Background: There are numerous variants of cutaneous neurofibroma reflecting its manner of growth and histologic composition. Lipomatous neurofibroma is the latest described variant with only eight cases reported.
Methods: A systematic study based on 320 consecutive specimens diagnosed of cutaneous neurofibroma was carried out. Conventional microscopy, immunohistochemistry, and statistical methods were used to determine the presence of fat cells, their amount, distribution, and frequency.
Results: Intratumoral fat was observed in 22 (6.9%) neurofibromas. All these were dermal neurofibromas. Intraneoplastic fat was divided into two groups: focal and diffuse (regularly interspersed). Eighteen tumors (5.6%) presented adipocytes focally intermingled with the spindle cells. There were four (1.3%) neurofibromas showing spindle cell proliferation with regularly scattered adipocytes. Lipomatous neurofibroma was more frequent located on head and neck than non-lipomatous neurofibroma ( p = 0.04). Neurofibromas without mature adipocytes were more frequently immunoreactive for CD34 compared with tumors showing intratumoral fat ( p = 0.02).
Conclusions: We suggest that both metaplasia and aberrant adipose differentiation from multipotential cells may result in lipomatous neurofibroma. Focal presence of adipose cells may be attributable to metaplasia as the pathogenic mechanism. The fatty tissue being intrinsic to the tumor structure in its diffuse form, the lesion represents a distinctive tumor of the peripheral nerve sheath.