancer)
108 CT units, 2081 patients

CT surveys in UK
• 1st in 1991
• 2nd in 2003
• 3rd in 2011
• 4th launched March 2019
Adult chest CT (lung cancer)
108 CT units, 2081 patients
6Max/min=8

FBP 150 mAs
FBP 75 mAs
FBP 40 mAs
IRIS 150 mAs
IRIS 75 mAs
IRIS 40 mAs
10 mGy
5 mGy
2.5 mGy
Example from the IAEA e-learning on
Radiation Dose Management in Computed Tomography

Nuclear medicine
• National survey of administered activities in NM in Australia:
• Bone scan: Tc-99m MDP/HDP: 73 facilities, 5244 individual patients
Individual patients Facility medians
https://www.arpansa.gov.au/research-and-expertise/surveys/national-diagnostic-reference-level-service/nm/statistics

• Data from 47 centre
• Ratio of maximum-to-minimum mean doses
between centers varied from 3.6 to 35 for a
given examination/CT purpose combination.
• Lack of standardization
Hybrid imaging

International recommendations
– ICRP first mentioned “DRLs” in Publication 60, 1990
– Elaborated in Publication 73, 1996
– Further in Publication 105, 2007
– Latest ICRP Publication 135, 2017

The IAEA and DRLs
• The International BSS, 1996
– Introduced Guidance Levels for medical exposure
– Concept same as DRLs
• New International BSS (GSR Part 3), 2014
• Accompanying Safety Guide (SSG-46)
– Diagnostic reference levels (DRLs)
as an important tool for optimization of
patient radiation protection in imaging

Optimization of protection (International BSS)
Registrants and licensees and radiological medical practitioners
shall ensure that protection and safety is optimized for each
medical exposure
Components to consider:
1.Design considerations for equipment
2.Operational considerations
3.Dosimetry of patients
4.Diagnostic Reference Levels (DRLs)
5.Calibration
6.Quality assurance
7.Dose constraints (for carers and comforters)
Monitoring patient dose
is a key requirement
toward optimization

Diagnostic Reference Level (DRL)
"A level used in medical imaging to indicate whether,
in routine conditions, the dose to the patient or the amount
of radiopharmaceuticals administered in a specified
radiological procedure for medical imaging is unusually
high or unusually low for that procedure”
DRL replaced previously used “guidance level”
Numerical values of DRLs not provided in the BSS
International BSS: DRL definition

• Dose metrics:
– easily measurable (not effective dose)
– must follow the ICRU recommendations (BSS)
Establishing DRLs
Nuclear medicine Administered activity,
Administered activity per body weight
Radiography Air kerma-area product,
Incident air kerma, or Entrance surface air kerma
Fuoroscopy Air kerma-area product
Image guided interventional
procedures
Air kerma-area product and Cumulative reference air kerma at the patient
entrance reference point
CT CT air kerma index, and
CT air kerma-length product
Mammography and tomosynthesis Incident air kerma and Mean glandular dose
Dentistry Incident air kerma for intra-oral radiography and
Air kerma-area product for panoramic radiography and CBCT

ICRU Report 74.
Patient Dosimetry for X-Rays used in
Medical Imaging. ICRU, 2005.
Measurable dose quantities
IAEA. Dosimetry in Diagnostic Radiology.
An International Code of Practice.
TRS 457, 2007

The measurements are a responsibility of qualified medical physicists

dFSD
Entrance surface air kerma
Ke [Gy]
Also known as
Entrance surface dose, ESD
1) Measured with phantom
2) With patients:
- Directly measured with TLD
- Calculated from the tube output
Radiography

Air kerma-area product, KAP
Also known as Dose Area Product, DAP
Unit: Gy·m2
Practical units:
1 Gy.m2 = 1 cGy.cm2
Kerma (dose)  (1/ f)2
Field area  f 2
КАР is independent on
distance from focus
f
Radiography and fluoroscopy

Radiography and fluoroscopy

• Cumulative air kerma (dose) at the Interventional reference point (mGy)
Interventional radiology

For one rotation:
Computed-tomography dose index, CTDIvol
Unit: mGy
For the whole exam:
Dose-length product, DLP
Unit: mGy.cm
26
Computed tomography

