PURPOSE: Patients with p16 positive tonsillar cancer (p16 + TC) have an excellent prognosis and long-life expectancy. Deintensification of therapy is a prevalent topic of discussion. Proton radiotherapy is one way to reduce radiation exposure and thus reduce acute and late toxicity. The aim is to evaluate treatment outcomes and toxicity of postoperative treatment with intensity-modulated proton therapy (IMPT). METHODS: Between September 2013 and November 2021, 47 patients with p16 + TC were treated postoperatively with IMPT. Median age was 54.9 (38.2-74.9) years, 31 were males and 16 were females. All patients had squamous cell carcinoma and underwent surgery as a primary treatment. Median dose of radiotherapy was 66 GyE in 33 fractions. Bilateral neck irradiation was used in 39 patients and unilateral in 8. Concomitant chemotherapy was applied in 24 patients. RESULTS: Median follow-up time was 4.2 (0.15-9.64) years. Five-year overall survival, relapse free survival and local control were 95.7%, 97.8% and 100%. The most common acute toxicities were dermatitis and mucositis, with grade 2 + in 61.7% and 70.2% of patients. No acute percutaneous gastrostomy insertion was necessary and intravenous rehydration was used in 12.8% of patients. The most common late toxicity was grade 1 xerostomia in 70.2% of patients and grade 2 in 10.6% of patients. Subcutaneous fibrosis of grades 2 and 3 occurred in 17.0% and 2.1% of patients, respectively. One patient developed late severe dysphagia and became PEG-dependent. CONCLUSION: IMPT for the postoperative treatment of p16 + TC is feasible with excellent efficiency and acceptable acute and late toxicity.

Department of Health Care Disciplines and Population Protection Faculty of Biomedical Engineering Czech Technical University Prague Sítná square 3105 Kladno 272 01 Czech Republic

Proton Therapy Center Czech Budínova 1a Prague 8 18000 Czech Republic

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