A case of refractory pelvic squamous cell carcinoma of unknown primary that responded to nivolumab.

Cancer of unknown primary (CUP) is a heterogeneous group of metastatic tumors with a usually unfavorable prognosis. A 33-year-old female was diagnosed with pelvic squamous cell carcinoma of unknown primary. The tumor was p16-positive, suggesting that it was human papillomavirus (HPV)-related. The tumor progressed for 4 months after concurrent chemoradiotherapy (initial treatment) and was refractory to paclitaxel plus carboplatin (second-line therapy). Liquid-based cancer genomic profiling identified five pathogenic variants, including Neurofibromin1 (NF1) (p.T1690Mfs*5); however, due to the lack of domestic clinical trials, the patient could not receive genome-based molecular-target therapies. Simultaneously, nivolumab was administered to the patient post its approval in Japan for CUP. The tumor responded to nivolumab, accompanied by decreased levels of tumor markers. NF1 mutations and HPV-related carcinogenesis may be associated with a favorable response to nivolumab treatment. It may therefore serve as a potential treatment against cancers of unknown primaries.