Multiple source-to-isocenter-distance (SID) optimization for 4 <i>π</i> robotic radiotherapy

While 4<i>π</i>radiotherapy has shown dosimetric advantages over coplanar delivery, prior studies have assumed a fixed source-to-isocenter distance (SID), as varying SID is cumbersome to implement on conventional C-arm linacs. Radiotherapy platforms based on articulated robotic arms eliminate this constraint, enabling non-coplanar, non-isocentric delivery with variable SIDs without moving the patient. Larger SIDs offer increased per-field tumor coverage with a larger field-of-view (FOV), while shorter SIDs improve modulation resolution. This study combines multiple SIDs to simultaneously achieve high target coverage and modulation resolution in robotic radiotherapy. A Multi-SID optimization framework was developed for simultaneous beam orientation optimization, isocenter selection, SID selection, and fluence map optimization. The planning cost includes a least-squares dose fidelity objective, a total variation term for fluence smoothness, and a group sparsity term for beam orientation, SID, and isocenter selection. The collision-free candidate beam pool includes over 1000 beam orientations, 10 SID levels (50-95 cm), and 5-9 isocenters. Multi-SID 4<i>π</i>intensity-modulated radiation therapy (IMRT) plans were compared against fixed-SID (SID-50 cm and SID-100 cm) 4<i>π</i>IMRT plans on 10 head-and-neck (H&N) cancer cases. Multi-SID consistently achieved superior plan quality across all 10 patients using varying beams (10-30). For 15-beam plans, the planning target volume homogeneity (defined as D95/D5) is (0.898, 0.893, 0.889), dose conformity (using the Paddick Conformity Index) is (0.867, 0.853, 0.837), the mean Organs-at-Risk (OAR) dose is (12.67, 15.05, 14.12), and the R50 is (2.429, 2.575, 2.485) for Multi-SID, SID-100, and SID-50 respectively. Wilcoxon signed-rank testing showed that the improvements in homogeneity, conformity, and OAR sparing with Multi-SID were statistically significant. Our Multi-SID method leverages both high target coverage and modulation resolution to achieve better dosimetry and/or more efficient treatment plans than fixed SID methods.