Germ cell immunoprofile and KIT exon 17 mutation guide rectification of seminoma misclassified as epithelioid gastrointestinal stromal tumor: A case report.

Seminoma, derived from germ cell neoplasia in situ, is the most common testicular germ cell neoplasm. Seminoma exhibits overlapping genetic mutations and immunohistochemical profiles with diverse tumors, substantially complicating diagnosis, particularly in biopsy specimens. In the present study a patient was reported, harboring a huge abdominal mass, who was initially diagnosed as an epithelioid gastrointestinal stromal tumor via biopsy pathology due to CD117 immunohistochemical expression and detection of KIT mutation in exon 17 (p.Y823D). After first-to fourth-line targeted therapies, the patient exhibited rapid tumor progression, promoting diagnostic reevaluation. Definitive diagnosis of seminoma was established through expanded immunohistochemistry (OCT4+/SALL4+/PLAP+/D2-40+) and molecular confirmation of the characteristic KIT p.D816Y mutation, which concurrently explains the observed imatinib resistance. In conclusion, the findings of the present study highlighted the imperative of integrating morphology, immunohistochemistry and context-specific molecular profiling to avoid diagnostic in CD117-positive tumors.