IAEA Training Material on Radiation Protection in Radiotherapy
Radiation Protection in
Radiotherapy
Part 10
Good Practice including Radiation
Protection in EBT
Lecture 2: Dosimetry
Dose in radiotherapy
Is the therapeutic agent
 Is high - radiotherapy means putting as
much dose into the target as possible
 Carries some risk of severe
complications
 Must be delivered very accurately

Radiation Protection in Radiotherapy
Part 10, lecture 2: Dosimetry
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Required dose
accuracy


Depends on
steepness of the dose
response curve
5% difference in dose
make a 15%
difference in tumour
control probability in
head and neck
patients - this is
clinically detectable
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Delivery of dose within +/-5%

Sources of uncertainty:
Absolute dosimetry/calibration
 Relative dosimetry (%depth dose, profiles,
output factors)
 Treatment planning (estimated uncertainty
of the order of +/- 2%)
 Machine performance on the day (+/- 2%)
 Patient set-up and movement (+/- 3%)

Not much room for error in dosimetry...
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Objectives
Understand the principles of beam
calibration
 Appreciate the objectives of clinical
dosimetry
 Identify methods for in vivo dose
verification on patients undergoing
external beam radiotherapy

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Contents
1. Calibration
2. Clinical dosimetry
Beam data acquisition
 Phantom measurements
 In vivo dosimetry

3. External audits
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Absolute and relative dosimetry

Absolute dosimetry is a technique that yields
information directly on absorbed dose in Gy. This
absolute dosimetric measurement is also referred to
as calibration. All further measurements are then
referenced to this standard geometry i.e. relative
dosimetry is performed. In general no factors are
required in relative dosimetry since it is only the
comparison of two dosimeter readings, one of them
being in reference conditions.
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1. Calibration




Determine absolute dose in Gy at one
reference point in the beam
Determines the beam on time or the number
of monitor units required to deliver a certain
dose
Very important - if this is wrong, everything
will be wrong
In the BSS framework part of the optimization
of medical exposure
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Optimization of protection in
therapeutic exposure

BSS appendix II.18.
“Registrants and
licensees shall ensure that:
(a) exposure of normal tissue during
radiotherapy be kept as low as reasonably
achievable consistent with delivering the
required dose to the planning target
volume, and organ shielding be used when
feasible and appropriate; ...
(e) the patient be informed of possible risks.”
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Important note on optimization
1. The dose only to normal tissues shall be kept
as low as reasonable achievable
2. In practice, the dose to the target in radical
radiotherapy shall be as high as possible to
maximize the chances of tumour control
 The two requirements may be seen at times
as incompatible - the key lies in the term
“reasonable”
 What is “reasonable” is a decision which the
patient and the clinician must make
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Important note
1. The dose only to normal tissues shall be kept
as low as reasonable achievable
2. The dose to the target in radical radiotherapy
shall be as high as possible to maximize the
chances of tumour control


In practice usually the second objective takes
precedence in radical treatments - if the tumour
cannot be controlled, there is no point sparing
normal tissues...
One must still protect normal tissues as much as
possible...
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Mis-calibration is an important
contributor to accident in EBT

Calibration of beams


Accidents due to mistakes in the determination of the
dose rate caused overdosage to as many as 115, 207,
426 patients… by as much as 60%
There were other accidents, related to misinterpretation
of a calibration certificate, of a reported pressure value
for correction, a change of physicist with poor
information transfer; a wrong use of a plane-parallel
ionization chamber
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Accidents due to calibration
mistakes

Contributing factors to accidents



Lack of understanding of beam calibration,
certificates, conversion factors and dosimetry
instruments… lack of training and expertise
within radiotherapy physics
Lack of redundant and independent
determination of the absorbed dose (mistakes
were not detected)
Lack of formal procedures for communication
and change of personnel
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Accidents due to calibration
mistakes

Contributing factors
to accidents

In one of the cases, for 22
months, there was no
verification of the beam; the
physicist was devoted to a new
accelerator and “ignored” the
Co-60 unit (There was a lack of
revision of the staff needs when
a new accelerator was installed)
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BSS appendix II.19.

