3184 - The First Investigation of Spot Size Impact on Spot Scanning Proton Arc Therapy across Multiple Disease Sites

Purpose/Objective(s):

Spot scanning proton arc (SPArc) therapy, delivering dose through continuous gantry rotation with pencil beam scanning, has attracted intensive attention due to its superior dose conformity, organs-at-risk (OAR) sparing and treatment efficiency compared to intensity-modulated proton therapy (IMPT). The resulting dose distribution depends on the underlying beam model, particularly lateral spot size. To facilitate the clinical implementation of SPArc, we conducted a systematic study on SPArc plan quality across multiple disease sites using a standardized composite scoring rubric.

Materials/Methods:

Three beam models were generated with lateral spot sizes from 70MeV to 227.7MeV (Gaussian s): small (1.7–4 mm), medium (3.4–8mm), and large (6.8–16 mm). Four clinical patient cases (brain, lung, liver, pancreas) were replanned for SPArc with the beam models using identical settings during plan optimization. Plan quality was quantified with a composite score incorporating target coverage/homogeneity and organ-at-risk (OAR) dose objectives. Delivery-related plan characteristics including total monitor units (MU), beam spot number, and estimated delivery time were investigated.

Results:

Across all four sites, the highest plan quality were generated with small spot model, reflecting improved target coverage and OAR sparing within the scoring rubric compared to medium and large spot models. Compared to the large spot model, plan score increased by 30.2±28.3% and 22.7±21.0%, total MU decreased by 29.5±13.3% and 21.5±9.9%, and the number of spots increased by 688±275% and 268±48% while total delivery time only increased by 7.1±4.2% and 1.1±7.6% (n=4) for the small spot and medium spot models, respectively. These results indicate a tradeoff between plan quality and delivery-related plan characteristics.

Conclusion:

Our study uses three distinct spot-size beam models to quantify how spot-size modeling impacts SPArc across four different clinical sites (brain, lung, liver, pancreas) using a standardized composite plan-quality scoring rubric rather than isolated DVH metrics for the first time. It provides practical, implementation-ready evidence that smaller modeled spots improve overall plan-quality scores and reduce MU while simultaneously increasing planned spot positions, motivating centers to consider both dosimetric and delivery-related metrics when commissioning and adopting SPArc. We expect this study to provide guidance on the trade-off between spot size and plan quality for proton arc therapy at existing spot-scanning proton therapy centers and future systems.