Skin tumors are a common and frequent reason for veterinary visits. About two thirds of skin tumors in dogs are benign (noncancerous) and one third are malignant (cancerous).
In cats, the opposite is true. Malignant tumors comprise two thirds, while one third are benign. Because skin tumors vary in their presentation and behavior, every growth should be identified. For example, the malignant mast cell tumor is called the “great imitator” because it can resemble so many other tumors that are commonly benign.
A growth should be tested either by a fine needle aspirate, biopsy, and/or complete surgical excision to determine more information about the growth and what plan of action is necessary thereafter. A fine needle aspirate entails the use of a needle and syringe to remove some cells from the growth. The aspirated cells are then transferred onto a glass slide and stained. By performing this test, a veterinarian can look at the cells microscopically to determine if the cell characteristics are supportive of a malignancy and then suggest the next step to be taken. The slide can be submitted to a board-certified veterinary pathologist for further study if the veterinarian deems necessary. In some cases, fine needle aspirates may not be enough to make a diagnosis, so the pathologist may request a biopsy or the entire tumor for review.
If surgical excision is determined to be the best plan of action, the tumor is then submitted to the pathologist to determine if the surgeon removed all of the cancerous cells. This is accomplished by viewing the width of all the margins from healthy to unhealthy tissue. It is the pathologist that will ultimately determine the type of tumor and whether it is malignant or not. Once you know the type of tumor, then the behavior of the tumor may be determined. Behavior of a tumor includes whether or not the tumor is one to metastasize (spread to other parts of the body) and how likely or quickly recurrence may occur.
Not all surgical excision recommendations are due to malignancies. A benign fatty tumor can grow to be so large and invasive that it interferes with a pet’s mobility. In these cases, the fatty tumor (lipoma) should be removed. Although there is one type of malignant fatty tumor, a liposarcoma, it is rare and does not normally spread to other parts of the body.
Skin tumors are not always easily felt and many times groomers and veterinarians find these growths that otherwise would have gone unnoticed by the animal caregiver. Two types that are very common are sebaceous adenomas and cysts. They can secrete an oily substance and cause crusting around the growths. The majority are benign and removed mostly for cosmetic reasons, constant bleeding from self-trauma, or because they get nicked during grooming.
Some tumors do not shed a lot of cells. Therefore, a fine needle aspirate may not always give the pathologist and/or the veterinarian enough information for a diagnosis. Not all growths that have suspicious looking cell characteristics of malignancy will come back from the pathologist as malignant, but it is always in your pet’s best interest to be cautious. A tumor that increases in size, regardless of previous cell findings, should be rechecked and/or removed and submitted to the pathologist. Always inform your veterinarian of any growths and if they have increased in size and how quickly. Identifying skin tumors early and determining whether to remove them or not is extremely important.
Kim Donovan, D.V.M., is an associate veterinarian and the medical director at Oakhurst Veterinary Hospital in Seminole with 16 years of experience and has a special interest in feline medicine.