Testicular Primary Well-Differentiated Neuroendocrine Tumor: Clinicopathologic, Immunohistochemical, and Molecular...

Well-differentiated neuroendocrine tumor rarely occurs as a testicular primary tumor, accounting for less than 1% of all testicular cancers, and is rarely reported with sufficient molecular profiles. After searching our departmental database (2003-2023), two testicular primary well-differentiated neuroendocrine tumors were identified in a 35-year-old man and a 23-year-old man, respectively, both of whom had normal serum level of tumor markers. Both tumors grossly exhibited solid, yellow-tan, and homogeneous appearance and histologically displayed a mixture of growth patterns, including organoid, tubular, cribriform, nests, cords, and single cells, were composed of eosinophilic tumor cells with salt-and-pepper chromatin and indistinct cell borders. Immunoreactivity for chromogranin and synaptophysin were detected, with Ki-67 labeling 9% and 2% of tumor cells on counting of 500 tumor cells, respectively. There was no germ cell neoplasia in situ in the background testicular parenchyma. Furthermore, fluorescence in situ hybridization failed to identify the presence of isochromosome 12p in both tumors. A panel-based next-generation sequencing was done in one of tumors and showed no reportable pathogenic variants with a mutation burden of 0.5 mutations per megabase. Although elevated mitotic figures (up to 6 per 10 high power fields), lymphovascular invasion and marked nuclear pleomorphism were present in this tumor, there was no evidence of disease detected in this patient via Dotatate positron emission tomography/computed tomography scan after the surgery. This report expands the spectrum of testicular primary well-differentiated neuroendocrine tumor. Considering its rarity, it may pose a diagnostic challenge or pitfall in certain clinical circumstances. In addition, the literature pertaining to this entity is herein reviewed.