Abstract:

Solitary circumscribed neuroma (SCN) [syn. palisaded encapsulated neuroma] is a benign tumour composed of axonal and nerve sheath elements. It typically occurs as a painless flesh-colored papule/nodule on the head/neck of an adult. Microscopically it is characterized by a well-circumscribed, partially encapsulated dermal nodule comprised of short fascicles of spindle-shaped cells, with interfascicular clefts. Plexiform, multinodular, and fungating variants of the lesion have been described. Our case was of a 66-year-old male with a papule on the upper abdomen. A clinical diagnosis of epithelioma was suspected and the lesion was excised. Histopathologically, a vertically oriented, folliculocentric tumor, with peripheral nodular components was observed. It consisted of small fascicles of bland spindle-shaped cells with interfascicular clefts. Circumscription was confined to the nodular elements of the lesion. An infiltrative pattern was observed elsewhere. Prominent mucin deposition conveyed a microcystic/pseudoglandular appearance to the tumour. The initial impression was of a melanocytic neoplasm (cellular blue or deep penetrating nevus). Immunohistochemistry revealed positivity for S100, SOX10, and HMB45 and negativity for Mart-1. The latter result and the unusual appearance of the lesion prompted additional investigations. These revealed (i) intralesional axons, highlighted by a neurofilament stain, (ii) EMA and Claudin-1-positive perineurial cells in capsular and intralesional perifascicular distributions and (iii) a network of CD34positive endoneurial cells throughout the lesion. The final diagnosis was of an unusual SCN exhibiting limited circumscription and a mucinous microcystic appearance on routine sections, spurious, misleading positivity for HMB45 on immunohistochemistry, and an under-recognized perifascicular intra-lesional distribution of perineurial cells.

Financial Disclosure:
No current or relevant financial relationships exist.

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Marc Ranson, MD