Recurrent Nevus--Sometimes a Difficult Distinction From Melanoma With Regression Recurrent Nevus Phenomenon: A Clinicopathologic Study of 357 Cases and Histologic Comparison With Melanoma With Regression

Background: Recurrent nevus is a melanocytic lesion developing at the site of the removal of a previous benign nevus. While most cases are easily diagnosed with traditional histopathologic criteria, some cases have considerable histopathologic overlap with cases of regressed malignant melanoma. Objective: Clinical and histopathologic findings of recurrent nevi are compared to melanoma with regression. Design: Retrospective study. Participants: 357 cases of recurrent nevi were compared to 34 cases of melanoma with regression. Methods: Only cases with the complete medical history and the original and recurrent lesion available for review were included. Results: The average age of the patients was 32 years, and 72% were female. The most common location was the back with an average time to recurrence of 8 months. Ordinary nevi comprised 64% of cases, dysplastic nevi comprised 27% of cases, and congenital nevi comprised 6% of cases. Four broad categories of recurrent nevi (Types 1 through 4) were defined, varying with the degree of epidermal retiform effacement and location of melanocytes. Melanocytic atypia was identified in 26% of cases. The retiform epidermal pattern was maintained in 15% of recurrent nevi. Histologic overlap with early regressed melanomas was identified in cases with a retiform epidermis and confluent melanocytic growth pattern, pagetoid spread, and cytologic atypia. Conclusions: While the majority of cases of recurrent nevi could be accurately diagnosed, in cases of partial biopsies in which the scar extends to the edge of the biopsy or if there is no prior knowledge of a previous biopsy, a distinction between a regressed melanoma and recurrent nevus was difficult, especially if the retiform epidermal changes are maintained. Melanomas with late regression resembled Types 1 and 2 recurrent nevi, while melanomas with scarring and preservation of the epidermal retiform pattern resembled Type 3 recurrent nevi. Reviewer's Comments: This interesting and comprehensive study investigates a very difficult histopathologic problem with excellent clinical-pathologic correlation. If the epidermal retiform pattern is maintained (observed in 15% of recurrent nevi), an absolute distinction from a primary melanoma with fibrosis may be difficult, especially if there is no prior clinical history of a previous melanocytic nevus of the scar extending to the edges of the biopsy. A somewhat unexpected finding was dysplastic nevi comprising 27% of the recurrent nevi. The authors state that with careful correlation with the original biopsy, a confident diagnosis can be reached. Many dermatopathologists grade the degree of atypia with dysplastic nevi, which was not mentioned in this current study. It would be instructive to determine whether the degree of cytologic and/or architectural atypia influences the frequency of recurrence. Finally, although possibly limited by the amount of residual melanocytes, it would be interesting to determine whether proliferation markers such as Ki-67 would be helpful to distinguish between recurrent nevi and regressed melanomas. (Reviewer-Paul K. Shitabata, MD).