SU‐E‐T‐566: Comparison of VMAT and IMRT for Whole Abdomen Radiation Therapy (WART)

Purpose: Whole Abdomen Radiation Therapy (WART) is used in the treatment of desmoplastic small round cell tumors as well as other tumors with peritoneal dissemination. Intensity modulated radiation therapy (IMRT) is conventionally used to cover the multiple planning target volumes (PTVs) while sparing nearby critical structures, but this approach often requires two isocenters and ≥20 individual treatment fields. Four‐field volumetric arc therapy (VMAT) can produce clinically similar treatment plans with the potential to reduce treatment time substantially. Here we examine treatment times and plan robustness for patients undergoing WART. Methods: Twenty patients undergoing WART at our institution were included in this study. Twelve IMRT and 8 VMAT plans utilized upper and lower abdominal isocenters and met similar dose constraints. Treatment times were tabulated from start of daily kV imaging to beam delivery end. Daily treatment couch coordinates were also recorded. From these couch coordinates, difference between actual and planned separation between isocenters was computed. Plan robustness with regard to PTV coverage within the region of field overlap was analyzed for 3 VMAT and 3 IMRT plans assuming a 2σ deviation in isocenter location. Results: The average treatment time for VMAT was 15 minutes shorter than for IMRT (27 minutes vs. 42 minutes). The average deviation in isocenter separation was 0.0 – 0.1 cm in each direction, with a standard deviation of 0.2 – 0.3 cm. Compared with IMRT plans, VMAT plans showed similar loss in PTV coverage for increasing isocenter displacement and similar dose inhomogeneity with decreasing isocenter displacement. Conclusion: Use of VMAT results in substantial time‐savings for 2‐isocenter whole abdomen radiotherapy plans. VMAT plans show robustness similar to IMRT with respect to isocenter displacement. VMAT should be considered for these very complicated treatments to minimize risk of patient movement during therapy and maximize treatment delivery efficiency.