Clinical evaluation of photon optimizer (PO) MLC algorithm for stereotactic, single-dose of VMAT lung SBRT

Recently implemented 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) algorithm for a single-dose of volumetric modulated arc therapy (VMAT) lung stereotactic body radiation therapy (SBRT). Clinical SBRT treatment 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 MLC algorithm with identical planning parameters and objectives. Average planning target volume derived from the 4D CT scans was 13.6 ± 12.0 cc (range: 4.3 to 41.1 cc) Patients were treated with 6 MV flattening filter free beam using Acuros-based calculations and 2.5 mm calculation grid-size (CGS). Both treatment plans were normalized to receive same target coverage and identical CGS to isolate effects of MLC positioning optimizers. Original PRO and re-optimized PO plans were compared via RTOG-0915 protocol compliance criteria for target conformity, gradient indices, dose to organs at risks and delivery efficiency. Additionally, PO-VMAT plans with a 1.25 mm CGS were evaluated. Both plans met RTOG protocol requirements. Conformity indices showed no statistical difference between PO 2.5 mm CGS and PRO 2.5 mm CGS plans. Gradient index (p = 0.03), maximum dose to 2 cm away from planning target volume in any direction (D_2cm) (p < 0.05), and gradient distance (p < 0.05) presented statistically significant differences for both plans with 2.5 mm CGS. Some organs at risks showed statistically significant differences for both plans calculated with 2.5 mm CGS; however, no clinically significant dose differences were observed between the plans. Beam modulation factor was statistically significant for both PO 1.25 mm CGS (p = 0.001) and PO 2.5 mm CGS (p < 0.001) compared to clinical PRO 2.5 mm CGS plans. PO-VMAT plans provided decreased beam-on time by an average of 0.2 ± 0.1 minutes (up to 1.0 minutes) with PO 2.5 mm and 1.2 ± 0.39 minutes (maximum up to 3.22 minutes) with PO 1.25 mm plans compared to PRO 2.5 mm plans. PO-VMAT single-dose of VMAT lung SBRT plans showed slightly increased intermediate-dose spillage but boasted overall similar plan quality with less beam modulation and hence shorter beam-on time. However, PO 1.25 mm CGS had less intermediate-dose spillage and analogous plan quality compared to clinical PRO-VMAT plans with no additional cost of plan optimization. Further investigation into peripheral targets with PO-MLC algorithm is warranted. This study indicates that PO 1.25 mm CGS plans can be used for RapidPlan modeling for a single dose of lung SBRT patients. PO-MLC 1.25 mm algorithm is recommended for future clinical single-dose lung SBRT plan optimization.