Clinical Implementation of Robust Multi-isocentric Volumetric Modulated Arc Radiotherapy for Craniospinal Irradiation

Aims: To determine if multi-isocentric volumetric modulated arc radiotherapy for craniospinal irradiation (CSI-VMAT) can be implemented safely and accurately using robust optimisation in a commercially available treatment planning system. Our initial clinical experience is reported for the first 20 patients treated with the technique.

Materials and methods: Patients received between 23.4 and 39.6 Gy (mode 23.4 Gy) in 13-22 fractions with CSI-VMAT. The heart mean dose was 4.2-10.3 Gy (median 5.3 Gy) for patients prescribed up to 24 Gy and 6.5-16.3 Gy (median 10.1 Gy) for patients receiving 35 Gy or more. The lung mean dose was 5.5-7.6 Gy (median 6.8 Gy) for patients prescribed up to 24 Gy and 6.9-11.1 Gy (median 10.0 Gy) for patients receiving 35 Gy or more. The robustness of the planning target volume D_0.1cm3 and D_99% to systematic errors in the isocentre superoinferior position of up to 5 mm was evaluated. These remained acceptable but were correlated to the length of the available beam overlap through the neck.

Results: As of January 2021, one patient was deceased after 508 days and one patient was lost to follow-up after completing treatment. The median follow-up was 399 days (range 175-756 days) and progression-free survival was 131 days (34-490 days). Acute toxicities at Common Terminology Criteria for Adverse Events v5.0 grade 3+ included lowered white blood cell count (16/20), decreased platelet count (8/20), nausea (5/20), vomiting (2/20), pharyngeal mucositis (1/20) and oral mucositis (1/20). Three patients developed grade 4 neutropenia or decreased white blood cell count.

Conclusions: CSI-VMAT can be implemented safely and accurately using robust optimisation functions in a commercially available treatment planning system.