The occurrence of smooth muscle differentiation in a liposarcoma is a very uncommon phenomenon, even in dedifferentiated liposarcomas. In dedifferentiated liposarcomas, the leiomyosarcomatous component frequently displays high-grade cytologic features, increased mitotic activity, and tumor necrosis. Even more unusual are rare reported cases of low-grade smooth muscle differentiation in atypical lipomatous tumors/well-differentiated liposarcomas (WDLS). The current case describes a 39-year-old female with a large retroperitoneal WDLS harboring a well-demarcated mass composed of benign-appearing smooth muscle fascicles completely lacking cytologic atypia and mitotic activity. In conjunction with the immunopositivity for estrogen and progesterone receptors, the morphology of this nodule was highly reminiscent of a uterine-type leiomyoma. Of note, the lipomatous component largely displayed a lipoma-like appearance with only rare foci of mildly atypical spindle cell proliferation among the adipocytes and few fibrous septae harboring atypical stromal cells. Immunohistochemical and fluorescence in situ hybridization studies revealed MDM2 gene amplification in both the lipomatous and leiomyoma-like areas, thus confirming the diagnosis of a WDLS with smooth muscle differentiation. A literature review on the subject of sarcomas with dual adipocytic and low-grade smooth muscle differentiation provided sufficient supporting evidence to categorize the tumor as a WDLS with "leiomyomatous" differentiation. Pathologists should be aware of the occurrence of uterine-type leiomyomatous differentiation in retroperitoneal WDLS to avoid potential diagnostic errors.
Keywords: dual; leiomyomatous; lipomatous; low-grade; sarcomas; smooth muscle differentiation; uterine-type; well-differentiated liposarcoma.