Microsatellite Instability‐High Somatic‐Type Malignancy Arising From a Primary Retroperitoneal Germ Cell Tumor: A Case Report

Introduction Immune checkpoint inhibitors have shown only limited clinical efficacy in germ cell tumors (GCTs), underscoring the need for predictive biomarkers such as microsatellite instability‐high (MSI‐high). We report a rare case of mixed‐type GCTs treated with chemotherapy, in which the somatic‐type malignancy (STM) component was found to be MSI‐high. Case Presentation A 55‐year‐old man presented with a retroperitoneal mass, and an open biopsy revealed a mixed GCT. He was diagnosed with primary retroperitoneal GCT and treated with four cycles of bleomycin, etoposide, and cisplatin chemotherapy. Fourteen months later, retroperitoneal lymph node enlargement was observed, and retroperitoneal lymph node dissection revealed an STM. Six months later, local recurrence had occurred. MSI testing of the resected recurrent tumor demonstrated an MSI‐high status. Conclusion This case illustrates that careful selection of tissue specimens and appropriate timing of MSI testing are essential to identify potentially actionable cases in patients with GCTs.