IAEA Training Material on Radiation Protection in Radiotherapy
Radiation Protection in
Radiotherapy
Part 10
Medical Exposure: Good Practice including
Radiation Protection in External Beam
Radiotherapy
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 welldefined 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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IAEA Training Material on Radiation Protection in Radiotherapy
Radiation Protection in
Radiotherapy
Part 10
Good Practice in EBT
Lecture 1: Equipment design
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 cooperation 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 cooperation 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 cooperation 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?
Blocks versus MLC
• Blocks
• More work
• Lifting of heavy
blocks required
• No leakage
• Divergence
covered
• Isolated blocks
(e.g. larynx
shield) possible
Radiation Protection in Radiotherapy
• 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
MLC pattern 2
MLC pattern 3
• either step-by-step
or
• dynamically while
dose is delivered
Radiation Protection in Radiotherapy
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 cooperation 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 cooperation 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
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