Solitary plexiform neurofibroma is not pathognomonic of von Recklinghausen's neurofibromatosis: a report of a case

A 42-year-old Caucasian man presented with a 1.8-cm nodule in the proximal thigh (Fig. 1) that had been slowly enlarging for approximately 5 years. The patient had no family history of neurofibromatosis. Physical examination revealed none of the other stigmata of this neurocutaneous disorder. D.A.F. performed an elliptical excision as an out-patient procedure. Grossly, the lesion was homogeneously opalescent and gelatinous. The tissue was fixed in 10% formalin and embedded in paraffin. Routine hematoxylin and eosin stained sections were prepared. The most notable feature on low power magnification was a large, circumscribed, lobular proliferation of spindle cells in a loose fibrillar background that distorted much of the dermis and subcutaneous fat (Fig. 2). On high power magnification, a population of cells with ovoid and S-shaped nuclei admixed within densely packed eosinophilic collagenous material was seen at the center of the lobule (Fig. 3A). A small amount of mucin was also present. Within this cellular area, a dense population of S-shaped spindle cells with eosinophilic cytoplasm was arranged in a parallel array forming microfascicles. At the periphery of this cellular component, a similar population of cells was arranged against a loose fibrillar background of abundant mucin (confirmed by colloidal iron stain) and scattered mast cells (Fig. 4). Lining the lobule was a compressed rim of thin, delicate collagen bundles and perineurial cells. Pleomorphic nuclei and mitotic figures were not observed. The surrounding dermis and subcutaneous tissue were unremarkable. Thickened collagen bundles at the center of the lobule could be seen with a frichrome stain. An S-100 protein immunoperoxidase stain highlighted scattered Schwann cells within the lobule (Fig. 3B), and nerve bundles adjacent to the neurofibroma served as infernal positive controls.