Hibernoma: an uncommon cause of a pleural mass

Here, we report the case of a 37-year-old asymptomatic male patient who was referred because of abnormalities seen on a routine chest X-ray. The physical examination findings and laboratory test results were normal. A new chest X-ray revealed a large opacity in the left lower hemithorax. Chest CT revealed a heterogeneous pleural mass in the left lower hemithorax (Fig. 1). The mass appeared to be an extrapulmonary lesion arising from the chest wall. There were no calcifications. Complete surgical excision was performed. The gross specimen showed a well-circumscribed, encapsulated, soft, brownto-yellow mass, measuring 10 × 9 × 5 cm (Fig. 2A). The microscopic findings were diagnostic of a hibernoma (Fig. 2B). At this writing, the patient remains asymptomatic and subsequent follow-up evaluations have been unremarkable. Hibernomas are rare benign tumors that take their name from their histological similarity to the brown fat of hibernating animals, but also seen in the human fetus and to a diminishing degree with age in adults.(1-3) The distribution of this tumor follows the sites of persistence of brown fat. The