Atypical spindle cell lipomatous neoplasm, also known as well-differentiated spindle cell liposarcoma, represents a newly discovered entity of adipocytic tumors. Recent research has shown this tumor variant to be more related to spindle cell lipoma, rather than the originally hypothesized atypical lipomatous tumor spectrum. Here we present a case of a 58-year-old man with a history of chronic lymphocytic leukemia with an enlarging mass on the posterior left shoulder, initially hypothesized to be a benign lipoma. However, magnetic resonance imaging showed a large, multiseptated, heterogeneous mass concerning for soft tissue sarcoma. After resection, pathologic analysis showed cells closely resembling spindle cell lipoma, with additional cellular and fascicular zones containing lipoblasts and mitotic figures. Molecular analysis showed no MDM2 amplification. This lack of amplification indicates this tumor is distinctly different from an atypical lipomatous tumor, which characteristically displays MDM2 amplification. However, tumor expression of RB1 was normal. The majority of atypical spindle cell lipomatous neoplasms are associated with RB1 deletions. We conclude that we have a unique example of an atypical spindle cell lipomatous tumor.
Keywords: atypical spindle cell lipomatous neoplasm; lipoma; well-differentiated spindle cell liposarcoma.