Medical Physics Support of Linear Accelerators

Overview of Physics Support Accelerator safety issues Accelerator safety issues Task Group Report #35 Task Group Report #35 Acceptance testing Acceptance testing Perform radiation protection survey Perform radiation protection survey Verify accelerator characteristics are within specifications Verify accelerator characteristics are within specifications Task Group Report #45 Task Group Report #45 Commissioning Commissioning Collect and prepare beam data for clinical use Collect and prepare beam data for clinical use Task Group Report #45 Task Group Report #45 Quality Assurance Quality Assurance Daily, Weekly, Monthly, Annual Daily, Weekly, Monthly, Annual Task Group Report #40 Task Group Report #40

Accelerator Safety AAPM Task Group Report #35 (TG-35) covers safety issues that the medical physicist should be aware of. AAPM Task Group Report #35 (TG-35) covers safety issues that the medical physicist should be aware of. Two FDA classifications of hazards Two FDA classifications of hazards Class I – causes serious injury or death Class I – causes serious injury or death Type A hazard are directly responsible for life-threatening complications Type A hazard are directly responsible for life-threatening complications Type B hazard increases probability of unacceptable outcome (complication or lack of tumor control) 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 Class II – hazards where the risk of serious injury are small

Accelerator Safety Most common hazards Most common hazards Incorrect radiation dose Incorrect radiation dose Dose delivered to wrong region Dose delivered to wrong region Collision between patient and machine Collision between patient and machine Incorrect beam energy or modality Incorrect beam energy or modality Electrical/mechanical problems Electrical/mechanical problems Class I, Type A hazard involves improper delivery of 25% of prescribed dose. Class I, Type A hazard involves improper delivery of 25% of prescribed dose.

Radiation Protection Regulation Regulatory bodies Regulatory bodies Linear Accelerators Linear Accelerators National Council on Radiation Protection and Measurements (NCRP) National Council on Radiation Protection and Measurements (NCRP) Individual states (Suggested State Regulations for Control of Radiation, SSRCR) Individual states (Suggested State Regulations for Control of Radiation, SSRCR) Cobalt-60 Cobalt-60 Nuclear Regulatory Commission (NRC) Nuclear Regulatory Commission (NRC)

Exposure Limits NCRP Report #116 replaces Report #91 NCRP Report #116 replaces Report #91 Occupation Limits (controlled areas) Occupation Limits (controlled areas) Whole body – 50 mSv / yr (1 mSv / wk) Whole body – 50 mSv / yr (1 mSv / wk) Infrequent / Planned – 100 mSv Infrequent / Planned – 100 mSv Lens of Eye – 150 mSv / yr Lens of Eye – 150 mSv / yr Pregnant Worker – 5 mSv / term (0.5 mSv / mo) Pregnant Worker – 5 mSv / term (0.5 mSv / mo) Lifetime – 10 mSv x Age (years) Lifetime – 10 mSv x Age (years) Public Limits (noncontrolled areas) Public Limits (noncontrolled areas) Whole body – 1 mSv / yr (0.02 mSv / wk) Whole body – 1 mSv / yr (0.02 mSv / wk) Infrequent / Planned – 5 mSv Infrequent / Planned – 5 mSv Extremities, Skin, Lens of Eye – 50 mSv / yr Extremities, Skin, Lens of Eye – 50 mSv / yr

Radiation Protection Survey Performed after accelerator is installed and beams are calibrated. Performed after accelerator is installed and beams are calibrated. NCRP Report #51 was the standard reference NCRP Report #51 was the standard reference NCRP Report #144 updates and expands on #51 NCRP Report #144 updates and expands on #51 Neutron leakage measurements should be done for nominal photon energies 15 MV and above. Neutron leakage measurements should be done for nominal photon energies 15 MV and above. NCRP Report #79 NCRP Report #79 AAPM Report #19 AAPM Report #19 Survey meter Survey meter Should be capable of detecting exposure levels from 0.2 mR/hr to 1 R/hr. 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. AAPM TG-45 recommends survey meter be calibrated once a year. Required by law if Cobalt-60 unit is present in facility. Required by law if Cobalt-60 unit is present in facility.

