Atypical spindle cell/pleomorphic lipomatous tumor.

Atypical spindle cell/pleomorphic lipomatous tumor (ASPLT) is a recently described morphologically low-grade and clinically indolent adipocytic tumor, which will be incorporated as a new tumor entity in the upcoming 5th edition of the WHO Classification of Soft tissue and Bone tumors. Histologically, ASPLTs are characterized by ill-defined tumor margins and the presence of variable proportions of mild-to-moderately atypical spindle cells, adipocytes, lipoblasts, pleomorphic multinucleated cells and a myxoid or collagenous extracellular matrix. ASPLTs can show a wide variety of microscopic appearances and there is histologic overlap with diverse mimics. The diagnosis of ASPLT can therefore be challenging. Molecular studies have shown a consistent absence of MDM2 or CDK4 amplification. On the other hand, deletions or losses of 13q14, including RB1, have been identified in a significant subset of cases. This review provides an overview of the currently known clinical and pathological features of ASPLTs, detailing its most relevant differential diagnoses.