“Registrants and licensees shall ensure
that:
(a)
the calibration of sources used for medical
exposure be traceable to a Standards
dosimetry laboratory; …”
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The IAEA/WHO SSDL Network
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Traceability of calibration
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Traceability

National Strategy
Frequency established by the Regulatory
Authority
If there is no SDL in the country, the national
strategy should include institutional
arrangements to facilitate quick import/export
and additional arrangements among several
countries
Redundancy in the calibration of new sources
and beams
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BSS appendix II.19.

“Registrants and licensees shall
ensure that: ...
(b)
radiotherapy equipment be
calibrated in terms of radiation quality
or energy and either absorbed dose
or absorbed dose rate at a predefined
distance under specified conditions,
e.g. following the recommendations
given in IAEA Technical Reports
Series No. 277 [20];
…”
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Calibration


Determination of the dose at a reference point
- correlation of treatment time or ‘monitor
units’ with absolute dose
Absolute dosimetry required:
Dose = const * Detector Signal

Const. must be well known and fundamental:



Calorimetry
Ionometry W/e
Chemical dosimetry g
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Calibration protocols


Calibration is a complex process requiring an
expert in radiation oncology physics
There are many protocols which can provide
guidance


international (e.g. IAEA TRS 277 or TRS 398)
national (usually developed by the national
medical physics association) – e.g. AAPM TG 21,
AAPM TG 51, DIN 68, ...
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Calibration protocols


It is essential to
follow ONE protocol
It is essential to
follow the protocol
BY THE LETTER there is no room for
error...
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Part 10, lecture 2: Dosimetry
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Forms are
available



Radiation Protection in Radiotherapy
Very helpful for
guidance
Available for
most protocols
Here shown for
IAEA TRS 398
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Calibration protocols
There has been a development from
protocols based on calibrations at the
national standard lab in air as air
KERMA or exposure, to calibration in
terms of absorbed dose to water…
 This development has been in parallel
at the IAEA and many national
associations (e.g. AAPM)

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Move to absorbed dose to water
calibration


Follows
improved
capability of
national
standard labs
Same in US by
moving from
AAPM TG21
(1983) to
AAPM TG51
(2000)
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Which protocol to use?
Depends on how the ionization chamber
has been calibrated in the standards
lab. If one has an air KERMA calibration
factor (NK) or an exposure factor (NX),
TRS-398 CANNOT be used…
 If also the dose to water factor (NDw)
can be provided by the laboratory, TRS
398 can be used.

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Advantages of absorbed dose
calibration
The exposure/



KERMA way
Easier for the
user
Less factors
required
Get NDw
directly - only
conversion for
beam quality
required
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A note on calibration
The process is beyond the scope of the
present course
 Calibration is a very critical process
 Calibration (in particular using
exposure/KERMA formalism) is
complex (>10 factors)
 It should always be checked by an
independent person

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A second note: Calibration can
link the absolute dose to a variety
of different reference conditions
It is essential to
know what your
reference
conditions are.
(They are
typically linked
to the treatment
planning system
in use)
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BSS appendix II.19.

“Registrants and licensees shall ensure
that: ...
(e)
the calibrations be carried out at the time
of commissioning a unit, after any maintenance
procedure that may have an effect on the
dosimetry and at intervals approved by the
Regulatory Authority.”
The maximum interval in practice for re-calibration is
1 year - less if there is any indication of problems
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Absolute dosimetry


Can be done in
principle using
calorimetry, chemical
dosimetry or ionization
chambers
For radiotherapy
practice all protocols
are based on ionization
chambers
Farmer type chamber
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Tools required for calibration

In practice a Farmer type ionization
chamber - air volume 0.6cc for photons
and high energy electrons
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Plane parallel chamber

Required for low energy
electrons (<5MeV) and
recommended for
electrons with energy
below 10MeV due to
steep dose gradients
PTW Markus chamber
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Plane parallel chamber
2mm
Adapted from Kron in VanDyk 1999
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Ionization Chamber reading
require correction for

