Radiation Protection in Radiotherapy

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Radiation Protection inRadiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 5
• Properties and safety of radiotherapy sources and
  equipment used for external beam radiotherapy

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IAEA Safety Series 120, Safety Fundamentals (1996)

• Source Anything that may cause radiation
  exposure an X-ray unit may be a source

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External Beam Radiotherapy
Beam 2
Beam 3
Beam 1
tumour
patient
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External Beam Therapy (EBT)

• Non-invasive
• Target localization important and beam placement
  may be tricky
• Usually multiple beams to place target in the
  focus of all beams

Multiple non- coplanar beams
Single beam
Three coplanar beam
patient
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External Beam Radiotherapy

• More than 90 of all radiotherapy patients are
  treated using EBT
• Most of these are treated using X Rays ranging
  from 20keV to 20MeV in peak-energy
• Other EBT treatment options include telecurie
  units (60-Co and 137-Cs), electrons from linear
  accelerators and accelerators for heavy charged
  particles such as protons

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Objectives

• To become familiar with different radiation types
  used for external beam radiotherapy
• To understand the function of different equipment
  used for EBT delivery
• To appreciate the implications of different
  treatment units and their design
• To be familiar with auxiliary equipment required
  and used for EBT
• To understand the measures used in this equipment
  to ensure radiation safety

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Contents

• Lecture 1 Radiation types and techniques
• Lecture 2 Equipment and safe design

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Radiation Protection inRadiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 5
• External Beam RT
• Lecture 1 Radiation types and techniques

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Objectives

• To be familiar with different radiation types
  used in EBT
• To appreciate the technical needs to make these
  radiation types applicable to radiotherapy
• To understand common external beam radiotherapy
  techniques

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Contents

• 1. External Beam Radiotherapy process
• 2. Radiation qualities in use
• 3. Delivery techniques
• 4. Prescription and reporting
• 5. Special procedures

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1. EBT process
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EBT processUse of radiation
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Note on the role of diagnosis

• The responsibility of clinicians
• Without appropriate diagnosis the justification
  of the treatment is doubtful
• Diagnosis is important for target design and the
  dose required for cure or palliation

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Note on the role of simulation

• Simulator is often used twice in the radiotherapy
  process
• Patient data acquisition - target localization,
  contours, outlines
• Verification - can the plan be put into practice?
  Acquisition of reference images for verification
• Simulator may be replaced by other diagnostic
  equipment or virtual simulation

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Simulator

• However, some functions can be replaced by other
  diagnostic X Ray units provided the location of
  the X Ray field can be marked on the patient
  unambiguously
• Other functions (isocentricity) can then be
  mimicked on the treatment unit

• Important to mimic isocentric treatment
  environment

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Virtual simulation

• All aspects of simulator work are performed on a
  3D data set of the patient
• This requires high quality 3D CT data of the
  patient in treatment position
• Verification can be performed using digitally
  reconstructed radiographs (DRRs)

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CT Simulation (Thanks to ADAC)
Marking the Patient already during CT
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Virtual Simulation
3D Model of the patient and the Treatment Devices
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Digitally Reconstructed Radiographs as reference
image for verification
View and print DRRs for all planned
fields Improved confidence for planning and
reference for verification
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Note on the role of treatment planning

• Links prescription to reality
• The center piece of radiotherapy
• Becomes more and more sophisticated and complex
• Extensive discussion in part 10

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2. External beam radiotherapy (EBT) treatment
approaches

• Superficial X Rays
• Orthovoltage X Rays
• Telecurie units
• Megavoltage X Rays
• Electrons
• Heavy charged particles
• Others

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External beam radiotherapy (EBT) treatment
approaches

• Superficial X Rays
• Orthovoltage X Rays
• Telecurie units
• Megavoltage X Rays
• Electrons
• Heavy charged particles
• Others

• 40 to 120kVp
• 150 to 400kVp
• 137-Cs and 60-Co
• Linear accelerators
• Linear accelerators
• Protons from cyclotron, C, Ar, ...
• Neutrons, pions

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Photon percentage depth dose comparison for
photon beams
Superficial beam
Orthovoltage beam
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Superficial radiotherapy

• 50 to 120kVp - similar to diagnostic X Ray
  qualities
• Low penetration
• Limited to skin lesions treated with single beam
• Typically small field sizes
• Applicators required to collimate beam on
  patients skin
• Short distance between X Ray focus and skin

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Superficial radiotherapy
Philips RT 100
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Superficial radiotherapy issues

