Heavily Pigmented Melanocytic lesions

Heavily Pigmented Melanocytic Lesions: A Spectrum of Lesions with a Different Prognosis

Are these lesions indeed a horse of a different color when it comes to outcomes?

Older gray horses frequently develop heavily pigmented lesions around the anus, lips, and eyelids. A human variant of malignant melanoma resembling these lesions is called "animal-type" or "equine-type melanoma." A small number of these lesions have been reported, with some authors indicating that the course is indolent compared with conventional melanoma. These authors examined the prognostic significance of the clinical and pathologic features of such lesions.
They identified 22 patients with a diagnosis of animal/equine/pigmented synthesizing melanoma or pigmented epithelioid melanocytomas from one institution. Diagnosis was unequivocal in 14 cases. Median patient age was 35. Dysplastic nevi were found in 41%; only 9% had blue nevi without evidence of Carney complex. Ulceration was unusual; 68% of the lesions were raised, and more than half were larger than 6 mm. The head, neck, trunk, and upper extremities were the most frequent sites. The median lesion depth was 2.22 mm; more than half were in the reticular dermis. Mitosis was seen in all lesions; more than one mitotic cell was found in 41%. Sentinel lymph node (SLN) biopsy samples were taken from 17 patients; eight were positive and involved more than 1% of the lymph node. These patients underwent complete lymph node excision, and only one had an additional positive nonsentinel lymph node. Lesion depth was not significantly associated with positivity, but the positive SLN patients were older. No histologic features predicted SLN involvement, distant metastasis, or death.
Comment: The differential diagnosis of a heavily pigmented melanocytic lesion is epithelioid blue nevus and animal-type melanoma. Histological overlap and wildly ranging behavior among these lesions prompts a suggestion that the term "pigmented epithelioid melanocytoma" be applied to the full spectrum of these tumors. Unlike in conventional melanoma, younger patients with heavily pigmented lesions have a lower risk for positive SLN, but with either lesion type, the low likelihood of distant metastasis confers a good prognosis for young patients. Whether dermatopathologists call heavily pigmented lesions in young patients animal-type melanoma or epithelioid melanocytomas, the evidence predicts an indolent course.
— Angelica Maria Selim, MD
Published in Journal Watch Dermatology January 22, 2010

Citation(s):

Ludgate MW et al. Animal-type melanoma: A clinical and histopathological study of 22 cases from a single institution. Br J Dermatol 2010 Jan; 162:129.