Radiation Protection in Radiotherapy

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Radiation Protection inRadiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 10
• Medical Exposure Good Practice including
  Radiation Protection in External Beam Radiotherapy

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IAEA BSS Medical Exposure

• Exposure incurred by patients as part of their
  own medical or dental diagnosis and treatment
• by persons, other than occupationally exposed,
  knowingly while voluntarily helping in support
  and comfort of patients
• by volunteers in a program of biomedical research
  involving their exposure

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Medical Exposure Patients

• In external beam radiotherapy (EBT) the quality
  of the treatment depends essentially on two
  parameters
• the accurate dose given, and
• the exact localization of that dose in a
  well-defined target (by avoiding normal
  surrounding tissues as much as possible)

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External beam radiotherapy

• Support persons during irradiation not
  applicable - no one must be in the treatment room
  during irradiation
• Children are to be sedated or anesthetized during
  irradiation

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External beam

• Volunteers not possible in the conventional way,
  however considered must be
• participants in clinical trials
• patients who undergo radiotherapy and are
  evaluated for other radiation effects
• patients who undergo radiotherapy and have their
  treatment modified to answer research questions.

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Objectives of the lecture(s)

• Be aware of the implications of optimization on
  equipment design
• Be able to discuss methods to ensure accurate
  dosimetry
• Be able to understand the optimization process in
  radiotherapy planning and the delivery of the
  dose to the appropriate target
• Be familiar with the radiotherapy treatment
  planning process

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Contents

• Lecture 1 Equipment and Design
• Lecture 2 Dosimetry
• Lecture 3 Radiotherapy Treatment Planning
• Lecture 4 Treatment Verification and Reporting

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Radiation Protection inRadiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 10
• Good Practice in EBT
• Lecture 1 Equipment design

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Equipment in radiotherapy

• Radiotherapy relies - probably more than many
  other medical specialties - on equipment
• Equipment design and performance is therefore of
  paramount importance for radiotherapy

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Objectives

• 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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Contents

• 1. Design considerations
• 2. Features of safe design in practice
• 3. Operational considerations

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Design Considerations (BSS appendix II.11)

• Equipment used in medical exposure shall be so
  designed that
• failure of a single component of the system be
  promptly detectable so that any unplanned medical
  exposure of patients is minimized
• the incidence of human error in the delivery of
  unplanned medical exposure be minimized

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Requires Defense in Depth

• Redundant safety features
• Interactive equipment interface
• Self checks
• Intuitive software

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Design Considerations BSS II.12

• Registrants and licensees shall
• (a) taking into account information provided by
  suppliers, identify possible equipment failures
  and human errors that could result in unplanned
  medical exposures
• (b) take all reasonable measures to prevent
  failures and errors, including the selection of
  suitably qualified personnel, the establishment
  of adequate procedures for the calibration,
  quality assurance and operation of diagnostic and
  therapeutic equipment, and the provision to
  personnel of appropriate training and periodic
  retraining in the procedures, including
  protection and safety aspects

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This would have prevented the following accident

• Maintenance problem...
• A misadjustment of the electron energy of an
  accelerator resulted in overdosage of 27
  patients the doses were between 3 and 10 times
  higher than intended, causing several deaths
• Frequent failures of an accelerator, with
  frequent interruption of the treatments led to
  the decision to operate in physical mode this
  decision resulted in one death

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Accident after maintenance

• Contributing factors to the accident (amongst
  others)
• It was possible to operate the machine with the
  energy selector disabled
• Conflicting displays and signals on the control
  panel were misinterpreted (the energy selector
  indicated different energy than an instrument on
  the control panel)

Both factors should have been eliminated by good
design
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Design Considerations BSS II.12 (continued)

• Registrants and licensees shall
• (c) take all reasonable measures to minimize the
  consequences of failures and errors that may
  occur
• (d) develop appropriate contingency plans for
  responding to events that may occur, display
  plans prominently, and periodically conduct
  practice drills

