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CSH076 > TPS and LINAC QA > Flashcards

TPS and LINAC QA Flashcards

1
Q

Important Features of a TPS

A
  • Dose calculation accuracy
  • Dose calculation algorithm
  • Geometric accuracy
  • Representation of the treatment beam
  • Characteristics of CT scanner used changes the electron density to CT number graphs
  • CT orientation cube
  • DVH, scorecard, isodose lines represented correctly
  • Connectivity to CT
  • Connectivity to R&V system
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2
Q

TPS Commissioning Process

A
  • Network integration
  • Data transfer
  • System setup and machine-source configuration
  • Patient anatomical representation
  • External beam commissioning
  • Plan evaluation tools
  • Plan output and data transfer (report)
  • Overall clinical test
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3
Q

List of TPS QA Tests

A
  • Back-up and recovery
  • CT data transfer – demographics, patient orientation, CT HU
  • CT density and geometry
  • Patient anatomy
  • External beam revalidation (using previously calculated plans – recalc to check still the same)
  • Monitor Units Check (using previously calculated plans) (update upgraded calc time by farming out to many cores and adding up at the end which resulted in rounding errors)
  • Plan Transfer
  • Patient Specific Quality Assurance (PSQA)
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4
Q

Most Common PSQA equipment

A
  • matrixX
  • Delta4
  • ArcCheck
  • Octavius
  • MapCheck
  • EPID
  • PerFraction
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5
Q

Factors to Consider when choosing a device for PSQA

A
  • Does it record in Absolute or Relative Dose
  • Treatment technique (IMRT, VMAT, FFF)
    o Can it handle FFF dose rate?
  • Resolution
    o Small fields will require smaller detector spacing
  • Gamma Pass Criteria
  • Cost – Upfront and Ongoing
  • Compatibility with existing equipment (TPS, Linacs etc.)
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6
Q

What is EPIQA

A
  • EPIQA is a commercial software that converts a dosimetric image acquired by an EPID into a dose map and compare it with a reference dose distribution from the TPS.
  • EPIQA is a pre-treatment verification tool – it is not capable of in vivo dose verification.
  • It can identify potential errors in the calculation of the dose delivery process i.e. MLC not performing
  • Specifically developed for Varian amorphous silicone a-Si500 EPID
  • Software can be used for verification of static and dynamically modulated fields
  • You can perform comparison between EPID vs EPID, or TPS vs TPS as well
    o Useful for QA of the EPID/EPIQA software
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7
Q

Common Causes for QA Failure

A
  1. The Plan
  2. Detector Limitations
  3. Treatment Equipment Failure
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8
Q

Common Causes for QA Failure: The Plan

A
  • Low MU efficiency (too many MU)
  • High modulation factor (MU/cGy)
    o Daily prescribed dose (i.e. 2Gy) should equal 200 MU
    o At 100cm SSD, 1Gy = 100MU for 10x10cm field
    o Modulation Factor = if 2Gy = 200MU; so if you need 800MU to deliver plan = 800/200 = 4MF (3MF is the standard for most VMAT plans)
  • Plan complexity (e.g., High complexity fields)
    o High MU/Gy (typically a MF of 3 produces a good VMAT plan)
    o Narrow MLC aperture
    o Higher uncertainty during delivery
    o Apertures smaller than minimum calibrated field for EPIQA
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9
Q

Common Causes for QA Failure: Detector Limitations

A

Example: highly elongated, high modulated spinal fields

  • Solution: commission a beam model specifically for spinal fields as a typical/standard beam model is optimised for “square fields” with low to moderate shielding
    o Specific beam model introduced a 1cm minimum leaf gap
  • Inherent build-up of the Varian EPID is 8mm but images are converted to dose at dmax
    o (1.5 cm 6 MV, 2.1 cm at 10 MV)
    o Algorithm converts dose measured at 0.8cm (Inherent build up of the Varian EPID) to an equivalent radiological depth of dmax of beam
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10
Q

Common Causes for QA Failure: Treatment Equipment Failure

A
  • MLC failure
  • Beam symmetry
  • Alignment of optical system to radiation isocentre
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11
Q

Plan Complexity Metrics

A

Small aperture score

Modulation factor

Modulation Complexity Score

Area/perimeter ratio (low = complex)

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12
Q

When is in-vivo dosimetry done?

A
  • Pacemakers/ICD
  • Pregnant patients
  • Lens dose
  • TBI
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13
Q

What detectors are used for in vivo dosimetry?

