Transcript IAEA Training Material on Radiation Protection in Radiotherapy

IAEA Training Material on Radiation Protection in Radiotherapy
Radiation Protection in
Radiotherapy
Part 10
Good Practice in EBT
Lecture 1 (cont.): Equipment design
2. Features of safe design in practice
A General considerations
B Kilovoltage radiation units
C Telecurie units
D Megavoltage units
E Other irradiation units
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A. General Safety Requirements
• Radiation Protection Measures include
• Protection of the patient during treatment
• Equipment shielding
• Collimation system
• Patient comfort and control
• Protection of others
• Room shielding (this was covered in part 7)
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Equipment shielding
• Part of dose
reduction
strategy for
patients
• Dose to
patient other
than target
due to scatter
and leakage
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Equipment shielding
• X Ray equipment - only needed when
machine is on
• protects the patient during treatment
• Telecobalt units - shielding needed all
the time
• protects patient and staff during set-up
General design limit - leakage should be less
than 0.1% of the primary radiation
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Testing of shielding integrity of a linac
head using film
About 2t of lead
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Collimation
• Creates outlines of the radiation field which
should conform to the target
• Can be done by a variety of different
measures depending on the treatment unit
type
• Always includes some leakage through the
collimation - typically <2% of the primary
beam
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Collimation
Customized blocks
or prefabricated blocks
in geometric shapes
• Aim to limit field to the
target only
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Collimation
• Applicators
• electron beams
• superficial beams
• Movable jaws
• Lead blocks
• fixed shapes
• customized
• Multileaf collimator
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Custom shielding may reduce the dose to
critical organs
• e.g. scrotal shields to
reduce dose to scrotum due
to scattered radiation
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Patient comfort and control
• The best collimation does not help if the
patient is not stable
• need good immobilization devices
• need to put patient in a reasonably comfortable
position (this is often difficult with very sick
patients)
• need to make them feel comfortable
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Immobilization/set-up devices
• There are innumerable systems - many of
them home built and designed
• A good mould room is essential - they are
responsible for both,
• immobilization and
• block making
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Immobilization/set-up devices
• Head rests
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Head and Neck Immobilization
Head rests to fit
Prone head rest
All MedTec
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Lateral Head position
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Immobilization/set-up devices
• The more accuracy is
required, the more
effort one must make
e.g.:
• Stereotactic head
frame with
repositioning accuracy
better than 2mm
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Immobilization/set-up devices
• Immobilization shells
for head
• Vacuum bag for body
immobilization
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Various body immobilisation devices
Body fix with external markers for set-up
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All MedTec
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Belly board for prone position
• Allows ‘belly’ to
move into space
• Some of the bowel
can be moved out
of the field
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Vacuum bags
Customized for every patient
All MedTec
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Immobilization/set-up devices
• Board for set-up of breast patients
Arm rest to get arm
out of the treatment
field
Head rest
Slope to straighten
sternum in order to
minimize lung dose
Leg rest
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… sometimes movement is difficult to
control...
• e.g. rectal and bladder filling in prostate
treatment
• determine location of the prostate prior to each
treatment fraction using ultrasound
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… sometimes movement is difficult to
control...
• e.g. lung motion due to
breathing
• determine motion and
gate radiation beam
External markers
on the patient
which can be
tracked by a video
system
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Low cost solutions
• Ask patients to
• hold still
• have reproducible bladder filling (e.g. always full
or always empty)
• provide dietary advise
• breath shallow
• Make patients feel comfortable and secure
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A note on intercom systems
• Need to be able to see the patient - is
he/she comfortable? Is she/he moving?
• Need to be able to talk to the patient
• Need to be able to hear if the patient is
in distress
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B. Kilovoltage Equipment (10 - 150 kV)
• Dose rate is approximately proportional to
the nth power of the accelerating potential
as kVn where 2 < n < 3
• Dose rate is approximately proportional to
current (mA)
• Therefore important that kV and mA are
stable.
• It is obviously important that the timer is
accurate and stable
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Kilovoltage Equipment (10 – 150 kV)
• Dose control is achieved by a dual timer
system as it is usually not practical to use a
transmission ionization chamber
• Interlocks should be present to prevent
incorrect combinations of kV, mA, and
filtration
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Quick Question
What are the fluctuations of the mains voltage in your
hospital? What would be the consequence in dose if
these would not be filtered out before generating the
high voltage for the X Ray tube?
