SU‐E‐T‐699: Evaluation of a Monte Carlo Based TPS for SBRT

Abstract

Purpose: Hypo‐fractionated dose regimens used in stereotactic body radiotherapy (SBRT) demand superior dose calculation and delivery accuracy. We evaluate the impact of using Monte Carlo (MC) treatment planning system (TPS) in SBRT of lung patients.Methods: A commercial MC TPS (iPlan 4.1.2, BrainLAB AG, Feldkirchen, Germany) validated in heterogeneous phantom geometry was employed. For lung and bone heterogeneities, iPlan has shown excellent agreement with measurements to within 2–3%/2mm for dose/distance‐to‐agreement. Five lung SBRT patient plans (PTV: 10 – 250 cc, mean = 67.5cc, S.D. = 85.9cc) originally computed with pencil beam (PB) algorithm were retrospectively analyzed using MC. Prescription dose was 50Gy to >99% of PTV in 5 fractions using 10–12 non‐coplanar 6MV beams. For each patient, four delivery modes were considered: 3DCRT, IMRT, dynamic‐conformal arc (DCA), and hybrid‐arc (HA). Monitor units obtained from PB plans were used to recalculate MC dose. All patient plans were compared using PTV and OAR dose indices (Dmin, Dmax, Dmean and D95).Results: MC calculated doses were significantly lower compared to PB plans for PTV surrounded by lung medium. Owing to electron disequilibrium, largest dose discrepancy was seen in superficial (<5mm) regions of target. For a 22mm target (PTV = 38.2cc): PTV Dmean and Dmin were 14.3% and 27.6% lower respectively. Corresponding differences were smaller for ITV (Dmean and Dmin 14% and 7.6% lower respectively) showing better agreement between PB and MC plans. For PTV surrounded by soft‐tissue or adjacent to chest‐wall, MC doses were closer to PB (<5% agreement). Results were relatively insensitive to target size and mode of delivery Conclusions: This study highlights the need for MC based dose calculations in lung SBRT. Closer agreement between PB and MC results for ITV calls for robust and reliable image‐guidance.