Case Report and Review of Literature of Atypical Spindle Cell/Pleomorphic Lipomatous Tumor in the Paratesticular Region

Abstract The term “atypical spindle cell/pleomorphic lipomatous tumor” was introduced in the WHO Classification of Soft Tissue Tumors in 2020. This tumor is an adipocytic neoplasm of benign or low-grade category, is clinically indolent, has poorly circumscribed margins, and composed of mature adipocytes, lipoblasts, atypical spindle-shaped cells, and multinucleated cells. A 75-year-old male presented with a paratesticular mass. On microscopic examination, the tumor showed a mixture of two components—adipose and fibrous tissue components—which blended with each other along with scattered atypical giant cells. Immunohistochemically, smooth muscle actin showed diffuse positivity in the spindle cells. S100 was negative in the spindle cells and giant cells but was positive in the nuclei of some adipocytes. The Ki-67 index was only 5%. CD34 and desmin were positive in the blood vessel walls—in endothelial cells and muscle layer respectively—and negative in the giant cells. Immunohistochemistry (IHC) with MDM2 (murine double minute 2) and Rb (retinoblastoma) was negative, while CDK4 (cyclin-dependent kinase 4) was variably positive in the nuclei of the spindle cells. The spectrum of adipocytic tumors that show overlapping morphologic features may pose diagnostic difficulty. Precise diagnosis of ASPLT is important, as it can be misdiagnosed as an intermediate grade or malignant lipomatous tumor. A tumor can be diagnosed as ASPLT when it is composed of a heterogeneous mixture of adipocytes, spindle cells with focal atypia, and multinucleated cells. Lipoblasts are not mandatory for diagnosis. IHC with MDM2, Rb1, Ki67, and molecular testing is helpful in differentiating benign ASPLT from other entities and in ensuring a better prognosis. CDK4 is not found to be useful.