Skin and Toe Cancer (Melanocytic) in Dogs

Melanocytic tumors are tumors that arise from the pigment cells in the skin. In humans, melanoma is always malignant, but dogs can also have a benign version called melanocytoma. Because they are based in the pigment cells, melanocytic tumors usually have a dark, hyper-pigmented appearance, which can help to distinguish them from squamous cell carcinomas. Occasionally, the pigment may be lacking and the two types of tumors can look very similar. Benign tumors are more common on the head and forelimbs. They may appear as flat discolored areas of skin or as a raised mass. Malignant melanoma tumors appear most commonly around the mouth or in the nail bed, but they may also be found in the stomach or scrotum. They are the second most common form of toe cancer, after squamous cell carcinoma, accounting for about 16% of all digital tumors. Malignant melanoma metastasizes quickly, especially from the nail bed region where they have quick access to internal bone and nerve cells. Metastasis to the local lymph nodes and eventually the lungs is common and can happen quite quickly. Oral melanoma is believed to be less aggressive. Benign and malignant tumors are both removed surgically. Survival time for dogs with malignant tumors can vary quite a bit based on the rate of cancer growth and the degree to which the dog’s immune system is able to fight the spread of the abnormal proteins. Dogs that are diagnosed in the early stages will have a higher chance of survival.

Melanocytic tumors originate in the pigment cells in the skin. In dogs, melanocytic tumors can be either malignant or benign. Benign tumors are called melanocytoma, while cancerous tumors are called melanoma. Melanoma is an aggressive form of cancer that can metastasize quickly to the lungs and lymph nodes and may often end up being fatal.

These are some of the symptoms you might see if your dog has a melanocytic tumor.

• Patches of discolored, hyper-pigmented areas of skin
• Tumors around the mouth
• Hyper-pigmented tumor in the toe digits or elsewhere on the body
• Limping or lameness
• Swollen lymph nodes
• Cough or difficulty breathing (from metastasis to the lungs)

Types

Benign melanocytoma tumor

• Non-cancerous tumors
• This type of tumor is more common in dogs
• Miniature Schnauzers, Standard Schnauzers, Doberman Pinschers, Golden Retrievers, Irish Setters, Vizlas

Malignant melanoma 

• Cancerous tumor that will metastasize and spread to other areas of the body, especially the lymph nodes and lungs
• Miniature Schnauzers, Standards Schnauzers, Scottish Terriers

The cause of any type of cancer is difficult to trace. Genetic and environmental factors are both believed to play a part. In contrast to humans, sun-damage is rarely associated with melanoma in dogs. There is a higher incidence in some breeds, and dark coated dogs are more at risk. Benign melanocytoma are more common in middle-aged to older dogs; the chance of malignancy increases with age. Both types of melanocytic tumors are found more commonly in males, but they could occur in any dog.

The veterinarian will examine your dog’s tumor. The coloring, shape, and location may suggest that it is melanocytic, but X-rays and biopsies will be needed for a definitive diagnosis. Biopsies can be performed with local anesthetic. The veterinarian will need to insert a needle into your dog’s tumor and extract a sample. For nail bed tumors, a deep biopsy can tell if the cancer has spread into the bone. Microscopic examination of the sample with cytology will identify the type of tumor as well as whether it is malignant or benign.

The veterinarian will also examine the surrounding lymph nodes and take X-rays of the lungs to check for metastasis. Blood work and urinalysis will evaluate your dog’s overall health. Dogs with a stronger immune system will have more resistance to metastasis. The veterinarian will need to know your dog’s breed, age, and family history to help make an accurate diagnosis.

Surgical excision is the best treatment for melanocytic tumors. Distinctive tumors are much easier to remove cleanly than those which infiltrate the surrounding cells, but the veterinarian or surgeon will try to make a wide enough margin to ensure that all the cancerous cells are removed. If the tumor is located in the nail bed, the entire digit will need to be removed surgically (unless the tumor is benign). Your dog’s foot will be bandaged for about a week after surgery and reduced activity will be needed for at least three weeks to give the area more time to heal. Post-surgery care could vary depending on the location of the tumor. A check-up may be necessary to remove any stitches and evaluate for signs of metastasis if the tumor was malignant.

Chemotherapy hasn’t proven to be very successful with melanoma, so it’s unlikely that this treatment would be ordered. A vaccine for canine melanoma was put on the market in 2010. This uses a human protein to activate the dog’s immune system and help it fight the metastasis process that takes place with melanoma. The success rate of this treatment hasn’t been well documented. With very aggressive cancers it is believed that metastasis takes place too fast for the activation of the immune system to have an effect, and 15% of the dogs treated still died within 6 months. However, the side effects of the vaccine are minimal, and your veterinarian might recommend it as part of a post-surgery treatment.

Benign melanocytic tumors have a very good rate of recovery. The tumor is usually removed cleanly and dogs don’t have any further recurrence. Recovery from malignant tumors will depend on the stage at which the cancer was discovered as well as the aggressiveness of the tumor. Median survival times vary from less than one year with the cancer in stage II or III, to 511 days in stage I. Oral tumors seem to be less aggressive than those which are located on the nail bed, but this can vary from tumor to tumor and dog to dog. The veterinarian will evaluate your dog’s chances upon diagnosis. The condition may be manageable for several years at most, but malignant melanoma is an aggressive cancer, so it’s unlikely your dog will make a complete recovery.