IMRT/VMAT Plan Eval & AQ Flashcards
What is the Homogeneity index?
Homogeneity Index (HI) is an objective tool to analyse the uniformity of dose distribution in the target volume.
HI = (D2-D98)/Dp is a homogeneity index proposed in ICRU-83, where D2 and D98 represent the doses received by 2% and 98% volumes of PTV, respectively.
What is the Conformity Index?
CI is defined as ratio of volume of the body
receiving the prescribed dose (Vp) to the volume of
the PTV receiving the same dose (PTVp),
CI = Vri/TV
e.g. V95%/PTV
What are the advantages and disadvantages of the CI?
adv: simplicity and integration of multiple parameters
disadv: Cannot provide practical information, also require visual analysis
What is QA?
procedures the ensure consistency and safe fulfilment of the medical prescription
ensure dose to the target volume, minimal dose to normal tissue, minimal exposure of personnel and adequate patient monitoring
Why is QA good?
reduces uncertainties and errors in dosimetry, treatment planning, equipment performance,
treatment delivery…, thereby improving dosimetric
and geometric accuracy and the precision of dose delivery
This improves RT results raising tumour control rates as well as reducing complication and recurrence rates
allows reliable inter-comparision of results among RT centres
Sources of Error?
- Equipment/ software (tolerances and error logs documented)
- Natural variability (internal and set-up errors included in margins, daily imaging helps further reduce)
- Human error (random, transcription/ omission/ oversight)
- Data transfer
What are some DICOM issues?
coordinates, movements and scales
What is our responsibility as RT’s?
responsible for accuracy of information recorded in medical charts
- we calculate and document the relevant treatment parameters
- adhere to safe practice procedures
What are the pre-planning checks?
CT sim
Primary and secondary datasets are imported into TPS
RT checks prior to treatment planning
What are the checks after planning?
treatment plan done
Treatment plan checked by second RT
Treatment plan checked by physicist
What are the additional checks for QA?
Day 1 checks
weekly checks
Boost/ Phase 2 checks
Final treatment checks
What is the QA checklist?
Treatment patient correct site plan matches prescription total dose, fractionation daily dose treatment machine correct beam type and energy OAR dose isocentre moves MLC shielding appropriate inhomogeneity correction applied correct bolus target volume and field size correlate DRR generated to the correct isocentre
What are the QA tools?
manual/ hand calculation
departmental programs-spreadsheets (excel)
Commercial software (IMSure QA software, MU check)
What are the Physics checks for IMRT/VMAT?
- Validate IMRT parameters
- More complex and time consuming
- Dose delivery in IMRT dependent on accurate position of MLC leaves
- Important to verify leaf position
- 2D,3D and 4D verification
What is patient specific QA?
- depicts the information of variation in planned and measured dose in the PTV
- Point dose measurements (Checks dose at isocentre)
- Verification of dose delivery of separate beams using 2D, 3D or 4D detectors.
- Gravitational errors affecting MLC present