Computed tomography

Mammography
• Incident air kerma, Ki - the air kerma from
the incident beam on the central x-ray beam
axis at the skin entrance plane, backscatter
excluded
• Mean glandular dose (MGD) - mean
absorbed dose in glandular breast tissue
Dg = Ki . g . c . s
g, c, s – MC derived correction factors to account for
breast thickness, HVL, breast granularity, and
anode/filter combination

Establishing DRLs
• The concept of a DRL is based on a typical patient, either:
– A phantom, or, preferably, patients selected on basis of some criteria
• “Standard” adult patient :
– sample of patients mass 70 kg ± 20 kg, aiming for a sample average
70 kg ± 5 kg, or
– all adults in the initial sample but excluding extreme outliers in terms of
patient size indices
• Children:
– several mass, size or age groups, defined unambiguously by using intervals;
e.g. body mass bands
– The number of groups should take into account the practical difficulty in
collecting a sufficient number of patient dose data in each group.

Weight grouping for paediatric DRLs
European Guidelines on Diagnostic Reference Levels for Paediatric Imaging. Radiation Protection 185. European Union, Luxembourg.
http://www.eurosafeimaging.org/wp/wp-content/uploads/2018/09/rp_185.pdf

Patient grouping by weight
• Large samples
– Use of four age groups, <1, >1–5, >5–10
and >10–15 y, is realistic and pragmatic
for dose surveys in less resourced
countries
– Data for >30 patients in a particular age
group should be collected
• Small samples
– Need to record weight
– Median weight within 5–10 % from the
median weight of the sample for which
the DRLs are established
Vassileva J, Rehani M. Patient grouping for dose surveys and establishment of diagnostic reference levels in paediatric
computed tomography. Radiat Prot Dosimetry. 2015 Jul;165(1-4):81-5

DLR quantity-weight curve
DRL-curves for CT chest,
32 cm CT dosimetry phantom
Järvinen H, et al. Indication-based national diagnostic reference
levels for paediatric CT: a new approach with proposed values.
Radiat Prot Dosimetry. 2015 , 165(1-4):86-90.

Establishing DRLs
• Procedures to be included?
– Diagnostic and interventional radiology and diagnostic NM
– Priority to most frequent and high dose procedures
– Anatomical region / clinical question (indication based)
• For hybrid imaging procedures (SPECT-CT, PET-CT)
– Radiation from very different modalities
– Different dose quantities
– It is appropriate to set and present DRL values
for each modality independently

Calibration and accuracy of a DRL quantity
• All dosimeters used for dosimetry of patients
• To confirm the accuracy of:
– PKA meters
– CT scanner displays of CTDIvol and DLP
– Thermoluminescent dosimeters
– Dosimetric quantity transferred from a X ray system

Setting typical dose values
• A value of a DRL quantity
• Obtained from a local survey or a review of
local data
• May be set for a single X ray room or a single facility
• Terms “typical value” and “typical dose” are also used
The median of the distribution of the data for a DRL
quantity for a clinical imaging procedure.

Image quality
IMPORTANT!
• Dose data for each contributing facility is only collected for
procedures where the image quality was confirmed as adequate
for the clinical purpose.

Establishing DRLs
• Representative survey
– Wide-spread in terms of various types and size of facilities
(rural, urban, private, public), equipment, and geographical locations.
– Sample size (number of rooms) depend on the size of the country and
total number of facilities

Establishing DRLs
• Sample size for each room/facility:
– depends on imaging procedure frequency and variability in patient doses
– sufficient to assure confidence in the determination of the typical dose.
• 10-20 patients for non-complex examinations such as radiography and CT;
• 10-20 patients for nuclear medicine;
• 20-30 patients for complex procedures (fluoroscopy and FGP);
• 40-50 patients for mammography.

