Imaging findings In general, imaging studies show a well-defined, heterogeneous mass, usually showing a mass which is hypointense to subcutaneous fat on magnetic resonance T1-weight images. Serpentine, thin, low signal bands (septations or vessels) are often seen throughout the tumor. Pathology findings From a macroscopic perspective, there is a well-defined, encapsulated or circumscribed mass, showing a soft, yellow tan to deep brown mass. The size ranges from 1 to 27 cm, although the mean is about 10 cm. A high power hematoxylin and eosin stained photograph of a<unk> . The tumors histologically resemble brown fat. There are four histologic types recognized, but one is the most frequently seen (typical). There is a background of rich vascularity. # Lobular type: Variable degrees of differentiation of uniform, round to oval cells with granular eosinophilic cells with prominent borders, alternating with coarsely multivacuolated fat cells (pale cells). There are usually small centrally placed nuclei without pleomorphism. The cells have large cytoplasmic lipid droplets interspersed throughout. # Myxoid variant: Loose, basophilic matrix, with thick fibrous septa, and foamy histiocytes # Lipoma-like variant: Univacuolated lipocytes, with only isolated<unk>  cells # Spindle cell variant: Spindle cell lipoma combined with<unk>  Histochemistry Oil Red O-positive droplets of cytoplasmic lipid can be seen in most cases Immunohistochemistry The neoplastic cells are S100 protein positive (approximately 80%), and show membrane and vacuole CD31 immunoreactivity. Uncoupling protein 1 (UCP1), a unique brown fat mitochondrial protein, is also positive. Cytogenetics There are structural rearrangements of 11q13-21, which are considered most characteristic. This alteration can be detected by metaphase fluorescent in situ hybridization (FISH). MEN1 gene (11q13.1) is most frequently deleted, while GARP gene (11q13.5) may also be involved. Cytology The fine needle aspiration smears show small, round, brown fat-like cells, with uniform, small cytoplasmic vacuoles and regular, small, round nuclei. There is usually a rich vascular background of branching capillaries. It is not uncommon to also have mature fat cells. Differential diagnoses It is important to separate<unk>  from adult rhabdomyoma, a granular cell tumor and a true liposarcoma. Classification This lesion has been called a fetal lipoma, lipoma of embryonic fat or a lipoma of immature fat.
Hibernoma
Lipoma
Glucagonoma
Sebaceous adenoma
Insulinoma
Gastrinoma
Granular cell tumor
Angiolipoma