The role of carbon-ion radiotherapy in the treatment of adenoid cystic carcinoma of the nasopharynx

BACKGROUND: Nasopharyngeal adenoid cystic carcinoma (NACC) is a distinct subgroup of adenoid cystic carcinoma (ACC) with limited surgical access but predilection of regional and distant metastasis. Although radiotherapy is an integral treatment for patients with NACC, photon-based radiotherapy yielded suboptimal local control. Because of its advantages in biology and physics properties, carbon-ion radiotherapy (CIRT) was attempted for the treatment of head and neck ACC; however, the use of CIRT specifically for NACC has not been investigated. METHODS: Patients with NACC that received CIRT alone or a combination of CIRT and proton beam therapy (PBT) at the Shanghai Proton and Heavy Ion Center (SPHIC) between July 2016 and March 2019 were included in the analysis. Patients with newly diagnosed NACC received combined therapy of CIRT (as boost) and PBT, and those with recurrent disease received CIRT alone. Overall survival (OS), local progression-free survival (LPFS), regional progression-free survival (RPFS), and distant metastasis-free survival (DMFS) were calculated by Kaplan-Meier method. RESULTS: A total of 22 patients were included in this analysis. Among those, 18 patients had newly diagnosed NACC (17 with locally advanced disease), and 4 had recurrent NACC including 2 failed previous irradiation. After a median follow-up of 30.9 months, the 2-year OS rate, PFS rate, LPFS rate, RPFS rate and DMFS rate were 100%, 84.8%, 94.4%, 100%, and 84.8%, respectively. Three patients experienced grade 3 mucositis or xerostomia. No late toxicity of grade ≥3 was observed. CONCLUSIONS: CIRT alone or in combination with PBT appeared to be a promising modality for the treatment of NACC and produced satisfactory local disease control and toxicity profile. Distant metastasis remained to be a substantial mode for treatment failure. Further follow-up is necessary to evaluate long-term survivals and late toxicity profile.