DOSIMETRY FOR MEDICAL APPLICATION
OF IONIZING RADIATIONS:
Calibration requirements and clinical
applications
Olivera Ciraj
-
Bjelac
, Milojko Kovacevic, Danijela Arandjic, Djordje Lazarevic
Vinca
Institute of Nuclear Sciences
Radiation and Environmental Protection Department
Laboratory for Radiation Measurements
Belgrade, Serbia
ociraj@vinca.rs
Vinca
Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
www.vinca.rs
Content
Global trends in medical exposures
Dosimetric quantities and units
Dosimetry in diagnostic radiology
Metrology and calibration requirements
Clinical application
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Radiation and Environmental Protection Laboratory
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Medical exposure
contributes 99% of
man
-
made
radiation exposure
to humans
The concept of risk
is used to quantify
possible
detrimental effects
Medical exposure to ionizing radiation
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Total dose from man
-
made sources of
radiation>
0.61
mSv
Medical:
0.6
mSv
(>
99.97%)
Source:
United Nations Scientific Committee for Effect of Atomic Radiation (UNSCEAR), 2010
Dose?
Medical exposure to ionizing radiation
The role of dosimetry is to determine the
amount of radiation received by a person
from the radiological examination
Dosimetry in diagnostic radiology
Patient dose assessment
Establishment of Diagnostic Reference Levels
(DRL), optimisation of protection
Assessment of x
-
ray equipment performance
Standards of good practice
Assessment of radiation detriment
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Radiation and Environmental Protection Laboratory
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Global trend
3,6 billion radiological examinations in the
period 1997
-
2007
Increase of 50% compared to previous decade
Significant increase of CT practice:
Examination frequency
Dose per examination
Interventional procedures
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Depends on the examination type
Variations for the same type of procedure
0.02
-
0.05
mSv
2
mSv
100CxR
5
-
20
mSv
400
-
1000
CxR
Dose to patient
50
chest radiographies
=
annual natural background radiation dose
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Proble
ms
Dose for the same
examination type varies up
to 2 orders of magnitude
Increased utilization of
high
-
dose procedures
CT
Interventional
procedures
Increase of probability for
stochastic effects, in
particular in the case of the
repeated examinations
Possible radiation injuries
in high
-
dose procedures
Ef
fects
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Radiation injuries
10
Vinca Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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ICRP 85
Basic metrology elements
International Measurements
System (IMS)
Framework for dosimetry in
diagnostic radiology
Consistency in radiation
dosimetry
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International Measurements System
(IMS)
Bureau International des
Poids
et
Mesures
(BIPM)
National Primary Standard Dosimetry
Laboratories (PSDL)
Secondary Standards Dosimetry Laboratories
(SSDL)
Users performing measurements
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Radiation and Environmental Protection Laboratory
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Traceability chain
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Radiation and Environmental Protection Laboratory
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Metrology and traceability
Dosimeters used to determine doses received
by individuals
Measurements need to be traceable though an
unbroken chain of comparisons
to national
and international standards
Traceability is needed to ensure accuracy and
reliability
Legal and economic implications
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Radiation and Environmental Protection Laboratory
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Role of the SSDL
The prime function:
to provide a service in
metrology
Designated by the competent national
authorities
SSDL
-
Secondary standards, calibrated against
the primary standards of laboratories
participating in the IMS
Vinca Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Journal of the ICRU
Vol
5
No 2 (2005) Report 74
Vinca Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Dosimetric quantities in units in
diagnostic radiology
Basic dosimetric quantity: Air
kerma
Easy to measure
Calibration:
Dosimeters calibrated in terms of air
kerma
Clinical application:
Quantities derived from air
kerma
for different
imaging modalities
Vinca Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Dosimetric quantities
Basic dosimetric quantities
Application specific dosimetric quantities
Quantities for risk assessment
Conversion coefficient for tissue and organ
dose assessment
