MEMBRANOUS FAT NECROSIS IN LIPOMAS

Membranous fat necrosis (MFN) is an under-recognized variant of fat ne crosis (FN) that is characterized by the presence of membranocystic fo ci in which cysts are lined by an eosinophilic, homogeneous membrane w ith pseudo-papillary luminal projections. Although MFN has been descri bed in systemic adipose tissue, it has not been described in lipomas. We report the rare occurrence of MFN in four lipomas. The lipomas rang ed 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, wh ereas in the other two cases MFN was only identified on histologic ass essment. Apart from the typical histologic appearance of the membranoc ystic foci, the membranes have a distinct histochemical profile that i ncludes consistent Sudan black positivity and either periodic acid-Sch iff or Ziehl-Neelsen positivity. In all cases, there was autofluoresce nce of the membranes on immunofluorescent examination of unstained sec tions. These membranocystic foci have the typical staining reactions o f ceroid pigment. The exact cause of MFN in lipomas is not known, but in view of their large sizes, traumatic and ischemic etiologies are pr oposed.