Answer: Fibrothecoma

Histology: The tumor abuts the tunica albuginea and is relatively well-circumscribed, although not encapsulated. The growth is predominantly mixed storiform and short fascicles. The lesion is very hypercellular, yet has a low mitotic rate. Nuclei are spindled, lacking prominent nucleoli. There are areas with intervening thin bands of collagen.

Discussion: Testicular fibrothecoma is similar in morphology to ovarian fibroma. The tumors frequently abut the tunica, as seen in this case. Follow-up from the literature suggests that these lesions, despite mitotic activity of up to 10 mitotic figures/10HPFs and hypercellularity, are indolent and are cured following excision. They appear, therefore, to have properties analogous to ovarian fibromas where hypercellularity and elevated mitotic rates have not been associated with malignant behavior. Fibrothecoma may closely resemble spindle cell lesions seen outside of the testis, including monophasic synovial sarcoma and solitary fibrous tumor. To our knowledge, monophasic synovial sarcoma has not been reported in the testis. Microscopically, testicular fibrothecoma shows collagen deposition around individual cells, which is not seen in monophasic synovial sarcoma. Frequent inhibin positivity also helps define the sex cord-stromal origin. In terms of differentiating fibrothecoma from solitary fibrous tumor, fibrothecomas show only occasional CD34 staining, and the rounded thin walled vessels seen in fibrothecoma are distinct from the hemangiopericytomatous vasculature that is typically present in solitary fibrous tumor.