• Methodology used in performing the initial survey:
– Manual recording (paper or electronic)
– Manual extraction from archive of exams (retrospective)
– Electronic data sheets
– Automatic dose monitoring software (at facility)
– Automatic collection (dose indexes registries)
Establishing DRLs

DRL value
DRL value
• 75th percentile (third quartile) of the distribution of the medians of
distributions of the DRL quantity

DRL value
• 75th percentile (third quartile) of the distribution of the
medians of distributions of the DRL quantity
A few healthcare facilities
National DRL valueMultiple facilities throughout a country
Regional DRL value
Representative sample of healthcare
facilities or national DRL value from a region
Local DRL value

National DRLs
• Based on data from a representative sample
of healthcare facilities in that country
• Nationwide use, to identify X ray facilities where optimization is
needed
75th percentile (third quartile) of the distribution of the
median values of the appropriate DRL quantity observed at
each healthcare facility

Establishing national DRLs - BSS
• Who should establish?
• Government as the facilitator
– Health Authority
– Professional Bodies
– Regulatory Body
BSS, Requirement 34:
The government shall ensure, that as a result of consultation between the health
authority, relevant professional bodies and the regulatory body, a set of DRLs is
established for medical exposures incurred in medical imaging, including image guided
interventional procedures.
Such DRLs shall be based, as far as possible, on wide scale surveys or on published values
that are appropriate for the local circumstances.

Local DRLs
• May be set:
– For procedures for which no national DRL is available
– Where there is a national value but local equipment or techniques are
different
• Local use, to identify X ray units requiring further optimization
75th percentile (third quartile) of the distribution of the
appropriate DRL quantity in a reasonable number of X ray rooms
(at least 10–20 X ray rooms) in a local area

Regional DRLs
• Several countries within a continent
• May be set using two approaches
• Regional use, for countries in the region without national DRLs or
which national DRLs are higher than regional
75th percentile (third quartile) of distribution of median values
for representative sample of healthcare facilities in a region
The median value of the available national DRLs1
2

Term Facilities surveys Parameter of distribution
used to set DRL
Application
Typical
value
A single room, or a single facility
consisting of small number of X ray
rooms (e.g. <10), or a small number
of facilities with small number of
rooms (e.g. <10 rooms in total)
Median value of the distribution
of the DRL quantity, as there
are insufficient data to use the
third quartile
Local use to identify X ray
units requiring further
optimization
Local
DRL
X ray rooms within a few healthcare
facilities (e.g. with at least 10–20 X
ray rooms) in a local area
Third quartile of median values
for
individual X ray rooms
Local use to identify X ray
units requiring further
optimization
National
DRL
Representative selection of facilities
covering
an entire country
Third quartile of median values
for
individual X ray rooms or of
national
values
Nationwide to identify X
ray facilities where
optimization is needed
Regional
DRL
Several countries within one
continent
1) The median value of the
available national DRLs
2) 75th percentile of distribution
for representative sample of
healthcare facilities in a region
Countries within region
without a relevant DRL or
for optimization when
national DRL is higher than
regional value.

Achievable dose
UK: NRPB, 1999
• Level of a DRL quantity
achievable by standard
techniques and technologies in
widespread use, without
compromising adequate image
quality
• Mean values observed for a
selected sample of departments
USA: NCRP, 2012
• Median value (the 50th
percentile) of the distribution of a
DRL quantity observed in a survey
of departments
Wardlaw, Health Canada, 2017

Analysis: in a particular X-ray room
• For the most frequently performed exams
• For a particular age/weight group
• Register dose values for a sample of 20-50 patients
• Calculate median/ average dose index
• Typical dose value = median of the sample
CT thorax (lung cancer)
Adult patient (average 70 kg)
Median = 10.8 mGy

Analysis: in a hospital
Asses typical dose values for all equipment
performing the same exams
CT thorax (lung cancer) Adult patient (average 70 kg)
CT scanner 1:
Median = 10.8 mGy
CT scanner 2:
Median = 15.4 mGy
CT scanner 3:
Median = 38.8 mGy
?

P C Shrimpton, M C Hillier, S Meeson and S J Golding. Doses from Computed Tomography (CT) Examinations in the UK – 2011 Review ю, PHE 2014
Min CTDI = 5.5 mGy
Max CTDI = 41.3 mGy
Median CTDI = 9.9 mGy
75th percentile = 12.2 mGy
Analysis: in a country
Collect data from a representative sample of
hospitals performing the same exam

P C Shrimpton, M C Hillier, S Meeson and S J Golding. Doses from Computed Tomography (CT) Examinations in the UK – 2011 Review ю, PHE 2014
Min CTDI = 5.5 mGy
Max CTDI = 41.3 mGy
Median CTDI = 9.9 mGy
75th percentile = 12.2 mGy
Analysis: in a country
Collect data from a representative sample of
hospitals performing the same exam
Investigation and actions
to reduce doses!

