Recurrent nevus phenomenon: a clinicopathologic study of 357 cases and histologic comparison with melanoma with regression.

Recurrent nevus phenomenon and regression in melanoma may have overlapping histologic features. The clinical findings and histologic changes in 357 cases of recurrent nevus phenomenon were compared with 34 cases of melanoma with regression. Regression was defined as (1) Early: dense lymphoid infiltrates replacing nests of melanocytes, (2) Intermediate: absence/ loss of tumor with replacement by mix of lymphocytes and melanophages and early fibrosis, and (3) Late: tumor absence with extensive fibrosis and telangiectasia, melanophages and epidermal effacement. Four broad histologic patterns of recurrent nevus were identified and classified into type 1: junctional melanocytic hyperplasia with effacement of the retiform epidermis and associated dermal scar, type 2: compound melanocytic proliferation with effacement of the retiform epidermis and associated dermal scar, type 3: junctional melanocytic hyperplasia with retention of the retiform epidermis, and type 4: compound melanocytic hyperplasia with retention of the retiform epidermis and scar. Melanomas with early and intermediate regression were recognizable due to the presence of residual melanoma. Melanomas with late regression had overlapping features of type 1 and 2 recurrent nevi. Type 3 recurrent nevi resembled primary melanoma with scar/fibrosis. Histologically, the vast majority of recurrent nevi are readily identifiable; however, partial biopsies or cases without prior knowledge of the original biopsy may lead to misdiagnosis. This is especially true in recurrent nevus and regression in malignant melanoma, where these two lesions share overlapping histologic features. Correlation with the clinical findings and prior biopsy will avoid these pitfalls.