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Medical Physics Support of Linear Accelerators. Overview of Physics Support. Accelerator safety issues Task Group Report #35 Acceptance testing Perform radiation protection survey Verify accelerator characteristics are within specifications Task Group Report #45 Commissioning

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overview of physics support
Overview of Physics Support
  • Accelerator safety issues
    • Task Group Report #35
  • Acceptance testing
    • Perform radiation protection survey
    • Verify accelerator characteristics are within specifications
    • Task Group Report #45
  • Commissioning
    • Collect and prepare beam data for clinical use
    • Task Group Report #45
  • Quality Assurance
    • Daily, Weekly, Monthly, Annual
    • Task Group Report #40
accelerator safety
Accelerator Safety
  • AAPM Task Group Report #35 (TG-35) covers safety issues that the medical physicist should be aware of.
  • Two FDA classifications of hazards
    • Class I – causes serious injury or death
      • Type A hazard are directly responsible for life-threatening complications
      • Type B hazard increases probability of unacceptable outcome (complication or lack of tumor control)
    • Class II – hazards where the risk of serious injury are small
accelerator safety1
Accelerator Safety
  • Most common hazards
    • Incorrect radiation dose
    • Dose delivered to wrong region
    • Collision between patient and machine
    • Incorrect beam energy or modality
    • Electrical/mechanical problems
  • Class I, Type A hazard involves improper delivery of 25% of prescribed dose.
radiation protection regulation
Radiation Protection Regulation
  • Regulatory bodies
    • Linear Accelerators
      • National Council on Radiation Protection and Measurements (NCRP)
      • Individual states (Suggested State Regulations for Control of Radiation, SSRCR)
    • Cobalt-60
      • Nuclear Regulatory Commission (NRC)
exposure limits
Exposure Limits
  • NCRP Report #116 replaces Report #91
    • Occupation Limits (controlled areas)
      • Whole body – 50 mSv / yr (1 mSv / wk)
      • Infrequent / Planned – 100 mSv
      • Lens of Eye – 150 mSv / yr
      • Pregnant Worker – 5 mSv / term (0.5 mSv / mo)
      • Lifetime – 10 mSv x Age (years)
    • Public Limits (noncontrolled areas)
      • Whole body – 1 mSv / yr (0.02 mSv / wk)
      • Infrequent / Planned – 5 mSv
      • Extremities, Skin, Lens of Eye – 50 mSv / yr
radiation protection survey
Radiation Protection Survey
  • Performed after accelerator is installed and beams are calibrated.
    • NCRP Report #51 was the standard reference
    • NCRP Report #144 updates and expands on #51
  • Neutron leakage measurements should be done for nominal photon energies 15 MV and above.
    • NCRP Report #79
    • AAPM Report #19
  • Survey meter
    • Should be capable of detecting exposure levels from 0.2 mR/hr to 1 R/hr.
    • AAPM TG-45 recommends survey meter be calibrated once a year.
      • Required by law if Cobalt-60 unit is present in facility.
acceptance testing
Acceptance Testing
  • Manufacturers have Acceptance Testing Procedures (ATPs) which engineers and physicist follow and sign off on.
    • Sometimes a machine might ordered with specifications beyond what the manufacturer provides.
  • Types of ATPs
    • Radiation safety tests
    • Mechanical tests
    • X-ray beam tests
    • Electron beam tests
    • Dose delivery performance tests
initial mechanical radiation tests
Initial Mechanical/Radiation Tests
  • Alignment of collimator axis and collimator jaws
  • Collimator axis, light localizer axis, and cross hairs congruence
    • Be aware of whether light source rotates with collimators.
    • Cross hair congruence very important because future quality assurance will depend upon it
  • Light field and radiation field congruence and coincidence
initial mechanical radiation tests1
Initial Mechanical/Radiation Tests
  • Mechanical isocenter location
    • Idealized intersection of the collimator, gantry, and couch rotation axes.
  • Radiation isocenter location
    • Star shot film exposure technique
    • With respect to collimator axis
    • With respect to treatment table axis
    • With respect to gantry axis
safety checks
Safety Checks
  • Emergency stops
  • Proper console operation
    • Mode selection and beam control
    • Readouts
    • Computer-controlled software validation
    • Record and verify
  • Patient support system
  • Anticollision systems and other interlocks
  • Video monitors and intercoms
radiation beam parameters
Radiation beam parameters
  • Beam output
    • Calibratioin
    • Adjustability and range
    • Stability
  • Monitor characteristics
    • Linearity and end effects
    • Dose rate accuracy
    • Dose rate dependence
    • Constancy of output with gantry position
radiation beam parameters1
Radiation beam parameters
  • Flatness
    • Maximum variation of dose in central 80% of the FWHM of the open field.
  • X-ray off-axis ratios (“horns”)
  • Symmetry
    • Maximum percent deviation of the “leftside” dose frm the “right-side” dose at the 80% of the FWHM.
