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Quality Assurance > TPS and Treatment QA > Flashcards

TPS and Treatment QA Flashcards

1
Q

Why is checking beam model important?

A
  • a perfectly commissioned TPS will not produce perfect results
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2
Q

What causes differences in beam models?

A
  • diode positioning and sensitivity
  • penumbra is detected by diode with isnt in TPS
  • beam parameters such as symmetry and FFF
  • biggest difference for surface dose so use 10% as TPS doesnt accuratly represent this
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3
Q

What is different about sterotactic beam modelling?

A
  • need small detector as large detector averages both high dose from field and low dose penumbra
  • commonly use farmer chamber
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4
Q

What is the biggest reason for QA failure?

A
  • large difference in field size and detector perimeter
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5
Q

What is the benefits of MU checks?

A
  • no measurement required
  • can predict dose to a point at various SSD, depth, field size, wedge, tray, compensator, energy using reference data
  • independent MU/dose calculation
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6
Q

What is a limitation of MU checks?

A
  • flat geometry
  • radiological depth dose not accurately account for heterogeneity
  • not good for heavily shielded fields or elongated fields
  • cant use for IMRT, VMAT or DCAT
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7
Q

What is a benefit of MapCheck?

A
  • slab geometry is simple
  • dose per beam is measured can pick up systematic error in specific beam
  • systematic errors can be discovered by patient specific QA due to non optimal modelling in TPS
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8
Q

What is a limitation of MapCheck?

A
  • performed under gantry 0 therefore unable to detect any uncertainty introduced by irradiation under other gantry angles
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9
Q

What are the limitations of doing QA on gantry angle 0?

A
  • MLC leaf position under gravity
  • output variation with gantry angle
  • beam symmetry with gantry angle
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10
Q

What are examples of 2D array?

A
  • MatrixX

- MapCheck

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11
Q

What are examples of 3D array?

A
  • Delta4
  • ArcCheck
  • Octavius
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12
Q

What is the tolerance for PSQA?

A
  • 3%/3mm
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13
Q

What are factors to consider when purchasing a QA device?

A
  • absolute vs relative dose
  • treatement technique
  • resolution
  • detector drift (sensitivity change)
  • gamma pass criteria
  • cost (upfront and ongoing)
  • compatibility with existing equipment
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14
Q

What is gamma calculations?

A
  • gamma index combines dose differences and distance difference to calculate dimensions metric for each point in the evaluated distribution
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15
Q

What is the gamma pass rate?

A
  • 90% gamma <1 within 3%, 3mm
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16
Q

What is the rationale for PSQA?

A
  • validate mechanical and dosimetric uncertainties (compared to MU which only check dose)
  • evaluate plan for potential plan failure (complex, highly modulated, jaw define field edge)
  • is panel detecting dose patient will recieve
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17
Q

What is EPIQA?

A
  • software that converts a dosimetric image acquired by an EPID into a dose map and compare with a reference dose distribution from TPS
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18
Q

What are common causes for QA failure?

A
  • plan evaluation showing low MU efficiency (MU/segment), modulation factor (MU/cGy) and plan complexity
  • detector limitation (elongated fields/field size limitations, detector non-uniformity)
  • treatment equipment failure (MLC failure, beam symmetry and alignment of opitcal system to radiation isocentre)
19
Q

What is the benefit of a good detector for highly modulated fields?

A
  • more accurate evaluation especially in modualted areas
20
Q

What are the charactersitics of high complexity fields?

A
  • high MU per Gy (factor of 3 is good)
  • narrow MLC aperture
  • higher undertainty during delivery
21
Q

What are 9 main treatment daily QA tests?

A
  1. collision interlocks
  2. optical distance indicator accuracy at isocentre
  3. wedge moving check on one angle
  4. laser localisation
  5. imaging isocentre coincidence
  6. x-ray and electon output
  7. audio visual
  8. light field
  9. door interlock
22
Q

What is the reason for collision interock failure?

A
  • sensors failure

- electronics board failure

23
Q

What happens if collisional interlock not working?

A
  • gantry/panel collision into patient causing injury
24
Q

What is the reasons for imaging positioning and isocentre coicidence failure?

A
  • calibration file may have been corrupted

- electronic incorrect or damaged

25
Q

What happens if treatment and imaging isocentre coicidence is not working?

A
  • incorrect source/detector positioning

- geometric misalignment for treatment

26
Q

What is the reason for wedge moving check failure?

A
  • EDW STT table may have been corrupted
27
Q

What happens if the wedge moving check is not working?

A
  • all wedge fields would potentially be incorretly treated
28
Q

What is the reason for x-ray and electron output failing?

A
  • electronic drfit of detector

- damage to MU chamber

29
Q

What happens if the x-ray and electron output is wrong?

A
  • incorrect dose delivered to all patients
30
Q

What is the reason for laser localisation failing?

A
  • laser drift from wall vibrations or someone bumping them
31
Q

What happens if the lasers are off?

A
  • patient levelling and positioning off isocentre

- large shifts from imaging

32
Q

What is the reason for optical distance indicator failing?

A
  • ODI indicator may have drifted or been bumped
33
Q

What happens if the ODI is off?

A
  • SSD checks incorrect

- SSD setups would be systematically out

34
Q

What is the reason for light field size failing?

A
  • light soucenot on collimator rotation axis
  • light source may not be at correct SSD
  • jaw calibration changed
35
Q

What happens if the light field size is wrong?

A
  • field size could be inconsistent depending on the SSD, border chekcks show wrong information
  • jaw incorrect calibration can lead to all patient field size treated incorrectly
36
Q

What is the reason for door interlock failing?

A
  • electronic compenent failed
37
Q

What happens if the door interlock is not working?

A
  • safety issues (unintended radiation dose)
38
Q

What is the reason for audio visual failing?

A
  • AV system failure

- power supply of AV system failure

39
Q

What happens if the audio visual is not fixed?

A
  • patient safety

- inability to coach RPM patients

40
Q

What are some limitations of MapCheck?

A
  • directional dependence

- field size limitation

41
Q

What are some reasons a plan might fail QA?

A
  • high modulation/complexity
  • off axis field
  • wrong phantom loaded
42
Q

Advantages of ArcCheck?

A
  • can dliver pt plan at the gantry, collimator and couch angles
  • MLC accounted for
  • measures entry and exit dose
  • absolute dose
43
Q

Disadvantages for ArcCheck?

A
  • doesn’t deposit dose at a depth, only assess dose in the periphery of the phantom
  • phantom cost and training
  • alignment of phantom must be accurate

Quality Assurance (5 decks)

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