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 ABCDE criteria:
- Border irregularity
- Colour variation
- Diameter more than 6 mm.
The clinical differential diagnosis includes blue naevus variants:
- Deep penetrating naevus
- Cellular blue naevus
- Malignant blue naevus
- Epithelioid blue naevus, which can be associated with the Carney complex, a familial association of skin tumours with cardiac myxoma and psammomatous melanotic schwannoma. See myxoma syndromes.
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:
- Heavily pigmented compound or dermal melanocytic tumour
- Epithelioid and spindled melanocytes.
- Bland or malignant cytological appearance
- Usually, low mitotic activity and infrequent ulceration,
- No features to suggest regression.
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.
- In a review of 22 cases, Ludgate et al found patients with equivocal animal-type melanoma were younger and sentinel lymph node biopsy tended to be negative, compared to those with unequivocal malignant animal-type melanoma.
- Antony et al reported that although animal-type melanoma has a tendency for regional lymphatic spread, it is very unlikely to lead to distant metastases and death.
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:
|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.
- Self skin examination
- Regular routine skin checks by patient's preferred health professional
- Follow-up intervals are preferably six-monthly for five years for patients with stage 1 disease, three-monthly or four-monthly for five years for patients with stage 2 or 3 disease, and yearly thereafter for all patients.
- Individual patient’s needs should be considered before appropriate follow-up is offered
- Provide education and support to help patient adjust to their illness
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:
- A check of the scar where the primary melanoma was removed
- A feel for the regional lymph nodes
- A general skin examination
- A full physical examination
- In those with many moles or atypical moles, baseline whole body imaging and sequential macro and dermatoscopic images of melanocytic lesions of concern (mole mapping)
In those with more advanced primary disease, follow-up may include:
- Blood tests, including LDH
- Imaging: ultrasound, X-ray, CT, MRI and/or PET scan.
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 melanoma of a similar Breslow thickness.
- It may spread to regional lymph nodes and recur locally
- Recurrences have been noted many years after presentation.
- Visceral metastases and death are extremely uncommon.
- Patients with equivocal animal-type melanoma have less risk of lymph node spread and death than those with unequivocal malignant disease