RADIATION PROTECTION IN RADIOTHERAPY

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RADIATION PROTECTION IN RADIOTHERAPY
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 10 Optimization of protection in External
  Beam Radiotherapy
• PRACTICAL EXERCISE

IAEA Post Graduate Educational Course Radiation
Protection and Safe Use of Radiation Sources
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Objectives of Part 10

• Be familiar with the design considerations as
  stipulated by appendix II in the BSS
• Be able to apply these design considerations in
  the context of radiotherapy equipment
• Be aware of relevant international standards and
  other documents which provide specification for
  external beam radiotherapy equipment

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Part 10 External Beam Radiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Practical 1 Determination of the isocentre of an
  external beam radiotherapy unit

IAEA Post Graduate Educational Course Radiation
Protection and Safe Use of Radiation Sources
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Gamma-ray equipment

• Isocentric set-up allows movement of all
  components around the same centre

• collimator
• gantry
• couch

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Gamma-ray equipment

• Isocentric set-up allows movement of all
  components around the same centre

• collimator
• gantry
• couch

Which of the rotations shown here is NOT
isocentric?
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Gamma-ray equipment

• Isocentric set-up allows movement of all
  components around the same centre

• collimator
• gantry
• couch

Isocentre
Which of the rotations shown here is NOT
isocentric?
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Contents

• Understand the concept of an isocentre for
  external beam radiotherapy
• Be able to determine the mechanical isocentre for
  an isocentric unit
• Be able to confirm coincidence of radiation and
  mechanical isocentre
• Interpretation of results

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What Minimum Equipment is Needed?

• Isocentric unit
• Linac
• 60-Cobalt unit
• Simulator
• Frontpointer
• Independent pointer
• Radiographic film (pre-packed)

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Isocentre finding
Courtesy Siemens

• First collimator rotation (determines rotational
  axis)
• Second gantry rotation (defines point in space)
• Third confirm point with couch rotation

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Mechanical frontpointer

• Must be calibrated during commissioning

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Independent pointer
Typically located on couch Can be of different
designs
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Collimator rotation

• Determine axis of rotation
• Best to be done close to nominal SAD
• Can be done with
• frontpointer
• light field or cross hair
• anything which moves with the collimator

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Gantry rotation

• Determine location of isocentre in vertical
  direction
• This often also shows gantry sag
• In other words the isocentre is not a point but
  a volume in space

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Mechanical isocentre accuracy

• Depends on unit specification
• Should be within 2mm for collimator and gantry
• Could (not should) be slightly larger for couch
  rotation

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Mechanical isocentre

• Verify that also other couch movements are
  through the isocentre
• longitudinal
• lateral
• vertical

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Radiation and mechanical isocentre

• Must coincide
• Must also agree with isocentre defined from
  lasers and optics

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Determination of radiation isocentre

• Can be done easily using spoke shots
• close one collimator set to lt5mm
• open second set up to maximum
• repeat with exchanged jaws

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Spokeshot also possible for gantry rotation

• Close jaws orthogonal to plane of gantry rotation
• Can see divergence of the fan beams in the film

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Questions?

• Lets get started...