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Radiation and Environmental Protection Laboratory
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Energy
fluence
Unit:J
/m
2
Kerma
Unit:J
/kg,
Gy
A
bsorbed
dose
Unit:J
/kg,
Gy
Basic dosimetric quantities
Vinca Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Basic dosimetric quantities
Charged
-
particle equilibrium
Absence of
bremsstrahlung
losses
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Radiation and Environmental Protection Laboratory
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Application specific dosimetric
quantities
Quantity
Symbol
Unit
Equation
Incident air
kerma
Ki
Gy
Entrance
-
surface
air
kerma
Ke
Gy
Air
-
kerma
area
product
P
KA
Gym
2
Air
-
kerma
length
product
P
KL
Gym
X
-
ray
tube output
Y(d)
Gy
/As
Vinca Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Vinca Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Application specific dosimetric
quantities: computed tomography
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Radiation and Environmental Protection Laboratory
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Quantity
Symbol
Unit
Equation
CT air
-
kerma
index
(free
in air)
Ca,100
Gy
CT air
-
kerma
index
(in standard
phantom
)
C
PMMA
,
100
Gy
Weighted CT air
kerma
index
Cw
Gy
Normalized
weighted CT air
kerma
index
nCw
Gy
/As
Air
-
kerma
length
product
P
KL
Gym
Application specific dosimetric
quantities: computed tomography
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Quantities describing risk
Organ and tissue dose
Equivalent dose
Effective dose
Dose
-
conversion
coefficients for
assessment of organ
and tissue doses
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The Use of Effective Dose (E)
E
is a risk
-
related quantity and should only be used in the
low
-
dose
range
Primary
use
:
to demonstrate compliance with dose limits
in regulation, for prospective planning of radioprotection
Not
for:
detailed retrospective dose and risk assessments after exposure
of individuals
epidemiological studies, neither in accidents.
In the last cases:
organ doses are needed
!
ICRP 103, ICRP 105
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Effective Dose in Medical Exposure
The relevant quantity for planning the exposure of patients and
risk
-
benefit assessments is the
equivalent dose or the absorbed
dose to irradiated tissues.
The assessment and interpretation of
E
is very problematic
when organs and tissues receive only partial exposure or a very
heterogeneous exposure (x
-
ray diagnostics
)
E
can be of value for comparing doses from
different diagnostic procedures
similar procedures in different hospitals and countries
different technologies for the same medical examination.
ICRP 103, ICRP 105
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Dosimeters in diagnostic radiology
Tube voltage 20
-
150
keV
, various A/F combinations,
various modalities
Ionization chambers
Accurate
Good energy dependence
Design for different
application (cylindrical,
parallel
-
plate, different
volumes..)
Semiconductor dosimeters
Compact
Energy dependant
Others
TLD
OSL
Film (
radiochromic
)
Scintillation
(
kVp
meters)
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PSDL/SSDL
user
Dosimetry standards in diagnostic
radiology
IEC 61674: Dosimeters with
ionization chambers and/or
semi
-
conductor detectors as
used in X
-
ray diagnostic imaging
Diagnostic dosimeter:
detector and measuring
assembly
IEC 60580: Dose area product
meters
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Requirements for
dosemeters
User
IEC 61674
Ionization chambers
Semiconductor
detectors
SSDL
Ionization chamber of
reference class
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Calibrations in diagnostic radiology
Air
kerma
:
Radiography and mammography
Kerma
-
length product
Dosimeters in CT
Kerma
-
area product
Radiography and fluoroscopy
PPV:
kVp
m
eters
Frequency: according to national
regulations
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Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Calibration in diagnostic radiology
Vinca
Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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SSDL
with relevant measurement capabilities
General requirements
:
beam qualities, tube
voltage and filtration measurements
Dosimeter of reference class (with
electrometer)
Calibrated
Quality control
Traceability for all beam qualities
Auxiliary equipment: electrometers,
thermometers, barometers…
Environmental conditions
Equipment
Dosimetry
Ionization chambers
Position system
HV supply for monitor and
reference class ionization
chamber
Electrometer
Radiation source
X
-
ray generator,
50
-
150
kVp
, 20
-
40
kVp
Ripple less than
10%
for
radiography and less than
4%
for mammography
Beam qualities according
IEC 61267
“
Shutter
”
mechanism
Filters and attenuators
Tube voltage meter
(
ppv
,
±
1.
