Many dogs will develop at least one skin tumor over the course of their lifetime. While many can be diagnosed and then closely monitored, some skin tumors are malignant and require prompt treatment. In this article, we discuss the most common skin tumors as well as how they’re diagnosed, some of the available treatment options and when to consult with your family veterinarian to determine whether a referral to a Medical Oncology specialist would be helpful to your pet.
Skin tumors can grow on the skin itself (these are called dermal or cutaneous tumors) or they can grow below the skin (these are called subcutaneous tumors). Most dermal skin tumors are hairless, and they can vary in color from pink, to red, to darkly pigmented. Subcutaneous tumors grow below the fur and skin, and typically, you’ll notice the bump while petting your dog. In some cases, the appearance or feel of a skin tumor can give your veterinarian a hint at the type of tumor it is, but there is considerable overlap between tumor types, so tumor sampling is always recommended.
Types of Common Skin Tumors
There are five common skin tumors common in dogs. These include lipomas, cysts, histiocytomas, mast cell tumors, and soft tissue sarcomas. A description of each is included below:
- A lipoma is a benign fatty tumor, and these are extremely common in middle-aged to older dogs. They develop subcutaneously and can vary in size from <1-inch diameter to extremely large lipomas (>6 inches in diameter). Common locations include the back, belly, and groin area. Lipomas are typically very slow-growing tumors that do not interfere with function or mobility, but in rare cases they can grow in a locations such as the armpit or between the muscles of the thigh; in these locations, lipomas can result in discomfort or lameness.
- Benign cysts (known as sebaceous adenomas) are also extremely common in middle-aged to older dogs. These tumors develop on the skin and typically appear as pink or pigmented, irregular growths that can grow anywhere on the body. Cysts are slow-growing and typically do not cause any health concerns. However, in some cases they can grow in a location where they are easily bumped or scraped (such as on the face) or a dog may lick or chew at the cyst, resulting in bleeding or infection.
- A histiocytoma is a benign dermal tumor that is most common in young dogs, although it can be seen at any age. These tumors commonly form on the ear, and they are small, pink growths. Interestingly, histiocytomas are associated with an unusual phenomenon known as spontaneous regression, in which the tumor shrinks and goes away on its own, typically after several months.
- Mast cell tumors are the most common malignant skin tumor seen in dogs. These tumors are sometimes referred to as the ‘great pretender,’ as they can have any appearance ranging from dermal to subcutaneous, variable color and firmness, and can arise anywhere on a dog’s body. Most mast cell tumors are low grade, meaning that they have a low potential for spread to other areas of the body. However, some mast cell tumors are high grade; these tumors often grow rapidly, become ulcerated and/or bruised, and have a high potential for cancer spread.
- Soft tissue sarcomas are another common malignant skin tumor in dogs. These tumors grow subcutaneously, and they are most common on the legs and trunk. Like mast cell tumors, soft tissue sarcomas are categorized as low or high grade, and this influences their growth rate and potential for cancer spread.
The most common method for diagnosing a skin tumor is with fine needle aspirate and cytology. This involves inserting a thin needle into the dog’s tumor to withdraw a small amount of tumor cells. In most dogs, this procedure can be performed quickly and without the need for sedation or anesthesia. The tumor cells are deposited onto a microscope slide and submitted to a veterinary clinical pathologist for microscopic evaluation (cytology). Most skin tumors are readily diagnosed with this procedure.
In some cases, tumor cells cannot be easily collected via fine needle aspiration, and cytology results are inconclusive. In these cases, a biopsy is recommended. This is a surgical procedure requiring sedation or anesthesia, in which a small piece of the tumor is removed surgically and submitted to a veterinary anatomic pathologist for microscopic evaluation (biopsy). Following a biopsy, a dog will typically have several stitches or skin staples placed at the surgical site.
Treatment is Often Not Necessary
In most cases of benign skin tumors (such as small lipomas, cysts, and histiocytomas), no treatment is necessary; these tumors can be closely monitored over time without surgical removal. However, in some cases, lipomas can grow to a large size or in a location where they interfere with comfort or mobility; in such cases, surgical removal is recommended and can dramatically improve a dog’s comfort and quality of life. Similarly, most benign cysts can be monitored without treatment. In some dogs with cysts, rubbing or licking can result in bleeding or infection, and surgical removal is recommended. Because histiocytomas typically resolve after several months on their own, no treatment is necessary.
In the cases of malignant skin tumors, including mast cell tumors and soft tissue sarcomas, surgery is recommended to remove the cancer. Cancer surgeries typically require wide surgical margins (removal of a margin of normal tissue around the tumor to ensure that all microscopic tumor cells are removed). In many cases, your dog’s general practitioner will perform this surgery, but in some cases, they may recommend that your dog is evaluated by a board-certified veterinary surgeon. Such specialized veterinarians have advanced training in cancer surgeries. Once a skin tumor is removed, it should be submitted for histopathology to confirm the diagnosis, for tumor grading (which helps to determine how aggressive the malignancy is, and how likely it is to spread to other areas of the body), as well as for assessment of tumor margins (was the entire tumor removed, or do microscopic tumor cells remain at the surgical site?)
Following the biopsy results, your veterinarian may refer your dog to a board-certified oncologist for additional diagnostics (lymph node sampling and/or imaging tests to screen for any evidence of cancer spread, also known as metastasis) and treatment. In some cases, additional treatment such as radiation therapy or chemotherapy may be recommended for your dog.
In support of your pet’s overall health and wellbeing, skin tumors should always be evaluated by your veterinarian. Please keep in mind, however, that because many skin tumors are benign and do not require treatment, it can be challenging to monitor changes in size or appearance over time.
For dogs with multiple skin tumors, a medical record called a “body map” can be extremely helpful for documentation of the location, size, and cytology or biopsy findings of skin tumors and/or scars from previous surgeries. At each visit, your veterinarian can compare your dog’s current tumors to those recorded on their body map to determine if there are any new tumors or any significant changes to previously documented tumors. You can also keep a copy of your dog’s body map at home for careful home monitoring. If you notice a new skin tumor prior to your dog’s next scheduled appointment, you can contact your veterinarian about having your pet evaluated sooner. Indications for concern, and urgent evaluation by your veterinarian, include tumors that are: growing rapidly, ulcerated, bruised, bleeding, or accompanied by signs of illness in your dog such as vomiting, diarrhea, weight loss, or poor appetite.
For more information about skin tumors in dogs, contact the Medical Oncology team at MedVet Cincinnati.
JEANNE LANE, DVM, DIPLOMATE, ACVIM (ONCOLOGY) is a board-certified Veterinary Medical Oncologist at MedVet Cincinnati and MedVet Lexington, where she has been part of the medical teams since 2018. Dr. Lane graduated Summa Cum Laude, earning her Bachelor of Science degree in Animal Science from the University of Connecticut in Storrs, CT. Following completion of her undergraduate studies, she earned her Doctor of Veterinary Medicine from the Tufts Cummings School of Veterinary Medicine in North Grafton, MA. Dr. Lane then spent one year with VCA Veterinary Care in Albuquerque NM, during which time she completed her rotating internship in Small Animal Medicine and Surgery. She next completed a three-year residency at the University of Tennessee College of Veterinary Medicine in Knoxville, TN.
Passionate about all aspects of veterinary medical oncology, Dr. Lane has special interest in novel therapies for lymphoma, advanced diagnostics, and the early detection of cancer. She finds developing a close relationship with the patient and client throughout the treatment protocol to be the most rewarding part of being a veterinarian.
When not caring for her patients, Dr. Lane enjoys kickboxing, photography, and spending time with family and friends.