Answer: Fibrothecoma

Histology: Slides show a tumor proliferation of fusiform cells with a fascicular pattern of growth within a mildly collagenized stroma with abundant small vessels. Nuclei are spindle-shaped, with finely distributed chromatin, small nucleoli, and no evident atypias.

Discussion: Testicular fibrothecoma is an extremely rare testicular stromal tumor which resembles its ovarian counterpart. Clinically, it presents as a unilateral, painless testicular mass, unassociated with hormonal alterations, usually in patients under 40 years old. Grossly, fibrothecomas form a well-circumscribed, solid, yellowish white mass without necrotic or hemorrhagic areas. Microscopic examination discloses a tumor composed of fusiform spindle cells resembling ovarian stromal cells, with a fascicular or storiform pattern of growth, within a slightly collagenous stroma and numerous small blood vessels. Chromatin is finely granular, small nucleoli might be seen, but evident atypias are absent and mitotic rate is very low. Tumor cells are positive for vimentin and smooth muscle actin, and occasionally positive for cytokeratins, desmin and S-100 protein. Immunostains for inhibin and CD99 are negative. Its main differential diagnoses include leiomyoma, solitary fibrous tumor, and primary testicular sarcomas. The prognosis is good and surgical excision is curative.