S359
Clinical - CNS
ESTRO 2026
Proton Beam Therapy, University College London Hospital, London, United Kingdom
Purpose/Objective: To evaluate the dosimetry, replans and early clinical outcomes of patients receiving dose-escalated pencil beam scanning Proton Beam Therapy (PBT) for base of skull (BoS) chordoma and chondrosarcoma treated at a single institute. Material/Methods: Thirty-four adult patients with a diagnosis of a BoS chordoma or chondrosarcoma treated with dose- escalated PBT between June 2022 to February 2025 were analysed. Radiotherapy dose metrics were extracted from the treatment planning system (Varian Eclipse v16.1), including target volume coverage, organs of interest (OOIs) doses, replans and the underlying reasons. Local control and survival data were sourced from the electronic medical records system or from the local clinical team following up patients post-PBT. Results: All chordomas received 73.8Gy in 41 fractions and all chondrosarcomas received 70.2Gy in 39 fractions. Single phase simultaneous integrated boost was used for all patients except one (97%), who was treated using a two-phase approach. Single Field Optimisation was used in 5/20 chordoma and 12/14 chondrosarcoma cases. The remaining cases were planned using Multi-Field Optimisation. A gross tumour residual remained in 19/34 (56%) patients. A treatment replan was required in 6/34 (17.6%) patients, 4 of whom were chordoma cases. Replans were necessary due to sinus filling (4/6) and emptying (2/6). These anatomical changes risked CTV under coverage and OOI dose increase above tolerance to the brainstem, optic structures and spinal cord. No patient required more than one replan. Acute effects during treatment included: Grade (G) one fatigue (82%), G1 pain/headache (47%), and G2 nausea and vomiting (12%). The predominant late adverse effect, as defined by those present longer than 6 months from PBT completion was cognitive disturbance (43%) affecting memory and cognitive processing. No G3 late effects were reported. On data analysis in September 2025, all treated patients had maintained local control of their disease and were still alive (N=21 for follow up local control data ranging between 12 to 36 months).
Conclusion: This analysis demonstrates that dose-escalated PBT for BoS chordoma and chondrosarcoma can achieve adequate dose coverage, including to gross residual disease, and meet the recommended dose guidance for OOIs. The plans were robust to potential patient anatomical changes during treatment with a lower than expected proportion of patients requiring a replan and experiencing minimal late effects. The early outcomes are promising, with all patients maintaining local control of their disease. Data will continue to be collected as the number of patients increase prior to presentation at ESTRO. References: 1. Basler, L. et al. (2020) ‘Dosimetric analysis of local failures in skull-base chordoma and chondrosarcoma following pencil beam scanning proton therapy’, Radiation Oncology, 15(1). doi:10.1186/s13014-020- 01711-3. 2. Holtzman, A.L. et al. (2022) ‘The incidence of brainstem toxicity following high-dose conformal proton therapy for adult skull-base malignancies’, Acta Oncologica, 61(8), pp. 1026–1031. doi:10.1080/0284186x.2022.2101900. 3. Weber, D.C. et al. (2016) ‘Long term outcomes of patients with skull- base low-grade chondrosarcoma and chordoma patients treated with pencil beam scanning proton therapy’, Radiotherapy and Oncology, 120(1), pp. 169– 174. doi:10.1016/j.radonc.2016.05.011. Keywords: Proton
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