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

Abstract 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.