Dosimetric Performance of a New Ring-Gantry Linear Accelerator for IMRT and VMAT Prostate Plans

Abstract

We aimed to evaluate dosimetric performance and delivery efficiency of a new ring-gantry linac characterized by a staggered dual-layer multi-leaf collimator (MLC). Performance of treatment plans generated using a cloud-based planning system is reported here for the first time. Using IMRT & VMAT prostate plans, we compared performance of the ring gantry linac with that of a C-arm based linac. We hypothesized that both IMRT and VMAT treatments on the new system would yield similar target coverage and dose conformity, albeit with differences in treatment time and organ-at-risk (OAR) sparing. Ten prostate cancer patients previously planned using a treatment planning system (TPS) and treated with VMAT on a conventional C-arm linac (2300EX) at a dose rate of 300 MU/min were retrospectively re-planned for the new linac using a newer version of the TPS. Each plan was re-planned for IMRT (7 and 9 fields) and VMAT (2 and 3 arcs). Prescription dose for the target was standardized to 66.6 Gy. Plans were normalized to deliver 100% prescription dose to 95% of the target volume. Due to limitations in the current version of the ring gantry system, plans were generated using a single layer MLC (leaf width of 1 cm). Metrics of plan quality such as dose conformity, OAR sparing, and monitor units (MU) for new IMRT & VMAT plans were compared with those of the clinically delivered plans. Wilcoxon signed-rank test was used to compare differences in plan quality. Compared to the clinically delivered plans, rectum/bladder sparing and dose conformity were comparable for the new 9-field IMRT & VMAT (2- and 3-arc) plans, but were inferior for the 7-field IMRT plan (p <0.05). Both new IMRT plans achieved significantly better femoral head sparing than the delivered plans and the new VMAT plans (p <0.05). A significantly higher MU was required for all new plans (P < 0.05), to compensate for the wider leaf width. However, delivery time for the new VMAT plans is about 3 times shorter on the new system due to faster gantry and MLC motion. The 9-field IMRT and 2- and 3-arc VMAT prostate plans generated for the new ring-gantry linac offer dose coverage and OAR sparing that is comparable to the VMAT plans delivered on the conventional linac. When comparing ring-gantry IMRT and VMAT plans, the 9-field IMRT plans achieved better OAR sparing while maintaining similar dose coverage.Abstract TU_28_3176; Table 1Dosimetric comparison between clinically treated plans vs. plans for the ring-gantry linacClinically DeliveredPlans for new ring-gantry linac7-field IMRT9-field IMRT2-arc VMAT3-arc VMATCI1.02 ± 0.021.10 ± 0.051.06 ± 0.031.05 ± 0.031.04 ± 0.02HI1.05 ± 0.011.06 ± 0.011.05 ± 0.011.06 ± 0.011.05 ± 0.01MU666 ± 179931 ± 216948 ± 2321123 ± 4651151 ± 271Rectum V70 (%)33.3 ± 11.435.8 ± 14.931.5 ± 11.631.7 ± 10.133.1 ± 13.4Bladder V70 (%)32.2 ± 21.433.9 ± 20.828.8 ± 20.030.6 ± 18.431.1 ± 20.3Left Femoral Head V30 (%)19.1 ± 15.27.6 ± 10.49.7 ± 9.419.0 ± 14.317.9 ± 15.0Right Femoral Head V30 (%)18.3 ± 17.98.83 ± 15.54.5 ± 7.921.7 ± 11.916.0 ± 11.4 Open table in a new tab