Answer: C. Well-differentiated liposarcoma

Histology: The lesion was a soft lobulated, well-defined mass. It consisted primarily of irregular small bundles of benign-appearing smooth muscle with small foci of mature adipose tissue. Scattered cells with large atypical hyperchromatic, yet degenerative appearing, nuclei were present in the lesion.

Discussion: The location of this tumor is key. The vast majority of paratesticular soft tissue lesions in adults are liposarcoma. Leiomyosarcoma is the 2nd most common paratesticular mesenchymal tumor in adults. In contrast to this case, leiomyosarcoma consists of well-developed fascicles of cellular spindle cells with nuclear atypia and increased mitotic figures. If the lesion does not have the typical morphology of a leiomyosarcoma and is high grade and pleomorphic, then the likely diagnosis is de-differentiated liposarcoma and one should look at the benign-appearing adipose tissue for the large atypical smudgy hyperchromatic nuclei, typical of a well-differentiated component. Classic lipoblasts with indentation by intracytoplasmic lipid vacuoles leading to a characteristic scalloping of nuclear membrane are often not seen and not necessary for the diagnosis of well-differentiated liposarcoma. If the tumor is low grade, even if the adipose component is focal, again one should look for atypical cells and do MDM2 immunostaining to rule out a well-differentiated liposarcoma. This case is unusual due to the prominent smooth muscle component, which can uncommonly occur in well-differentiated liposarcoma. In the current case, MDM2 immunohistochemistry was positive verifying the diagnosis.

Reference: Folpe AL, Weiss SW. Lipoleiomyosarcoma (Well-Differentiated Liposarcoma With Leiomyosarcomatous Differentiation) A Clinicopathologic Study of Nine Cases Including One With Dedifferentiation. Am J Surg Pathol 2002; 26: 742-9.