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Mitigation and emergency plans

• Discussed in part 13 of the course

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Design Considerations BSS II.13

• Registrants and licensees, in specific
  co-operation with suppliers
• (a) the equipment conform to applicable standards
  of the International Electrotechnical Commission
  (IEC) and the ISO or to equivalent national
  standards
• (b) performance specifications and operating and
  maintenance instructions, including protection
  and safety instructions, be provided in a major
  world language understandable to the users and in
  compliance with the relevant IEC or ISO standards
  with regard to 'accompanying documents', and that
  this information be translated into local
  languages when appropriate

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IEC standards

• In reference list
• Often adopted into national standards - these may
  contain additional requirements
• http//www.iec.ch/

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Design Considerations BSS II.13 (cont.)

• Registrants and licensees, in specific
  co-operation with suppliers
• (c) where practicable, the operating terminology
  (or its abbreviations) and operating values be
  displayed on operating consoles in a major world
  language acceptable to the user
• (d) radiation beam control mechanisms be
  provided, including devices that indicate clearly
  and in a fail-safe manner whether the beam is
  on or off

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In radiotherapy practice

• Clear Indicators shall be provided at the control
  console and in the treatment room to show when
  the equipment is in operation
• A secondary independent indicator (e.g. a
  radiation monitor inside the treatment room) may
  also be useful

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General Safety Requirements

• Warning Signals and Signs

RADIATION DO NOT ENTER When RED LIGHT is on.
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Design Considerations BSS II.13 (cont.)

• Registrants and licensees, in specific
  co-operation with suppliers
• (e) as nearly as practicable, the exposure be
  limited to the area being examined or treated by
  using collimating devices aligned with the
  radiation beam
• (f) the radiation field within the examination or
  treatment area without any radiation beam
  modifiers (such as wedges) be as uniform as
  practicable and the non-uniformity be stated by
  the supplier
• (g) exposure rates outside the examination or
  treatment area due to radiation leakage or
  scattering be kept as low as reasonably
  achievable

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In radiotherapy practice

• From a practical point these are the most
  important stipulations regarding medical exposure
  within the BSS
• They direct the user towards conformal (limit the
  area) intensity modulated (homogenous dose
  distribution) radiotherapy using best possible
  equipment (leakage is minimized)

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Conformity

• Shielding of areas which shall not be irradiated
• Use of blocks - best customized for each
  individual patient

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Customization of blocks

• Use block outline on simulator film to cut the
  block shape into a Styrofoam block

Huestis
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Customization of blocks

• Pour low melting alloy into foam
• Customized blocks include divergence of the beam
• Blocks are mounted on trays

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Conformal radiotherapy

• Conform the treated volume (receiving a
  therapeutic dose) to the planning target volume
• shield all areas surrounding it
• MLC is an option for this

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Quick Question

• What are the differences between blocks and MLC
  for shielding in practice?

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Blocks versus MLC

• Blocks
• More work
• Lifting of heavy blocks required
• No leakage
• Divergence covered
• Isolated blocks (e.g. larynx shield) possible

• MLC
• Interleaf leakage needs to be considered
• Flexible
• Dynamic shielding possible
• Required for most IMRT
• High initial investment, no additional cost per
  patient

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Intensity modulation

• Optimize the dose distribution
• Make dose in the target homogenous
• Minimize dose out of the target
• Different techniques
• physical compensators
• intensity modulation using multileaf collimators

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Physical Compensator
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Compensator manufacturing

• Sheets of lead glued together (compare previous
  slide)
• Automatic milling into foam - this can be filled
  with low melting alloy or steel shot

Par Scientific
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Compensator manufacturing

• Several manufactures of automatic compensator
  cutters
• QC for each compensator is required

Huestis
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Intensity Modulation
MLC pattern 1

• Achieved using a Multi Leaf Collimator (MLC)
• The field shape can be altered
• either step-by-step or
• dynamically while dose is delivered

