ROS1-rearranged putative lung adenocarcinoma presenting as carcinoma of unknown primary site: a case report.

Carcinoma of unknown primary site (CUP) is diagnosed only in 2-9% of all cancer cases. Adenocarcinomas account for approximately 60% of CUP, and some of these are putative lung adenocarcinomas. The frequency of driver oncogene positivity in the putative lung adenocarcinomas is unknown, and the efficacy of targeting therapies for the driver oncogene is also unknown. This is the first case report of C-ros oncogene 1 (ROS1)-rearranged putative lung adenocarcinoma presenting as CUP showing a good response to ROS1 inhibitor therapy. A 55-year-old woman presented with neck lymphadenopathy. Computed tomography and [18F]-fluorodeoxyglucose (FDG) positron emission tomography (PET) showed swelling of the bilateral supraclavicular, left accessory, mediastinal, and abdominal lymph nodes. The pathological analysis of the lymph node specimen biopsy indicated adenocarcinoma with cytokeratin 7 and thyroid transcription factor-1 positivity. Thus, this case was identified as ROS1- rearranged putative lung adenocarcinoma presenting as CUP. Oral crizotinib, an ROS1 inhibitor, was administered at a dose of 250 mg twice daily. Four weeks later, several swollen nodes showed marked improvement, and eight weeks later, FDG PET showed almost no uptake. In conclusion, putative lung adenocarcinoma presenting as CUP may involve ROS1 rearrangement, and ROS1 inhibitor therapy may be effective.