Transcript IAEA Training Material on Radiation Protection in Radiotherapy

IAEA Training Material on Radiation Protection in Radiotherapy
Radiation Protection in
Radiotherapy
Part 5
External Beam Radiotherapy
Lecture 2 (cont.): Equipment and safe design
3. Medical Linear Accelerators



Short: “linac”
Most radiotherapy
patients are treated
using linacs
Several
manufacturers
Courtesy Siemens
Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
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Photon percentage depth dose
comparison

PHOTONS

ELECTRONS
Linac beams
Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
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Different designs
Touch guard
Wall panel to
hide stand
Couch with
controls
Hand control
Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
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The problem: require >4MeV
electrons
Not possible to achieve
this conventionally
using a potential
difference
 Electrons are
accelerate using
microwaves

Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
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Schematic drawing of a linac
Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators

6 MV short waveguide
Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
No bending
magnet
7
Electron Accelerators

18 MV long waveguide
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators

Waveguides for
acceleration of
electrons using
microwaves
Short standing
wave guide
Buncher for initial
acceleration of
electrons
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators

Bending the electron beam
Achromatic magnet:
All energies are
focused onto the
target
Slits for selection
of electron energy
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Part 5, lecture 2 (cont.): Equipment - linacs
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Treatment
head
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Electron Accelerators
Radiation exposure is controlled by two
independent integrating transmission
ionization chamber systems.
 One of these is designated as the
primary system and should terminate
the exposure at the correct number of
monitor units
 These also steer the beam via a
feedback loop

Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
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Monitor ion chamber design
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Two independent chambers - redundant check of
dose delivered
Each chamber segmented - allows feedback for
flatness and symmetry
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators
The other system is termed the
secondary system and is usually set to
terminate the exposure after an
additional 0.4 Gy
 Most modern accelerators also have a
timer which will terminate the exposure
if both ionization chamber systems fail

Defense in depth at work!!!
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators

Modern accelerators have a
lot of treatment options, for
example
X Rays or electrons (dual
mode)
 multiple energies
 2 X Ray energies
 5 or more electron energies

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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators

Head complexity
to handle multiple
energies and
multiple
modalities



different flattening
filters and scattering
foils on a ‘carousel’
monitor chambers
collimators
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators

X Ray Collimators may be (1)
 rectangular (conventional)
 the transmission through the collimators
should be less than 2% of the primary (open)
beam
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators

X Ray Collimators may be (2)
 Multi-Leaf collimators (MLC)
 the transmission through the collimators
should be less than 2% of the primary (open)
beam
 The transmission between the leaves should
be checked to ensure that it is less than the
manufacturer’s specification
Siemens MLC
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators
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Electron applicators, these may be
 open sided for modern accelerators using double
scattering foils or scanned beams
 enclosed for older accelerators using single
scattering foils
both types should be checked for
leakage
 adjacent to the open beam
 on the sides of the applicators
Varian open sided
electron cone
Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
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Important Accessories
Wedges
 Dynamic wedges
 Blocks
 Multileaf Collimator (MLC)
 Electronic Portal Imaging (EPID)

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Electron Accelerators

angle
Wedges
 3 or more fixed
wedges
 auto-wedge
 dynamic wedge

Modify dose
distribution
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators

Dynamic wedge
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Part 5, lecture 2 (cont.): Equipment - linacs
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Wedges
‘standard’ treatment accessory
 required for example in breast and head
and neck treatment
 dynamic wedge most popular because:

no weight
 any wedge angle possible
 but difficult to commission

Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators

Asymmetric collimator
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators

Asymmetric collimators

Read-outs on the linac gantry
Collimator
rotation
read-out
Right collimator
read-out
Lower collimator
read-out
Left collimator
read-out
Gantry
read-out
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Multileaf Collimator (MLC)
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
Used to define any
field shape for
radiation beams
Several variations to
the theme:
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different leaf widths
(1cm to 0.4cm)
replaces collimators
or additional to
normal collimators
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Linac mounted MLC
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MLC
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Radiation Protection in Radiotherapy
The quality of the
field definition
depends on the width
of the leafs
There is always
some interleaf
leakage
Typically the
transmission through
the MLC is larger
than through a
standard collimator
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Multileaf Collimator (MLC)
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Dynamic MLC
Concept similar to dynamic wedge
 When MLC moves during treatment
different parts of the field are shielded
resulting in different overall radiation
levels delivered in different parts of the
beam: Intensity modulated radiotherapy

