Radiation Protection in Radiotherapy

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Radiation Protection inRadiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 10
• Good Practice including Radiation Protection in
  EBT
• Lecture 3 Radiotherapy Treatment Planning

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In BSS Treatment Planning is part of 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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and 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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Treatment planning is the task to make sure a
prescription is put into practice in an optimized
way
Prescription
Planning
Treatment
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Objectives

• Understand the general principles of radiotherapy
  treatment planning
• Appreciate different dose calculation algorithms
• Understand the need for testing the treatment
  plan against a set of measurements
• Be able to apply the concepts of optimization of
  medical exposure throughout the treatment
  planning process
• Appreciate the need for quality assurance in
  radiotherapy treatment planning

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Contents of the lecture

• A. Radiotherapy treatment planning concepts
• B. Computerized treatment planning
• C. Treatment Planning commissioning and QA

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The need to understand treatment planning

• IAEA Safety Report Series 17 Lessons learned
  from accidental exposures in radiotherapy
  (Vienna 2000)
• About 1/3 of problems directly related to
  treatment planning!
• May affect individual patient or cohort of
  patients

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A. Basic Radiotherapy Treatment Planning Concepts

• i. Planning process overview
• ii. Patient data required for planning
• iii. Machine data required for planning
• iv. Basic dose calculation

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i. Planning process overview

• Combine machine parameters and individual patient
  data to customize and optimize treatment
• Requires machine data, input of patient data,
  calculation algorithm
• Produces output of data in a form which can be
  used for treatment (the treatment plan)

Patient information
Treatment unit data
Planning
Treatment plan
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The treatment planning process
Individual patient
Radiotherapy treatment units
Beam data radiation quality, PDD, profiles, ...
Patient data CT scan, outlines
Localization of tumor and critical structures
Optimization of source or beam placement
Simulation
Dose calculation
Preparation of treatment sheet and record and
verify data
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Individual patient
Radiotherapy treatment units
Choose for each patient
Beam data radiation quality, PDD, profiles, ...
Patient data CT scan, outlines
Localization of tumor and critical structures
Optimization of source or beam placement
For all patients
For each patient
Simulation
Dose calculation
Preparation of treatment sheet and record and
verify data
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ii. Patient information required

• Radiotherapy is a localized treatment of cancer -
  one needs to know not only the dose but also the
  accurate volume where it has been delivered to.
• This applies to tumour as well as normal
  structures - the irradiation of the latter can
  cause intolerable complications. Again, both
  volume and dose are important.

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One needs to know

• Target location
• Target volume and shape
• Secondary targets - potential tumour spread
• Location of critical structures
• Volume and shape of critical structures
• Radiobiology of structures

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It all comes down to the correct dose to the
correct volume

• Dose Volume Histograms are a way to summarize
  this information

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Dose Volume Histograms
Comparison of three different treatment
techniques (red, blue and green) in terms of dose
to the target and a critical structure
Critical organ
Target dose
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The ideal DVH

• Tumour
• High dose to all
• Homogenous dose

• Critical organ
• Low dose to most of the structure

volume
volume
100
100
dose
dose
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Dose Volume Histograms
Comparison of three different treatment
techniques (red, blue and green) in terms of dose
to the target and a critical structure
Critical organ
Target dose
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Need to keep in mind

• Always a 3D problem
• Different organs may respond differently to
  different dose patterns.
• Question Is a bit of dose to all the organ
  better than a high dose to a small part of the
  organ?

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Organ types

• Serial organs - e.g. spinal cord

• Parallel organ - e.g. lung

High dose region
High dose region
Parallel organ
What difference in response would you expect?
Serial organ
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In practice not always that clear cut

• ICRU report 62
• Need to understand anatomy and physiology
• A clinical decision

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In many organs, dose and volume effects are
linked - e.g.
Boersma et al., classified the following
(Dose,Volume) regions to be regions of high risk
for developing rectal bleeding
Int. J. Radiat. Oncol. Biol. Phys., 1998
4184-92.
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In EBT practice

• Need to know
• where to direct beam to, and
• how large the beam must be and how it should be
  shaped

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Target design and reference images

• In radiotherapy practice the target is localized
  using diagnostic tools
• Diagnostic procedures - palpation, X Ray,
  ultrasound
• Diagnostic procedures - MRI, PET, SPECT
• Diagnostic procedures - CT scan, simulator
  radiograph

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BSS appendix II.18.

