Dosimetric impact of arc simulation angular resolution in single-isocentre multi-target stereotactic radiosurgery

This study evaluates the dosimetric impact of arc simulation angular resolution in VMAT-based Single Isocentre Multiple Target (SIMT) SRS, focusing on their dependence on target size, isocentre distance, number of arcs, and arc type. A phantom study analysed angular resolution (0.5°, 1°, 2°) effects on dosimetric accuracy for PTVs of 0.5 cm, 1 cm, and 2 cm at distances of 2.5 cm, 5 cm, and 7.5 cm from the isocentre using conformal arc and VMAT plans. Clinical validation involved 32 patients with 2-8 brain metastases, comparing plans recalculated at 1° and 2° resolutions. Dosimetric parameters included: D_near-Min, D_near-Max, D_mean, D_median, TV_PIV, CI_Paddick, GI, and Brain-GTV 12 Gy. For the 0.5 cm diameter target located at 7.5 cm distance from isocentre, phantom results showed TV_PIV, D_mean, and GI deviations of 7.91%, 1.8%, and 0.85 for single-conformal arcs, which decreased to 4.84%, 1.3%, and 0.77 with 4-conformal arcs, and 3.4%, 0.96%, and 0.5 for 4-arc VMAT. Deviations varied based on target size, isocentre distance, number of arcs, and arc type. Clinical results mirrored the phantom study, with maximum TV_PIV and GI deviations of 2.76% and 0.65 for the smallest target (0.6 cm) located at 7.5 cm distance for four-arc VMAT. Other dosimetric parameters showed minimal variations (< 1%). Correlation analysis revealed strong associations between dosimetric differences, target size, and distance (r = 0.6-0.78 for small targets). MANOVA identified TV_PIV as the only significant parameter (p = 0.01). A 1° angular resolution significantly improves dosimetric accuracy for small, distally located targets in SIMT SRS.