Skin melanomas occur mostly in the face (especially the eyelids), trunk and limbs.

Melanomas may also appear on the sole of the feet, nailbed, eye, oral cavity, the gastro-intestinal system and transitions between mucosae and the skin (such as the lip and anus). Depending on the location melanomas have a less or more malignant behaviour.

Behavior

Melanomas that occur in hairy places are usually not malignant, in contrast to those on the transition between skin and mucous membrane. Benign melanomas of the skin are well defined, highly pigmented, small and mobile. Malignant skin melanomas appear to be larger (> 2.5 cm), grow quickly and can ulcerate.

Melanomas that occur on hairy skin make up 5-7% of all skin tumors in dogs. These tumors arise from pigment cells in the epidermis, dermis, structures closely associated with the skin, but mainly arise from the hair follicles.

Prevalence

This tumor type affects mainly dogs between 5-11 years of age (average age: 9 years).

Breed predisposition

The following breeds were reported to be at higher risk to develop this tumour type: i.a. Airedale Terrier, Boston Terrier, Cocker Spaniel, Miniature Schnauzer, Doberman Pinscher, Scottish Terrier, Magyar Vizsla.

Symptoms

Skin melanomas vary in their appearance. Generally, a melanoma forms one solid lump, and the colour of the melanoma can take on a variety of colours, ranging from black to shades of brown, gray, and red.

Generally, a thorough physical examination is performed, possibly followed by a blood or urine test.

The mass itself

The diagnosis can sometimes be difficult because not all melanomas contain pigment (up to 30% of melanoma’s do not contain melanin). In case of melanomas, a cytologic examination can suffice to make a diagnosis. When the veterinarian punctures the tissue mass and observes round cell tumour cells containing pigment, this usually is enough to diagnose the tissue mass as a melanoma. If this is not the case, a biopsy should be taken for further examination.

Via immunophenotyping (via PNL2/tyrosinase/Melan A/S100-staining) typical proteins for pigment cells can be coloured. If these proteins can be found on locations where they normally do not occur, this is indicative of cancerous pigment cells (melanoma).

Estimation of metastases

Fine needle aspiration of the regional or draining lymph nodes is recommended. Draining lymph nodes may be identified by lymph node mapping in complex locations. For this tumour type, the size of the local lymph nodes does not accurately indicate whether they contain metastases.

Because of the diverse behaviour of melanomas, it can be indicated to take X-rays of the bones and lungs, as well as ultrasound of the abdomen to detect metastases on other locations than lymph nodes.

Surgery

Surgery is the first treatment for treating melanomas on hairy skin. Long-term responses for fractional radiotherapy have also been described.

The prognosis is good if the melanoma is benign, but reserved if the melanoma shows malignant features (metastatic rate varies from 30-75%).

Several characteristics contribute to the prognosis, such as the breed of the dog, the tumour size, histopathological markers (mitotic index, Ki-67 content, staining for the canine melanoma antigen with monoclonal antibody MAB IBF9) and penetration into the surrounding tissue.

Breed

Depending on the dog's breed, melanoma may behave more favourably / malignantly. For example, it was found that 75% of the melanomas in Dobermanns and Miniature Schnauzers are benign, while 85% of the melanomas in Miniature Poodles are malignant.

Tumour size

Malignant skin melanomas of <4 cm show significantly better survival time than tumours ≥4 cm (12 months vs 4 months) and after 2 years, 46% vs 0% of these dogs are still alive.

Histopathological markers

The proliferation marker Ki-67, which indicates how fast the cancer cells divide, has a higher predictive value for the biological behaviour of skin melanomas than classical histology.A higher index for Ki-67 indicates a more aggressive behaviour of the tumour.