SU-F-T-567: Sensitivity and Reproducibility of the Portal Imaging Panel for Routine FFF QC and Patient Plan Dose Measurements.

PURPOSE: The purpose of this work was to see if the EPID is a viable alternative to other QA devices for routine FFF QA and plan dose measurements.

METHODS: Sensitivity measurements were made to assess response to small changes in field size and beam steering. QA plans were created where field size was varied from baseline values (5-5.5cm, 20-20.5cm). Beam steering was adjusted by altering values in service mode (Symmetry 0-3%). Plans were measured using the Varian portal imager (aS1200 DMI panel), QA3 (Sun Nuclear), and Starcheck Maxi (PTW). FFF beam parameters as stated in Fogliata et al were calculated. Constancy measurements were taken using all 3 QC devices to measure a MLC defined 20×20cm field. Two clinical SABR patient plans were measured on a Varian Edge linac, using the Portal Dosimetry module in ARIA, and results compared with analysis made using Delta4 (ScandiDos).

RESULTS: The EPID and the Starcheck performed better at detecting clinically relevant changes in field size with the QA3 performing better when detecting similar changes in beam symmetry. Consistency measurements with the EPID and Starcheck were equivalent, with comparable standard deviations. Clinical plan measurements on the EPID compared well with Delta4 results at 3%/1mm.

CONCLUSION: Our results show that for FFF QA measurements such as field size and symmetry, using the EPID is a viable alternative to other QA devices. The EPID could potentially be used for QC measurements with a focus on geometric accuracy, such as MLC positional QA, due to its high resolution compared to other QA devices (EPID 0.34mm, Starcheck 3mm, QA3 5mm). Good agreement between Delta4 and portal dosimetry also indicated the EPID may be a suitable alternative for measurement of clinical plans.