SMARCB1/INI1-deficient Undifferentiated Carcinoma Presenting as Cancer of Unknown Primary with Massive Hepatic Involvement: Transient Response to FOLFOX plus Nivolumab.

SMARCB1/INI1-deficient carcinomas are rare and aggressive, and their responses to immune checkpoint inhibitors remain incompletely characterized. A 38-year-old woman presented with massive bilobar liver metastasis, ascites, bone lesions, and a small pancreatic tail lesion without an identifiable primary tumor. Liver biopsy revealed an undifferentiated carcinoma with rhabdoid features and complete loss of INI1. Comprehensive genomic profiling revealed microsatellite stability, low tumor mutational burden (TMB), and SMARCB1 copy number loss. The programmed death ligand 1 combined positive score was 18. Chemotherapy plus nivolumab induced transient regression of the hepatic lesions; however, progression occurred and the patient died 8 months after presentation.