Melanoma arising in exophytic melanocytic nevus (Unna's nevus). A clinico-pathological study

Background. Melanocytic nevi are considered as precursors of malignant melanoma (MM). However, no data concerning the association between MM and Unna's nevus have been reported yet. Objective. This study was undertaken with the following aims: 1) to determine the prevalence of MM in association with Unna's nevus; 2) to define the clinical and histopathologic features of this variety of MM; 3) to establish whether Unna's nevus associated MM is a biologically distinct entity. Methods. Histological specimens from 898 MM were reviewed. Clinical data and follow-up were obtained. Result. We identified 25 patients who had MM in association with pre-existing Unna's nevus. In most cases the clinical diagnosis was MM, but in three cases the lesions had the clinical appearance of Unna's nevus. The most common site for these tumors was the trunk. Depending upon the anatomical relationship between MM and residual nevus three groups of lesions have been recognized. In most lesions the tumor thickness was high. Prognosis was severe. No significant differences in prognosis were observed between Unna's associated MM and other MM, if groups of the same Breslow thickness were compared. Conclusions. MM can rarely occur in association with Unna's nevus. Clinical changes of Unna's nevus may be not only due to inflammation or trauma but also to malignant transformation.