A
  • Film, TLD, Alanine and OSLD
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14
Q

Daily Linac QA Checks

A

Xray output constancy

electron output constancy (weekly)

Laser localisation

distance indicator (ODI) at isocentre (SSD)

Field Light

Door interlocks

AV monitors

Beam on indicator

Beam hold and Beam off

Backup MU Counter

SGRT QA

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15
Q

LINAC Daily QA: Xray Output Constancy: Rationale, Action Level, and Potential QA Tools

A

Rationale:
Ensure dosimetric treatment accuracy

Action Level:
3% variance

Potential QA Tools
* Constancy meter
* Daily QA 3
* BeamChecker
* MPC Phantom

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16
Q

LINAC Daily QA: Xray Output Constancy: Reasons for Failure and Potential Errors

A

Reasons for Failure:
* Electronic Drift of Detector
* Damage/Change to the MU Chamber (requiring replacement or recalibration)

Potential Errors:
* Incorrect dose delivered to all patients

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17
Q

LINAC Daily QA: Laser Localisation: Rationale, Action Level, and Potential QA Tools

A

Rationale:
* Treatment set-up accuracy to ensure accurate tumour localisation for optimal outcome

Action Level:
* 2mm variance

Potential Tools:
* Cube Phantom
* Quasar Penta Guide and Tilt Plate

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18
Q

LINAC Daily QA: Laser Localisation: Reasons for Failure and Potential Errors

A

Reasons for Failure:
* Laser Drift
* Wall vibrations from surroundings
* Someone bumped it

Potential Errors
* Patient Levelling and Positioning off Isocentre
* Large shifts during imaging

19
Q

LINAC Daily QA: ODI Accuracy at Isocentre: Rationale, Action Level, and Potential QA Tools

A

Rationale:
* Treatment Setup Accuracy

Action Level:
* 2mm

Potential QA Tools:
* Cube Phantom

20
Q

LINAC Daily QA: ODI at Isocentre: Reasons for Failure and Potential Errors

A

Reasons for Failure:
* ODI may have been drifted/damaged/bumped

Potential Errors:
* SSD checks would be incorrect
* SSD setups would be systematically out

21
Q

LINAC Daily QA: Field Light: Rationale, Action Level, and Potential QA Tools

A

Rationale:
* Setup Accuracy

Action Level:
* 2mm

Potential Tools:
* Beamchecker
* Daily QA 3

22
Q

LINAC Daily QA: Field Light: Reasons for Failure and Potential Errors

A

Reasons for Failure:
* Light source may not be the correct SSD
* Jaw calibration changed
* Light source may not be on collimator rotation axis

Potential Errors:
* Jaw incorrect - can lead to all patient field size treated correctly
* Field Border Checks show incorrect information

23
Q

LINAC Daily QA: Door Interlock/Beam on Indicator: Rationale, Action Level, and Potential QA Tools

A

Rationale:
* Staff and Patient Safety

Action Level:
* Functioning

Potential Tools:
* Visual observation

24
Q

LINAC Daily QA: Door Interlock/Beam on Indicator: Reasons for Failure and Potential Errors

A

Reasons for Failure:
* Electronic component failure
* Circuit board failure

Potential Errors:
* Safety issue - someone could enter the room during treatment without the knowledge of operator

25
Q

LINAC Daily QA: AV Monitors: Rationale, Action Level, and Potential QA Tools

A

Rationale:
* Staff and Patient Safety

Action Level:
* Functioning

Potential Tools:
* Visual observation

26
Q

LINAC Daily QA: AV Monitors: Reasons for Failure and Potential Errors

A

Reasons for Failure
* AV system failure
* Power supply of AV system failure

Potential Errors
* Patient safety
* Inability to coach RPM

27
Q

AAPM TG 142: LINAC QA Recommendations (tests and frequency)

A
  • Recommendation from this task group are not intended to be used as regulation, but merely guidelines
  • Tests performed must be relevant to individual institution and clinical setting, which may include site specific
  • Daily tests include parameters that affect dose to the patient by dosimetric (output constancy) or geometric (lasers, ODI, field size)
  • Monthly tests include those that have a lower likelihood of changing over a month, or more involved and quantitative tests, which would require a physicist i.e. Winston Lutz, MLC function test, detector array QA
  • Likelihood to change dictates frequency
28
Q

Imaging positioning and iso coincidence reasons for failure and potential errors

A

Calibration file may have been corrupted, changed/calibrated
Electronics may be incorrect or damaged

Errors:
Incorrect source/detector positioning
Geometric misalignment for treatment

29
Q

What are gamma calculations?

