Advancing stereotactic body radiotherapy through off-axis beam optimization to enable safer treatment of multi-isocenter coplanar targets

Background and purpose: Oligometastatic lung cancer often requires multiple stereotactic body radiotherapy (SBRT) courses for spatially distinct lesions, increasing the risk of field overlap and side effect. While beam optimization can reduce this risk, posterior lesions remain challenging due to gantry–couch collision constraints. Hence, an off-axis beam optimization framework was developed for posterior lung lesions with direct clinical relevance for coplanar, multi-isocenter SBRT, enabling safer and more efficient dose delivery. Materials and methods: Twenty-five posterior lung lesions were retrospectively analyzed. For each case, non-optimized, on-axis optimized, and off-axis optimized plans were generated. On-axis optimization preserved central-axis geometry, while off-axis optimization shifted the isocenter toward the midsagittal plane to avoid gantry–couch collision. Dose–volume metrics were compared, and plan efficiency was assessed using isodose line volume (IDLV, 2.5–25 Gy). In selected multi-isocenter cases, overlap between adjacent fields was evaluated. Results: Optimized beamsets reduced mean lung dose (2.5 ± 1.03 vs 2.3 ± 1 Gy), V_{5 Gy} (445.1 ± 183.7 vs 385.0 ± 168.0 cm³), V_{10 Gy} (251.3 ± 123.9 vs 210.6 ± 107.4 cm³), and V_{20 Gy} (104.7 ± 63.4 vs 98.2 ± 59.5 cm³), with improved organs at risk (OAR) sparing and preserved target coverage. Off-axis plans matched standard plans while eliminating collision risk. In a representative case, IDLV_{25 Gy} and IDLV_{20 Gy} decreased from 406.9 to 270.1 cm³ and 668.9 to 502.1 cm³, with chest wall V_{30 Gy} reduced from 79.5 to 44.0 cm³. Conclusions: Beam optimization reduces field overlap and OAR dose in multi-lesion SBRT while maintaining plan quality, with off-axis approaches enabling safe, collision-free treatment of posterior lesions.