By Gregory A. Hosler, M.D., Ph.D.
Nevoid melanoma is a diagnostic challenge in Dermatopathology. It shares many clinical and histopathologic features of benign nevi, yet it assumes an aggressive clinical course. We present two cases, one of a nevoid melanoma and the other of an atypical benign nevus, to illustrate the problem.
One patient, a 57-year-old man, presented to his dermatologist with a raised 1.0 cm well-circumscribed pigmented papule on his trunk. An outside facility diagnosed the lesion as an intradermal nevus. Unfortunately, three years later he developed a matted axillary mass, diagnosed as metastatic melanoma. One of the following images is from the original truncal pigmented lesion.
A second patient, a 36-year-old woman, presented with a slightly raised pigmented lesion on her thigh. It was mildly asymmetric and had mild pigment heterogeneity, but was otherwise unremarkable. One of the following images is from this thigh lesion. There were no sequelae.
Answer: Case A represents the nevoid melanoma and Case B represents a compound nevus.
Figure 3. Case A – Nevoid Melanoma Figure 4. Case B – Atypical Benign Nevus
Nevoid melanoma is an unusual variant of malignant melanoma, accounting for less than 5% of all melanomas when using strict definitions. Since Schmoeckel’s original description in 1985, several published series have reiterated the malignant behavior of nevoid melanoma, reporting 14%-70% of patients with ultimate metastatic disease and/or death. This entity is recognized by few, but feared by many, especially the purely “low power” diagnostician. To paraphrase Dr. Philip H. McKee in his Elson B. Helwig Memorial Lecture at a recent American Society of Dermatopathology meeting, “Nevoid melanoma is a nevus you called a nevus…but wish you hadn’t.”
Nevoid melanoma, as the name implies, closely recapitulates a benign nevus, both clinically and histologically. The lesion is often dome-shaped or polypoid with sharp lateral circumscription (Figure 1). An irregular, infiltrative base is helpful for diagnosis, but is not always present. The proliferation may abut the overlying epidermis but intraepidermal pagetoid growth is conspicuously absent. Another benign histologic feature is the apparent maturation, i.e. the melanocytes become smaller, and nests break up into small cords and single cells, as they descend into the dermis.
With inspection at higher power, the malignant nature of the lesion is uncovered. Although relatively bland compared to other melanoma subtypes, the melanocytes in nevoid melanoma show more atypia than in banal nevi, including mild nuclear enlargement, irregular nuclear borders, and conspicuous nucleoli. Mitotic activity is often the first noticed evidence for deviant behavior, and prompts closer inspection of the lesion. In our nevoid melanoma case, the cells show cytologic atypia in the papillary dermis with several mitoses (Figure 3a). As the lesion descends from the papillary dermis to the reticular dermis (Figure 3b), there is a hint of maturation. However, the melanocytes are more haphazardly arrayed and show more pleomorphism when compared to their benign nevic counterparts (compare Figures 3 and 4).
Immunohistochemical analysis may aid in the diagnosis of nevoid melanoma when the features are subtle. HMB-45 stains melanocytes in a patchy or diffuse pattern within the dermis in nevoid melanoma while it stratifies benign nevi (Figures 3c and 4c). Ki67 will highlight cycling cells and would stain numerous dermal melanocytes in nevoid melanoma, while only the rare superficial melanocyte would stain in benign nevi (Figures 3d and 4d). Consultation is recommended for these difficult lesions.
1. Schmoeckel C, Castro CE, Braun-Falco O. Nevoid malignant melanoma. Arch Derm Res 1985;277:362-9.
2. Magro CM, Crowson AN, Mihm MC. Unusual variants of malignant melanoma. Mod Pathol 2006;19;S41-70.
3. McKee PH. Melanoma variants – wolves in sheep’s clothing. Elson B. Helwig Memorial Lecture, The American Society of Dermatopathology 2003.