Medical Physics Medical Physics Support of Linear Support of Linear

AcceleratorsAccelerators

Overview of Physics Overview of Physics SupportSupport

Accelerator safety issuesAccelerator safety issues Task Group Report #35Task Group Report #35

Acceptance testingAcceptance testing Perform radiation protection surveyPerform radiation protection survey Verify accelerator characteristics are within Verify accelerator characteristics are within

specificationsspecifications Task Group Report #45Task Group Report #45

CommissioningCommissioning Collect and prepare beam data for clinical useCollect and prepare beam data for clinical use Task Group Report #45 Task Group Report #45

Quality AssuranceQuality Assurance Daily, Weekly, Monthly, AnnualDaily, Weekly, Monthly, Annual Task Group Report #40Task Group Report #40

Accelerator SafetyAccelerator Safety

AAPM Task Group Report #35 (TG-35) AAPM Task Group Report #35 (TG-35) covers safety issues that the medical covers safety issues that the medical physicist should be aware of.physicist should be aware of.

Two FDA classifications of hazardsTwo FDA classifications of hazards Class I – causes serious injury or deathClass I – causes serious injury or death

Type A hazard are directly responsible for life-Type A hazard are directly responsible for life-threatening complicationsthreatening complications

Type B hazard increases probability of unacceptable Type B hazard increases probability of unacceptable outcome (complication or lack of tumor control)outcome (complication or lack of tumor control)

Class II – hazards where the risk of serious Class II – hazards where the risk of serious injury are smallinjury are small

Accelerator SafetyAccelerator Safety

Most common hazardsMost common hazards Incorrect radiation dose Incorrect radiation dose Dose delivered to wrong regionDose delivered to wrong region Collision between patient and machineCollision between patient and machine Incorrect beam energy or modalityIncorrect beam energy or modality Electrical/mechanical problemsElectrical/mechanical problems

Class I, Type A hazard involves Class I, Type A hazard involves improper delivery of 25% of improper delivery of 25% of prescribed dose.prescribed dose.

Radiation Protection Radiation Protection RegulationRegulation

Regulatory bodiesRegulatory bodies Linear Accelerators Linear Accelerators

National Council on Radiation Protection National Council on Radiation Protection and Measurements (NCRP)and Measurements (NCRP)

Individual states (Suggested State Individual states (Suggested State Regulations for Control of Radiation, Regulations for Control of Radiation, SSRCR)SSRCR)

Cobalt-60Cobalt-60 Nuclear Regulatory Commission (NRC)Nuclear Regulatory Commission (NRC)

Exposure LimitsExposure Limits

NCRP Report #116 replaces Report #91NCRP Report #116 replaces Report #91 Occupation Limits (controlled areas)Occupation Limits (controlled areas)

Whole body – 50 mSv / yr Whole body – 50 mSv / yr (1 mSv / wk)(1 mSv / wk) Infrequent / Planned – 100 mSvInfrequent / Planned – 100 mSv Lens of Eye – 150 mSv / yrLens 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 mSvInfrequent / Planned – 5 mSv Extremities, Skin, Lens of Eye – 50 mSv / yrExtremities, Skin, Lens of Eye – 50 mSv / yr

Radiation Protection Radiation Protection SurveySurvey

Performed after accelerator is installed and Performed after accelerator is installed and beams are calibrated.beams are calibrated. NCRP Report #51 was the standard reference NCRP Report #51 was the standard reference NCRP Report #144 updates and expands on #51NCRP Report #144 updates and expands on #51

Neutron leakage measurements should be done Neutron leakage measurements should be done for nominal photon energies 15 MV and above.for nominal photon energies 15 MV and above. NCRP Report #79NCRP Report #79 AAPM Report #19AAPM Report #19

Survey meter Survey meter Should be capable of detecting exposure levels from 0.2 Should be capable of detecting exposure levels from 0.2

mR/hr to 1 R/hr.mR/hr to 1 R/hr. AAPM TG-45 recommends survey meter be calibrated AAPM TG-45 recommends survey meter be calibrated

once a year.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 TestingAcceptance Testing Manufacturers have Acceptance Testing Manufacturers have Acceptance Testing

Procedures (ATPs) which engineers and Procedures (ATPs) which engineers and physicist follow and sign off on.physicist follow and sign off on. Sometimes a machine might ordered with Sometimes a machine might ordered with

specifications beyond what the manufacturer specifications beyond what the manufacturer provides.provides.

