Answer: Dedifferentiated liposarcoma

Histology: The cellular nodules consist of cells with moderate to marked pleomorphism arranged in a storiform pattern. Mitotic figures are relatively scarce. There is an admixed lymphoplasmacytic infiltrate. In the adjacent adipose tissue and in dense fibrosis there are scattered cells with large hyperchromatic nuclei, many of them appearing multinucleated.

Discussion: The most common paratesticular sarcoma in adult males is liposarcoma. The majority of these are well differentiated liposarcomas. In terms of distinguishing lipomas from liposarcomas in this region, it is important to know the location of the lesions. Almost all adipose tumors located in the distal spermatic cord immediately adjacent to the testes are liposarcomas. In contrast, smaller adipose lesions in the upper spermatic cord near the inguinal canal are typically lipomas. Even at scanning magnification, one should be able to identify scattered, large, hyperchromatic nuclei within the adipose tissue and often within the fibrous septae within the adipose tissue in well differentiated liposarcoma. It is not necessary to identify lipoblasts to establish the diagnosis of a well differentiated liposarcoma. Rather, the finding of atypical hyperchromatic nuclei within the adipose or dense fibrous tissue in the paratesticular region is sufficient for the diagnosis. If there is some difficulty in determining whether a lesion is a lipoma or a liposarcoma one can perform immunohistochemistry for MDM2 which will label the nuclei of liposarcomas and not lipomas. In the current case there is also an area of malignant spindle cells without adipose differentiation. The tumor does not conform to the second most common sarcoma in adults which is leiomyosarcoma. The tumor lacks the long sweeping fascicles of cells cut in different planes of section that characterize leiomyosarcomas. This more cellular solid area could out of context be malignant fibrous histiocytoma, yet its juxtaposition to well differentiated liposarcoma is diagnostic of a de-differentiated liposarcoma. A common pitfall of pathologists in dealing with paratesticular sarcomas is to only sample the firm solid areas and not the mature fat exterior to the solid areas such that the more well differentiated component is not identified, and the diagnosis of de-differentiated liposarcoma is difficult to make. There is rather limited data on the prognosis of de-differentiated liposarcomas. With relatively short follow-up in relatively few cases, the prognosis has been favorable. It is also important to rule out extension from a retroperitoneal primary liposarcoma if the proximal spermatic cord margin shows tumor, and this possibility should be raised in the pathology report.