Vassileva, ICDA-3 2019, Lisbon 52
DRL и AD in USA
• Head – AD 49 mGy, DRL 57 mGy
• Neck – AD 15 mGy, DRL 20 mGy
• C-Spine – AD 21 mGy, DRL 28 mGy K. Kanal

1. Establish an working group; appoint coordinator and administrator of the
database
2. Decide for which imaging procedures DRLs to be established
3. Decide which dose quantities (modality specific, measurable)
4. Decide on the methodology - patients (groups, sample size, etc.), or a phantom
5. Prepare standardised data collection forms (electronic or paper-based)
6. Collect data in a particular contributing X-ray room
7. Perform statistical data analysis for a given X-ray room and set typical dose (=
median of the DRL value)
8. In a big hospitals: collect data from different X-ray rooms, performing the same
procedure (establish local DRLs)
9. Collect data from different X-ray rooms (representative sample)
10.Perform statistical analysis and set DRL (usually at 3rd Q (75th percentile)
10 steps to establish DRLs

• Many players: the imaging facilities, the health authority,
the professional bodies, and the regulatory body
• Collective ownership of the DRLs – deciding on:
– what procedures,
– what age groups,
– how to collect the data,
– who will manage the data, and
– when to review and update the DRLs.
• Administrator of the national database:
– a national governmental body
– regulatory body or
– a professional body.
Establishing DRLs

Using DRLs
• At each medical radiation facility
– Local assessments of typical doses for common procedures
– Typical dose: median/average value of the distribution of data collected
from a representative sample
– Results compared with relevant DRLs, and if:
• Exceed the relevant DRLs; or
• Substantially below the relevant DRL and images not of diagnostic quality
– Review of adequacy of optimization of patient radiation protection
• Corrective action, if indicated

How DRLs work – a trigger for review
• National DRLs have been established
• Typical doses at a facility are periodically
compared with the relevant DRLs
– If exceeds DRL, or
– If significantly below DRL and there are IQ
problems
• Investigate and if needed improve
optimization
DRL based on 75th
percentile
Average ESD
Room AA = 4.4 mGy
Average ESD
Room BB = 6.9 mGy

For what purpose we use of DRLs?
• DRLs are an important tool for optimization of
protection and safety
• Optimization of protection and safety should
be reviewed if the comparison shows that:
• the facility’s typical dose exceeds the DRL
• the facility’s typical dose is substantially
below the DRL

International BSS requirements
What? Who is responsible?
DRLs should be established and
updated periodically
Government: Consultation
between the health authority,
relevant professional bodies and
the regulatory body
Patient dose audits should be
performed in each medical
facility and local typical doses
compared to DRLs
Registrants and licensees: Team
of medical physicists,
radiographers and radiologists
Local review if doses above or
substantially below DRL
Registrants and licensees: Team
of medical physicists,
radiographers and radiologists

Patient dose audit and DRLs

Lajunen A. Indication-based diagnostic reference levels for adult CT-examinations in Finland. Radiat Prot Dosimetry. 2015, 165(1-4):95-7
Adult CT-examinations in Finland from 2000 to 2013
Trends in DRLs with time

DRLs are…
• DRLs are typical dose values for „standard-sized patient” at
particular examination
• DRLs are representative for the practice in the country or region
• DRLs are indication of amount of radiation used for specified
procedure
• DRLs are a tool for optimization of protection

DRLs are not …
• DRL values are not intended for individual patients
• DRLs are not dose limits
• DRLs are not border between good and bad practice

• DRLs are not static – they should be revised periodically
• Dose audits should be performed in each medical facility and
local typical values compared to DRLs
• Local review if doses above or substantially below DRL
• Intervention / optimization of clinical protocols
• DRLs should be included in the education and training programs
of the health professionals involved in the medical imaging
• Patient dose audit and DRLs are important tools
for optimization of patient protection
Summary

Vassileva, ICDA-3 2019, Lisbon j.vassileva@iaea.org
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Diagnostic Reference Levels (DRLs): The concept and use

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