A Carcinoma of Unknown Primary (CUP) Patient Treated Successfully With Immunotherapy Upon Recognition of Deficient Mismatch Repair Signature on Liquid Biopsy: A Case Report

Carcinoma of unknown primary (CUP) refers to a heterogeneous group of metastatic tumors in which the origin of the tumor is undetermined. Patients with CUP have limited treatment options and an overall poor prognosis. Genomic profiling of the tumor tissue sometimes reveals actionable mutations leading to targeted treatment opportunities. However, sufficient tumor tissue is not often available for genomic profiling. In such situations, circulating tumor DNA (ctDNA)-based tumor genomic profiling may be valuable, although robust data do not exist to support the role of ctDNA testing in patients with CUP. Herein, we report a patient with CUP who was successfully treated with an immune checkpoint inhibitor (ICI) after ctDNA testing revealed a deficient mismatch repair (dMMR) tumor.