5%)
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Application
Type of
chamber
Range tube
voltage (kV)
Intrinsic
uncertainty
(k=2)
Maximum variation
of response (%)
Range of air
kerma
rate
Unatte
-
nuated
beam
Attenuated
beam
General
radiography
cylindrical or
plane parallel
60
-
150
3.2
±
2.6
1
mGy
/s
-
500
mGy
/s
10
μGy
/s
-
5
mGy
/s
Fluoroscopy
cylindrical or
plane parallel
50
-
100
3.2
±
2.6
0.1
μGy
/s
-
100
μGy
/s
Mammogra
-
phy
plane parallel
22
-
40
3.2
±
2.6
10
μGy
/s
-
10
mGy
/s
CT
cylindrical
100
-
150
3.2
±
2.6
0.1
mGy
/s
-
50
mGy
/s
Dental
radiography
cylindrical or
plane parallel
50
-
90
3.2
±
2.6
1
μGy
/s
-
10
mGy
/s
Reference class dosemeter
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Specification of the x
-
ray beam
Spectrum
X
-
ray beam quality
:
First half
-
value layer
(HVL
1
)
Second half
-
value layer
(HVL
2
)
Homogeneity coefficient
:
Tube voltage
Total filtration
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Radiation beam qualities (IEC 61267)
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Radiation quality
Radiation origin
Phantom material
Application
RQR
Unfiltered beam
emerging from x
-
ray
assembly
No phantom
General radiography,
fluoroscopy, dental
radiology
RQA
Radiation beam from
an added filter
Aluminium
Measurements
behind the patient
(on the image
intensifier)
RQT
Radiation beam from
an added filter
Copper
CT applications (free
in air)
RQR
-
M
Unfiltered beam
emerging from x
-
ray
assembly
No phantom
Mammography (free
in air)
RQA
-
M
Radiation beam from
an added filter
Aluminium
Measurements
behind the
patient
Typical calibration set up
X
-
ray tube
window
Focal
spot
S
hutter
A
pertures
Additional
filtration
Monitor
chamber
Test point
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Calibration procedures
Procedures before calibration (acclimatization, positioning,
stabilization…)
Calibration procedures (methods, number of
measurements, interval between
measurements…corrections…
Procedures following calibration (uncertainty budget,
certificate…)
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Institute of Nuclear Sciences
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Dosimetry formalism
Air
kerma
:
Reference conditions
:
set of influencing quantities
Influencing condition
:
quantities that are not subject of
mesusremst but have an impact on the result
Air density correction
:
Beam quality correction
:
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Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Calibrations of
dosemeters
for CT
Traditionally, irradiation of the whole volume
Contras
:
Information on chamber response only
Size on active volume only assumed
Far from real situation
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Institute of Nuclear Sciences
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Calibration for CT
:
air
kerma
length
product
Cylindrical chamber
, 100 mm
Non
-
uniform irradiation
Uniform response
RQT 9
(120
kVp
, HVL: 8.5 mm Al)
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Fo
cal
spot
M
onitor
chamber
A
pertur
e
Ionization
chamber
w
d
a
d
r
In laboratory (SSDL)
F
ield
calibration
Film
10 cm
10 cm
Ref.
