Dose Comparison between Proton Pencil Beam and Monte Carlo Dose Calculation Algorithm in Lung Cancer Patients

Abstract

Proton pencil beam dose calculation algorithms (PBA) have limitations in modeling proton scatter dose and its impact on depth dose properties through heterogeneous tissues present when treating lung cancer patients. Monte Carlo dose calculations (MC) may provide more accurate dose calculations when compared with the current algorithms that are in clinical use. This study investigates clinical pencil beam scanning (PBS) planned dose differences between PB and MC in lung cancer patients. With IRB approval, a comparative dosimetric analysis between RaystationTM Monte Carlo v4.0 (pending FDA 510 clearance) and RaystationTM pencil beam v4.0 dose algorithms was performed on 10 patient plans. PBS gantry plans were generated using single-field optimization technique to maintain target coverage under 3% range and 3 mm setup uncertainty. Dose prescriptions ranged from 60 Gy(RBE) to 66.6 Gy(RBE). Dose differences between PBA and MC were recorded for the following plan metrics: CTV V95, CTV homogeneity index (HI), total lung V20, total lung VRX (relative volume of lung receiving the prescribed dose or higher), and global maximum dose. Non-parametric Wilcoxon signed-rank testing was carried out to evaluate pairwise differences in planned dose and dose-volume parameters between PBA and MC. In this 10 patient sample, CTV V95 ranged from 99% to 100% with a median of 100% for PBA. When recalculated with MC, CTV V95 ranged from 77% to 94% with a median of 90% (p=0.002). MC treatment plans showed a reduction of dose homogeneity within the target volume. The median CTV HI (D95/D5) was 0.98 for PBA and 0.91 for MC (p = 0.002). Total lung V20 distributions were statistically identical to within 1% between dose calculation algorithms (p=1.0). Median lung VRX was 7% and 1% for PBA and MC, respectively (p = 0.002), indicating a reduction in lung volume receiving high dose when using MC. Lastly, the global maximum patient dose increased from a median value of 106% for PBA (range 101-113%) to 109% for MC (range 104-122%, p = 0.002). A retrospective analysis of 10 lung cancer PBS plans revealed a planned dose reduction in coverage to the CTV when recalculated using MC. This suggests that the use of PBA dose calculation in this disease site may lead to underdosing of the target volume and may support the clinical integration of MC dose calculation during treatment planning.Abstract 3654Pencil BeamMonte CarloMedianRangeMedianRangeSign Rank pCTV V95100%99% - 100%90%77% - 94%0.002CTV HI0.980.95 - 1.000.910.87-0.940.002Lung V2025%15% - 35%25%16% - 34%1.000Lung VRX7%2% - 11%1%0% - 6%0.002Max Dose106%101% - 113%109%104% - 122%0.002 Open table in a new tab