Adjuvant Therapy with Immune Checkpoint Inhibitors after Carbon Ion Radiotherapy for Mucosal Melanoma of the Head and Neck: A Case-Control Study

Simple Summary Mucosal malignant melanoma is a type of head and neck cancer with a high mortality rate. Although favorable local control has been reported when using carbon ion radiotherapy (CIRT) to treat mucosal malignant melanoma of the head and neck, the prognosis remains unsatisfactory because of the tendency for early distant metastasis. In recent years, favorable outcomes have been reported for the use of immune checkpoint inhibitors (ICIs) as adjuvant therapy for cutaneous malignant melanoma, indicating their potential applicability to mucosal malignant melanoma. In addition to achieving favorable local control with CIRT, the control of distant metastasis using ICIs is expected to prolong survival. The findings of this analysis indicate that the use of ICIs as adjuvant therapy can improve prognosis following CIRT, offering a new treatment option.Abstract The development of new treatment strategies to improve the prognosis of mucosal malignant melanoma of the head and neck (MMHN) after carbon ion radiotherapy (CIRT) is essential because of the risk of distant metastases. Therefore, our objective was to evaluate the outcomes of immune checkpoint inhibitor (ICI) treatment to justify its inclusion in the regimen after CIRT. Thirty-four patients who received CIRT as an initial treatment were included in the analysis and stratified into three groups: those who did not receive ICIs (Group A), those who received ICIs after recurrence or metastasis (Group B), and those who received ICIs as adjuvant therapy after CIRT (Group C). In total, 62% of the patients (n = 21) received ICIs. The 2-year local control and overall survival (OS) rates for all patients were 90.0% and 66.8%, respectively. The 2-year OS rates for patients in Groups A, B, and C were 50.8%, 66.7%, and 100%, respectively. No significant differences were observed between Groups A and B (p = 0.192) and Groups B and C (p = 0.112). However, a significant difference was confirmed between Groups A and C (p = 0.017). Adjuvant therapy following CIRT for MMHN may be a promising treatment modality that can extend patient survival.