• Due to short FSD high output and large influence
  of inverse square law
• Calibration difficult (strong dose gradient,
  electron contamination)
• Dose determined by a timer - on/off effects must
  be considered
• Photon beams may be contaminated with electrons
  from the applicator

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Orthovoltage radiotherapy

• 150 - 400kVp
• Penetration sufficient for palliative treatment
  of bone lesions relatively close to the surface
  (ribs, spinal cord)
• Largely replaced by other treatment modalities

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Orthovoltage Equipment (150 - 400 kVp)

• Depth dose dramatically affected by the FSD

FSD 6cm, HVL 6.8mm Cu
FSD 30cm, HVL 4.4mm Cu
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Orthovoltage patient set-up

• Like for superficial irradiation units the beam
  is set-up with cones directly on the patients
  skin

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Megavoltage radiotherapy

• 60-Cobalt (energy 1.25MeV)
• Linear accelerators (4 to 25MVp)
• Skin sparing in photon beams
• Typical focus to skin distance 80 to 100cm
• Isocentrically mounted

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Photon percentage depth dose comparison

• PHOTONS

• ELECTRONS

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Typical locations of tumor and normal tissues

• PHOTONS

• ELECTRONS

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Build-up effectResult of the forward direction
of secondary electrons - they deposit energy down
stream from the original interaction point
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Build-up effect

• Clinically important as all radiation beams in
  external radiotherapy go through the skin
• Is reduced in large field sizes and oblique
  incidence and when trays are placed in the beam
• Can be avoided by the use of bolus on the patient
  if skin or scar shall be treated

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Isocentric set-up
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Isocentric set-up

• Result of the large FSDs possible with modern
  equipment
• Places the tumour in the centre - multiple
  radiation beams are easily set-up to deliver
  radiation from many directions to the target

Image from VARIAN webpage
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Common photon treatment techniques

• Two parallel opposed fields
• lung
• breast
• head and neck

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Common photon treatment techniques

• Four field box
• cervix
• prostate

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Isocentric or not?

• All the beam arrangements discussed so far can be
  set-up with a fixed distance (e.g. 80cm) to the
  patients skin or isocentrically with a fixed
  distance to the centre of the target.

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Photon beam modification

• Blocks
• Wedges
• Compensators

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Shielding blocks
Customized shielding block

• Beam shaping
• Conform the high dose region to the target
• Fixed blocks
• Customized blocks made from low melting alloy
  (LMA)
• Partially replaced now by Multi Leaf Collimator
  (MLC)

Siemens MLC
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Physical wedge
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Wedges

• One dimensional dose modification
• Different realizations
• Now often a dynamic wedge

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Use of wedges

• Wedged pair
• Three field techniques

Isodose lines
patient
patient
Typical isodose lines
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Compensators

• Physical compensators
• lead sheets
• brass blocks
• customized milling
• Intensity modulation
• multiple static fields
• arcs
• dynamic MLC

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Intensity modulation

• Can be shown to allow optimization of the dose
  distribution
• Make dose in the target homogenous
• Minimize dose outside the target
• Different techniques
• physical compensators
• intensity modulation using multileaf collimators

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Intensity Modulation
MLC pattern 1
MLC pattern 2

• Achieved using a Multi Leaf Collimator (MLC)
• The field shape can be altered
• either step-by-step or
• dynamically while dose is delivered

MLC pattern 3
Intensity map
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Dynamic treatment techniques

• Arcs
• Dynamic wedge
• Dynamic MLC
• increasing complexity with increasing
  flexibility in dose delivery. Verification
  becomes essential

patient
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Electron radiotherapy

• Finite range
• Rapid dose fall off

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Characteristics of an electron beam
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Electron beam isodoses (20MeV)
Watch dose increase (115!) due to oblique
incidence
Watch bulging of isodoses at depth
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Other issues with electron beams
Dose distribution significantly affected by
surface contour changes - this must be considered
when using bolus to shape dose distribution at
depth.
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Inhomogeneities affect the dose distribution
Air cavity
Monte Carlo Calculations
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Use of electrons

• Skin lesions
• Scar boosting
• Avoidance of deep lying sensitive structures
  (e.g. spinal cord)

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More issues with the use of electrons for
radiotherapy

• Computer prediction of dose distribution more
  difficult
• Small fields difficult to predict
• Dosimetry somewhat more difficult than in photons
  due to strong dose gradients and variation of
  electron energy with depth

