Abstract
Aims
Spindle cell differentiation is not an uncommon finding in common acquired naevi and may represent a form of neurotisation with Schwannian differentiation of melanocytes. Perineurial differentiation in this context appears to be distinctly rare and is only poorly documented in the literature.
Methods and results
We have identified six melanocytic tumours showing spindle cell morphology and perineurial differentiation from routine and referral material. Clinical data and follow-up were obtained, and the histological and immunohistochemical features were analysed. The tumours affected middle aged adults (median: 48; range: 26-74 years) with a wide anatomical distribution and benign follow up (median: 13; range: 6 to 48 months). All tumours were nodular and circumscribed but asymmetrical with extension into deep dermis and superficial subcutis. A characteristic finding was a biphasic growth pattern with a lentiginous compound naevus in the superficial aspect and abrupt transition to a prominent nodular spindle cell proliferation in the deeper reaches. Spindle cells were bland and uniform and arranged singly and in short fascicles in a loose fibromyxoid stroma. In areas, a whorled arrangement of slender spindle cells with wavy nuclei was appreciated. Distinctive intratumoral hypocellular nodules and peripheral lymphoid aggregates were additional features. By immunohistochemistry, the spindle cells were mainly S100-positive melanocytes. In areas, S100-/EMA+ spindle cells showing co-expression of glut-1 and claudin-1 were closely admixed.
Conclusion
This perineurial differentiation likely represents a rare and unusual form of neurotisation. The tumours are benign but may be mistaken for desmoplastic melanoma. Awareness and careful attention to the clinico-pathological and immunohistochemical features allows reliable separation.
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