  • Penumbra
    • Film is choice because of spatial resolution
radiation beam parameters2
Radiation beam parameters
  • X-ray beam energy
    • Specified as depth of dmax and/or %dd at 10-cm depth for a 10x10-cm2 field.
  • Electron beam energy
    • Usually specified at depth of 80% and 50% dose for a 10x10-cm2 field.
  • Contamination – surface dose
    • Measure with TLDs
  • Commissioning is the gathering and processing of measured data needed to deliver a prescribed dose with a clinical setup.
    • Handbook tables of relative measurements so that monitor units can be calculated.
  • Each machine energy/modality is commissioned separately.
  • Special procedures usually require additional commissioning.
    • IMRT, electron arc therapy, stereotactic
  • 3D treatment planning systems (TPS) require a specific set of commissioning data to model clinical beams.
  • Records of the machine data measured for commissioning should be properly maintained at the time of commissioning.
  • 3D water phantoms are preferable, but 2D water phantoms can be used.
    • Will have to turn 2D water phantom during measurements to obtain profiles in each orthogonal direction.
commissioning depth dose
Commissioning – Depth Dose
  • X-rays
    • 3x3-cm2 to 40x40-cm2 field sizes
      • Be sure to measure small fields with appropriate detector size.
      • Buildup should be measured with plane-parallel chambers.
  • Electrons
    • 2x2-cm2 to maximum field size for each electron cone.
      • Be sure to convert ionization to dose because the mass stopping power ratio of air to water changes with energy.
other measurements
Other measurements
  • Output measurements at reference depth
    • Can measure x-ray output at any depth and correct back to the reference depth using PDD.
    • Electrons should be measured at or close to R100 due to high-gradient dose falloff.
    • Measure electron output at several different SSDs to obain air gap correction factors.
  • Output measurements with beam modifiers.
    • Wedge factors, block tray factors
  • Cross beam profile measurements for isodose charts and as needed for TPS.
quality assurance
Quality Assurance
  • In general, QA involves three steps
    • The measurement of performance
    • The comparison of the performance with a given standard
    • The actions required to maintain or regain the standard
  • Tolerances (standards) are specified in two ways
    • a tabulated value
      • Light field / radiation field coincidence should be within 2 mm.
    • percentage change in the nominal value
      • Output should be within 2% of some measured value.
quality assurance1
Quality Assurance
  • In addition to tolerance level, there is an action level that when exceeded, appropriate actions are initiated to regain parameter values within the tolerance level.
  • Some have proposed two different tolerance levels.
    • Level I – when exceeded, the parameter might be either remeasured with additional tests or monitored closely over a period of time
    • Level II – Machine is taken out of service until physicist advises otherwise.
  • The QA test procedure should be able to distinguish parameter changes smaller than the tolerance and action levels.
    • For example, test should precise enough so that two standard deviations in the measurement is less than the action level.
qa testing frequency
QA - Testing frequency
  • Testing frequency should be related to
    • Possible patient consiquence
    • Likelihood of malfunction
    • Experience
    • Cost-benefit assessment
  • Daily tests relate to the most critical parameters
    • Patient positioning and the registration of the radiation field and target volume
      • Lasers, optical density indicator
    • Dose to the patient
      • Output
    • Safety features
      • Door interlock, patient audio-visual contact
qa testing frequency1
QA - Testing frequency
  • Monthly tests relate to less critical parameters that should be checked regularly, or tolerances that are less likely to be exceeded.
    • For example, light/radiation field coincidence, beam flatness and symmetry, PDD constancy
  • Annual tests are usually comprehensive
    • Some measurements are done to verify parameters are within tolerances associated with acceptance testing.
      • Collimator, gantry, table, radiation field isocenter coincidence
    • Some measurements are done to set up standards for the following year.
      • Output and PDD constancy
tg 40
  • TG-40 is a comprehensive report on quality assurance in the clinic.
  • Lists recommended and suggested tolerances and frequency of tests for a multitude of clinical equipment
    • Cobalt-60 units, linacs, simulators, dosimetry equipment, TPS and monitor unit calcs, brachytherapy sources and equipment
    • Patient QA – chart checks/reviews, portal imaging
tg 401
  • Table II (pg 589) lists QA checks for linacs.
    • Daily output constancy – 3%
    • Monthly/Annual output constancy – 2%
    • Most other checks have tolerances of 2% or 2 mm.
  • TG-40 is not binding, but should be a guideline for a QA program because it is based on a vast amount of experience.
tg 53
  • TG-40 report covered QA for “traditional” treatment planning systems.
  • TG-53 report needed because treatment planning systems became much more complex (e.g., 3D TPS, image-based, IMRT).
    • Very comprehensive, covers all steps in planning process.
    • Do not read it while operating machinery or driving a vehicle (will put you to sleep).
clinical treatment planning process
Clinical Treatment Planning Process
  • Steps in the process
    • Patient positioning and immobilization
    • Image acquisition and transfer
    • Anatomy/target volume definition
    • Beam/source technique
    • Dose calculations and dose prescription
    • Plan evaluation
    • Plan implementation
    • Plan review