Melanoma in Dogs: A Potentially Malignant Form of Canine Cancer

Canine melanoma is a form of cancer. These cancers are termed melanomas because the tumors are derived from melanocytes, a specific type of cell. Unlike melanomas seen in people, those seen in dogs are not related to increased exposure to sunlight.

Locations of Melanomas in Dogs

Canine melanomas can occur in many different locations. The most commonly seen are in the mouth, on the skin, on the digits (feet) and in the eyes of the dog.

  • Oral melanomas (those within the mouth) can occur on the lips, the gums or the tongue. Melanoma is the most common form of cancer in the mouth of the dog.
  • Ocular melanomas (those within the eye) are the most common type of tumor to affect the eye of the dog.
  • Some skin melanomas are raised and do not deeply invade the tissue, while other invade the underlying tissue to a much greater extent.
  • Melanomas can mimic many other types of cancer and a biopsy is necessary to accurately diagnose a melanoma.

Malignant Melanoma Versus Benign Melanoma in the Dog

There are many factors that affect the behavior of a melanoma in a dog. These factors are used to determine if the tumor is likely to behave in a benign or a malignant fashion.

  • The location of the tumor plays a role in determining whether the melanoma is likely to be benign or malignant. Melanomas of the skin that are in a haired region are often, but not always, benign. Melanomas in the mouth are likely to be malignant. Melanomas of the digit are more likely to be malignant as well.
  • Canine melanomas are graded based on size and spread to local lymph nodes. A stage I melanoma is less than 2 cm and does not invade the local lymph nodes. A stage II melanoma is between 2-4 cm and does not invade the local lymph nodes. A stage III melanoma is between 2-4 cm and does invade local lymph nodes. A stage IV melanoma is any size tumor that has spread to other parts of the body. Stage I melanomas are more likely to have a curative effect with surgical excision whereas melanomas of stage II and higher are more likely to act in an aggressive manner.
  • Histological (microscopic) characteristics of the tumor are also often predictive of the behavior of the cancer. Histologic characteristics that help determine whether the melanoma is likely to act in a malignant and/or aggressive fashion include the mitotic index, the degree of invasiveness and the amount of differentiation in the cells within the tumor.

Diagnosis of Canine Melanoma

Initially, fine needle aspirates (FNA) are often used to determine whether a suspect mass may be a melanoma or another form of cancer. Fine needle aspirates involve using a small needle to retrieve cells from a mass and examining the cells under a microscope.

If a fine needle aspirate indicates the potential for a melanoma, surgical removal of the mass is usually planned, if possible. A biopsy allows examination of the entire mass under a microscope and is necessary to accurately diagnose the tumor type. Fine needle aspirates are a good screening test but do not remove the need for a biopsy.

Local lymph nodes should be sampled at the time of surgical removal of the mass in order to stage the cancer. In some cases, radiographs (x-rays) of the lungs may be recommended to check for metastasis as well.

Treatment of Canine Melanoma

Treatment of a melanoma in a dog will depend on the location, the stage of the tumor and the ability to surgically remove the entire growth with wide margins that are free of tumor cells.

Surgical removal is always the initial recommended treatment, if possible. It is important for the veterinarian to plan to allow a wide surgical margin. If the tumor can be removed entirely with a wide margin that is free of tumor cells, no additional treatment may be necessary, assuming that the cancer has not spread to the lymph nodes or other areas.

If the tumor cannot be removed or is removed with “dirty margins” (the area surrounding the tumor still has tumor cells that extend to or near the edge of the surgical incision), then further treatment is required and may include chemotherapy, radiation therapy or immunotherapy.

  • Radiation therapy and immunotherapy have shown greater promise than chemotherapy.
  • Immunotherapy includes a melanoma vaccine that may be effective in controlling some cases of melanoma.

Canine melanoma is a form of cancer seen in dogs. Some cases of melanoma can be extremely malignant and factors such as location, grading and histological characteristics of the tumor need to be considered to determine the severity of a given melanoma.