Air pressure: require an accurate
barometer for calibration purposes


an error of 10 mBar will give an error of 1%
in the calibration
Temperature: accurate thermometer

an error of 3 degrees centigrade will give
an error of 1% in the calibration
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Calibration
Records

BSS appendix II.32.
“Registrants and
licensees shall keep and make
available, as required, the results of the
calibrations and periodic checks of the
relevant physical and clinical
parameters selected during treatments.”
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2. Clinical
dosimetry

In the context of BSS,
dosimetry has two
components:
a) dose measurements
(dealt with in the present
lecture) and
b) dose planning
discussed more
extensively in the fourth
lecture of part 10
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There are
multiple
objectives
for dose
measurements in
radiotherapy
practice
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Roles of clinical dose
measurements in radiotherapy
Data collection for treatment planning in
general
 Data collection for individual patients
 Dose verification

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Clinical Dosimetry

BSS appendix II.20. “Registrants and
licensees shall ensure that the following
items be determined and documented:
...
(b)
for each patient treated with external beam
radiotherapy equipment, the maximum and minimum
absorbed doses to the planning target volume
together with the absorbed dose to a relevant point
such as the centre of the planning target volume, plus
the dose to other relevant points selected by the
medical practitioner prescribing the treatment; …”
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In radiotherapy practice:

This means dose
measurements are
required as


as dose
determination for
the treatment of
individual patients
input for treatment
planning systems
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Dose measurement for individual
patients
In vivo dosimetry
 Determination of output for electron cutouts or compensators
 Assessment of dose distribution in
complex treatments (e.g. IMRT)

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Dosimetry as part of
commissioning of equipment


In the past this has been more the
determination of unknown dose rather than
verification, however, these days most beam
parameters are within tight specifications and
known prior to commissioning.
Commissioning affects both:
 Treatment units
 Treatment planning
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Treatment unit
commissioning

Aspects:





Safety
Verification that specs are
met
Other bits and pieces
required for planning
Many protocols and
guidelines available
Usually done using a
water phantom and slab
phantoms
Radiation Protection in Radiotherapy

Significant time
commitment - however,
access is usually not a
problem
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Tools for
commissioning


Mainly scanning
water phantom
Determine all
properties of all
radiation beams




depth dose, TPR
profiles
wedges
blocks,...
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Phantoms


In radiotherapy the term "phantom" is used to
describe a material and structure which
models the radiation absorption and
scattering properties of human tissues of
interest.
There are many different phantoms for a
variety of purposes available in radiotherapy
dosimetry. Phantoms are an essential part of
the dosimetric process.
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Commissioning of treatment
planning






Non-dose related components
Photon dose calculations
Electron dose calculations
Brachytherapy
Data transfer
Compare lecture 4
Special procedures
in the present part 10
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Typical dosimetric accuracy required
(examples)




From AAPM TG53
Radiation Protection in Radiotherapy

Square field CAX:
1%
MLC penumbra: 3%
Wedge outer beam:
5%
Buildup-region: 30%
3D inhomogeneity
CAX: 5%
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Typical accuracy required (examples)


The required
accuracy depends
on situation and
purpose
Uncertainty has two
components: dose
uncertainty AND
spatial localization
uncertainty
Radiation Protection in Radiotherapy





Square field CAX:
1%
MLC penumbra: 3%
Wedge outer beam:
5%
Buildup-region: 30%
3D inhomogeneity
CAX: 5%
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Requirements for
dosimetry


Required accuracy
depends on
situation and
purpose
Uncertainty has two
components: dose
uncertainty AND
spatial localization
uncertainty
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Clinical Dosimetry is not
only applicable to the tumor

BSS appendix II.20. “Registrants and
licensees shall ensure that the following
items be determined and documented:
...
(e)
in all radiotherapeutic treatments, the absorbed
doses to relevant organs….”
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Dose measurements in
phantoms


Phantoms mimic
radiological
properties of
patients
Different complexity


from slabs of tissue
equivalent material
to anthropomorphic
phantoms
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Examples for phantoms
Slab phantom for consistency measurements
IMRT verification
phantom
Small water phantom for
calibration
Anthropomorphic
head phantom
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Phantoms are available to mimic all
aspects of patients and all types of patients

Radiation Protection in Radiotherapy
Example: Pediatric
phantom and CT scans of
the phantom
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…but no phantom mimics everything
Therefore, one must be aware of the
limitations of each material and
phantom
 This means also other - and often
cheaper - materials can be used to test
a particular property of the radiation
beam.

