Melanoma: criteria for histological diagnosis and its reporting

The diagnosis of malignant melanoma requires clinical recognition of suspect lesions, biopsy, and histologic examination. Histological features which serve to distinguish malignant melanoma from their benign counterparts can be found in both the epidermis and dermis. The intraepidermal component of a common acquired nevus usually consists of more or less uniform theques of melanocytes located at or near the tips of rete ridges. Most melanomas are characterized by less orderly intraepidermal growth with areas in which melanocytes lose their nesting characteristics and are distributed more diffusely, sometimes replacing the basal keratinocytes by confluent growth and sometimes by invading upwards either as single cells or small nests into the upper reaches of the epidermis. Nested melanocytes can be found along the basal layer in malignant melanoma, but these nests are usually quite variable in size and location with respect to the tips of the rete ridges, and they are often irregularly distributed along the breadth of the lesion. The dermal component of malignant melanoma usually shows little tendency towards maturation, unlike that of benign nevi. Mitotic figures are unusual to find in the dermal component of common acquired nevi. When they are present, the possibility of melanoma should be considered. Other cytological features can also be useful in the diagnosis of malignant melanoma, particularly when there is marked cytological atypia; however, in some lesions, the cytological changes are not so pronounced and correct diagnosis depends on evaluation of growth pattern. While distinguishing between melanoma and atypical moles can be difficult, problems also arise in distinguishing melanoma from other neoplastic processes. The most common differential diagnosis includes melanoma, paget's disease, and pagetoid Bowen's disease. Desmoplastic melanoma is frequently difficult to distinguish from spindle cell squamous cell carcinoma and atypical fibroxanthoma. Histochemical and immunocytochemical stains are useful in resolving these problems. The pathology report of a melanoma should include the diagnosis, the maximum thickness of the tumor, the adequacy of the surgical margins (if the lesion has been excised), the presence or absence of ulceration, tumor regression, angiolymphatic invasion, and satellitosis. The inclusion of patients in treatment protocols may require additional information such as the host response of tumor-infiltrating lymphocytes, mitotic index, and Clark's level of invasion.