Atypical spindle cell lipoma: a clinicopathologic, immunohistochemical, and molecular study emphasizing its relationship to classical spindle cell lipoma

We studied a series of spindle cell lipomas arising in atypical sites and showing unusual morphologic features (which we called atypical spindle cell lipoma) to assess if these lesions have the same chromosomal alterations as classical spindle cell lipoma but different from those found in atypical lipomatous tumor/well-differentiated liposarcoma. We investigated alterations of different genes in the 13q14 region and the amplification status of the MDM2 and CDK4 genes at 12q14-15 by multiplex ligation-dependent probe amplification (MLPA) and fluorescence in situ hybridization (FISH) analysis. In the atypical spindle cell lipomas, MLPA revealed deletions in the two nearest flanking genes of RB1 (ITM2B and RCBTB2) and in multiple important exons of RB1. In contrast, in classical spindle cell lipomas, a less complex loss of RB1 exons was found but no deletion of ITM2B and RCBTB2. Moreover, MLPA identified a deletion of the DLEU1 gene, a finding which has not been reported earlier. We propose an immunohistochemical panel for lipomatous tumors which comprises of MDM2, CDK4, p16, Rb, which we have found useful in discriminating between atypical or classical spindle cell lipomas and other adipocytic neoplasms, especially atypical lipomatous tumor/well-differentiated liposarcoma. Our findings strengthen the link between atypical spindle cell lipoma and classical spindle cell lipoma, and differentiate them from atypical lipomatous tumor/well-differentiated liposarcoma.