Case Report: An exceptionally large spindle cell lipoma of the popliteal fossa

Background: Spindle cell lipoma (SCL) is an uncommon subtype of lipomas. It usually occurs in the posterior side of the trunk such as neck, back and shoulders and rarely touches the extremities. Only four cases of SCL of the knee and no cases of SCL of the popliteal fossa have been reported in the literature. We present herein the largest case of SCL of the popliteal fossa and knee to our knowledge. Case presentation: A 75-year-old woman presented with a slow growing swelling of the left knee. Physical examination showed a 12 cm well-limited painless mass of the left popliteal fossa. The mass was pediculated with a central ulceration. MRI scan showed a tumoral mass of the left popliteal fossa with some non-dilated vessels draining to the long saphenous vein. The mass had no evidence of invasion of the underlying tissues. The patient underwent a surgical resection of the mass with postoperative recovery. The histological examination revealed a tumoral proliferation diffusely expressing CD34 without malignancy evidence, and a Ki67 proliferation index <1%. This morphological and immunohistochemical aspect is typical of a SCL. Clinical discussion: SCL is a rare histological variant of lipoma occurring mostly in the posterior side of the upper trunk. SCL of the knee or the popliteal fossa are exceptional. Clinically, it presents as a gradually increasing painless mass with a long evolving history. Histologically, these tumors are made of spindle-shaped cells, rope-like collagen fibers and mature adipocytes surrounded by a fibrous capsule. Immunohistologically, spindle cell lipomas usually show diffuse and strong expression of CD34, and to a lesser degree vimentin. The optimal treatment of SCL is a surgical marginal resection. Conclusions: SCL represents an infrequent subtype, and its atypical presentation should push clinicians to further investigations in order to rule out more worrisome malignancies.