Pigment-producing melanoma (animal-type melanoma; equine type melanoma) is a rare variant that resembles melanocytic tumors occurring in horses and other animals. It is thought to be distinct entity and not related to malignant blue nevus. Most cases present as brown, blue-black, or black solid nodules in 2nd to 4th decades with no sex or site predilection. They are considered to be low-grade lesions with a better prognosis than ordinary melanomas of comparable thickness. The tumors show a dense infiltrate of melanocytes with copious amounts of melanin in the papillary and reticular dermis. A junctional component may be present. The tumor cells are epithelioid or spindle shaped and may be arranged in fascicles or nodules. The melanin granules are fine or coarse and obscure the nuclear details. Morphologic features can be appreciated better in bleached sections. The tumor cells are uniform, display little or no atypia, and mitotic activity is low. This allows their distinction from malignant blue nevus which shows greater cytologic atypia. The prognosis of pigment-producing melanoma appears to be better than stage-matched classic melanoma; however, it can produce nodal and systemic metastases with resultant death in rare instances.
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