• Therapeutic exposure 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 ...

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Optimization of protection

• One part of the optimization of radiotherapy
• Strategies
• Employ shielding where possible
• Use best available radiation quality
• Ensure that plan is actually followed in practice
  verification

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Selection of treatment approach

• Requires training and experience
• May differ from patient to patient
• Requires good diagnostic tools
• Requires accurate spatial information
• May require information obtained from different
  modalities

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Minimum patient data required for external beam
planning

• Target location
• Patient outline

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Diagnostic tools which could be used for patient
data acquisition

• Ruler, calipers, many homemade jigs
• CT scanner, MRI, PET scanner, US,
• Simulator including laser system, optical
  distance indicator (ODI)
• Many functions of the simulator are also
  available on treatment units as an alternative -
  simulator needs the same QA! (compare part 15)

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Simulator
Rotating gantry
Diagnostic X Ray tube
Radiation beam defining system
Simulator couch
Image intensifier and X Ray film holder
Nucletron/Oldelft
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Radiotherapy simulator

• Obtain images and mark beam entry points on the
  patient

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Patient marking
Marks on shell

• Create relation between patient coordinates and
  beam coordinates

Tattoos
Skin markers
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There may be a need to combine images from
different modalities

• Look at them next to each other...

MR angiogram
MR image
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Fusion of images

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Fusion of images

• overlay
• adjust
• scale,
• location,
• orientation

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Registration

• Identify identical points/lines on each image -
  these features could either be part of the
  patient (e.g. bones) or external reference frames
• Computer registers these points/lines -thereby
  changing scale, location and orientation of one
  image

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Image registration

• external frames
• stereotactic procedures

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Image registration with external markers
CT scan
MRI
Leksell fiducial markers on both
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Combination of images

• Look at them next to each other
• Fusion
• Registration
• Matching/molding

The computer automatically finds structures of
interest and combines two images
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Image matching using internal reference points

• Complex computing problem
• Still in its infancy

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Combination of images

• Useful for optimization of the target volume and
  therefore the beam design
• Important for comparison of reference and
  treatment images for verification
• Useful for follow up of patients - compare
  diagnostic scans prior and post treatment...

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Beam placement and shaping
DRR with conformal shielding
simulator film with block
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Tools for optimization of the radiotherapy
approach

• Choice of radiation quality
• Entry point
• Number of beams
• Field size
• Blocks
• Wedges
• Compensators

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Optimization approaches
Choice of best beam angle
beam
beam
target
patient
target
patient
wedge
target
Use of a beam modifier
patient
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Beam number and weighting
Beam 1
beam
50
100
50
target
patient
Beam 2
patient
40
30
10
20
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A note on weighting of beams
Different approaches are possible 1. Weighting
of beams as to how much they contribute to the
dose at the target 2. Weighting of beams as to
how much dose is incident on the patient These
are NOT the same
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40
25
25
30
10
20
25
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Use of wedges

• Wedged pair
• Three field techniques

Isodose lines
patient
patient
Typical isodose lines
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Beam placement and shaping

• Entry point
• Field size
• Blocks
• Wedges
• Compensators

• a two-dimensional approach?

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Beam placement and shaping

• Entry point
• Field size
• Blocks
• Wedges
• Compensators

• Multiple beams
• Dynamic delivery
• Non-coplanar
• Dose compensation (IMRT) not just missing tissue
• Biological planning

This is actually a 3D approach
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Target Localization

• Diagnostic procedures - palpation, X Ray,
  ultrasound
• Diagnostic procedures - MRI, PET, SPECT
• Diagnostic procedures - CT scan, simulator
  radiograph

Allows the creation of Reference Images for
Treatment Verification Simulator Film, Digitally
Reconstructed Radiograph
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Simulator image