MLC pattern 2
MLC pattern 3
Intensity map
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BSS appendix II Requirements for radiation
generators and irradiation installations for
radiotherapy

• II.15. Registrants and licensees, in specific
  co-operation with suppliers, shall ensure that
• (a) radiation generators and irradiation
  installations include provisions for
  selection, reliable indication and
  confirmation (when appropriate and to the
  extent feasible) of operational parameters
  such as type of radiation, indication of
  energy, beam modifiers (such as filters),
  treatment distance, field size, beam
  orientation and either treatment time or
  preset dose

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Two important thoughts

• Radiotherapy depends on manufacturers and should
  be working with them to achieve common goals
• Many parameters determine a particular radiation
  treatment - equipment should both allow
  unambiguous selection of these parameters AND
  indication of what has been selected

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BSS appendix II Design criteria specific to
radiotherapy

• II.15. Registrants and licensees, in specific
  co-operation with suppliers, shall ensure that
• ... (b) irradiation installations using
  radioactive sources be fail-safe in the sense
  that the source will be automatically shielded in
  the event of an interruption of power and will
  remain shielded until the beam control mechanism
  is reactivated from the control panel ...

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Fail to safety concept

• If something fails the unit automatically turns
  the beam OFF
• This includes power failures
• X Ray units / linacs turn off
• radioactive sources must be withdrawn
  automatically e.g. via a spring mechanism or
  gravity (fail safe)

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BSS appendix II Design criteria specific to
radiotherapy

• II.15. Registrants and licensees, in specific
  co-operation with suppliers, shall ensure that
• ...
• (c) high energy radiotherapy equipment
• (i) have at least two independent 'fail to
  safety' systems for terminating the irradiation
  and
• (ii) be provided with safety interlocks or other
  means designed to prevent the clinical use of the
  machine in conditions other than those selected
  at the control panel

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In radiotherapy practice

• These 'fail to safety' systems for terminating
  the irradiation could be for example
• two independent integrating in-beam dosimeters
  (e.g. most linacs)
• or two independent timers (e.g. 60-Co units)
• or an integrating dosimeter and timer
• Each system should be capable of terminating the
  exposure

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A linac control example
Additional timer
Two independent dosimeters MU1 and MU2
Varian
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A kV X Ray unit
Two independent timers elapsed time and
remaining time
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BSS appendix II Design criteria specific to
radiotherapy

• II.15. Registrants and licensees, in specific
  co-operation with suppliers, shall ensure that
• ...
• (d) the design of safety interlocks be such that
  operation of the installation during maintenance
  procedures, if interlocks are bypassed, could be
  performed only under direct control of the
  maintenance personnel using appropriate devices,
  codes or keys

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Safety requirement

• All modes of the equipment must be accessible
  only via a key and password
• This affects in particular physics or
  maintenance modes, in which interlocks can be
  overwritten and system parameters changed
• No treatment must be performed in service mode

NEVER give your password to anyone...
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BSS appendix II Design criteria specific to
radiotherapy

• II.15. Registrants and licensees, in specific
  co-operation with suppliers, shall ensure that
• ...
• (e) radioactive sources for either teletherapy
  or brachytherapy be so constructed that they
  conform to the definition of a sealed source and
  
• (f) when appropriate, monitoring equipment be
  installed or be available to give warning of an
  unusual situation in the use of radiation
  generators and radionuclide therapy equipment.

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In radiotherapy practice...

• This is a wide ranging requirement
• Usually it is sufficient if conditions which
  affect correct operation do just make the
  treatment unit inoperable
• One may then check later WHY this is the case...

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A note on suppliers

• All the BSS quotes so far have included the
  phrase in specific co-operation with suppliers
• This emphasizes the close collaboration which is
  required between manufacturers and operators of a
  radiotherapy facility
• The collaboration is essential in regards to
• documentation
• service/maintenance
• training