Recurrent microsatellite instability-high thymic carcinoma showing complete response to immune checkpoint inhibitor: A case report.

Microsatellite instability-high (MSI-H) tumors, which exhibit somatic hypermutations due to defects in the DNA mismatch repair system, are found in several cancer types. These tumors are known for their responsiveness to immune checkpoint inhibitors (ICIs), regardless of tumor origin. Thymic carcinomas (TCs) are characterized by high programmed death ligand 1 expression and low levels of somatic mutations. Consequently, the incidence of MSI-H TCs is extremely rare. To the best of our knowledge, the present report is the first to describe a case of recurrent MSI-H TC achieving complete remission with pembrolizumab monotherapy, distinguishing it from prior reports of partial responses. The patient in the present case was a 78-year-old woman with solitary pulmonary and thymic tumors. Thymoma, lung cancer or breast cancer with lung metastasis were suspected, and simultaneous videoscopic resection was performed. The patient was diagnosed with TC with pulmonary metastasis. Adjuvant platinum-based doublet chemotherapy was only administered twice due to side effects, and extensive recurrent lesions were found in the thoracic cavity 8 months after surgery. Genetic testing of the tumor tissue revealed MSI-H, and pembrolizumab was administered as second-line chemotherapy. A total of 4 years and 9 months after the initial surgery, the metastatic lesions had completely disappeared, with no signs of recurrence. In conclusion, the present report describes a rare case of recurrent TC presenting with MSI-H, in which a complete remission was achieved with immune checkpoint inhibitor (ICI) treatment; however, further research is needed to explore the efficacy of ICIs in rare cases of MSI-H TCs.