Evaluation of Biological Dose Enhancement in Mucosal Surfaces of Oropharyngeal Cancer Patients Treated With Ipsilateral Discrete Spot-Scanning Proton Therapy.

Abstract

PURPOSE/OBJECTIVE(S)\nProton therapy plans for head and neck cancer patients receiving ipsilateral neck radiotherapy have limited angle options given desired avoidance of contralateral tissue and other at-risk organs. This may lead to dose enhancement at the mucosal surfaces at the end of range. Herein, we study the relationship between potential areas of mucosal toxicity and biological dose (BD).\n\n\nMATERIALS/METHODS\nThe patient cohort comprised of 5 patients with oropharyngeal carcinoma treated between 2019 and 2020 with adjuvant proton therapy to doses of 60-66 Gy in 30-33 fractions to the tumor bed with/without ipsilateral neck treatment after radical resection. Monte-Carlo based linear energy transfer (LET), relative biological effectiveness (RBE), and BD distributions of clinical plans were generated using the research version of a commercial treatment planning system. BD and RBE were calculated using the McNamara (MCN) model, which includes dependencies with dose-weighted LET (LETd), alpha/beta ratio, and physical dose (PD). α/β\u202f=\u202f3.1 Gy and a reference dose of 2 Gy per fraction, were used to perform RBE computations. 10 mm rings around the mucosal targets were used to quantify biological dose enhancement outside the target structure. Mean and max RBE, LETd Volume Histograms, as well as D2cc and near-maximum (D1%) values in comparison to conventional PD\u202f×\u202f1.1 were analyzed.\n\n\nRESULTS\nAs summarized in Table 1, the rings showed an increase of 8-10 % when comparing D2cc numbers between conventional PD\u202f×\u202f1.1 and BD. The RBE predicted by the McNamara model showed an average of 1.207 and a maximum (RBE1%) of 1.358 for this group of patients. LETd2cc and LETd1% had average values of 5.998 and 6.413 keV/µm, respectively.\n\n\nCONCLUSION\nBD enhancement was observed at the periphery of mucosal targets in these patients. An additional study is needed to evaluate the correlation of this dose enhancement with clinical/radiologic outcomes. In the future, use of additional beam angles and/or incorporating LET in treatment planning may mitigate this effect.