Advances In Intensity Modulated Neutron Therapy (IMNT): Improvement In Plan Quality and Reduction In Treatment Time through Beam Angle Optimization (BAO) and Field Co-Optimization
Abstract
Purpose
High linear energy transfer (LET) IMNT faces practical delivery constraints: low output rates (~55 MU/min), manual gantry and couch rotations (~3-5 min/field), and mandatory wait times (~1-2 min/field) for neutron activation concerns. Our standard-of-care (SOC) plans independently optimizes sequential initial and boost fields with manual beam angle selection, which may be operationally inefficient and dosimetrically suboptimal. This study investigates the potential of beam angle optimization (BAO) with field co-optimization (FCO) to improve plan quality, facilitate delivery of simultaneous integrated boost (SIB), and reduce treatment times for complex head and neck cancers (HNC).
Methods
A Python script implementing BAO was developed within a commercial treatment planning system. To improve delivery efficiency, we investigated the dosimetric impact of: (1) reducing total field count (12-field SOC vs. 9-field BAO-FCO), (2) restricting the BAO search space to coplanar angles to minimize collision risk, and (3) restricting to anterior angles to avoid couch traversal and opening the vault floor for the large gantry.
Results
Retrospective re-planning of 5 HNC treatment plans demonstrated that BAO-FCO, with an equal number of fields, decreased mean organ at risk (OAR) dose (parotids: 35%, brainstem: 13%, oral cavity: 11%, cochlea: 4%) while improving high-risk target V95: 2.9%. BAO-FCO of 9-field IMNT further reduced OAR dose compared to our SOC plan (parotids: 45%, cochlea: 14%, brainstem: 16%, oral cavity: 21%, temporal lobes: 12%). Limited coplanar and anterior search spaces still achieved acceptable plan quality. On average, BAO-FCO plans reduced delivery time by 38% (range: 16%-49%).
Conclusion
BAO-FCO improves the therapeutic ratio and delivery efficiency of SIB-IMNT for complex HNC cases, while respecting operational facility constraints. With a modest increase in hypofractionation (e.g., daily dose of 1.5Gy instead of 1.15Gy), BAO-FCO could increase the capacity of our facility to treat IMNT patients can be increased >30-40% through more efficient delivery.