A

Method to check if a planned radiation treatment matches the actual dose that a patient receives during treatment

  • Compare planned dose with the measured dose
  • To pass, each point of measured dose must be within a tolerance of typically 2% variance
  • To be considered accurate, approx. 80-90% need to match for treatment to be considered accurate (varies from department to department)
  • Each planned dose point is checked against nearby measured points (within a small radius, typically 2 mm) to ensure it closely matches in both distance and dose
30
Q

Limitation of Gamma Calculations

A

Does not measure the actual dose delivered to the patient in real-time

Compares the planned dose against a measured dose delivered to a water phantom

31
Q

Six Step Methodology for PSQA

A
  1. Verification that the intensity field boundary matches the planning boundary
  2. An independent calculation, verification that the machine instructions driving the leaves produce the planned absorbed-dose distribution
  3. Comparison of the absorbed-dose distribution in a phantom with that
    calculated by the treatment planning computer for the same irradiation
    condition.
  4. Comparison of the planned leaf motions with that recorded on the MLC log files.
  5. Confirmation of the initial and final positions of the MLC for each field by a
    record-and-verify system
  6. In vivo dosimetry
32
Q

Rationale for PSQA

A

Verifying that each patient’s treatment plan will deliver the correct dose accurately and safely

33
Q

Key PSQA Concepts: Definition and Benefits of DTA

A

DTA= Distance to Agreement
* Used to compare how well the dose distributions overlap between planned and measured doses

In high dose gradient areas (regions where dose changes quickly (i.e. OARs in proximity to target)) –> DTA is beneficial
* DTA specifically is used to check spatial alignment between calculated dose and measured dose
* Smaller DTA = measured dose is close to planned –> confirming machine delivered the treatment in the exact location as intended
* DTA ensures that dose conforms tightly to targets
* Effective in identifying small shifts that can affect dose delivered to sensitive areas

34
Q

Key PSQA Concepts: Limitations of DTA

A
  • DTA is not as effective in areas of low dose gradient (regions where dose gradually changes or remains uniform across a relatively large area)
  • Within these regions, small shifts (mm’s) won’t cause a large variance in dose –> may lead to DTA showing position is aligned
  • However, a mismatch in absolute dose (how much radiation is actually delivered) can indicate that the delivery of dose is not accurate, even if the spatial alignment (DTA) appears to be correct
  • For example, in the low dose regions, the dose should be uniform or change very slowly–> if measured dose varies, this could be a problem with dose expected to be relatively consistent
35
Q

Key PSQA Concepts: Validation of Mechanical and Dosimetric Uncertainties

A

Mechanical uncertainties can come from components like gantry angles, collimator positions, or couch positions, which all need to align correctly.

Dosimetric uncertainties come from factors in the TPS, such as dose calculation algorithms or machine calibration settings.

PSQA checks that both types of uncertainties are within acceptable limits.

36
Q

Key PSQA Concepts: Dose to Panel vs Dose to Patient

A

Dose to Panel
* Refers to the measured dose in a phantom setup using detector panel or measuring device

Dose to Patient
* Refers to the actual dose a patient would receive when treated, based on the treatment plan

  • Dose to panel check serves to validate both the machines mechanical accuracy ad dosimetric accuracy
  • PSQA uses dose to panel as a surrogate to ensure that the treatment plan is accurate and will deliver the correct dose to the patient during the actual treatment
37
Q

PSQA checks

A
  • delivery of plan dose in a phantom
  • validation of photon fluency using portal imaging system
  • point dose measurement
  • independent MU validations using MU check software
38
Q

3 types of PSQA checks

A
  • point dose measurement in virtual water phantoms
  • 2D array checks
  • 3D array checks
39
Q

Mapcheck3

A

2D patient plan QA - for iMRT and 3DCRT
2 diode detectors uniform throughout array - high sensitivity and less drift
20x20cm field size
Gantry mount for VMAT

40
Q

What effects QA pass rates

A

FS -> different pSQA machines have different field sizes
Collimator angles

41
Q

Process of QA for VMAT plan

A
  1. Generate QA plan in TPS
  2. Export to RV system
  3. Physics - set up phantom
  4. Physics - deliver treatment
  5. Physics - check outcome (pass/fail)

If fail - then replan

42
Q

ArcCheck

A

1386 diodes
Cylinder - 21cm long and diameter
3D nature
Real time acquisition 50mc

43
Q

EPID

A

40x30cm (100FDD)
25x25cm (160FDD)
2D - rotates with beam so always perpendicular to beam
Software: EPIQA, EPIdose, PerFraction

CSH076 (11 decks)

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