Types of ATPsTypes of ATPs Radiation safety testsRadiation safety tests Mechanical testsMechanical tests X-ray beam testsX-ray beam tests Electron beam testsElectron beam tests Dose delivery performance testsDose delivery performance tests

Initial Initial Mechanical/Radiation TestsMechanical/Radiation Tests Alignment of collimator axis and Alignment of collimator axis and

collimator jawscollimator jaws Collimator axis, light localizer axis, and Collimator axis, light localizer axis, and

cross hairs congruencecross hairs congruence Be aware of whether light source rotates Be aware of whether light source rotates

with collimators.with collimators. Cross hair congruence very important Cross hair congruence very important

because future quality assurance will because future quality assurance will depend upon itdepend upon it

Light field and radiation field Light field and radiation field congruence and coincidencecongruence and coincidence

Initial Initial Mechanical/Radiation TestsMechanical/Radiation Tests Mechanical isocenter locationMechanical isocenter location

Idealized intersection of the collimator, Idealized intersection of the collimator, gantry, and couch rotation axes.gantry, and couch rotation axes.

Radiation isocenter locationRadiation isocenter location Star shot film exposure techniqueStar shot film exposure technique With respect to collimator axisWith respect to collimator axis With respect to treatment table axisWith respect to treatment table axis With respect to gantry axisWith respect to gantry axis

Safety ChecksSafety Checks

Emergency stopsEmergency stops Proper console operationProper console operation

Mode selection and beam controlMode selection and beam control ReadoutsReadouts Computer-controlled software validationComputer-controlled software validation Record and verifyRecord and verify

Patient support systemPatient support system Anticollision systems and other interlocksAnticollision systems and other interlocks Video monitors and intercomsVideo monitors and intercoms

Radiation beam Radiation beam parametersparameters

Beam outputBeam output CalibratioinCalibratioin Adjustability and rangeAdjustability and range StabilityStability

Monitor characteristicsMonitor characteristics Linearity and end effectsLinearity and end effects Dose rate accuracyDose rate accuracy Dose rate dependenceDose rate dependence Constancy of output with gantry positionConstancy of output with gantry position

Radiation beam Radiation beam parametersparameters

Flatness Flatness Maximum variation of dose in central 80% Maximum variation of dose in central 80%

of the FWHM of the open field.of the FWHM of the open field. X-ray off-axis ratios (“horns”)X-ray off-axis ratios (“horns”) SymmetrySymmetry

Maximum percent deviation of the Maximum percent deviation of the “leftside” dose frm the “right-side” dose at “leftside” dose frm the “right-side” dose at the 80% of the FWHM.the 80% of the FWHM.

PenumbraPenumbra Film is choice because of spatial resolutionFilm is choice because of spatial resolution

Radiation beam Radiation beam parametersparameters

X-ray beam energyX-ray beam energy Specified as depth of dSpecified as depth of dmaxmax and/or %dd at and/or %dd at

10-cm depth for a 10x10-cm10-cm depth for a 10x10-cm22 field. field. Electron beam energyElectron beam energy

Usually specified at depth of 80% and Usually specified at depth of 80% and 50% dose for a 10x10-cm50% dose for a 10x10-cm22 field. field.

