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Pelvic sarcomas are challenging to treat due to growth extent and location. Proton beam therapy (PBT) is increasingly used for their management, especially in the paediatric/TYA group to reduce late toxicity, but there is a dearth of published data. This study aimed to evaluate the acute toxicity and early outcomes of paediatric and TYA patients with pelvic Ewing sarcoma (ES) and rhabdomyosarcoma (RMS) treated with PBT in a national proton treatment centre.

We retrospectively reviewed the records of 20 patients aged ≤25 years old with pelvic ES and RMS treated with PBT and systemic anti-cancer therapy (SACT) according to national protocols between April 2019 and December 2020. The staging, treatment details, toxicity data and disease status at each follow-up were recorded and exploratory analysis undertaken. Toxicity was reported as per Common Terminology Criteria for Adverse Events, version 4.0.

The median age was 13 years (range, 2-25). 14 patients had ES and 6 patients had RMS. 16 patients had non-metastatic disease, 4 patients had lung only metastases. The median primary tumour size at diagnosis was 303.3cc (range, 54.8-1051.7). 11 patients had definitive PBT, 6 patients had pre-operative PBT and 3 patients had post-operative PBT. The median prescribed dose was 50.4Gy (range, 41.4-59.4), and all patients had concurrent SACT. The median follow-up time was 12.5 months (range, 4-28). All patients experienced grade 3/4 lymphopenia during their course of PBT. At completion of PBT, all patients had an acute skin reaction. Only 3 patients had acute/subacute grade 3 skin reactions, and there were no grade 4 skin reactions. The other common toxicities were grade 1/2 fatigue (7 patients), constipation (5 patients) and diarrhoea (3 patients). One patient had grade 3 fatigue and another patient had grade 3 anorexia. One patient had grade 1 dysuria at completion of treatment which resolved by 6 weeks after PBT. Another patient had grade 1 haematuria at 6 weeks after PBT, which resolved by 1 year after PBT. Two patients had grade 2 lymphoedema at 6 weeks and 1 year after PBT. Lymphoedema severity improved for one patient, and was stable for the other. An asymptomatic left-sided sacral insufficiency fracture was found on routine imaging for one of the 20 patients at 17 months after completion PBT. Six months later, the same patient had an asymptomatic benign 2cm lesion in the right ischium detected on imaging. Three out of 20 patients had disease progression. One patient with pelvic ES treated with definitive PBT had local progression of the primary tumour and lung metastases 3 months after completion of PBT. Two patients relapsed distally 9 months after completion of PBT.

PBT for paediatric and TYA patients with pelvic ES and RMS has acceptable acute toxicities and offers good local control rates consistent with existing literature. Further long term follow-up is required to assess late toxicities and outcomes.