• During verification session the treatment is
  set-up on the simulator exactly like it would be
  on the treatment unit.
• A verification film is taken in treatment
  geometry

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Simulator Film

• Shows relevant anatomy
• Indicates field placement and size
• Indicates shielding
• Can be used as reference image for treatment
  verification

Field defining wires
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An alternative reference image

• Simulator image - a real image of the patient

• Computer generated digitally reconstructed
  radiograph (DRR) - shows what the planning
  computer thinks the portal image should look like

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Digitally reconstructed radiographs (DRRs)

• Computer generated virtual images
• Requires patient CT dataset
• Choice of image quality - diagnostic or therapy
  type image
• Depends significantly on the number of CT slices
  available

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DRRs can mimic any geometry

• Divergent beams
• 3D
• Dose pictures

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An alternative reference image

• Simulator image - a real image of the patient

• Computer generated digitally reconstructed
  radiograph (DRR) - shows what the planning
  computer thinks the portal image should look like

A good test Do simulator image and DRR show the
same
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iii. Machine data requirements for treatment
planning

• Beam description (quality, energy)
• Beam geometry (isocentre, gantry, table)
• Field definition (source collimator distance,
  applicators, collimators, blocks, MLC)
• Physical beam modifiers (wedges, compensator)
• Dynamic beam modifiers (dynamic wedge, arcs, MLC
  IMRT)
• Normalization of dose

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Machine data required for planning

• Depends on
• complexity of treatment approaches
• resources available for data acquisition
• May be from published data or can be acquired
• MUST be verified...

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Quick Question

• Who is responsible for the preparation of beam
  data for the planning process in your center?

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Acquisition of machine data

• from vendor or publications (e.g. BJR 17 and 25)
  - this requires verification!!!
• Done by physicist
• Some dosimetric equipment must be available
  (water phantom, ion chambers, film, phantoms,)
• Documentation essential

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Machine data availability

• Hardcopy (isodose charts, output factor tables,
  wedge factors,) - for emergencies and computer
  break downs
• Treatment planning computer (as above or beam
  model) - as standard planning data
• Independent checking device (e.g. MU checks) -
  should be a completely independent set of data

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Machine data availability

• Hardcopy (isodose charts, output factor tables,
  wedge factors,)
• Treatment planning computer (as above or beam
  model)
• Independent checking device (eg. mu checks)

The data must be dated, verified in regular
intervals and the source (including the person
responsible for it) must be documented
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Machine data summary

• Need to include all beams and options (internal
  consistency, conventions, collision protection,
  physical limitations)
• Data can be made available for planning in
  installments as required
• Some data may be required for individual patients
  only (e.g. special treatments)
• Only make available data which is verified

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Quick Question

• What data is available for physical wedges in
  your center?

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iv. Basic dose calculation

• Once one has the target volume, the beam
  orientation and shape one has to calculate how
  long a beam must be on (60-Co or kV X Ray units)
  or how many monitor units must be given (linear
  accelerator) to deliver the desired dose at the
  target.

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Normalization

• Specifies what absolute dose
  should be given to a relative dose value in a
  treatment plan - e.g. deliver 2Gy per fraction to
  the 90 isodose
• Often the reason for misunderstanding
• Should follow recommendation of international
  bodies (compare e.g. ICRU reports 39, 50, 58 and
  62)

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Components of dose calculation for a single beam

• Calibration method - what is the reference
  condition?
• Dose variation with depth and field size -
  covered in percentage depth dose or TPR/TMR data
• Off axis ratio - if the normalization point is
  not on central axis

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Variation of percentage depth dose with field size
10MV photons
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Variation of percentage depth dose with FSD
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TMR, TPR

• Mimic isocentric conditions
• Vary with field size - the larger the field, the
  more scatter, the more dose at depth...

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Dose calculation

• Scatter corrections for field size changes with
  blocking
• Attenuation factors for wedges and trays
• difference between physical and dynamic wedges
• the thicker the wedge, the higher the attenuation
  at central axis

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From single to multiple beams

• Mainly an issue for megavoltage photons where we
  have significant contribution of dose to the
  target from many beams

Beam weighting must be factored in !!!