Contamination – surface doseContamination – surface dose Measure with TLDsMeasure with TLDs

CommissioningCommissioning

Commissioning is the gathering and Commissioning is the gathering and processing of measured data needed to processing of measured data needed to deliver a prescribed dose with a clinical deliver a prescribed dose with a clinical setup.setup. Handbook tables of relative measurements so that Handbook tables of relative measurements so that

monitor units can be calculated.monitor units can be calculated. Each machine energy/modality is Each machine energy/modality is

commissioned separately.commissioned separately. Special procedures usually require Special procedures usually require

additional commissioning.additional commissioning. IMRT, electron arc therapy, stereotacticIMRT, electron arc therapy, stereotactic

CommissioningCommissioning

3D treatment planning systems (TPS) 3D treatment planning systems (TPS) require a specific set of commissioning data require a specific set of commissioning data to model clinical beams.to model clinical beams.

Records of the machine data measured for Records of the machine data measured for commissioning should be properly commissioning should be properly maintained at the time of commissioning.maintained at the time of commissioning.

3D water phantoms are preferable, but 2D 3D water phantoms are preferable, but 2D water phantoms can be used.water phantoms can be used. Will have to turn 2D water phantom during Will have to turn 2D water phantom during

measurements to obtain profiles in each measurements to obtain profiles in each orthogonal direction. orthogonal direction.

Commissioning – Depth Commissioning – Depth DoseDose

X-raysX-rays 3x3-cm3x3-cm22 to 40x40-cm to 40x40-cm22 field sizes field sizes

Be sure to measure small fields with Be sure to measure small fields with appropriate detector size.appropriate detector size.

Buildup should be measured with plane-parallel Buildup should be measured with plane-parallel chambers.chambers.

ElectronsElectrons 2x2-cm2x2-cm22 to maximum field size for each to maximum field size for each

electron cone.electron cone. Be sure to convert ionization to dose because Be sure to convert ionization to dose because

the mass stopping power ratio of air to water the mass stopping power ratio of air to water changes with energy.changes with energy.

Other measurementsOther measurements Output measurements at reference depthOutput measurements at reference depth

Can measure x-ray output at any depth and Can measure x-ray output at any depth and correct back to the reference depth using PDD.correct back to the reference depth using PDD.

Electrons should be measured at or close to RElectrons should be measured at or close to R100100 due to high-gradient dose falloff.due to high-gradient dose falloff.

Measure electron output at several different Measure electron output at several different SSDs to obain air gap correction factors.SSDs to obain air gap correction factors.

Output measurements with beam modifiers.Output measurements with beam modifiers. Wedge factors, block tray factorsWedge factors, block tray factors

Cross beam profile measurements for Cross beam profile measurements for isodose charts and as needed for TPS.isodose charts and as needed for TPS.

Quality AssuranceQuality Assurance In general, QA involves three stepsIn general, QA involves three steps

The measurement of performanceThe measurement of performance The comparison of the performance with a given The comparison of the performance with a given

standardstandard The actions required to maintain or regain the standardThe actions required to maintain or regain the standard

Tolerances (standards) are specified in two waysTolerances (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 valuepercentage change in the nominal value

Output should be within 2% of some measured value.Output should be within 2% of some measured value.

Quality AssuranceQuality Assurance In addition to tolerance level, there is an action In addition to tolerance level, there is an action

level that when exceeded, appropriate actions level that when exceeded, appropriate actions are initiated to regain parameter values within are initiated to regain parameter values within the tolerance level.the tolerance level.

Some have proposed two different tolerance Some have proposed two different tolerance levels.levels. Level I – when exceeded, the parameter might be Level I – when exceeded, the parameter might be

either remeasured with additional tests or monitored either remeasured with additional tests or monitored closely over a period of timeclosely over a period of time

Level II – Machine is taken out of service until Level II – Machine is taken out of service until physicist advises otherwise.physicist advises otherwise.

