Transcript IAEA Training Material on Radiation Protection in Radiotherapy

IAEA Training Material on Radiation Protection in Radiotherapy
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
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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IAEA Training Material on Radiation Protection in Radiotherapy
Part 10: External Beam
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
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 <5mm
open second set up
to maximum
repeat with
exchanged jaws
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Spokeshot also possible for
gantry rotation


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Close jaws
orthogonal to plane
of gantry rotation
Can see divergence
of the fan beams in
the film
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Questions?
Let’s get started...
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