Introducing a New Spot-Scanning Proton Arc Optimization Algorithm With a Variable Tolerance Window to Improve Plan Quality For Bilateral Breast Cancer

Document Type

Conference Proceeding

Publication Date

9-2026

Publication Title

International Journal of Particle Therapy

Abstract

Background and Aims: Breast cancer is the most common non-cutaneous cancer in women. Despite state-of-the-art Intensity Modulated Proton Therapy (IMPT), achieving satisfactory performance of target coverage and OAR dose remains a challenge, especially for complex target geometries such as bilateral breast cancer. It has been challenging for energy layer selection in evenly-distributed control points, resulting in suboptimal plan quality. We hypothesize that giving more degrees of freedom by allowing extra energy layers across specific arc trajectories will improve treatment delivery. Therefore, we introduce a new SPArc algorithm with a variable tolerance window (SPArc-variable) to further improve plan quality for patients receiving bilateral breast and regional nodal irradiation. Methods: The SPArc-variable optimization algorithm is based on 0- 1 knapsack problem. Based on the multifield-IMPT plan with 20- degree spacing, the algorithm iteratively selects a subset of energy layers and resamples them according to the gantry’s motion. Finally, spot optimization is applied, fine-tuning the dose distribution. Five cases with bilateral breast and lymph nodes treatment are retrospectively selected. Planning groups include IMPT with 2 isos, SPArc planning with a fixed distance of 2.5 degrees between adjacent control points(SPArc-original), and SPArc planning with non-fixed distance between adjacent control points(SPArc-variable), with a prescription of 5000cGy(RBE). DVH metrics are included to evaluate performance over OARs and targets, and dynamic delivery is simulated via a published dynamic arc system controller. Results: Both SPArc-original and SPArc-variable plans showed slightly superior target coverage compared to IMPT plans, but not statistically significant. In addition, SPArc-variable showed significantly better performance than IMPT and SPArc-original in sparing the heart (P< 0.01 for D1%) and significantly better performance in sparing the left and right lungs(P< 0.01 for V20Gy and V5Gy) than both IMPT and SPArc-original. In terms of delivery efficiency, the SPArc-variable shows slightly prolonged treatment delivery time compared to SPArc-original and superior to IMPT(SPArc-variable:434.99±52.95sec, IMPT: 506.47±14.65sec, P=0.02, SPArcoriginal:382.11±49.31sec, P=0.01). Conclusions: This study introduces a novel SPArc optimization algorithm with a variable window to enhance dosimetric performance for the bilateral breast cancer patient population. More specifically, it reduces the dynamic delivery time compared to IMPT, while achieving equivalent target coverage and significantly superior heart and lungs sparing compared to IMPT and SPArc-original.

Volume

21

Issue

Suppl

First Page

101793

Last Page

101793

Comments

PTCOG 64 Particle Therapy Co-Operative Group 64th Annual Conference, June 8-13, 2026, Deauville, France

DOI

10.1016/j.ijpt.2026.101793

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