A Challenging Case of Pleomorphic Lipoma of the Shoulder Mimicking Liposarcoma

BACKGROUND Pleomorphic lipoma is a rare benign adipocytic tumor that can closely mimic, radiologically and histologically, malignant soft tissue neoplasms, particularly atypical lipomatous tumor or liposarcoma. Despite is benign nature, pleomorphic lipoma poses a diagnostic challenge because of its significant histologic overlap with malignant entities. Accurate diagnosis is essential to prevent unnecessary wide surgical resections or overtreatment. Recognition of its distinctive clinical, histologic, and immunohistochemical features is therefore critical. CASE REPORT A 68-year-old man presented with a progressively enlarging, painful mass in the left upper arm. Ultrasound demonstrated a lobulated subdermal soft tissue lesion measuring 4.7×1.0×2.5 cm with a small hypoechoic component, raising concern for an atypical lipomatous tumor. Surgical excision was performed for definitive diagnosis and symptom relief. Gross examination demonstrated a well-circumscribed, yellow, lobulated mass. Histopathologic evaluation showed a lipomatous lesion composed of atypical spindle cells with pleomorphism and multinucleation, intermixed with variably sized adipocytes within a fibrotic and myxoid stroma. Immunohistochemical studies were positive for CD34 and negative for MDM2 and CDK4, with loss of RB1 expression and a low Ki-67 proliferation index (