The QA test procedure should be able to The QA test procedure should be able to distinguish parameter changes smaller than the distinguish parameter changes smaller than the tolerance and action levels. tolerance and action levels. For example, test should precise enough so that two For example, test should precise enough so that two

standard deviations in the measurement is less than standard deviations in the measurement is less than the action level.the action level.

QA - Testing frequencyQA - Testing frequency Testing frequency should be related toTesting frequency should be related to

Possible patient consiquencePossible patient consiquence Likelihood of malfunctionLikelihood of malfunction ExperienceExperience Cost-benefit assessmentCost-benefit assessment

Daily tests relate to the most critical Daily tests relate to the most critical parametersparameters Patient positioning and the registration of the Patient positioning and the registration of the

radiation field and target volumeradiation field and target volume Lasers, optical density indicatorLasers, optical density indicator

Dose to the patientDose to the patient OutputOutput

Safety featuresSafety features Door interlock, patient audio-visual contactDoor interlock, patient audio-visual contact

QA - Testing frequencyQA - Testing frequency Monthly tests relate to less critical Monthly tests relate to less critical

parameters that should be checked regularly, parameters that should be checked regularly, or tolerances that are less likely to be or tolerances that are less likely to be exceeded.exceeded. For example, light/radiation field coincidence, For example, light/radiation field coincidence,

beam flatness and symmetry, PDD constancybeam flatness and symmetry, PDD constancy Annual tests are usually comprehensiveAnnual tests are usually comprehensive

Some measurements are done to verify parameters Some measurements are done to verify parameters are within tolerances associated with acceptance are within tolerances associated with acceptance testing.testing.

Collimator, gantry, table, radiation field isocenter Collimator, gantry, table, radiation field isocenter coincidencecoincidence

Some measurements are done to set up standards Some measurements are done to set up standards for the following year.for the following year.

Output and PDD constancyOutput and PDD constancy

TG-40TG-40

TG-40 is a comprehensive report on TG-40 is a comprehensive report on quality assurance in the clinic.quality assurance in the clinic.

Lists recommended and suggested Lists recommended and suggested tolerances and frequency of tests for a tolerances and frequency of tests for a multitude of clinical equipmentmultitude of clinical equipment Cobalt-60 units, linacs, simulators, Cobalt-60 units, linacs, simulators,

dosimetry equipment, TPS and monitor unit dosimetry equipment, TPS and monitor unit calcs, brachytherapy sources and equipmentcalcs, brachytherapy sources and equipment

Patient QA – chart checks/reviews, portal Patient QA – chart checks/reviews, portal imagingimaging

TG-40TG-40

Table II (pg 589) lists QA checks for Table II (pg 589) lists QA checks for linacs.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 Most other checks have tolerances of 2% or

2 mm.2 mm. TG-40 is not binding, but should be a TG-40 is not binding, but should be a

guideline for a QA program because it is guideline for a QA program because it is based on a vast amount of experience.based on a vast amount of experience.

TG-53TG-53

TG-40 report covered QA for “traditional” TG-40 report covered QA for “traditional” treatment planning systems.treatment planning systems.

TG-53 report needed because treatment TG-53 report needed because treatment planning systems became much more planning systems became much more complex (e.g., 3D TPS, image-based, complex (e.g., 3D TPS, image-based, IMRT).IMRT). Very comprehensive, covers all steps in Very comprehensive, covers all steps in

planning process.planning process. Do not read it while operating machinery or Do not read it while operating machinery or

driving a vehicle (will put you to sleep).driving a vehicle (will put you to sleep).

Clinical Treatment Clinical Treatment Planning ProcessPlanning Process

Steps in the processSteps in the process Patient positioning and immobilizationPatient positioning and immobilization Image acquisition and transferImage acquisition and transfer Anatomy/target volume definitionAnatomy/target volume definition Beam/source techniqueBeam/source technique Dose calculations and dose prescriptionDose calculations and dose prescription Plan evaluationPlan evaluation Plan implementationPlan implementation Plan reviewPlan review