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