Malignant melanoma


Nesta, the senior yellow Labrador, arrived for his annual visit. We hadn’t seen much of the old man this year. He had been remarkably healthy, despite his severe chronic arthritis, which his owner was managing beautifully. I began my physical exam. Ears — nice and clean. Eyes — bright and normal. Mouth. I flipped back his lip to check teeth and gums. Inside, way back toward his molars, dangled a one-and-a-half centimeter lump, like a cluster of tiny black grapes hanging from a thick stalk. “Have you noticed this before?” I asked. Here was a classic example of the benefit of annual checkups. Even this most observant owner would not have seen this growth. It was too large to remove without general anesthesia, but I was able to pinch off one of the “grapes” for biopsy, so we could have a diagnosis, and prognosis, before proceeding with further recommendations.

When your veterinarian tells you your dog has a tumor, that can be pretty scary. Before you panic, collect information. Many growths are benign. This means they are unlikely to spread or cause serious illness or death. Although it may be prudent to remove benign tumors, many can be left untreated, especially in elderly patients. To definitively determine if a tumor is benign, we need to send a section for evaluation by a pathologist. With an “excisional” biopsy, the entire mass is removed. With an “incisional” biopsy, only a small piece of the growth is cut out. There are pros and cons for each type of biopsy. In Nesta’s case, we wanted to know whether this growth was dangerous enough to warrant anesthesia and surgery in such an old man, so I nipped off a small piece in the exam room, without general anesthesia. Voilȁ! Incisional biopsy.

Melanoma. Most people are familiar with the word. Skin cells that contain the pigment melanin are called melanocytes, and an abnormal growth of melanocytes is called a melanoma. It is a common form of skin cancer in people, associated with sun exposure. Many melanomas are benign, but in dogs there is a particularly tricky situation unique to this kind of cancer. A melanoma can appear benign, based on the usual criteria used when evaluating a tumor microscopically, but canine melanomas don’t always behave the way their biopsy suggests they should. The behavior is highly dependent on the region of the body where the melanoma develops. If the growth is on haired skin, they usually behave in a benign fashion, not spreading or causing life-threatening illness. But if they are in the mouth, on the toe, especially at the nail bed, or within about a half-inch of a mucous membrane (like skin near the lips), then they behave far more malignantly than expected from what the pathologist sees under the microscope. They may be locally invasive and destructive, and spread to lung, liver, and other areas.

In many types of cancer, the size of the tumor does not necessarily correlate with how dangerous it is. For example, lipomas are fatty tumors that can grow to be huge and still be completely benign. But with oral melanomas, prognosis is closely linked to the size of the growth. Oral melanomas are therefore classified by “stage” at time of diagnosis. Stage I tumors are less than two centimeters in diameter. Stage II are two to four centimeters. Stage III are four centimeters or larger, or have spread to regional lymph nodes. Stage IV have spread farther, to lungs, liver, and/or kidneys. Median survival time is about a year for Stage I and declines with each progressive stage. Stage IV dogs may live only three months, even with surgery. Nesta’s tumor appears to be Stage I based on size, but we needed to check for metastasis. “Let’s do a lymph node aspirate and chest X-rays,” I advised.

Malignant melanomas also may occur in dogs on the toe or toenail bed. These tend to invade the bone, and are painful, aggressive tumors that have often already metastasized by the time of diagnosis. Black dogs are particularly prone. If we can catch it before it has spread, the best treatment is amputation of the toe. In cats, the most common site for malignant melanoma is the iris, i.e. the colored portion of the eye. There are other benign conditions that can appear similar, so don’t panic if you see a black spot in Kitty’s eye, but do consult your veterinarian. Iris melanoma can be life-threatening, and removal of the eye may be recommended to prevent the tumor spreading.

For Nesta, his lymph node aspirate and chest films were clean. No evidence of spread. His surgery is scheduled. If we can cut out all the cancer, he may be cured. We hope this will be the case, though recurrence is not uncommon even when it looks like the tumor has been completely excised. For dogs in which the tumor is clearly not completely removed by surgery or when there has been spread to regional lymph nodes but no further, owners can consider radiation therapy. Remission rates are very good with radiation, but many tumors ultimately recur anyway. Radiation therapy involves treatment at an oncologist’s, under general anesthesia, five days a week for three weeks, and costs many thousands of dollars. Many owners find this too difficult to navigate or too hard on their pet. Unfortunately, chemotherapy is not effective for malignant melanoma.

A relatively new form of therapy that can be tried after surgery is the Melanoma Vaccine. Available only through veterinary oncologists, the vaccine uses an enzyme called tyrosinase to prompt the dog’s own immune system to attack the melanoma. Treatment is a shot every two weeks for four injections, then a booster every six months. Whether this will be a good option for Nesta remains to be seen. The bottom line is that early intervention gives the best outcome. Nesta’s owner may have saved his life simply by making an appointment for an annual physical.