Acceptance Testing Manufacturers have Acceptance Testing Procedures (ATPs) which engineers and physicist follow and sign off on. 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. Sometimes a machine might ordered with specifications beyond what the manufacturer provides. Types of ATPs Types of ATPs Radiation safety tests Radiation safety tests Mechanical tests Mechanical tests X-ray beam tests X-ray beam tests Electron beam tests Electron beam tests Dose delivery performance tests Dose delivery performance tests

Initial Mechanical/Radiation Tests Alignment of collimator axis and collimator jaws Alignment of collimator axis and collimator jaws Collimator axis, light localizer axis, and cross hairs congruence Collimator axis, light localizer axis, and cross hairs congruence Be aware of whether light source rotates with collimators. Be aware of whether light source rotates with collimators. Cross hair congruence very important because future quality assurance will depend upon it Cross hair congruence very important because future quality assurance will depend upon it Light field and radiation field congruence and coincidence Light field and radiation field congruence and coincidence

Initial Mechanical/Radiation Tests Mechanical isocenter location Mechanical isocenter location Idealized intersection of the collimator, gantry, and couch rotation axes. Idealized intersection of the collimator, gantry, and couch rotation axes. Radiation isocenter location Radiation isocenter location Star shot film exposure technique Star shot film exposure technique With respect to collimator axis With respect to collimator axis With respect to treatment table axis With respect to treatment table axis With respect to gantry axis With respect to gantry axis

Safety Checks Emergency stops Emergency stops Proper console operation Proper console operation Mode selection and beam control Mode selection and beam control Readouts Readouts Computer-controlled software validation Computer-controlled software validation Record and verify Record and verify Patient support system Patient support system Anticollision systems and other interlocks Anticollision systems and other interlocks Video monitors and intercoms Video monitors and intercoms

Radiation beam parameters Beam output Beam output Calibratioin Calibratioin Adjustability and range Adjustability and range Stability Stability Monitor characteristics Monitor characteristics Linearity and end effects Linearity and end effects Dose rate accuracy Dose rate accuracy Dose rate dependence Dose rate dependence Constancy of output with gantry position Constancy of output with gantry position

Radiation beam parameters Flatness Flatness Maximum variation of dose in central 80% of the FWHM of the open field. Maximum variation of dose in central 80% of the FWHM of the open field. X-ray off-axis ratios (“horns”) X-ray off-axis ratios (“horns”) Symmetry Symmetry Maximum percent deviation of the “leftside” dose frm the “right-side” dose at the 80% of the FWHM. Maximum percent deviation of the “leftside” dose frm the “right-side” dose at the 80% of the FWHM. Penumbra Penumbra Film is choice because of spatial resolution Film is choice because of spatial resolution

Radiation beam parameters X-ray beam energy X-ray beam energy Specified as depth of d max and/or %dd at 10-cm depth for a 10x10-cm 2 field. Specified as depth of d max and/or %dd at 10-cm depth for a 10x10-cm 2 field. Electron beam energy Electron beam energy Usually specified at depth of 80% and 50% dose for a 10x10-cm 2 field. Usually specified at depth of 80% and 50% dose for a 10x10-cm 2 field. Contamination – surface dose Contamination – surface dose Measure with TLDs Measure with TLDs

Commissioning Commissioning is the gathering and processing of measured data needed to deliver a prescribed dose with a clinical setup. 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. Handbook tables of relative measurements so that monitor units can be calculated. Each machine energy/modality is commissioned separately. Each machine energy/modality is commissioned separately. Special procedures usually require additional commissioning. Special procedures usually require additional commissioning. IMRT, electron arc therapy, stereotactic IMRT, electron arc therapy, stereotactic

Commissioning 3D treatment planning systems (TPS) require a specific set of commissioning data to model clinical beams. 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. 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. 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. Will have to turn 2D water phantom during measurements to obtain profiles in each orthogonal direction.