chamber
Calibration for fluoroscopy
:
air
kerma
area product
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Cekerevac
at al, Poster B3
Calibration in terms of practical peak
voltage
X
-
ray tube voltage
measurements
Practical
Peak
Voltage
(
ppv
):
Property of the whole
exposure cycle
Related to image contrast
Invasive or non
-
invasive
measurements
Voltage divider
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Uncertainty of the reference standard
Uncertainty of user’s instrument
Uncertainty due to calibration set up
Uncertainty of the evaluation procedure
Uncertainty budget
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Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Uncertainty budget
Air
kerma
:
±
2.7 %
Air
kerma
length product:
±
3.0 %
Air
kerma
area product:
±
15 %
Non
-
invasive tube voltage measuring devices:
2.5 %
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Institute of Nuclear Sciences
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Goal: minimal uncertainty
Assurance and control of traceability
Quality manual: technical details, methods, traceability,
uncertainty budget, QC, safety….
Continuous improvements and reviews
External peer review/audit
Quality Management System
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Patient dose assessment
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Clinical dosimetry
Direct measurement on
patients or phantoms
Indirect measurements
on patients or phantoms
Output of the X
-
ray tube,
scaled for exposure and
geometry
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Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Dosimetric
quantites
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Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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BSF
KAP
Ke
Quantities
a)
incident air
kerma
, entrance
surface air
kerma
and
kerma
-
area product (radiography);
b)
kerma
-
area product and
entrance surface air
kerma
rate (fluoroscopy);
c)
incident and entrance surface
air
kerma
(mammography);
and
d)
kerma
-
length product
(computed tomography)
Patients and phantoms
Patient
Real situation
Phantoms
Objects that simulate real
patients in terms of
interaction of radiation
with matter
Easy to perform
Standardized
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Radiography
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Fluoroscopy
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Institute of Nuclear Sciences
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Mammography
Vinca
Institute of Nuclear Sciences
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Computed tomography
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Institute of Nuclear Sciences
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Patient dose levels
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Institute of Nuclear Sciences
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Uncertainty of clinical dose
assessment
Radiographers taking the
x
-
ray images
Determining tube output
Calculation of individual
patient doses
Determining dose to an
average patient
Use of k=2 for expression
of uncertainty of dose
assessment
Typically >10% and close
to 25%*
*if correction for beam quality and for individual patient is not applied
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Institute of Nuclear Sciences
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Form measurements towards risk
assessment
Conversion coefficients
Conversion of measured quantity into organ doses
and effective dose
Ratio of the dose to a specified tissue or effective
dose divided by the normalization quantity
Measured using phantoms or calculated using
computer models
Voxel
phantoms based on images of human
anatomy
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Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Organ dose
assesment
Vinca
Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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ICRU 74
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Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Optimization of protection
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Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Application of DRLs
Values
of
measured
quantities
above
which
some
specified
action
or
decision
should
be
taken
Values
must
be
specified
Action
must
be
specified
DRLs
will
be
intended
for
use
as
a
convenient
test
for
identifying
situations
where
the
levels
of
patient
dose
are
unusually
high
.
Quantities
that
are
easily
measured!
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Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Diagnostic Reference Levels
Doses to patients from radiographic and fluoroscopic X
-
ray imaging procedures
in the UK
—
2005 review. HPA RPD
-
029, HPA; 2007.
Vinca
Institute of Nuclear Sciences
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Re
-
cap
Diagnostic radiology is major contribution to total dose
from man
-
made sources of radiation
Dose measurements: population dose assessment,
optimization of practice
Application
-
specific dosimetric quantities (patients,
phantoms)
Calibration of dosimeters in the conditions that are similar
to the clinical environment, in terms of
air
kerma
kerma
-
area product
kerma
-
length product
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Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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Olivera
Ciraj
-
Bjelac
,
PhD, research associate, dosimetry and radiation physics
Milojko Kovacevic,
MSc
, Head of MDL, radiation physicist
Danijela Arandjic,
MSc
, PhD student, dosimetry and radiation physics
Djordje Lazarevic,
MSc
, PhD student, dosimetry and radiation physics
Dragana
Divnic
, technician
Milos
Jovanovic
, technician
Nikola
Blagojevic
, technician
Vinca
Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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