Perineuriomatous nevi: a series of 8 cases highlighting unifying pathologic features to avoid misdiagnosis.

BACKGROUND Perineuriomatous nevi are rare and diagnostically problematic. We report a series of 8 perineuriomatous nevi to highlight the diagnostic features. METHODS Cases were retrospectively reviewed and characterized. RESULTS Median age was 42.5 years (range 25-64), with equal sex distribution. Lesions occurred on the arm (n = 4), trunk (n = 2) and head/neck (n = 2). Median size was 7.5 mm (range 5-12 mm). Clinical differential diagnoses included atypical nevus (3), blue nevus (1), neurofibroma (1) and dermatofibroma (1). Lesions were circumscribed, dome-shaped (5/8), and biphasic (8/8) with nested epithelioid cells and wavy spindled cells arranged in whorled fascicles in a myxocollagenous stroma. When present, junctional growth was lentiginous (4/8). No cases displayed pleomorphism or mitoses. The perineuriomatous component stained positively for EMA (8/8 focal to diffuse) and CD34 (4/5 focal to diffuse). SOX10 and S100 protein stained all nevoid cells and in some cases a subset of intermingled spindled cells in perineuriomatous areas, where other melanocytic markers were negative. p16 protein expression was uniformly retained (3/3), and p53 negative (0/2). Nevoid cells in most lesions were positive for BRAFV600E (5/7). Ki67 was mildly elevated (~5%) in 3/3 cases. CONCLUSIONS Recognizing the histopathologic and immunophenotypic features in these unusual nevi helps avoid over diagnosis. This article is protected by copyright. All rights reserved.