In summary, a 50-year-old male with a past medical history of a right testicular mass status post orchiectomy presented with abdominal discomfort and a 9.2 cm retroperitoneal mass causing mass effect on surrounding structures, including the inferior vena cava, aorta, duodenum, and pancreas. Endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) revealed large cells in loose clusters and singly dispersed that were positive for OCT3/4, CD117, and PLAP with rare HCG positivity. A final diagnosis of metastatic seminoma was rendered, based on the clinical, cytologic, and immunohistochemical findings.

Seminoma, a subtype of germ cell tumor, typically presents as a painless, firm testicular mass, often discovered incidentally.¹ Serum tumor markers play a vital role in its evaluation; while alpha-fetoprotein (AFP) is not elevated in pure seminomas, lactate dehydrogenase (LDH) and beta-human chorionic gonadotropin (β-hCG) may be mildly elevated. On cytopathology, seminomas display uniform, large, round to polygonal cells with clear cytoplasm and distinct cell boarders, prominent central nucleoli, often accompanied by a lymphocytic infiltrate.² The background may show a wavy, reticular gray pattern, also known as a “tigroid” background.³ Metastatic spread primarily occurs via lymphatic channels, with the retroperitoneal lymph nodes being the most common site of metastases¹, though hematogenous spread to lungs and other organs may occur in advanced cases.

Rendering a primary diagnosis on a metastatic focus (or rarely a primary extratesticular seminoma) can be challenging, especially in tumors that have acquired more aggressive cytologic features, such as pleomorphism, increased mitotic activity, necrosis, and apoptosis.