Parosteal lipoma of the proximal humerus mimicking atypical lipomatous tumor: A diagnostic challenge.

Parosteal lipomas are a rare benign fat-containing neoplasm that originates from mature adipose tissue near the bone periosteum that can often be difficult to differentiate from malignant tumors. We present the case of a 75-year-old female presenting with several months of a growing, uncomfortable right shoulder mass without neurological or motor deficits. Computerized tomography (CT) demonstrated a well-defined mass located deep to the deltoid along the proximal humerus with peripheral fat attenuation while follow-up magnetic resonance imaging (MRI) demonstrated internal central enhancement and thin fibrous septae. Such findings were most consistent with a parosteal lipoma, and given the indolent nature of the mass, absence of aggressive features, and stable size over a 5-month period, conservative management with observation and interval imaging was recommended. This case emphasizes that while histological confirmation remains the gold standard, magnetic resonance imaging (MRI) is currently the most valuable tool in differentiating parosteal lipoma from atypical lipomatous tumors and other fat-containing lesions and guiding management decisions.