TPS and Treatment QA Flashcards
Why is checking beam model important?
- a perfectly commissioned TPS will not produce perfect results
What causes differences in beam models?
- 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
What is different about sterotactic beam modelling?
- need small detector as large detector averages both high dose from field and low dose penumbra
- commonly use farmer chamber
What is the biggest reason for QA failure?
- large difference in field size and detector perimeter
What is the benefits of MU checks?
- 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
What is a limitation of MU checks?
- 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
What is a benefit of MapCheck?
- 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
What is a limitation of MapCheck?
- performed under gantry 0 therefore unable to detect any uncertainty introduced by irradiation under other gantry angles
What are the limitations of doing QA on gantry angle 0?
- MLC leaf position under gravity
- output variation with gantry angle
- beam symmetry with gantry angle
What are examples of 2D array?
- MatrixX
- MapCheck
What are examples of 3D array?
- Delta4
- ArcCheck
- Octavius
What is the tolerance for PSQA?
- 3%/3mm
What are factors to consider when purchasing a QA device?
- absolute vs relative dose
- treatement technique
- resolution
- detector drift (sensitivity change)
- gamma pass criteria
- cost (upfront and ongoing)
- compatibility with existing equipment
What is gamma calculations?
- gamma index combines dose differences and distance difference to calculate dimensions metric for each point in the evaluated distribution
What is the gamma pass rate?
- 90% gamma <1 within 3%, 3mm
What is the rationale for PSQA?
- 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
What is EPIQA?
- software that converts a dosimetric image acquired by an EPID into a dose map and compare with a reference dose distribution from TPS
What are common causes for QA failure?
- 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)
What is the benefit of a good detector for highly modulated fields?
- more accurate evaluation especially in modualted areas
What are the charactersitics of high complexity fields?
- high MU per Gy (factor of 3 is good)
- narrow MLC aperture
- higher undertainty during delivery
What are 9 main treatment daily QA tests?
- collision interlocks
- optical distance indicator accuracy at isocentre
- wedge moving check on one angle
- laser localisation
- imaging isocentre coincidence
- x-ray and electon output
- audio visual
- light field
- door interlock
What is the reason for collision interock failure?
- sensors failure
- electronics board failure
What happens if collisional interlock not working?
- gantry/panel collision into patient causing injury
What is the reasons for imaging positioning and isocentre coicidence failure?
- calibration file may have been corrupted
- electronic incorrect or damaged
What happens if treatment and imaging isocentre coicidence is not working?
- incorrect source/detector positioning
- geometric misalignment for treatment
What is the reason for wedge moving check failure?
- EDW STT table may have been corrupted
What happens if the wedge moving check is not working?
- all wedge fields would potentially be incorretly treated
What is the reason for x-ray and electron output failing?
- electronic drfit of detector
- damage to MU chamber
What happens if the x-ray and electron output is wrong?
- incorrect dose delivered to all patients
What is the reason for laser localisation failing?
- laser drift from wall vibrations or someone bumping them
What happens if the lasers are off?
- patient levelling and positioning off isocentre
- large shifts from imaging
What is the reason for optical distance indicator failing?
- ODI indicator may have drifted or been bumped
What happens if the ODI is off?
- SSD checks incorrect
- SSD setups would be systematically out
What is the reason for light field size failing?
- light soucenot on collimator rotation axis
- light source may not be at correct SSD
- jaw calibration changed
What happens if the light field size is wrong?
- 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
What is the reason for door interlock failing?
- electronic compenent failed
What happens if the door interlock is not working?
- safety issues (unintended radiation dose)
What is the reason for audio visual failing?
- AV system failure
- power supply of AV system failure
What happens if the audio visual is not fixed?
- patient safety
- inability to coach RPM patients
What are some limitations of MapCheck?
- directional dependence
- field size limitation
What are some reasons a plan might fail QA?
- high modulation/complexity
- off axis field
- wrong phantom loaded
Advantages of ArcCheck?
- can dliver pt plan at the gantry, collimator and couch angles
- MLC accounted for
- measures entry and exit dose
- absolute dose
Disadvantages for ArcCheck?
- 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
