Three-year insights from a proton therapy centre: Evolving strategies in ependymoma treatment planning

Treatment of posterior fossa ependymoma with proton therapy (PT) is challenging due to the proximity to the brainstem and potential risk of brainstem toxicity.

This retrospective study included patients with intracranial ependymoma treated with PT or a hybrid proton-photon (PT-XT) approach at our centre between July 2020 and December 2023 with a minimum 12 month follow-up. Treatment approach was determined based on tumour relationship to the brainstem. Patient and tumour characteristics and treatment planning approaches were evaluated, and dosimetric data between PT and PT-XT approaches were compared. Outcomes including symptomatic brainstem injury (SBI) and asymptomatic radiographic changes (ARC) were assessed.

Fourteen patients were included, of which nine had posterior fossa ependymoma, with five treated using a hybrid PT-XT planning approach and four with PT alone. The hybrid PT-XT approach involved delivering 75 % of the treatment (0-45 Gy(RBE)) with PT and the remaining portion (45-59.4 Gy) with XT, addressing concerns regarding increased relative biological effectiveness of PT in the brainstem. While all plans adhered to the European Particle Therapy Network D_0.03cc brainstem dose constraint, the University of Florida constraints were not always met. Comparison of the hybrid PT-XT and PT-only replans showed a trend towards improved brainstem dose parameters with the hybrid approach. After a median follow-up of 22.2 months, no patients developed symptomatic brainstem injury and three showed asymptomatic radiographic changes.

Tailored planning approaches, depending on location and relationship to the brainstem, are essential when treating ependymoma patients. A hybrid PT-XT approach, is feasible in well-selected patients, demonstrating acceptable target coverage, brainstem doses, and to date no occurrence of symptomatic brainstem injury.