[Cutaneous neural neoplasms--an update]

Until recently, benign cutaneous neural tumours which do not fulfill criteria for either neurofibroma or schwannoma often were lumped into the broad category of benign peripheral nerve sheath tumours (PNST). However, during the last years a number of new entities of neural tumours has been described, and advances in immunohistochemistry and electronmicroscopy have helped us better to understand the cytological differentiation in these neoplasms. The knowledge of these distinctive neoplasms is necessary in order to avoid diagnostic pitfalls and misdiagnosis of more aggressive neoplasms. These distinctive lesions include: Neurothekeoma, which can be divided into classical myxoid and cellular types showing characteristic histological and immunohistochemical features. Typical neurothekeoma (nerve sheath myxoma) is a lobular or nodular dermal neoplasm composed of plump spindled or stellated S-100 positive tumour cells set in a myxoid stroma. In contrast cellular neurothekeoma is characterized as an ill-defined dermal neoplasm composed of concentric nests and fascicles of spindle-shaped and epitheloid tumour cells, which are S-100 negative but stain positively for NKIC3. The evidence of intermediate forms of neurothekeoma showing features of ordinary, hypocellular neurothekeoma and cellular neurothekeoma, as well as ultrastructural studies emphasize, that both variants represent a spectrum of neurothekeoma. Solitary circumscribed neuroma ("palisaded encapsulated neuroma") manifests mainly as a skin-colored or pink papule or nodule, and is most often located on the face. Histologically, solitary circumscribed neuroma is a well-circumscribed round or ovoid dermal neoplasm composed of interwoven fascicles of schwann cells, which stain positively for S-100 protein, and numerous neurofilament positive axons surrounded partly by fibroblasts and EMA-positive perineurial cells. Perineurioma is a rare well-circumscribed neoplasm which occurs mainly in subcutaneous tissue and only rarely in the dermis and in deep soft tissue. Perineurioma is composed of elongated bipolar spindle-shaped tumour cells which are arranged in a storiform, linear or lamellated growth pattern. The tumour cells stain positively for vimentin and EMA, and for CD34 in a number of cases, but lack positivity for S-100 protein, neurofilament and desmoplakin. In addition unusual forms of schwannoma including cellular schwannoma, solitary plexiform schwannoma, and melanocytic schwannoma are briefly discussed.