CLINICAL OUTCOMES AND FAILURE PATTERNS IN PEDIATRIC PATIENTS WITH EWING SARCOMA TREATED WITH PROTON BEAM THERAPY

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Background and Aims: External auditory canal (EAC) and middle ear carcinomas are rare malignancies with poor prognoses in advanced stages, where surgery or chemoradiotherapy yields suboptimal control and morbidity.Carbon-ion radiotherapy (CIRT), which has superior dose distribution and higher relative biological effectiveness, has shown promise in the treatment of head and neck tumors; however, evidence for its efficacy in treating EAC and middle ear carcinomas remains limited.This study reports the clinical outcomes and toxicity profiles of patients with EAC or middle ear carcinoma treated with definitive CIRT.Methods: Patients with histologically confirmed EAC or middle ear squamous cell carcinoma who treated at Gunma University between 2019 and 2024 were retrospectively reviewed.CIRT was administered at a total dose of 64.0 Gy (RBE) in 16 fractions, with specific dose constraints for brain and brainstem.The Pittsburg staging system was employed to determine the T stage, while lymph node (N category) and distant (M category) metastases were classified using the 8th UICC TNM staging system for head and neck cancers.Local control (LC), overall survival (OS), and treatmentrelated toxicities (CTCAE v5.0) were evaluated.Survival rates were estimated using the Kaplan-Meier method.Results: This retrospective analysis included seven patients with ECOG PS 0-1.All patients were ineligible for surgery or chemoradiotherapy because of factors such as age, comorbidities, or advanced disease stage.The median age was 80 years (range, 47-84).One patient had T3, and six had T4.The median gross tumor volume was 35.4 cm 3 (range, 0.6-100.9).One patient was N1, and six were N0.No distant metastases were detected.The median follow-up period was 26.0 months (range, 4.0-62.0).Two-year LC and OS rates were 57.1% (95% CI 30.0-100%) and 85.7% (95% CI 63.3-100%), respectively.Acute and late toxicities included grade 1-2 dermatitis and otitis media, without any grade ≥3 toxicity.Conclusions: These findings suggest that CIRT can be a valuable treatment option for inoperable patients with these rare tumors, despite the small sample size and retrospective design of the study.Further multi-institutional studies are warranted to validate these results.

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