Room: Exhibit Hall | Forum 3
Purpose: Photon Optimizer (PO) MLC optimization algorithm is mandatory for RapidPlan modeling in Eclipse. This report quantifies and compares the dosimetry and treatment delivery parameters of PO vs its predecessor progressive resolution optimizer (PRO) for a single-dose of volumetric modulated arc therapy (VMAT) lung SBRT.
Methods: Clinical SBRT plans for 12 early-stage non-small-cell-lung cancer patients receiving 30 Gy in 1 fraction using PRO-VMAT were re-optimized using the PO-VMAT algorithm with identical planning parameters. Average PTVs derived from 4D-CT scans were 4.3-41.1cc. Patients were treated with 6MV-FFF beam using Acuros-based calculations and 2.5mm calculation grid-size (CGS). Both treatment plans were normalized to receive same target coverage and identical CGS to isolate the MLC positioning optimizer effects. Original PRO and re-optimized PO plans were compared via RTOG-0915 protocol compliance for target conformity, heterogeneity, gradient index and dose to critical structures. Additionally, PO-VMAT plans with a 1.25mm CGS were evaluated.
Results: Both plans met RTOG protocol requirements. Conformity indices showed no statistical difference between PO-VMAT and PRO-VMAT plans for both CGS. Gradient distance showed a statistical difference for 1.25mm CGS(p=0.001) and 2.5mm CGS(p<0.001). Gradient index(p=0.028) and D2cm(p=0.009) presented statistically significant differences for a 2.5mm CGS. Some OARs showed statistically significant differences, however no clinically significant dose differences were observed (average absolute difference <1.0Gy). Modulation factor was statistically significant for both 1.25mm CGS(p=0.001) and 2.5mm CGS(p<0.001). PO-VMAT plans provided decreased beam-on time for an average of 1.0Â±0.7min with 2.5mm CGS and 1.2Â±0.9min with 1.25mm CGS.
Conclusion: PO-VMAT single-dose lung SBRT treatments showed slightly increased intermediate-dose spillage but boasted similar plan quality with less beam modulation. PO-VMAT plans with 1.25mm CGS had less intermediate-dose spillage and analogous plan quality compared to PRO-VMAT plans with no additional cost. This indicates that RapidPlan modeling can be performed using PO-VMAT plans with 1.25mm CGS for single-dose lung SBRT.