Postpubertal-Type Teratoma of the Retroperitoneal Space as a Late Recurrence of a Testicular Germ Cell Tumor: A Case Report With Clinical Insight

Postpubertal-type teratomas are rare malignant tumors derived from germ cell neoplasia in situ (GCNIS). This case report presents a rare instance of a retroperitoneal postpubertal-type teratoma as a late recurrence of a testicular germ cell tumor (GCT) that was initially diagnosed as a seminoma. A 48-year-old male who had undergone left inguinal orchidectomy for a testicular mass was diagnosed with a seminoma (stage I) six years prior and presented with an asymptomatic 6-cm retroperitoneal tumor near the left renal hilum. Initial blood tests at presentation for the retroperitoneal tumor were normal, except for a mild elevation of lactate dehydrogenase. Computed tomography (CT), magnetic resonance imaging, and fluorine-18-fluorodeoxyglucose positron emission tomography (18F-FDG PET)/CT imaging revealed a well-defined tumor with calcification and high glucose metabolic activity. A CT-guided biopsy for the retroperitoneal tumor suggested a high-grade malignant tumor of neuroendocrine origin. The patient underwent neoadjuvant chemotherapy, which resulted in tumor shrinkage and decreased metabolic activity. Subsequent surgical resection and histopathological examination revealed a postpubertal-type teratoma with a concomitant neuroendocrine tumor. A reexamination of the previous testicular tumor specimen revealed a small amount of embryonal carcinoma within the seminoma, indicating that the initial diagnosis should have been a mixed germ cell tumor rather than a pure seminoma. This explains the unusual recurrence pattern observed and highlights the importance of a thorough histological examination for testicular GCTs, as microscopic non-seminomatous components can significantly affect prognosis and recurrence patterns. When encountering suspected recurrences of GCNIS-derived GCTs, clinicians should consider the possibility of an initially undetected mixed GCT, particularly in cases with atypical presentation or recurrence patterns.