Treatment planning for in vivo multi-fraction, multi-port, multi-modal Synchrotron Radiotherapy

Treatment planning systems (TPSs) for microbeam radiotherapy (MRT) have seen rapid development over the past few years, however very few TPSs are equipped to handle the growing complexity of in-vivo MRT studies. Current literature lacks sufficient detail for dosimetry, treatment planning, evaluation, and delivery to make meaningful links between dose and treatment outcomes. In a recent study, we investigated treatment outcomes in tumour-bearing mice through modifying temporal placement of MRT fields in a daily-fractionated synchrotron BB schedule. Here, we detail dosimetry, treatment planning, delivery, and evaluation to meaningfully correlate physical dose with observed treatment outcomes.

The Eclipse TPS (Varian Medical Systems) was adapted to accumulate dose for fractionated mixed-modality treatments. Manual plan optimisation was used to meet dose prescriptions and constraints. Volumetric dose data for the primary tumour, organs-at-risk, and locoregional metastasis sites were calculated and compared to treatment outcomes.

Manual plan optimisation generally met treatment requirements but was limited by the delivery system's physical limits. Dose accumulation for BB and MRT valleys was achieved, and MRT peaks were assessed separately. Tumour control was maintained in all mice, and all failed to treat out-of-field locoregional metastases. Mice receiving at least one MRT fraction had a >32 % reduction in primary tumour volume. The temporal placement of MRT beams did not affect treatment outcomes or metastatic burden.

This study represents the first detailed use of the Eclipse TPS for multi-fraction, multi-port, multi-modal synchrotron radiotherapy, providing insights into treatment outcomes and biological end-points in relation to physical dose.