Room: Exhibit Hall
Purpose: In the case of VMAT SRS treatments the TG-218 recommend true composite patient specific QA is not possible because there are large couch kicks which will tend to irradiate the electronics of collecting devices such as Mapcheck/ArcCheck. The results of using the perpendicular field-by-field (PFF) collection method with true composite 3D trajectory log calculation for VMAT SRS QA with PerFraction are presented.
Methods: The PerFraction software was commissioned according the vendorâ€™s guidelines for 2D and 3D dose calculation. Each patientâ€™s treatment plan was delivered to the portal imager on a TrueBeam linear accelerator in integration mode. The imager distance was 160 cm to prevent saturation of the imager with the 6X FFF beam. A verification plan was created in Eclipse to calculate the dose in a 40Ã—40Ã—10 cmÂ³ phantom with 0 gantry angle, mimicking the FZAD calculation. The integrated image and trajectory logs for each beam were automatically imported and analyzed in the PerFraction software. The resulting field-by-field 2D planar phantom doses and true composite 3D patient dose were analyzed.
Results: The per field central axis dose calculated by Eclipse in the FZAD phantom was within 3% on average. TG-218 FZAD field-by-field and 3D PTV Î³ evaluations had an average passing rate >99%. The average Î³ passing rates dropped to 98% (2D) and 90% (3D) when the evaluation criteria was (2%/1mm/global normalization). The 3D passing rate was dependent on PTV size, 80% and 97% for PTV volumes <1 cc and >1 cc respectively. DVH comparison at the minimum, maximum, and mean was with 3% of the planned values.
Conclusion: The PFF method suggested by TG-218 works well for VMAT SRS plans using the PerFraction software. The trajectory log based 3D patient dose calculations are an added benefit and alleviate the disadvantages of the PFF method indicated by TG-218.