Interplay Effect during Lung SBRT with VMAT Delivered

Interplay Effect during Lung SBRT with VMAT Delivered by Flattening Filter Free Beams Min Rao Swedish Cancer Institute Seattle, WA Poster#: 1082

Purpose Ø VMAT delivered by flattening filter free (FFF) beams can significantly reduce treatment time for SBRT. Ø High dose rate raises the concern regarding the impact of the interplay effect between MLC movement and target motion. Ø 4 D dose calculations were performed to investigate the interplay effect on the delivered dose in lung SBRT cases treated using FFF beams on Elekta Versa. HD. Poster#: 1082

Methods Ø ITV: MIP 4 DCT dataset Ø PTV = ITV + 5 mm Ø Prescription: 12 Gy x 4 Ø Elekta Versa. HD, 10 MV Ø Two dose rates § Conventional dose rate: 400 MU/min § Maximum dose rate for FFF: 2300 MU/min Ø 10 patients with tumor motion 1~3 cm Ø 4 D dose calculation based on deformable image registration Poster#: 1082

Results and Summary Ø Comparable plan quality were achieved by using 10 MV FFF beams compared to 6 MV conventional beams. Ø Larger interplay effect was observed in high dose rate FFF plans. § High dose rate: segment delivery time shorter than breathing cycle Ø Dosimetric impact of interplay effect is unlikely to be clinically significant. § Maximum dose discrepancy due to solely interplay effect was less than 3% within the VOI. Poster#: 1082