Skin lumps can be tricky. we stick them with a needle, suck up some cells, smear the sample on a slide and take a peek down a microscope, but sometimes we still can’t make a diagnosis. This leaves us with the option of surgery on a lump of unknown provenance and the dilemma of how wide an excision to make. If the culprit is benign, the surgery need not be radical. But what if the lump is malignant? Should the surgeon be aggressive, just in case? How on earth do we strike the perfect balance?
From time to time, I am presented with precisely this kind of case. Hannah, a Shar-Pei, was seven years old when her owner noticed an innocuous but slightly pigmented and raised skin mass on her left forelimb. The referring veterinarian had performed a local excision and in doing so, achieved his foremost objective: a diagnosis — in this instance, the troubling discovery of melanoma.
“The margins were dirty,” said Hannah’s owner, Barbara. “Do you think you can get what was left behind, given the location?”
On the extremities of the body, particularly the face and limbs, loose skin is at a premium compared to the chest and abdomen. When doing surgery, a veterinarian’s natural tendency is to take less to ensure that the hole left behind will close. On occasion, some owners will question whether the original surgeon was at fault. Barbara was not one of those owners.
“Sure,” I said, pinching an inch around the scar. “It’s always difficult to decide how aggressive you should be when the lump is a mystery. But now that we know it’s a melanoma, Hannah can also reap the rewards of a treatment not even available in human medicine.”
Barbara looked confused.
I was talking about the first fully licensed anti-cancer vaccine to be approved by the USDA: the canine melanoma vaccine, Oncept™.
“You mean Hannah could have been vaccinated against melanoma?”
“Not quite,” I said.
“The vaccine is therapeutic, not preventive.”
I went on to tell Barbara that melanoma cells express large quantities of a protein called tyrosinase. The vaccine incorporates DNA that expresses a gene for the human version of tyrosinase. When the dog’s immune system recognizes this human protein as a foreign substance, it mounts a response. Fortunately, the human version is similar enough to its canine counterpart that the stimulated immune system attacks the melanoma tumor cells.
“Why not vaccinate all dogs?” asked Barbara.
“Malignant melanoma is quite common, affecting about one in every 20 dogs with cancer, but it’s still not common enough to justify widespread vaccination.”
“So when do we get started?”
“We don’t, or at least I don’t — I’m not allowed. For now, only board-certified oncologists can administer the vaccine: four doses, inside of the thigh, every two weeks, and then a booster every six months. I’ll have you meet with one of our oncologists and she’ll give Hannah her first treatment once she recovers from the surgery.” And this was what we did for Hannah after the pathologist confirmed that my surgical margins were clean.
While the vaccine is primarily aimed at oral melanoma, especially where the tumor is locally invasive or has spread to local lymph nodes, many oncologists are keen to reap its benefits for melanomas in other locations. One of the appealing features of the treatment is its lack of side effects. Some dogs may develop a low-grade fever, but that’s about it. The biggest deterrent may be the price; in our hospital, an initial course of vaccinations will set you back around $2,000.
It is important to bear in mind that the vaccine is not a cure. For dogs with early-stage melanoma, like Hannah, survival times greater than three years can be expected. For dogs with more aggressive types, a median survival of one year might be more realistic (but much improved from the previous expectation of only two to three months with other therapies). Oncologists have also seen periods of remission in dogs with visible spread of the cancer to the lungs.
One of the lessons I learned from my experience with Hannah is that the vaccine appears to be more effective the earlier it is given; sometimes the disease will progress while you are waiting for the immune system to kick in. In removing a bleeding but benign skin lump on a 16-year-old Jack Russell named Charlotte, I snipped off a small lip mass in the spirit of “while you’re at it.” As so often happens, the seemingly incidental lip mass turned out to be the more serious concern when the pathology report came back: malignant melanoma.
“Charlotte’s way too old for chemotherapy or radiation therapy,” said her owner, Ann. I didn’t even try to argue, but I did mention the vaccine.
Guess who booked the first available appointment with oncology!
*Bergman, P. J., et al. 2006. Development of a xenogeneic DNA vaccine program for canine malignant melanoma at the Animal Medical Center. Vaccine 24(21): 4582–85.