Tumors of the skin and subcutaneous (tissue just underneath the skin) are the most common tumors affecting dogs and the second most common tumors affecting cats.

There are a wide variety of tumors that can occur within the skin, and it is very important to remember that not every skin tumor is cancerous. In fact, the vast majority — 80 percent — of skin tumors in dogs are considered to be benign, meaning they do not metastasize (spread) to other locations in the body.This is in contrast to skin tumors in cats, where 50-65 percent of tumors are malignant, meaning they grow as very locally invasive masses and have a higher chance of metastasizing to distant sites.

Unfortunately, a veterinarian cannot tell if a tumor is benign or malignant just by visualizing or palpating the mass. Further tests are necessary to be able to determine exactly what kind of tumor the lump or bump could be.There are two main ways to determine whether a skin tumor is benign or malignant. The first involves performing what is known as a fine needle aspirate with cytological analysis. This non-invasive procedure generally entails introducing a small gauge needle (about the same size that is used to administer a vaccination) into the tumor, obtaining some of the cells into the needle. The cells are then dispersed onto a microscope slide, special stains are applied to the sample, and the slide is then evaluated under a microscope. The sample is sent to a laboratory where a cytopathologist (veterinarian with specialized training in the evaluation of samples of this nature) will examine the slides and make a diagnosis

There are several advantages to this type of sampling.  It is considered a rapid, non-painful, simple procedure to perform, and is relatively inexpensive. In most cases, fine needle aspirates can be performed while the patient is awake. If the tumor is located in a particularly sensitive area (e.g., around the eyes or anus), a veterinarian may recommend that the patient be lightly sedated to facilitate sampling in a safe manner. Fine needle aspirates will give information about the type of the individual cells comprising a tumor helping to determine whether it is cancerous or not.  For those tumors where removal is recommended, the information provided can be invaluable to help determine what type of preliminary tests should be done before surgery to rule out whether or not the tumor has spread, and provides information needed to determine how aggressive the surgical removal will need to be. The main disadvantage to this form of sampling is it may not prove to be the most accurate because this type of analysis examines only individual cells. There is also the possibility that the sample may return non-diagnostic, meaning no cellular material could be obtained. Finally, since the size of the needle used to sample the tumor is very small, it is possible to miss the portion of the tumor containing the cancerous cells and a misdiagnosis could be made.

A more accurate way of sampling skin tumors from dogs and cats involves performing what is known as a tissue biopsy. There are several ways to obtain a tissue biopsy; all of which usually involve either heavy sedation or general anesthesia. The main advantage of performing a biopsy is the higher degree of an accurate final diagnosis. The main disadvantages are that biopsy procedures require heavier sedation or anesthesia, the results take longer to return, they are considered slightly more invasive, and can be more costly.

If you notice a new lump or bump on your pet, you should have it evaluated by your veterinarian as soon as possible. During the visit, the tumor should be measured and its location “mapped,” either by physically drawing a picture of the location of the tumor on your pet, or by taking a photograph of the tumor and making it a part of your pet’s medical record. You and your veterinarian can discuss what would be the best plan for evaluating the tumor.

If the tumor is determined to be benign, you will need to continue monitoring it for any signs of change in size, shape, or consistency, as this could indicate transformation to a more malignant behavior. If the tumor is determined to be malignant, your veterinarian may recommend referring you to a veterinary surgeon or veterinary oncologist for further testing. If noticed early, some malignant skin tumors can be treatable and the prognosis excellent. The best way to examine your pet for skin tumors is simply by petting them or grooming them, and also by scheduling regular physical examinations with your veterinarian.