Organ at risk sparing by non-coplanar prone breast radiotherapy on Halcyon/Ethos linacs utilizing breast couch slewing

Organ at risk (OAR)-dose reductions are needed to minimize radiation toxicity in settings with high OAR-doses, like adjuvant breast and lymph node radiotherapy. Non-coplanar techniques, combined with prone positioning and deep inspiration breath hold (DIBH) techniques, could accomplish this. A planning study was performed in patients requiring left-side adjuvant breast and axillary/periclavicular lymph node irradiation (n = 8). Simulation was performed in prone crawl position using DIBH. Coplanar and non-coplanar short-arc VMAT plans were compared. Non-coplanar plans featured ≤ 20° angular separation between planes. Feasibility was tested using a CAD model, including a prone crawl breast couch positioned in an up to 20° angle (transverse plane) with the couchtop. Non-coplanar techniques can further reduce radiation exposure. Adding beams in planes with ± 15° to ± 20° angular separation with the transverse plane yielded > 20% mean-dose reductions simultaneously to heart, lungs and esophagus, compared to coplanar plans. Plans with ≤ ± 20° angular separation with the transverse plane proved feasible on linacs lacking couch isocenter rotations. Angular separation of ± 15° seems the best compromise between OAR-sparing and technical challenges. Non-coplanar radiotherapy options yielded superior OAR sparing compared to coplanar techniques in patients requiring adjuvant breast and lymph node irradiation and should be considered for improved radiation toxicity prevention.