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Intensity
Modulation
MLC pattern 1
MLC pattern 2
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
Achieved using a
Multi Leaf Collimator
(MLC)
The field shape is altered
step-by-step or dynamically
while dose is delivered
Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
MLC pattern 3
Intensity
map
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Two approaches to IMRT
Linac based IMRT

Multiple individual
fields, each of them
intensity modulated in
two dimensions
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Two approaches to IMRT

Continuous rotation of
a one dimensional fan
beam which consists
of many beamlets
which can be turned
on or off
Tomotherapy
Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
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Components of
Helical Tomotherapy
Binary
MLC
Ring detector at exit side
Radiation Protection in Radiotherapy
Helical Scanning
Part 5, lecture 2 (cont.): Equipment - linacs
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Comments on IMRT
Best possible dose distribution with
photons
 No intuitive link between MLC settings,
monitor units and the delivered dose
distribution
 Impossible without computers in
diagnostics, planning and delivery
 Challenges for QA

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Electronic Portal Imaging
Imaging device at the beam exit side of
the patient to record the treatment field
 Allows to verify that the field was
delivered to the correct location in the
patient
 Many different systems available...

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Part 5, lecture 2 (cont.): Equipment - linacs
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Electronic portal imaging devices
in practice
Siemens
Radiation Protection in Radiotherapy
Varian
Part 5, lecture 2 (cont.): Equipment - linacs
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Electronic portal image
Comparison of
simulator and portal
image (right)
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron beams
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No target required
Scattering foil used to
produce larger beam alternative would be to
scan the pencil beam
using electromagnetic
fields
Applicator required to
produce good field
delineation on the
patient
Radiation Protection in Radiotherapy
Electron
Beam
Primary
Collimator
Scattering Foil
Part 5, lecture 2 (cont.): Equipment - linacs
Ion Chamber
Secondary
Collimator
Electron
applicator
Patient
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Electron
applicator on
a modern
Varian linear
accelerator
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron applicators

may be
open sided for modern accelerators using
double scattering foils or scanned beams
 enclosed for older accelerators using single
scattering foils


must be checked for leakage
adjacent to the open beam
 on the sides of the applicators

Radiation Protection in Radiotherapy
Part 5, lecture 2 (cont.): Equipment - linacs
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Model of an
electron
applicator for
Monte Carlo
Calculations
Electron
applicator
patient
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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron
Accelerators
With such a large number of possible
settings it is essential that interlocks be
provided to prevent inappropriate
combinations from being selected
 It is also essential that the control
console provides a clear indication of
what functions have been set

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Part 5, lecture 2 (cont.): Equipment - linacs
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Electron Accelerators
Complex control system
 Reliance on computers

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Part 5, lecture 2 (cont.): Equipment - linacs
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Varian Clinac operation screens

Clinical mode
Radiation Protection in Radiotherapy
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Service mode
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Electron Accelerators
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Verification systems

All accelerator manufacturers now produce
computer controlled verification systems
which provide an additional check that the
settings on the accelerator console are
correct for
 proper accelerator function and
 correspond exactly with the parameters
determined for the individual patient during the
treatment planning process
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Electron Accelerators

Head leakage

the Air Kerma Rate (AKR) due to leakage
radiation at any point outside the maximum useful
beam, but inside a plane circular area with a
radius of 2 meters centered around, and
perpendicular to, the central axis of the beam at
the normal distance of treatment shall not exceed
0.2% of the AKR at the central axis of the open
beam. The measurement shall be done with a
thick shielding block covering the open beam
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Electron Accelerators

Head leakage

Except in the area defined in the previous
slide the Air Kerma Rate (AKR) due to
leakage radiation (excluding neutrons) at
any point 1 meter from the path of the
electrons between their origin and the
target or electron window shall not exceed
0.5%
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Electron Accelerators

Neutrons
these will only be a problem if the X Ray
energy is equal to or greater than 15 MV
 issues which need to be considered when
neutrons are presents include:

 neutron activation
 shielding problems
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