Dermal-Based Spindle Cell Lipoma: A Case Series of 12 Patients.

Spindle cell lipoma (SCL) is a benign adipocytic neoplasm that typically presents as a subcutaneous mass in the "shawl" distribution of older men. This classic prototype is well recognized; however, the clinicopathologic features of dermal variants remain incompletely characterized. This case series includes 12 dermally located SCLs from our archives. Morphologic features were evaluated by consensus, and immunohistochemistry for RB1 and CD117 was performed on available tissue. The cohort included eight men and four women, with a mean age of 55 years. Half of the cases (50%) arose in atypical locations, most commonly the face. Histologically, tumors frequently deviated from the classic morphology: 50% were fat-free or fat-poor, 83% were poorly circumscribed, 33% showed myxoid change, and 16% displayed pseudoangiomatous features. All cases where immunohistochemistry could be performed showed RB1 loss (6 cases) and abundant CD117-positive mast cells (7 cases). Dermal SCL differs from the classic subcutaneous prototype. They are more likely to affect women, occur in atypical sites, be poorly circumscribed, and have fat-free or fat-poor morphologies. These deviations from the prototypical SCL illness script are diagnostically challenging, since they mimic other spindle cell neoplasms, such as neurofibroma, PRRX1-rearranged fibroblastic tumor, and a variety of spindle cell lesions that lack RB1 expression. CD117-positive mast cells may be helpful diagnostically.