A case of pseudoangiomatous spindle-cell lipoma with unique MRI manifestations corresponding to distinct histological features.

A 50-year-old man presented with a subcutaneous mass on the posterior neck. A skin biopsy suggested lipoma, leading to observation. Over the next 5 years, the tumor gradually enlarged. Clinical evaluation found the tumor to be a soft, mobile mass of 10 × 7 cm. Magnetic resonance imaging showed the lesion to be cystic, with hyperintense mural nodules on T2-weighted sequences, and with scattered, hyperintense foci indicative of fatty components on T1-weighted imaging. Fat-suppressed contrast-enhanced T1-weighted imaging revealed moderately enhanced mural nodules. Surgical resection revealed a well-circumscribed cystic mass containing fluid, surrounded by a fibrous capsule. We suspected spindle cell lipoma associated with fluid and performed a marginal resection. Gross examination showed a heterogeneous mixture of reddish-brown and yellowish zones interspersed with fissures. Histological analysis revealed a leaf-like or reticular pattern, with slender, spindle-shaped cells and ropey collagens within a mucinous matrix, along with mature adipocytes. There were many irregular, branching, cleft-like vascular spaces lined by flattened cells. Immunohistochemical staining revealed tumor cells positive for CD34 and negative for retinoblastoma protein. Luminal endothelial-like cells showed positivity for D2-40, CD31, and erythroblastosis transformation-specific regulated gene 1. Based on these findings, the diagnosis of pseudoangiomatous spindle-cell lipoma was confirmed, with the luminal endothelial-like cells exhibiting characteristics of lymphatic endothelium.