Membranous fat necrosis in lipomas.

Membranous fat necrosis (MFN) is an under-recognized variant of fat necrosis (FN) that is characterized by the presence of membranocystic foci in which cysts are lined by an eosinophilic, homogeneous membrane with pseudo-papillary luminal projections. Although MFN has been described in systemic adipose tissue, it has not been described in lipomas. We report the rare occurrence of MFN in four lipomas. The lipomas ranged in size between 9 and 22 cm and occurred in menopausal women in the left shoulder, right upper back, right thigh, and gluteal regions. In two cases the membranocystic foci were identified macroscopically, whereas in the other two cases MFN was only identified on histologic assessment. Apart from the typical histologic appearance of the membranocystic foci, the membranes have a distinct histochemical profile that includes consistent Sudan black positivity and either periodic acid-Schiff or Ziehl-Neelsen positivity. In all cases, there was autofluorescence of the membranes on immunofluorescent examination of unstained sections. These membranocystic foci have the typical staining reactions of ceroid pigment. The exact cause of MFN in lipomas is not known, but in view of their large sizes, traumatic and ischemic etiologies are proposed.