SU-FF-T-157: A Comparison of Treatment Planning and Delivery of VMAT Using Anatomy-Based and Fluence-Based Inverse Planning with Step and Shoot IMRT

Purpose: In this study, we compare the plan quality, treatment delivery efficiency and accuracy of VMAT and fixed‐field IMRT. VMAT plans were created using both anatomy‐based and fluence‐based inverse planning techniques. Method and Materials: Nine cases including 3 prostate, 3 pancreas, 2 head and neck (HN) and 1 brain were selected for this study. The patients were previously treated with step‐and‐shoot IMRT plans developed in the CMS Xio treatment planning system (TPS). For each case, two VMAT plans were generated retrospectively. The first VMAT plan was generated using ERGO++ R1.7 TPS (Elekta), which uses an anatomy‐based inverse planning method. The second VMAT plan was generated by converting the optimized fluence maps calculated by the Pinnacle3 TPS into deliverable arcs using an in‐house arc sequencer. VMAT plans were verified on an Elekta Synergy and plan comparisons were made in terms of delivery efficiency and accuracy. Results: For pancreatic, prostate and brain cases, anatomy‐based VMAT provided comparable target coverage and sparing of OARs as compared with fluence‐based VMAT and fixed‐field IMRT. For HN, fluence‐based VMAT improved target coverage and sparing of OARs compared with anatomy‐based VMAT and IMRT. For the 9 cases tested, the mean delivery time for the fluence‐based VMAT was 20% less than the anatomy‐based VMAT and 78% less than IMRT. The number of monitor units for the anatomy‐based VMAT was approximately 10% less than fluence‐based VAMT and 56% less than IMRT. The average passing rates for the plan QAs were 98%, 99% and 91% for IMRT, fluence‐based VMAT, and anatomy‐based VMAT, respectively. Conclusion: VMAT is able to reduce treatment times by 70% while producing equivalent or better dose distributions as fixed‐field IMRT. For challenging HN cases, VMAT using a fluence‐based inverse planning method improves the dose distribution quality as compared with IMRT and anatomy‐based VMAT. Research supported by Elekta