Energy Layer Spacing Effects on Target Coverage and Homogeneity In Dynamic ARC Pencil Beam Scanning (PBS) Proton Lung Plans
Abstract
Purpose
DynamicARC is a pencil beam scanning (PBS) proton therapy technique designed to improve delivery efficiency and plan quality. This study evaluated how energy layer spacing (ELS) affects target coverage, dose homogeneity, and organ-at-risk (OAR) sparing in lung DynamicARC planning.
Methods
Four anonymized lung cancer patients were replanned to 50 Gy(RBE) in 5 fractions using a single partial arc for IBA ProteusOne in RayStation (research version;2023B). ELS was varied from 0.10–0.40 cm (0.05 cm increments) while all other parameters were held constant. Robust optimization incorporated setup (±5 mm) and range (±3%) uncertainties. Metrics included CTV_D99%, CTV_D95%, CTV_Dmax, CTV_D1%, CTV_D95wcs, homogeneity index (HI), and OAR doses.
Results
CTV_D99% demonstrated minimal variation across ELS values (mean≈49.48 Gy(RBE)), with the lowest dose at 0.10 cm and the highest at 0.35 cm. All ELS values met the CTV_D95%, except 0.15 cm, which was slightly lower (49.96 Gy(RBE)). High-dose regions increased with larger ELS; CTV_Dmax ranged from 55.19 to 55.33 Gy(RBE). Worst-case coverage (CTV_D95wcs) remained stable at smaller spacings (≈49.05 Gy(RBE)), with greater fluctuation observed at ELS≥0.30 cm. HI indicated improved dose uniformity at smaller ELS, with patient HI values of approximately 1.09–1.10 at fine spacing, increasing to 1.12–1.13 at 0.40 cm. For OARs, total lung V20 remained low across all plans (3.87–3.95%). Mean lung dose ranged from 2.20 to 2.32 Gy(RBE), with the highest value at ELS 0.35 cm. Spinal cord and esophagus constraints were satisfied for all ELS values, although larger spacings produced greater inter-plan variability.
Conclusion
ELS influenced dose homogeneity and high-dose regions, with limited impact on nominal target coverage and OAR constraints. ELS 0.20–0.25 cm provided the most consistent balance of uniformity and robust coverage. A follow-up study will assess respiratory motion interplay across ELS values using 4DCT-based simulations and/or motion phantoms.