Hibernoma: Rare Case of An Intense 18F-FDG Uptake Mediastinal Lesion Mimicking Thymic Carcinoma

Abstract Hibernoma is a rare benign neoplasia derived from remnants of brown adipose tissue. Here, we describe an unusual case of mediastinal hibernoma which radiologically mimicked a thymic malignancy. The patients had presented with chest pain, associated with palpitations and dizziness suggestive of cardiac pathology. Investigation of coronary anatomy by CT scan, incompletely revealed the presence of the lesion situated in the region of the thymus gland, 48 mm x 94 mm. The mass was smooth walled with evidence of invasion of local structures. Whole-body positron-emission tomography (PET) demonstrated the mass had a very high uptake, SUV 55.2. The mass was excised by total thymectomy through a sternotomy. Histopathological analysis revealed it to be fibro-fatty in nature. Light microscopy demonstrated the presence of large multi-vacuolated cells, with appearances typical of brown fat, a hibernoma. This is one of only 7 reported cases. Its findings highlight the importance of careful interpretation of PET studies in hibernoma cases. We propose hibernoma should be included as a differential diagnosis of growing mediastinal masses.