Bone marrow core biopsy performed in a 74-year-old man with untreated lymphoplasmacytic lymphoma showed persistent lymphoma and a large area of loosely aggregated adipose cells containing multiple cytoplasmic vacuoles, suggestive of brown adipocytes (panels A and B; hematoxylin and eosin stain; original magnifications ×40 [A] and ×500 [B]). Immunostains for S100 (panel C, original magnification ×400) and uncoupling protein 1 (UCP-1) (panel D, original magnifications ×200 and ×500 [inset]) confirmed this impression. Brown adipocytes contain numerous small lipid droplets and have eosinophilic cytoplasm resulting from high numbers of mitochondria.

Brown adipose tissue (BAT) is abundant in infants and hibernating animals in which its primary function is thermoregulation. BAT-associated nonshivering thermogenesis is due to UCP-1, a unique mitochondrial enzyme that generates heat by uncoupling adenosine triphosphate energy production via promoting proton leakage across the inner mitochondrial membrane, which reduces the transmembrane gradient. Although its prevalence was thought to decrease with age, BAT is now known to be present and metabolically active in adults. Hibernomas may develop as neoplastic growths of BAT. Incidental BAT has only been reported in bone marrow once, and only sporadic cases of intraosseous hibernomas are reported. In our case, BAT was discovered incidentally, was not seen on imaging studies, and was largely separate from surrounding marrow. On the basis of the latter, we believe this is an incidental intraosseous hibernoma. However, distinguishing BAT from hibernoma may be difficult.