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Clinical Dosimetry

BSS appendix II.21.: “In radiotherapeutic
treatments, registrants and licensees shall
ensure, within the ranges achievable by good
clinical practice and optimized functioning of
equipment, that:
(a) the prescribed absorbed dose at the prescribed
beam quality be delivered to the planning target
volume; and
(b) doses to other tissues and organs be minimized.
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Minimization of dose to normal
tissues



Optimization of
beam direction
Beam shaping using
blocks or MLC
Conformal therapy =
conform high dose
envelope to target
region
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Optimization of beam direction

Avoid critical
structures - e.g.


spine in a lung
cancer treatment
lung in breast
radiotherapy,
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Blocks to avoid lung

Radiation Protection in Radiotherapy
Could be
customized or
prefabricated
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Customized shielding

Depends on
approach and
radiation quality
Multi Leaf Collimator for
megavoltage photons
Radiation Protection in Radiotherapy
Eye shields for
superficial radiation
beams
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Organ specific
shielding


Radiation Protection in Radiotherapy
Scrotal shields for
megavoltage photon
treatments
Suitable for scattered
radiation not primary beam
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Verification of dose for
individual patients
Each patient is different
 Each (radical) treatment is different

Routine verification of single fields, e.g.
electron output factors, compensator
factors
 Now increasingly verification of full 3D
dose distribution for complex high-tech
treatments, e.g. IMRT, HDR brachytherapy

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Dosimetry for special procedures

Difficult to model in treatment planning
Unusual geometry
 Good patient data is not available
 Rare occurrence


Examples:
Most of brachytherapy
 Total Body Irradiation (TBI)
 Total electron skin irradiation (TBSI)

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Total body irradiation (TBI)
Target: Bone marrow
 Different techniques available

2 lateral fields at extended FSD
 AP and PA
 moving of patient through the beam

Typically impossible to do a
computerized treatment plan
 Need many measurements

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TBI: one possible patient position
Rice bags
Radiation field
at >3m FSD;
collimator rotated
Placed all around body
to achieve two distinct
separations
Breast board
Angle of breast board
adjusted for individual
patients
Couch top
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Issues with TBI
In vivo dosimetry essential
 May need low dose rate treatment
 Shielding of critical organs (e.g. lung)
and thin body parts may be required


this can be only for parts of the treatment
to achieve the best possible dose
uniformity
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Total electron skin irradiation
Treat all skin to very shallow depth
 Different techniques available

4 or 6 fields
 rotating patient

Impossible to plan using a computer
 Requires many measurements for beam
characterization

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Total Body Skin Irradiation


Multiple electron
fields at extended
FSD
Whole body skin as
target
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Many of these applications also
benefit from IN VIVO dosimetry
ICRU report 24 (1976):
“An ultimate check of the actual treatment given
can only be made by using in vivo dosimetry.”
Single Quality Assurance
Activities: Quality Control
Hand calculation of
treatment time
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Part 10, lecture 2: Dosimetry
Check source
activity
70
Treatment Verification: in vivo
dosimetry
Treatment
verification
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In vivo dosimetry
Checks large parts of the treatment
chain at once – one detects if
something is wrong but not necessarily
what the problem is.
 Good strategy when things are mostly
OK and within tight tolerances
 Requires resources
 Can prevent accidents

