A Case of Myxoid Pleomorphic Liposarcoma with Rhabdoid Cells: A Diagnostic Pitfall.

Myxoid pleomorphic liposarcoma (MPLS) is an extremely rare tumor listed in the fifth edition of the WHO classification (2020). Histologically, it mainly comprises a mixture of myxoid and pleomorphic liposarcoma-like components. Genetically, it lacks FUS/EWSR1::DDIT3 fusion and MDM2 amplification. Herein, we describe an example of MPLS with rhabdoid cells in a 10-year-old girl who presented with a growing mass in the right inguinal region. The specimen from the wide excision measured 68 mm × 55 mm × 43 mm, and a circumscribed and lobulated mass was observed in the subcutaneous tissue. Histologically, oval-to-short, spindle-shaped, proliferating tumor cells with moderate nuclear atypia and mesh-like capillaries against a myxoid background were noted. Adipocytes were observed focally, while rhabdoid cells were observed multifocally. Immunohistochemically, the tumor showed inconsistent reactivity for desmin but was negative for MYOD1, myogenin, MDM2, and CDK4. Fluorescence in situ hybridization revealed no DDIT3 rearrangement. Despite adjuvant chemotherapy, the tumor metastasized to the thoracic cavity 24 months after excision. The metastatic lesions contained abundant lipoblasts rather than rhabdoid cells, and we concluded this tumor was a MPLS. The presence of rhabdoid cells could be a diagnostic pitfall, and recognizing such a variation in histology would help improve diagnostic accuracy.