Atypical Spindle Cell Lipomatous Tumor: Clinicopathologic Characterization of 232 Cases Demonstrating a Morphologic Spectrum.

The classification of atypical adipocytic neoplasms with spindle cell features remains challenging. To better define this category of low-grade lipomatous neoplasms, we present herein the clinical, histologic, and immunohistochemical characteristics of a large series of 232 atypical spindle cell lipomatous tumors. The lesions affected 140 males and 92 females, at an average age of 54 years (range, 6 to 87 y), clinically presenting as a persistent or enlarging mass with a median size of 5 cm. The anatomic distribution of the tumors was wide, predominating in the limbs and limb girdles (147 cases, 63%), mainly in the hands and feet (17% and 11%, respectively), with equal distribution between subcutaneous and deeper locations. Microscopic examination revealed a spectrum of histologic appearances. All cases consisted of a poorly marginated proliferation of mildly atypical spindle cells set in a fibrous or myxoid stroma, with a variably prominent admixed adipocytic component showing variation in adipocyte size and scattered nuclear atypia, frequently with univacuolated or multivacuolated lipoblasts. Tumor cellularity and the relative proportion of the different components were very variable. Tumor margins were often ill defined with invasion into surrounding tissues. Two tumors showed morphologic features reminiscent of dedifferentiation. By immunohistochemistry, the neoplastic spindle cells expressed CD34 (64%), S100 protein (40%) and, less frequently, desmin (23%). Expression of Rb was lost in 57% of cases examined. MDM2 and CDK4 were never coexpressed and FISH for MDM2 amplification was consistently negative, highlighting critical biological differences from atypical lipomatous tumor/dedifferentiated liposarcoma. The morphologic differential diagnosis of atypical spindle cell lipomatous tumor is broad, and includes spindle cell lipoma, diffuse neurofibroma, mammary-type myofibroblastoma, dermatofibrosarcoma protuberans, fat-forming solitary fibrous tumor, and morphologically low-grade malignant peripheral nerve sheath tumor. Most patients underwent surgical excision of the primary mass. With a median follow-up of 4 years (range, 1 mo to 20 y), 87% of patients (63/72) were alive with no evidence of recurrence or metastatic disease. Local recurrence of the tumor was observed in 12% of patients (9 out of 72, multiple in 3 of them) at intervals between 6 months and 17 years after resection of the primary tumor. None of the patients developed tumor metastasis or died of disease. Identification of the neoplastic adipocytic component admixed with spindle cells, and recognition of the range of histologic appearances are key for the diagnosis of atypical spindle cell lipomatous tumor. Whereas the risk of metastatic dissemination is minimal, there is a non-negligible risk for local recurrence (13%) which warrants surgical resection with clear margins whenever feasible.