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Why do in vivo dosimetry


Quality Control – Treatment Verification
Measure because we don’t know



Limitations of dose planning
Patient movement
Verify dose for the record



Critical organs
Legal aspects
Clinical trials
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Methods for in vivo dosimetry
Thermoluminescence dosimetry
 Semiconductors

diodes
 MOSFETs


Exit dose measurements
portal films
 electronic portal imaging devices

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Thermoluminescence
Dosimeters
 Small physical size
 Tissue equivalence (at
least some materials)
 No cables, high voltage
or bias required
 High sensitivity - wide
dosimetric range
 Cheap, reusable
 Many physical forms
and materials available
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Example for TLD in vivo dosimetry:
Lens dose measurements
TLD detector
7 mm of wax bolus
to mimick the position
of the lens under the lid
TLD
detectors
lens of
eye
arangement in AP or PA
radiation fields
lens of
eye
arangement in lateral radiation fields
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Semiconductors
diodes
MOSFETs
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Features of semiconductors
+ Small
+ On-line
+ Easy to use
+ Small - versatile
+ Small - arrays
-
Temperature
dependence
Cables needed
Generally not
tissue equivalent
possible
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Documentation of all
dosimetric measurements?
Absolutely essential
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3. Dosimetric audits
No one is infallible…
 Dosimetry may be a difficult and
complex task
 Defense in depth requires redundant
check
 A fresh look from outside can verify
dosimetry

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WHO/IAEA photon dose
Dose Quality Audit
TLD capsules
Level 1 Dose Quality Audit:
Dose in Reference Conditions
FS 10x10, d5cm
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Aim: The dose at reference conditions
should be the same all over the world
1Gy
1Gy
1Gy
1Gy
1Gy
1Gy
1Gy
Radiation Protection in Radiotherapy
1Gy
Part 10, lecture 2: Dosimetry
1Gy
1Gy
83
Participation
in IAEA/WHO
postal dose
quality audit
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Results of IAEA/WHO postal
dose quality audit
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IAEA/WHO postal dose quality audit
Important result: Centres which have
participated previously in the audit are
significantly less likely to have a
deviation of measured from expected
dose.
 Audits are not only a check but also a
tool for improvement...

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Prostate Radiotherapy
Are we as sure about the correct dose?
CTV dose 2Gy
CTV
dose 2Gy
?
CTV
dose 2Gy
Radiation Protection in Radiotherapy
Part 10, lecture 2: Dosimetry
CTV dose 2Gy
87
Level III dosimetric
intercomparisons
Use of ‘anthropomorphic’ phantom
 Check entire treatment chain

Radiation Protection in Radiotherapy
Part 10, lecture 2: Dosimetry
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Dosimetric Intercomparison



Level 1: Absolute calibration at reference
point (e.g. IAEA/WHO postal TLD service)
Level 2: Include simple physical phantom to
gather additional information (e.g. wedge
factors, %DD, profiles)
Level 3: Check of whole treatment chain
using an anthropomorphic phantom (e.g.
TROG study)
Radiation Protection in Radiotherapy
Part 10, lecture 2: Dosimetry
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Anthropomorphic phantom CAN
travel...
ART
radiotherapy
phantom in
TROG
study
(Kron et al.
IJROBP 2002)
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Part 10, lecture 2: Dosimetry
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Summary




Dose determines treatment outcome and should be
controlled within 5%
Calibration of treatment units must be traceable to
national standards and should be performed by
qualified experts following appropriate protocols
There are a wide variety of tasks and techniques
available for clinical dosimetry
In vivo dosimetry and external audits are valuable
verifications of dose delivery in a radiotherapy centre
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Part 10, lecture 2: Dosimetry
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Where to Get More Information
Radiotherapy textbooks
 Calibration protocols

Radiation Protection in Radiotherapy
Part 10, lecture 2: Dosimetry
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Any questions?
Question:
Please comment from your experience on the
advantages and disadvantages of radiographic film
as a radiotherapy dosimeter.
Radiographic film as a
radiotherapy dosimeter
Advantages
 Two dimensional
 Widely available
 Relatively cheap
 Provides a dose
record
 Highly sensitive
Radiation Protection in Radiotherapy
Disadvantages
 Depends on
developing
 Not very accurate
 Could be too
sensitive
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