Animal melanoma: Rare disease variant appears to be more benign in humans

Key Points

Detroit - Little is known about the extremely rare variant of melanoma known as animal-type melanoma. The striking clinical presentation of this tumor can be the cause of concern and worry to the inexperienced dermatologist, and, therefore, biopsy is always warranted.

Originally termed equine melanocytic disease, this curious tumor occurs in horses and is highly lethal. However, animal-type melanoma appears to take a much more benign course in humans.

The tumor typically has a jet-black appearance and is composed of a confluent proliferation of melanocytes with striking melanin synthesis reminiscent of vertical growth-phase melanoma.

Animal-type melanomas are significantly darker than classic melanomas. Because their occurrence is very rare, it is difficult to establish generalizations about the tumors.

Case study

Recently, an 89-year-old black woman presented to Dr. Moiin with a black nodule located on the back with a six-month growth history. Other than the extremely dark color of the mole, the lesion was completely inconspicuous.

An excisional biopsy was done, and histopathology revealed a 4 mm deep tumor with sheets of heavily pigmented polygonal, rounded and spindled-shaped melanocytes filling the dermis and extending to the subcutaneous fat. The tumor cell cytoplasm contained fine to coarse melanin granules and pleomorphic nuclei with coarse chromatin and prominent nucleoli.

The pathological picture was diagnostic of a malignant melanoma, animal type. Sentinel lymph node biopsy did not reveal any metastasis, and the patient is alive and well two years later.

Presentation

Animal-type melanoma has no predilection for age, sex or site, and to date only 10 such tumors have been reported in the literature, with seven having occurred in light-skinned patients and three in black patients.

Dr. Moiin says he is "fortunate" enough to have seen two cases of this strange animal-type variant of melanoma.

"Both of the lesions I saw looked like large, darkly pigmented moles and had a Breslow depth of over 4 mm. Interestingly, the sentinel lymph node biopsies performed in both patients did not reveal any metastases.

"Both patients are alive and well today, two years after initial presentation, diagnosis and treatment," Dr. Moiin says.

It's possible to confuse animal-type melanoma with a blue nevus, which is also characterized by very dark pigmentation. However, blue nevi are typically bluish in pigment and look different from a typical benign blue nevus.

According to Dr. Moiin, the experienced dermatologist can easily distinguish the two lesions both macroscopically and dermatoscopically.

"Blue nevi are dark with a homogenous bluish hue, whereas the animal-type melanoma is 'wild-looking' and looks like something you have never seen before. They are much darker than blue nevus, with a blue-black hue, and are not typically homogenous," he says.

Caution

Animal-type melanoma is classically staged as malignant melanoma, but it seems to be less aggressive and have less of a malignant potential. Nevertheless, generalizations cannot be made due to the extremely few cases reported.

As a result, most experts suggest performing an excisional biopsy with or without subsequent sentinel lymph node biopsy, then taking a wait-and-see approach, as standard chemotherapy regimens are not indicated for animal-type melanoma unless there is spread to the lymph nodes.

"Dermatologists should keep a keen eye out for any unusual-looking lesions, such as darkly pigmented moles or seborrheic keratosis with a black-bluish hue. If a definitive diagnosis can't be made, an excisional biopsy should be performed with a suspect diagnosis of animal-type melanoma in mind," Dr. Moiin says.

Disclosures: Dr. Moiin reports no relevant financial interests.