Commissioning – Depth Dose X-rays X-rays 3x3-cm 2 to 40x40-cm 2 field sizes 3x3-cm 2 to 40x40-cm 2 field sizes Be sure to measure small fields with appropriate detector size. Be sure to measure small fields with appropriate detector size. Buildup should be measured with plane-parallel chambers. Buildup should be measured with plane-parallel chambers. Electrons Electrons 2x2-cm 2 to maximum field size for each electron cone. 2x2-cm 2 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. Be sure to convert ionization to dose because the mass stopping power ratio of air to water changes with energy.

Other measurements Output measurements at reference depth Output measurements at reference depth Can measure x-ray output at any depth and correct back to the reference depth using PDD. 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 R 100 due to high- gradient dose falloff. Electrons should be measured at or close to R 100 due to high- gradient dose falloff. Measure electron output at several different SSDs to obain air gap correction factors. Measure electron output at several different SSDs to obain air gap correction factors. Output measurements with beam modifiers. Output measurements with beam modifiers. Wedge factors, block tray factors Wedge factors, block tray factors Cross beam profile measurements for isodose charts and as needed for TPS. Cross beam profile measurements for isodose charts and as needed for TPS.

Quality Assurance In general, QA involves three steps In general, QA involves three steps The measurement of performance The measurement of performance The comparison of the performance with a given standard The comparison of the performance with a given standard The actions required to maintain or regain the standard The actions required to maintain or regain the standard Tolerances (standards) are specified in two ways Tolerances (standards) are specified in two ways a tabulated value a tabulated value Light field / radiation field coincidence should be within 2 mm. Light field / radiation field coincidence should be within 2 mm. percentage change in the nominal value percentage change in the nominal value Output should be within 2% of some measured value. Output should be within 2% of some measured value.

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. 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. 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 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. 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. 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. For example, test should precise enough so that two standard deviations in the measurement is less than the action level.

QA - Testing frequency Testing frequency should be related to Testing frequency should be related to Possible patient consiquence Possible patient consiquence Likelihood of malfunction Likelihood of malfunction Experience Experience Cost-benefit assessment Cost-benefit assessment Daily tests relate to the most critical parameters Daily tests relate to the most critical parameters Patient positioning and the registration of the radiation field and target volume Patient positioning and the registration of the radiation field and target volume Lasers, optical density indicator Lasers, optical density indicator Dose to the patient Dose to the patient Output Output Safety features Safety features Door interlock, patient audio-visual contact Door interlock, patient audio-visual contact

QA - Testing frequency Monthly tests relate to less critical parameters that should be checked regularly, or tolerances that are less likely to be exceeded. 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 For example, light/radiation field coincidence, beam flatness and symmetry, PDD constancy Annual tests are usually comprehensive Annual tests are usually comprehensive Some measurements are done to verify parameters are within tolerances associated with acceptance testing. Some measurements are done to verify parameters are within tolerances associated with acceptance testing. Collimator, gantry, table, radiation field isocenter coincidence Collimator, gantry, table, radiation field isocenter coincidence Some measurements are done to set up standards for the following year. Some measurements are done to set up standards for the following year. Output and PDD constancy Output and PDD constancy

TG-40 TG-40 is a comprehensive report on quality assurance in the clinic. 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 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 Cobalt-60 units, linacs, simulators, dosimetry equipment, TPS and monitor unit calcs, brachytherapy sources and equipment Patient QA – chart checks/reviews, portal imaging Patient QA – chart checks/reviews, portal imaging

TG-40 Table II (pg 589) lists QA checks for linacs. Table II (pg 589) lists QA checks for linacs. Daily output constancy – 3% Daily output constancy – 3% Monthly/Annual output constancy – 2% Monthly/Annual output constancy – 2% Most other checks have tolerances of 2% or 2 mm. 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-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-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). 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. Very comprehensive, covers all steps in planning process. Do not read it while operating machinery or driving a vehicle (will put you to sleep). Do not read it while operating machinery or driving a vehicle (will put you to sleep).

Clinical Treatment Planning Process Steps in the process Steps in the process Patient positioning and immobilization Patient positioning and immobilization Image acquisition and transfer Image acquisition and transfer Anatomy/target volume definition Anatomy/target volume definition Beam/source technique Beam/source technique Dose calculations and dose prescription Dose calculations and dose prescription Plan evaluation Plan evaluation Plan implementation Plan implementation Plan review Plan review