DOSIMETRY FOR MEDICAL APPLICATION OF IONIZING RADIATIONS: Calibration requirements and clinical applications

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


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs


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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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


Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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





Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs




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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Radiation injuries

10

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

ICRP 85

Basic metrology elements


International Measurements
System (IMS)


Framework for dosimetry in
diagnostic radiology


Consistency in radiation
dosimetry


Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Traceability chain

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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


Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

www.vinca.rs

Journal of the ICRU
Vol

5

No 2 (2005) Report 74


Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

www.vinca.rs

Dosimetric quantities


Basic dosimetric quantities


Application specific dosimetric quantities



Quantities for risk assessment



Conversion coefficient for tissue and organ
dose assessment

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs



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

www.vinca.rs

Basic dosimetric quantities




Charged
-
particle equilibrium


Absence of
bremsstrahlung

losses

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

www.vinca.rs

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Application specific dosimetric
quantities: computed tomography

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Quantities describing risk


Organ and tissue dose


Equivalent dose


Effective dose





Dose
-
conversion
coefficients for
assessment of organ
and tissue doses

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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)


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Requirements for
dosemeters

User


IEC 61674


Ionization chambers


Semiconductor
detectors

SSDL


Ionization chamber of
reference class

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Calibration in diagnostic radiology

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs


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%)


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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




Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Radiation beam qualities (IEC 61267)

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Calibration procedures


Procedures before calibration (acclimatization, positioning,
stabilization…)


Calibration procedures (methods, number of
measurements, interval between
measurements…corrections…


Procedures following calibration (uncertainty budget,
certificate…)

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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
:

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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)



Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs


Uncertainty of the reference standard


Uncertainty of user’s instrument


Uncertainty due to calibration set up


Uncertainty of the evaluation procedure

Uncertainty budget

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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 %


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs


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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Patient dose assessment


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Dosimetric
quantites

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Radiography

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Fluoroscopy

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Mammography

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Computed tomography

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Patient dose levels

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Organ dose
assesment

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

ICRU 74


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Optimization of protection

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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!

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

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

Radiation and Environmental Protection Laboratory

www.vinca.rs

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


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs


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

www.vinca.rs