Medical physics support of linear accelerators
This presentation is the property of its rightful owner.
Sponsored Links
1 / 26

Medical Physics Support of Linear Accelerators PowerPoint PPT Presentation

  • Uploaded on
  • Presentation posted in: General

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

Download Presentation

Medical Physics Support of Linear Accelerators

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript

Medical physics support of linear accelerators

Medical Physics Support of Linear Accelerators

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

  • Login