Adenoid Cystic Carcinoma Treated With Salvage ZAP-X Radiosurgery in Elderly Patients: A Report on a Case Series

Management of recurrent adenoid cystic carcinoma (ACC) in elderly patients remains challenging due to comorbidities, functional impairments, and anatomically complex tumor locations that complicate surgical access and increase operative risk. The ZAP-X Gyroscopic Radiosurgery System (ZAP Surgical Systems, Inc., San Carlos, CA, USA) offers a highly precise, non-invasive treatment modality, potentially suitable for salvage therapy in previously irradiated fields and in medically inoperable patients. The article presents two elderly female patients (aged 84 and 89 years) with histologically confirmed recurrent ACC of the nasopharynx and nasal cavity/ethmoid sinus, respectively, who were treated with salvage ZAP-X radiosurgery. The first patient presented with a recurrent lesion measuring 2.1 cm³, while the second patient's tumor volume was significantly larger at 34.9 cm³, approaching the upper end of published single-fraction volume limits typically deemed manageable for radiosurgical intervention. MRI-based simulation and treatment planning were performed to ensure optimal target coverage and sparing of organs at risk, particularly the optic structures. The first lesion was prescribed 16 Gy to the 52.5% isodose line, while the sinonasal tumor was treated with 10 Gy to the 60% isodose line, both delivered in a single fraction without margins. Treatment plans prioritized steep dose gradients and high conformity indices. Both patients underwent treatment without CTCAE ≥ Grade 2 toxicities. In the first patient, follow-up MRI over 18 months showed significant tumor reduction and sustained clinical improvement. In the second patient, early follow-up showed disease stabilization and symptom palliation. Both patients experienced relief from vertigo, tinnitus, and ocular motility disturbances, respectively, and no new adverse events were noted. WHO performance status remained excellent in both patients. Notably, despite the larger tumor burden in the second patient, effective local control was achieved, demonstrating the potential of ZAP-X-based radiosurgery even in very large-sized lesions. Salvage ZAP-X radiosurgery appears feasible, safe, and effective for the treatment of recurrent ACC in elderly patients, including tumors exceeding the upper size limits traditionally considered for radiosurgical management. The system's high precision and ability to generate steep dose gradients facilitate safe reirradiation in previously treated, anatomically complex regions. These early findings encourage further prospective studies with larger cohorts and longer follow-up to validate efficacy, monitor late toxicity, and better define the role of radiosurgery in the multidisciplinary management of recurrent head and neck malignancies.