Answer
• A +/- 10% voltage variation is not uncommon
due to loading of the net at different times of
the day or heavy occasional uses on the
same mains (e.g. a lift)
• This translates into 40% dose variation
which is unacceptable
• Mains stabilization is a MUST
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Kilovoltage Equipment (10 - 150 kV)
• Leakage from the tube housing, the Air
Kerma Rate (AKR) shall not exceed
• 10 mGy h-1 at 1 metre from focus
• 300 mGy h-1 at 5 cm from housing or accessory
equipment
• if the tube is designed to operate in the range
10 - 50 kV then a special housing is required
with a maximum leakage of 1 mGy h-1
• Testing for hot spots should be carried out
using film-wrap techniques
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Patient shielding
• May be done on the
skin using lead sheets
cut into customized
shapes
• Special shields may be
used - e.g. eye shields
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Kilovoltage Equipment (150 - 400 kV)
Orthovoltage irradiation units
• It is practical to use
a transmission
ionization chamber
with this equipment
and the primary
dose control system
should be an
integrating
dosemeter.
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• The backup (secondary) dose
control system can be either an
independent integrating
dosemeter or a timer
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Kilovoltage Equipment (150 - 400 kV)
• Leakage from the tube housing, the Air
Kerma Rate (AKR) shall not exceed
• 10 mGy h-1 at 1 metre from focus
• 300 mGy h-1 at 5 cm from housing or accessory
equipment (including the beam collimation
system such as cones)
• Testing for hot spots should be carried out
using film-wrap techniques
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C. Telecurie units
• 137-Cs or more
importantly 60-Co
• High activity in
treatment head
• Termination of
exposure is usually
by dual independent
timers
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Timers
• Need two completely
independent timers
• One should count time
up, one down
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Gamma-ray equipment
• The source should be sealed such that the
container can withstand temperatures likely to
be obtained in building fires.
• Wipe tests should be carried out initially at
installation and at regular intervals to check
for surface contamination. This test need not
be carried out directly on the source surface
and can be carried out on a surface which
comes into contact with the source during
normal operation of the equipment.
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Cobalt unit designs
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Gamma-ray equipment
• At commissioning, cross-sectional drawings of the
head should be examined to identify possible
locations where radiation leakage could be a
problem.
• Film wrap techniques can be used to identify
positions of ‘hot’ spots.
• Accurate integrated ionization chamber readings
should be made at the location of any hot spots
and also in a regular pattern around the head.
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Gamma-ray equipment
• Leakage from the
head with the
source in the Off
position: the Air
Kerma Rate (AKR)
shall not exceed
• 10 Gy h-1 at 1 metre
from source
• 200 Gy h-1 at 5 cm
from housing or
accessory equipment
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Gamma-ray equipment
• Leakage from the head with the source in
the On position: the Air Kerma Rate (AKR)
shall not exceed
• 10 mGy h-1 at 1 metre from source or
• 0.1% of the useful beam AKR
• whichever is the greater
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Gamma-ray equipment
• The beam control mechanism shall be
of the ‘fail to safety’ type and will return
to the Off position in the event of:
• end of normal exposure
• any breakdown situation
• interruption of the force holding the beam
control mechanism in the On position, for
example failure of electrical power or
compressed air supply
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Gamma-ray equipment
Mechanical source
position indicator
• In case of failure of the automatic source return
section of the beam control mechanism, it shall be
possible to interrupt the exposure by other means,
for example, a manual return system
• It shall be possible to unload or repair the
treatment head without exceeding the dose limit for
occupational exposure recommended by regulation
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Gamma-ray equipment
• Collimation, patient immobilisation and
blocking as described in first section of part
10 and the case of linacs.
