Animal-type melanoma

Author: Dr Susannah Fraser, MBChB, FRCP (Edin), Consultant Dermatologist, NHS Fife, Scotland. Chief Editor: Dr Amanda Oakley, Dermatologist, Hamilton, New Zealand, August 2015.

What is animal-type melanoma?

Melanoma is a skin cancer that arises from pigment cells (melanocytes).

Animal-type melanoma is a very rare form of melanoma with dark brown/black appearance. The diagnosis is made from its histological appearance on biopsy. The name "animal-type" arises from the close resemblance to the heavily pigmented melanocytic tumours found in grey horses. It is also known as equine-type melanoma, pigment synthesising melanoma, and pigmented epithelioid melanocytoma.

Animal-type melanoma was first described by Darier in 1925.

Who gets animal-type melanoma?

Animal-type melanoma may occur at any age including during childhood. Some studies have found it to be most common in young adults, but another reported it to be more common in middle-aged and older adults.

Males and females appear to be equally affected.

Patients with animal-type melanoma are no more likely than the general population to have well-known risk factors for common types of melanoma (such as fair skin, family history and sun damage).

What does animal-type melanoma look like?

Animal-type melanoma can develop on any body site. It arises from normal skin (de novo), rather than from a pre-existing naevus (mole).

Animal-type melanoma usually presents as a dark brown/back papule or nodule. By the time of diagnosis, it is likely to have been present for a year or longer.

Typically, animal-type melanoma has the ABCD melanoma criteria:

The clinical differential diagnosis includes blue naevus variants:

What tests should be done?

After clinical assessment has suggested a skin lesion to be suspicious of melanoma, the lesion should be examined using a dermatoscope. The dermatoscopic appearance of animal-type melanoma may show a structureless blue pattern, irregular whitish structures, and irregular, large blood vessels.

After the lesion is removed by excision biopsy, a histology report of animal-type melanoma may report:

The pathologist may find it difficult to make a definite diagnosis of melanoma, as the features of animal-type melanoma can resemble those of blue naevi. Thus, there are equivocal and unequivocal cases.

What is the treatment for animal-type melanoma?

Confirmed animal-type melanoma is widely excised, with a clinical margin depending on Breslow thickness.

Many centres offer sentinel lymph node biopsy if the melanoma has a Breslow thickness of 1 mm or over, or invasive tumours that are less than 1 mm in thickness but having ulceration or a mitotic rate of 1 or more.

Staging

Melanoma staging means finding out if the melanoma has spread from its original site in the skin. Most melanoma specialists refer to the American Joint Committee on Cancer (AJCC) cutaneous melanoma staging guidelines (2009). In essence, the stages are:

StageCharacteristics
Stage 0 In situ melanoma
Stage 1 Thin melanoma <2 mm in thickness
Stage 2 Thick melanoma > 2 mm in thickness
Stage 3 Melanoma spread to involve local lymph nodes
Stage 4 Distant metastases have been detected

What happens at follow-up?

The main purpose of follow-up is to detect recurrences early but it also offers an opportunity to diagnose a new primary melanoma at the first possible opportunity. A second invasive melanoma occurs in 5–10% patients with melanoma; an unrelated melanoma in situ affects in more than 20% of melanoma patients.

The Australian and New Zealand Guidelines for the Management of Melanoma (2008) make the following recommendations for follow-up for patients with invasive melanoma.

The follow-up appointments may be undertaken by the patient's general practitioner or specialist or they may be shared.

Follow-up appointments may include:

In those with more advanced primary disease, follow-up may include:

Tests are not typically worthwhile for stage 1/2 melanoma patients unless there are signs or symptoms of disease recurrence or metastasis. And no tests are thought necessary for healthy patients who have remained well for 5 years or longer after removal of their melanoma, whatever stage.

What is the outlook for patients with animal-type melanoma?

As animal-type melanoma is rare, there is less information about prognosis available compared to other types of melanoma. It is thought to have a better prognosis than superficial spreading malignant melanoma of a similar Breslow thickness.

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