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Other radiation types

• Neutrons
• Complex radiobiology
• Complex interactions
• Potential advantages for hypoxic and
  radioresistant tumors
• Not widely used
• Protons - probably the most promising other
  radiation type

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Comparison to other radiation types
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Potential Advantage of Proton radiotherapy dose
sparing before and behind the target due to Bragg
peak
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X Rays versus protons
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4. Prescription and reporting

• Prescription is the responsibility of individual
  clinicians, depending on the patients condition,
  equipment available, experience and training.
• The prescription should follow protocols which
  are established by professional organizations and
  modified and adopted by radiotherapy departments.
• The prescription must be informed - as far as
  possible - by clinical evidence

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Prescription and reporting

• Prescription may vary within reason depending on
  equipment available
• Reporting must be uniform - any adequately
  educated person must be able to understand what
  happened to the patient in case of
• need for a different clinician to continue
  treatment
• re-treatment of the patient
• clinical trials
• potential litigation

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Recommendations by the ICRU

• International Commission on Radiation Units and
  Measurements
• ICRU reports provide guidance on prescribing,
  recording and reporting

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Target delineation

• ICRU report 50

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Definitions form ICRU 50

• Gross Tumour Volume (GTV) clinically
  demonstrated tumour
• Clinical Target Volume (CTV) GTV area at risk
  (e.g. potentially involved lymph nodes)

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Definitions form ICRU 50

• Planning Target Volume (PTV) volume planned to
  be treated CTV margin for set-up
  uncertainties and potential of organ movement

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Strategies for margins

• Margins are most important for clinical
  radiotherapy - they depend on
• organ motion - internal margin
• patient set-up and beam alignment - external
  margin
• Margins can be non-uniform but should be three
  dimensional
• A reasonable way of thinking would be Choose
  margins so that the target is in the treated
  field at least 95 of the time

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Definitions form ICRU 50

• Treated Volume volume that receives dose
  considered adequate for clinical objective
• Irradiated volume dose considered not
  negligible for normal tissues

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• The concept of margins was expanded on by ICRU
  report 62
• Internal margin due to organ motion
• Set-up margin
• The two are often combined as independent
  uncertainties

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5. Special procedures

• Total body irradiation
• Total electron skin irradiation
• Stereotactic radiosurgery

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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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Issues with TBSI

• Use low energy electrons (4 or 6MeV)
• Spoiler in front of patient improves dose
  distribution
• in vivo dosimetry required
• shielding of nails and eyes
• boost of some areas (e.g. under arms) may be
  required

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Stereotactic procedures

• Target usually brain lesions
• External head frame used to ensure accurate
  patient positioning
• Invasive or
• Re-locatable

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Image registration

• Variety of systems
• Many frame attachments to allow for different
  diagnostic modalities (MRI, CT, angiography)

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Image registration
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Stereotactic procedures
Both systems MedTec

• Spatial accuracy around 1mm
• High dose single fraction (e.g. for
  arterio-venous malformations) stereotactic
  radiosurgery using an invasively mounted head
  frame
• Multiple fractions for tumour treatment
  stereotactic radiotherapy using a re-locatable
  head immobilisation

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EBT verification tools

• Correct location
• portal films
• electronic portal imaging
• Correct dose
• phantom measurements
• in vivo dosimetry

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EBT verification tools

• Correct location
• portal films
• electronic portal imaging
• Correct dose
• phantom measurements
• in vivo dosimetry

• Part 10 with some comments in second lecture part
  5 (now)
• Parts 2 and 10

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Summary

• A wide variety of radiation qualities are
  available for the optimization of radiotherapy
  for individual patients
• The choice depends on patient and availability of
  equipment
• Given adequate understanding of radiation
  properties and patient requirements many highly
  specialized procedures have been developed to
  address problems in radiotherapy

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Have we achieved the objectives?

• To be familiar with different radiation types
  used in EBT
• To appreciate the technical needs to make these
  radiation types applicable to radiotherapy
• To understand common external beam radiotherapy
  techniques

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Where to Get More Information

• Part 10 relates directly to this part
• References
• Karzmark, C, Nunan C and Tanabe E. Medical
  electron accelerators. McGraw Hill, New York,
  1993.
• Site visit of ...

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Any questions?
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Question

• Please put together a table comparing electron
  and X Rays produced by linear accelerators

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X Rays and electrons in EBT
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Acknowledgments

• John Drew, Westmead Hospital, Sydney
• Patricia Ostwald, Newcastle Mater Hospital