Medical Physics Support of Linear Accelerators

1
Medical Physics Support of Linear Accelerators
2
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

3
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

4
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.

5
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)

6
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

7
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.

8
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

9
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

10
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

11
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

12
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

13
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

14
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

15
Commissioning

• 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

16
Commissioning

• 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.

17
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.

18
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.

19
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.

20
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.

21
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

22
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

23
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

24
TG-40

• 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.

25
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).

26
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