This form of melanoma can be found on the toe or sole of the foot.

Melanomas may also appear on the haired skin, eye and 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.

Behaviour

In terms of malignant behaviour, this tumour type is in between oral melanomas and skin melanomas. Metastases to local lymph nodes and lungs occur in 58-71% of cases, 5% of the melanomas on the toes lead to bone damage to the underlying bone.

Appearance

These melanomas can grow in the nail bed or on the sole of the foot. It can take different forms: a small growth, a poorly demarcated swelling of the involved toe, an ulcerated growth.

Symptoms

Swelling of the toes is often one of the first symptoms to notice. Some dogs will lose a nail or limp on that paw. In general, these tumours are black.

A fine needle aspirate from the mass as well as from the local lymph nodes should be performed.

X-rays of the affected toe should be taken to estimate whether the melanoma has affected the bone. X-rays of the lungs are also important to estimate the presence of metastases.

Surgery

Surgery is the most appropriate treatment for local melanomas. If the melanoma is on the nail bed, it may be necessary to amputate the entire toe due to bone damage. Footpad melanoma can lead to full limb amputation, as full weight may be resting on the affected footpad and removal of the footpad will result in a loss of use of that leg.

The functional outcome of single digit amputation is excellent and partial foot amputations (requiring excision of more than one digit) are also tolerated very well and result in a good functional outcome.

For tumours that are not amenable to wide resection or for which resection results in incomplete histologic margins, the combination of surgery with radiotherapy or other adjuvant therapies should be considered.

Radiotherapy

Radiotherapy is an efficient alternative for tumours that cannot be (completely) surgically removed. Melanomas generally respond well to radiotherapy administered in high doses (1x / week, four times). A possible side effect of this treatment is loss of nails or sloughing of the sole of the foot. The nails and soles of the feet will regrow and will not cause long-term problems.

Chemotherapy

Chemotherapy does not seem to have a role in the management of dogs with malignant melanoma. Carboplatin chemotherapy may be useful to delay or prevent the spread of metastases, although the majority of responses are short lived. So far, there are no published studies yet that show that chemotherapy leads to an extended survival time. The standard chemotherapy protocol is a combination of carboplatin with surgery and / or radiotherapy. Carboplatin is administered intravenously every 3 weeks for a total of 4 treatments. The side effects are minimal.

Melanoma vaccine

Melanomas express the pigmentation enzyme tyrosinase and its expression increases as the tumour grows. Therefore, treatments that target tyrosinase will also allow targeting of melanomas. Oncept® is a therapeutic cancer vaccine that produces a foreign (human) tyrosinase. This vaccine teaches the dog's immune system to better recognize melanoma cells and attack them. This vaccine could be interesting as an additional treatment in a minimal residual disease setting (when as little cancer tissue as possible is present (e.g. after surgery)) and this approach is still under investigation. Vaccination can be indicated for malignant tumours and cases where the lymph nodes are affected.

About 50% of the melanomas of the nail bed and the soles of the feet spread to the other parts of the body. If the mitotic index is <3, the tumour is more likely to behave benignly and there is a lower risk of metastasis. The treatment of a local relapse has a less good prognosis than the treatment of the original tumour. The chance of local relapse is 14-30%. The mean survival time is 12 months, statistically 1 year after diagnosis an average of 42-66% of the dogs are alive, 2 years after diagnosis an average of 12-22%.