Radiation Protection in Radiotherapy

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Radiation Protection inRadiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 10
• Good Practice in EBT
• Lecture 1 (cont.) Equipment design

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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 lt2 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
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 lt n lt 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?

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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.

• 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
  manufacturers 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 levelsin 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 patients
  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