Radiation Protection in Radiotherapy

1
Radiation Protection inRadiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 10
• Good Practice including Radiation Protection in
  EBT
• Lecture 2 Dosimetry

2
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

3
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

4
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

13
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
• Const. must be well known and fundamental
• Calorimetry
• Ionometry W/e
• Chemical dosimetry g

Dose const Detector Signal
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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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Forms are available

• 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 (gt10 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 (lt5MeV) 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
measure-ments 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 cut-outs 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

• 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
• Special procedures

Compare lecture 4 in the present part 10
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Typical dosimetric accuracy required (examples)

• Square field CAX 1
• MLC penumbra 3
• Wedge outer beam 5
• Buildup-region 30
• 3D inhomogeneity CAX 5

From AAPM TG53
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Typical accuracy required (examples)

• Square field CAX 1
• MLC penumbra 3
• Wedge outer beam 5
• Buildup-region 30
• 3D inhomogeneity CAX 5

• The required accuracy depends on situation and
  purpose
• Uncertainty has two components dose uncertainty
  AND spatial localization uncertainty

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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

• 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

• Could be customized or prefabricated

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Customized shielding

• Depends on approach and radiation quality

Eye shields for superficial radiation beams
Multi Leaf Collimator for megavoltage photons
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Organ specific shielding

• 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
Radiation field at gt3m FSD collimator rotated
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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.

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Single Quality Assurance Activities Quality
Control
Check source activity
Hand calculation of treatment time
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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 dont 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
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Semiconductors
diodes
MOSFETs
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Features of semiconductors

• Small
• On-line
• Easy to use
• Small - versatile
• Small - arrays possible

• Temperature dependence
• Cables needed
• Generally not tissue equivalent

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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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(No Transcript)
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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
1Gy
1Gy
1Gy
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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 RadiotherapyAre we as sure about the
correct dose?
CTV dose 2Gy
?
CTV dose 2Gy
CTV dose 2Gy
CTV dose 2Gy
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Level III dosimetric intercomparisons

• Use of anthropomorphic phantom
• Check entire treatment chain

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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)

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Anthropomorphic phantom CAN travel...
ART radiotherapy phantom in TROG study (Kron et
al. IJROBP 2002)
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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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Where to Get More Information

• Radiotherapy textbooks
• Calibration protocols

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Any questions?
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Question

• Please comment from your experience on the
  advantages and disadvantages of radiographic film
  as a radiotherapy dosimeter.

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Radiographic film as a radiotherapy dosimeter

• Advantages
• Two dimensional
• Widely available
• Relatively cheap
• Provides a dose record
• Highly sensitive

• Disadvantages
• Depends on developing
• Not very accurate
• Could be too sensitive