ESTRO 2025 - Abstract Book

S947

Clinical – Head & neck

ESTRO 2025

Five patients in the without MS-CIRT (n=33) and one patient in the MS-CIRT group (n=23) developed late grade 3 mucosal toxicity/ oro-antral fistula (OAF). Average LEM-I, mMKM doses and LETd after filtering out RBE-weighted doses (D RBE- filtered LETd) for mucosa were higher in patients with late grade 3 mucosal toxicity (without MS-CIRT) compared to those treated with MS-CIRT. MS-CIRT did not significantly compromise the doses to HD-CTV. None of the patients in the MS-CIRT group developed local recurrences in the spared regions. Out of 6 patients with OAF, 3 patient showed significantly mucosal doses > 70 Gy in both LEM-I and mMKM models was a predictive factor for development of late >/= G3 mucosal toxicity. On univariate analysis concurrent high D RBE and LETd on mucosa was one of the predictive factors for development of OAF. For patients with OAF and mucosal doses < 70 Gy, we identified, the D RBE and LETd threshold using ROC and KMS analysis [LEM-I: D RBE : 66 Gy, LETd: 75 keV/um ≥ 5% voxel of mucosa] (Figure 2a-c) which predicted late mucosal toxicity risk. Conclusion: The concurrent high D RBE and LETd in the same voxels influenced severe late toxicities like OAF. The multi-RBE optimization, LETd optimization could reduce dose and LETd and help in minimizing late toxicities. References: 1. Sasahara G, Koto M, Ikawa H, Hasegawa A, Takagi R, Okamoto Y, Kamada T. Effects of the dose-volume relationship on and risk factors for maxillary osteoradionecrosis after carbon ion radiotherapy. Radiat Oncol. 2014 Apr 3;9(1):92. doi: 10.1186/1748-717X-9-92. PMID: 24708583; PMCID: PMC3992144. 2. Bhattacharyya T, Koto M, Ikawa H, Hayashi K, Hagiwara Y, Tsuji H. Assessment of risk factors associated with development of oronasal fistula as a late complication after carbon-ion radiotherapy for head and neck cancer. Radiother Oncol. 2020 Mar;144:53-58. doi: 10.1016/j.radonc.2019.10.015. Epub 2019 Nov 13. PMID: 31733488. Digital Poster Clinical outcomes and QOL in 39 patients with head and neck mucosal melanoma after hypofractionated spot scanning proton therapy: retrospective study Koichiro Nakajima 1 , Hiromistu Iwata 1 , Gaku Takano 2 , Yukiko Hattori 1 , Yusuke Tsuzuki 1 , Shuou Sudo 1 , Kensuke Hayashi 3 , Toshiyuki Toshito 4 , Hideo Fukano 5 , Hiroyuki Ogino 1 , Akio Hiwatashi 6 1 Department of Radiation Oncology, Nagoya Proton Therapy Center, Nagoya City University West Medical Center, Nagoya, Japan. 2 Department of Otorhinolaryngology, Nagoya City University West Medical Center, Nagoya, Japan. 3 Department of Proton Therapy Technology, Nagoya Proton Therapy Center, Nagoya, Japan. 4 Department of Proton Therapy Physics, Nagoya Proton Therapy Center, Nagoya, Japan. 5 Department of Oral and Maxillofacial Surgery, Nagoya City University West Medical Center, Nagoya, Japan. 6 Department of Radiology, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan Purpose/Objective: Head and neck malignant mucosal melanoma (HNMM) is an extremely rare tumor with a poor prognosis. This study aims to analyze the clinical outcomes and patient-reported quality of life (QOL) measures following hypofractionated spot scanning proton therapy (HF-SSPT) for HNMM. Material/Methods: A single-institutional retrospective study was conducted on patients who underwent HF-SSPT for HNMM between December 2013 and December 2023. The key eligibility criteria were: 1) histologically confirmed malignant melanoma, 2) a primary site in the head and neck region (pharynx, oral cavity, or sinonasal cavity), and 3) no evidence of distant metastasis. HF-SSPT was delivered at 60 GyRBE in 15 fractions, or 60.8–64 GyRBE in 16 fractions. The primary endpoint was overall survival (OS), with secondary endpoints including local control (LC), progression- Keywords: Carbon ion therapy, late mucosal toxicity 903