A Case of Unknown Primary Cancer with a Tumor Formed in the Retroperitoneum: A Case Report

Cancer of unknown primari (CUP) are said to account for 2% of all carcinomas. Here we report a rare case of CUP confined to the retroperitoneum.

A 51-year-old man consulted a nearby physician for back pain. The malignant tumor could not be denied by MRI, and she was referred to our hospital. CT and MRI revealed uneven enhanced tumor structures protruding into the L2/3 disc. Part of the tumor was continuous with the left iliopsoas muscle. A CT-guided needle biopsy was performed. Histologically, the sheet-like proliferation of atypical cells was observed. Immunohistochemistry showed that atypical cells were positive for cytokeratin AE1&3, CK7, CD10, GATA3, glypican 3, Hep Par 1, carbonic anhydrase 9 (focal), and vimentin (focal) but negative for CK20, CD56, chromogranin A, synaptophysin, TTF1, HMB45, melan A, and PSA. The pathological diagnosis was poorly differentiated carcinoma. However, it was difficult to determine the primary site from the pathological findings. Positron emission tomography (PET) scan showed no distant metastases. He was diagnosed as poorly differentiated cancer localized to the lumbar spine from the retroperitoneum. Paclitaxel plus carboplatin (TC) was started. After completing 3 kr of TC, she was hospitalized urgently due to worsening lumbago. CT and MRI at admission showed an increase in the main lesion and exacerbation of bone invasion. Radiation therapy was given for curative purposes. Eventually, he died seven months after visiting our hospital and five months after starting TC therapy.

CUP has various disease states, and it is necessary to finish the examination immediately and shift to treatment. More effective treatment including immune checkpoint inhibitor for CUP is needed in the future.