• Two particularities
• No commercial MLC available (but several
home built systems)
• Due to large source size and wide penumbra:
penumbra trimmers (collimation close to the
patient can be employed)
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Specific design for Co units
• Penumbra trimmers - collimation close
to patient reduces penumbra width
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Beam stopper
• Metal disk at the exit side:
• reduces primary beam shielding requirements
• may make set-up of patients more cumbersome
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D Megavoltage units
• Electron linear accelerators - linacs
• Capable of X Ray (4 to 25MV) and
electron (4 to 25MeV) irradiation
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Linacs
• Radiation exposure is usually 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
• The other system is termed the secondary system and is
usually set to terminate the exposure after an additional
dose, typically set around 0.25 Gy
• Most modern accelerators also have a timer which will
terminate the exposure if both ionization chamber systems
fail
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Linacs
• Modern accelerators have a lot of treatment
options as discussed in part 6, for example
• X Rays or electrons (dual mode)
• multiple energies
• 2 X Ray energies
• 5 or more electron energies
• wedges
• 3 or more fixed wedges
• auto-wedge
• dynamic wedge
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Linacs
• 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
provide a clear indication of what functions
have been set
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A linac control example
Active selection
Parameter display
Varian
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Linacs
• 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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Linacs - a note on MLCs
• X Ray Collimators may be
• rectangular (conventional)
• 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 - this can be done
using radiographic film
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Linacs - electrons
• 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
• should be checked for leakage
• adjacent to the open beam
• on the sides of the applicators
Cut-out at the
end of the applicator
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Electron collimation
• Done at the end of the
applicator using
customized cut-outs
Pour
LMA
around
it
Cut-out foam where field should be
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IEC 601.2
• Limit values
at different
locations
around the
useful field
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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
metres centred 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
metre from the path of the electrons between
their origin and the target or electron window
shall not exceed 0.5%
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IEC standard 601.2
• Leakage in from linac
head particularly of
concern if the radiation
can reach the patient
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Guidance on leakage levels
in different parts of the field
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Also consider
• Treatment in different patient positions –
e.g. sitting or standing next to the linac for
treatment of a hand
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Linacs - a note on neutrons
• Neutrons will only be a problem if the X Ray
energy is greater than 10 MV - in practice
consideration MUST be given to neutrons if the
energy is greater than or equal to 15MV
• The rate of equivalent dose of the neutrons should
not exceed 1% of the dose-equivalent rate of the X
Rays - measured in sievert
• The radiation weighting factor for the neutrons
should be taken as 20. The above limit means that
the neutron absorbed dose rate is always less than
the X Ray absorbed dose rate
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Accidents due to equipment design...
• An operator of an accelerator quickly
selected X Ray mode and quickly changed
to electron mode before the machine was
able to complete the first request (to operate
in X Ray mode) and it operated with hybrid
instructions. The same accident occurred in
six different hospitals and two patients died
due to doses as high as about 160-180 Gy
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This should not have happened...
• Contributing factors:
• The computer controlled accelerators were not
tested for the extreme conditions that occurred
in practice at the hospitals.
• It took too long for the manufacturer to identify
the problem and disseminate the information
and by then six hospitals had experienced the
same failure and two patients had died from
radiation
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E Other irradiation units
• Diagnostic units in radiotherapy
• CT scanner
• Simulator
• Other therapy irradiation units
• heavy particles
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Diagnostic units in radiotherapy
• Essential and often integral part of a modern
radiotherapy department
• Essential for adequate target definition - therefore
important also for optimization of medical exposure
from a radiation protection point of view
• Includes not only X Ray equipment but may be
MRI, ultrasound and nuclear medicine
• Beyond the scope of this course - however,
covered in separate courses on diagnostic
radiology and nuclear medicine
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A note on simulators
• The simulator
should be capable
of reproducing all
motions and X Ray
exposure types
(not radiation
energy and dose
though) of the
treatment units
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Simulator control
Patient clearly visible through
large lead glass window
Fluoroscopy screen
Varian Medical Systems
Control screen similar to linac
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Simulators and other diagnostic equipment
• Often the most important aspect of design is
to ensure that the simulator patient set-up
can be transferred without any modifications
to the treatment unit. This includes
‘imperfections’ of the systems such as couch
sag under patient’s weight.
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Heavy Particles
• These could include
• Neutrons
• Protons
• Helium nuclei (alpha particles)
• Other heavy nuclei (Carbon nuclei)
• Negative pi mesons
• Protons are most common and
increasing in their use
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Heavy Particles treatment facilities
• These are very specialized installations
• shielding with high neutron fluxes can be extensive
and complex
• neutrons require hydrogen rich materials for good
energy absorption for example wood and or
plastics
• many neutron interactions produce high energy
gamma rays requiring large thicknesses of
concrete , or steel to absorb them
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Additional note on heavy particles
• Many of the points covered for electron accelerators
are also applicable for these installations
• Specialized systems for positioning patients may be
required
• The charged particle accelerators are often
multipurpose facilities which also serve research
objectives (e.g. material research). These
applications may require entirely different beam
parameters (e.g. high particle flux) than medical
treatment. More care has to be taken to ensure that
only the correct beam can reach the patient.
• There may be several treatment rooms for one
accelerator
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