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Case Reports

Abstract

A 53-year-old man presented with a slow-growing, painless, firm nodule (1.5 cm) on the distal left fourth finger, first noticed after a childhood finger injury. His medical and family histories were unremarkable. A deep shave biopsy revealed an unencapsulated, multilobular dermal tumor composed of spindle cell fascicles separated by clefts, with diffuse central gland-like structures and intraluminal mucin, initially raising suspicion for a primary adnexal tumor or metastatic carcinoma. Immunohistochemistry (IHC) was performed, with S100 showing strong positivity in spindle cells and EMA highlighting the surrounding perineurial cells. The Ki-67 proliferation index was low. A diagnosis of palisaded encapsulated neuroma (PEN) with microcystic changes was made – the first reported case with diffuse central microcystic degeneration to the author’s knowledge. Both neurofibroma and schwannoma are known to occasionally exhibit microcystic changes. However, schwannoma typically arises in the deep subcutis with encapsulation, Antoni A/B areas, and Verocay bodies. Neurofibromas are typically more superficial and feature scattered mast cells, wispy collagen, fibroblasts, and patchy S100 staining. Unlike these entities, PEN is surrounded by perineurial cells, aiding in differentiation. Clinically, distinguishing PEN is crucial, as it is not associated with neurofibromatosis and has no malignant potential. Conservative excision is curative with low recurrence risk. However, PEN in mucosal sites should raise suspicion for mucosal neuroma, a feature of multiple endocrine neoplasia type 2B. Recognizing this uncommon variant of PEN can prevent diagnostic confusion and unnecessary IHC panels.