Congenital pauci-melanotic cellular blue nevus.

Unusual or atypical melanocytic nevi can be confused with malignant melanoma. Two patients are presented here with a rare variant of melanocytic nevus. Both were men. One was 39 years old and sought medical attention after trauma of a "congenital mole". The other was 24 years old and presented with a history of a slowly growing lesion, which had been known since childhood. In both patients, the lesion occurred on the buttock. They were dermal and superficial subcutaneous nodules measuring 1.5 and 2.3 cm in greatest dimension, respectively. The tumors were composed of densely cellular fascicles of melanocytes arranged in a lobulated growth pattern. Rare nests of small epithelioid melanocytes were also seen. No melanin pigment was seen on hematoxylin and eosin-stained sections. Focal minimal pigment was noted by Fontana-Masson stain in one case. Involvement of numerous peripheral nerve trunks by fusiform melanocytes was a prominent feature. Rare mitotic figures were seen in melanocytes [1-2 mitoses per 50 high-power fields (HPF)]. The MIB-1 labeling index was low (less than 5% of the lesional cell population was immunopositive). Both tumors were excised with negative surgical margins. One patient underwent sentinel lymph node biopsy because there was controversy regarding the biologic potential of the lesion. No melanocytic tumor deposits were found in the lymph nodes. On clinical follow up of 11 years and 18 months after complete excision, both patients are alive and well with no evidence of recurrence. We regard these lesions as congenital monophasic and pauci-melanotic variants of cellular blue nevus. The nevi are presented here to enhance our knowledge of the morphologic spectrum of melanocytic tumors and to help avoid confusion with malignant melanoma.