Desmin expression in spindle cell lipomas:: a potential diagnostic pitfall

The immunohistochemical analysis of the spindle cell lipomas has been for the most part limited to the study of S-100 protein, CD34, and Bcl-2 reactivity. To evaluate the immunoexpression of desmin and actins in spindle cell lipomas of different histological subtypes a retrospective immunohistochemical study of 25 spindle cell lipomas using archival formalin-fixed, paraffin-embedded tissue was performed. Strong positivity of the spindle cell component for desmin was found in 4 of 25 cases (16%). The expression was diffuse in two cases and focal (in up to 25% of the spindle cells) in the other two. Two of these cases were of the classical type and the other two were angiomatous spindle cell lipomas. Desmin-positive and desmin-negative spindle cells showed no morphological differences. The spindle cell component expressed CD34 in all cases and Bcl-2 in 14 of the 25 cases. There was no immunoreactivity for smooth muscle actin, muscle-specific actin, or S-100 protein. We conclude that a significant proportion of spindle cell lipomas express desmin, and therefore that the immunoreactivity for this antigen does not exclude the diagnosis, even in lesions with nonclassical histological features and/or atypical locations.