Persistent/Recurrent Melanocytic Nevi, Traumatized Nevi, and Nevi Changing Under Therapy

Clinical Findings.

Persistent nevi are benign neoplasms that appear at the site of prior surgery, usually presenting as “recurrent” pigmentation. The term persistent or regenerating nevus is favored over recurrent nevus or pseudomelanoma to avoid confusion with recurrent melanoma.

The incidence of persistent nevi varies from 0.3% to 27%, depending on the initial procedure type, being most common after a shave biopsy. Persistent nevi typically appear in adolescents or young adults, especially females, and on the back. The nevi reappear typically within a few weeks to months, most commonly within the first 8 months, after the initial surgery. Dysplastic nevi may reappear later, based on some studies, after approximately 2 years. Clinically, a persistent nevus manifests as repigmentation in the area of the scar of a previously biopsied or excised melanocytic nevus ( Fig. 8.1 ). Typically, pigment is visible centrally and is confined to the area within the scar. A small macule is present in the center of a shave biopsy scar but can be off center in a larger excision scar. The color of the macule is often darker than that of the original lesion. The pigment is usually contiguous. Expansion of the pigmentation develops centrifugally. A subset of persistent nevi may display variability of color, jagged borders, and poor circumscription. Dermoscopically, these lesions may exhibit segmental radial lines, as also observed in melanoma and basal cell carcinoma. After an initial period of growth, persistent nevi typically become stable in appearance. Over time, the pigment may fade.

Histopathologic Findings.

Classically, a persistent nevus shows three histologic zones (a trizonal pattern) ( Figs. 8.1–8.3 ). There is a proliferation of melanocytes within the epidermis, underlying horizontal fibrosis consistent with a scar, and in some cases a residual proliferation of melanocytes below the scar within the dermis. Melanocytes may also be found within the dermal scar (see Fig. 8.1 ) or adjacent to it ( Fig. 8.2 ).

The proliferation of melanocytes within the epidermis may mimic melanoma in situ. It may consist of predominantly singly disposed melanocytes of variable confluence (see Fig. 8.3 ) with or without occasional nests. If nests are present, one may find elongated and horizontal nests of various sizes and shapes displaying a complex architecture ( Fig. 8.4 ). Suprabasilar ascent of melanocytes may be present ( Fig. 8.5 ), although florid pagetoid scatter is uncommon. Importantly, pagetoid growth should be limited to the epidermis directly above the scar. In addition to the epidermis, the melanocytes are often present along the eccrine and follicular epithelia ( Fig. 8.6 ). The proliferation is initially limited to the epidermis, but melanocytes can also be found in the papillary dermis, separated from the original nevus by fibrosis.

The melanocytes within the epidermis are often enlarged, with epithelioid morphology including abundant cytoplasm, vesicular chromatin, and prominent nucleoli. Occasionally, however, melanocytes are elongated or dendritic, with hyperchromasia. Although cytologic atypia is present, the degree of atypia is less than in melanoma.

The melanocytes within papillary dermis are arranged in irregular nests and often plump with abundant pale cytoplasm (see Fig. 8.6 ). Maturation is present, although not as well developed as in common nevi. A gradient of HMB45 expression is typically seen, although sometimes HMB45 expression is retained in the deeper aspects of the lesion. Therefore HMB45 is not considered reliable as a diagnostic marker distinguishing a persistent nevus from melanoma. An occasional mitosis may be seen.

Corresponding to the darker pigmentation observed clinically, pigmentation is typically prominent within the basal layer of the epidermis and within melanocytes and/or melanophages. The underlying dermis is characterized by scar fibrosis parallel to the epidermis, with often prominent fibroblasts and vertically oriented vessels. The epidermis may show attenuation of rete ridges, acanthosis, and hyperkeratosis. By contrast, regressing melanoma typically shows epidermal thinning. Below the scar, a nevus remnant may be found, often around an adnexal structure and discontinuous with the scar.