Like the skin covering the rest of the body, the skin of the breast gives rise to commonplace nevi. Those arising in the mammary skin do not differ from nevi of other locations in their general histologic classification (junctional, compound, and intradermal types) or their division into well-recognized variants. Rongioletti et al.¹ compared 101 nevi from mammary skin with 97 nevi from other sites, mainly the back and extremities, excluding those of the palms of the hands and soles of the feet. There was no significant difference in the frequencies of different types. The four most common types of mammary cutaneous nevi (Clark nevi, compound nevi, lentiginous compound nevi, and congenital nevi) accounted for two-thirds of the lesions. Eighty percent of mammary cutaneous nevi were from women. The age distribution of patients with mammary cutaneous nevi (range, 8 to 81 years; mean, 31.5 6 12.7 years) was not significantly different from patients with nevi at other sites. In this study, nevi of the skin of the breast had atypical features significantly more often than nevi from other sites. For this reason, dermatopathologists regard such nevi as members of a group termed “nevi of special sites,” a category that also includes nevi of the genital, acral, and flexural locations.^2,3 Special site nevi may show architectural features overlapping with melanoma; consequently, pathologists can misinterpret these nevi as dysplastic nevi or even melanomas. Special site nevi of the breast more frequently display intraepidermal melanocytes above the basal layer, melanocytic atypia, and dermal fibrosis than do nevi in other locations.¹ The presence of these features does not seem to foretell any unusual biologic behavior. Pathologists should take note of these peculiarities so as to avoid raising undue alarm.

Cutaneous melanoma represents the most common malignant melanocytic lesion involving the breast. It occurs more often in men than in women.^4,5,6,7 The ages of the patients range from 16 to 86 years. The average age of women is the mid-30s and of men the 40s.⁶ Any region may be affected, but the upper quadrants and especially the upper inner quadrant represent favored locations in women.⁸ Approximately two-thirds of melanomas arise medial to the midclavicular line or in the central region, where exposure to the sun is greatest.^6,8 The left and right breasts of women are involved with equal frequency.⁸ Origin in the nipple-areola complex (Figs. 42.1A, B) is relatively uncommon.^9,10,11,12 When the patient reports a recent change in a long-standing pigmented lesion, the presence of an antecedent nevus is suspected and is often confirmed histologically. Some patients with malignant melanoma of the nipple or areola reported a change in an existing “mole,” but in most cases a new pigmented lesion was described.^4,9,11 Origin of a malignant melanoma of the mammary skin at the site of a tattoo in the skin of the breast has been reported.¹³

There is no predilection for any subtype of malignant melanoma to arise in the skin of the breast. Superficial spreading melanomas occur most frequently, but nodular and ulcerated types have been described, also.^8,14 Melanomas of the skin of the breast demonstrate the macroscopic features seen in melanomas arising in other cutaneous sites, and the histologic characteristics of mammary cutaneous melanomas do not differ from those of other cutaneous melanomas. The diagnosis of melanoma of the skin of the breast is based on the same criteria generally employed to assess cutaneous melanocytic lesions at other sites.

Melanomas involving the mammary skin do not always represent primary tumors, for melanomas arising in extramammary sites can spread to the skin of the breast.¹⁴ Attention to the clinical history will keep one from mistaking the cutaneous metastasis of a melanoma for a primary tumor.