Abstract

Pleomorphic lipoma is a type of rare benign soft tissue tumor that may clinically resemble an ordinary lipoma. However, on histology they are diverse in their appearance and have several lipomatous tumor histological mimics, making their diagnosis more complex. In this case report, we describe an unusual presentation of a posterior neck pleomorphic lipoma. Here, while on histology the presentation was more consistent with pleomorphic lipoma than spindle cell lipoma, a higher degree of cellular atypia than is typically encountered in pleomorphic lipoma was present, raising suspicion for histological mimics including well-differentiated liposarcoma or pleomorphic liposarcoma. We utilized modern characterization methods including gross appearance, immunohistochemical analysis, and FISH amplification to arrive at diagnosis. Given the varying pathologic clinical course, management, and potential for recurrence across these lesions, it is important that we continue to understand and classify lipomatous tumors to provide the best treatment to affected patients. As dermatopathologists learn more and can better characterize the similar but distinct entities of lipomatous tumors, their differential list is growing. This presentation of pleomorphic lipoma adds to the literature of lipomatous tumors and demonstrates how modern molecular characterization tools help distinguish benign tumors from malignant histological mimics.