Clinical Outcomes of Dual-Beam Particle Therapy in Head and Neck Adenoid Cystic Carcinoma

<b>Background and Purpose</b>: This study retrospectively evaluates the outcomes of head and neck adenoid cystic carcinomas (ACCs) treated with particle therapy, including carbon ion radiotherapy (CIRT) alone or combined with photon therapy, at a single institution. <b>Methods and Materials</b>: Patients with ACC who underwent CIRT alone or a combination of CIRT and photon therapy at the Marburg Ion Therapy Center between February 2017 and December 2023 were included. Radiation therapy was administered postoperatively in surgically resectable patients and as definitive treatment in unresectable patients. Newly diagnosed patients received CIRT as a boost in combination with photon intensity-modulated radiation therapy (IMRT), while those with recurrent disease received CIRT alone. Prognostic factors were analyzed using Kaplan-Meier analysis and proportional hazards regression for multiple regression. Late toxicities (grade 3 or higher) were recorded according to the National Cancer Institute Common Terminology Criteria for Adverse Events, version 4. <b>Results</b>: A total of 73 patients were included, with a median age of 57 years (range: 16-86 years) and a median follow-up of 20 months (range: 3-70 months). The cohort included 28 males (38%) and 45 females (62%). The median CIRT dose was 24 Gy (relative biological effectiveness (RBE)) (range: 15-60 Gy) in a median of 8 fractions (range: 5-20), and the median photon dose was 50 Gy (range: 45-54 Gy) in 25 fractions (range: 15-30). Locoregional recurrence-free survival rates at 1 and 3 years were 89.6% and 75.4%, respectively, while distant metastasis-free survival rates were 82.1% and 61.4%, respectively. LC was significantly influenced by T stage, with patients with T4 tumors showing worse outcomes. Treatment was generally well tolerated, with acute side effects including mucositis and skin erythema. Severe chronic toxicities were rare, with only 1% of patients experiencing grade 3 dysphagia and grade 3 xerostomia. <b>Conclusions</b>: CIRT, particularly when combined with photon therapy, demonstrates favorable local control and promising efficacy in head and neck ACC, though distant metastasis remains the primary pattern of failure. Tumor stage is a significant negative prognostic factor for local control and overall survival.