Portal dosimetry of small unflattened beams

We developed and validated a dedicated small field back-projection portal dosimetry model for pretreatment andin vivoverification of stereotactic plans entailing small unflattened photon beams. For this purpose an aSi-EPID was commissioned as a small field dosimeter. Small field output factors for 6 MV FFF beams were measured using the PTW microDiamond detector and the Agility 160-leaf MLC from Elekta. The back-projection algorithm developed in our department was modified to better model the small field physics. The feasibility of small field portal dosimetry was validated via absolute point dose differences w.r.t. small static beams, and 5 hypofractionated stereotactic VMAT clinical plans measured with the OCTAVIUS 1000 SRS array dosimeter and computed with the treatment planning system Pinnacle v16.2. Dose reconstructions using the currently clinically applied back-projection model were also computed for comparison. We found that the latter yields underdosage of about -8% for square beams with cross section near 10 mm x 10 mm and about -6% for VMAT treatments with PTV volumes smaller than about 2cm³. With the methods described in this work such errors can be reduced to less than the ±3.0% recommendations for clinical use. Our results indicate that aSi-EPIDs can be used as accurate small field radiation dosimeters, offering advantages over point dose detectors, the correct positioning and orientation of which is challenging for routine clinical QA.