Mast Cell Tumors

Laurie Anne Walden, DVM

Mast cell tumors are common in dogs and somewhat common in cats. In dogs, mast cell tumors are usually lumps on or under the skin. Mast cell tumors in cats can affect the skin or the internal organs.[1]
 
Mast cells are a normal part of the immune system. They are most often found in the skin, digestive tract, and other areas that are exposed to substances from the environment. Mast cells are full of granules that contain histamine and other chemicals that are released as part of allergic and inflammatory responses. You’ve seen mast cells in action yourself: the itchy lump you get after a mosquito bite is caused by mast cells in the skin releasing histamine and other substances. Mast cell tumors are cancerous growths made up of mast cells.
 
Signs in Dogs
Mast cell tumors are most common in older dogs but occur in dogs of all ages. Dogs of any breed can develop mast cell tumors. Breeds at higher risk than others include retrievers, shar-peis, and brachycephalic (flat-faced) breeds like boxers, pugs, and Boston terriers.
 
In dogs, mast cell tumors can look similar to other conditions. Some look and feel just like benign fatty lumps. Mast cell tumors in dogs vary in appearance and behavior. A mast cell tumor might be a smooth, round, raised skin lump; a red, itchy lump; or a soft lump under the skin. Some are solitary growths, and some are clusters of lumps or small bumps. Some grow very slowly, with no apparent change for months; others grow quickly. Mast cell tumors sometimes get bigger (or pinker or itchier) and then return to their normal appearance. This change happens when they release histamine. More serious effects of histamine release include decreased appetite, vomiting, diarrhea, blood in the stool, and collapse.[1,2]
 
Signs in Cats
Most cats with mast cell tumors are older, although an atypical form is most common in young cats. Siamese may be more likely than other cat breeds to have mast cell tumors.[3]
 
In cats, the signs of mast cell tumors depend on their location (skin or internal organs) and the tumor subtype, which determines how aggressive they are. As in dogs, mast cell tumors in cats can mimic other conditions and do not all look and behave the same way. Mast cell tumors in the skin can be smooth, round growths or flat red patches, and they might grow quickly or very slowly. Mast cell tumors inside the body most often affect the spleen or digestive tract and can cause decreased activity, decreased appetite, vomiting, diarrhea, and other general signs of illness.[1,3]
 
Diagnosis
Most mast cell tumors of the skin can be diagnosed during a veterinary appointment with a needle aspirate, in which a small sample of cells is removed with a needle and syringe and examined under a microscope. Because putting a needle in a mast cell tumor can cause histamine release, veterinarians often give an antihistamine when they aspirate a mast cell tumor.
 
A needle aspirate can show that a patient has a mast cell tumor, but it doesn’t give enough information to know the prognosis. Some mast cell tumors are single lumps that don’t spread to lymph nodes or distant parts of the body. Others are more invasive and have a high risk of spreading through the body. Currently the only way to know the grade or type of a mast cell tumor, and hence the prognosis, is by surgically removing all or part of the tumor and sending this biopsy sample to a laboratory for analysis.
 
Patients with high-grade or aggressive mast cell tumors, including cats with mast cell tumors in internal organs, benefit from further tests to find out whether the cancer has spread or is causing other problems. This workup can include blood and urine tests, lymph node aspiration, imaging (such as ultrasound or computed tomography), and bone marrow analysis. Whether to do these tests before or after biopsy depends on the patient and the tumor.
 
Treatment
Surgical removal is recommended for most mast cell tumors and might be the only treatment (other than antihistamines) needed for low-grade mast cell tumors of the skin. Cancerous mast cells extend past the edges of the visible lump, so a wide area of normal-appearing skin around the tumor must be removed. The biopsy report indicates whether all of the tumor was removed during the procedure.
 
Chemotherapy, other medications, and radiation therapy are available for patients with aggressive tumors, cancer in internal organs, tumors in areas where wide removal isn’t possible, or tumors that aren’t completely removed during surgery. Your veterinarian is likely to recommend referral to an oncologist if your pet might need these types of treatment.
 
Prognosis
The prognosis is good for patients with mast cell tumors that haven’t spread and are completely removed with surgery, radiation therapy, or both. These patients sometimes develop another mast cell tumor later, so all new lumps warrant a visit to the veterinarian. For patients with other types of mast cell tumor, the prognosis depends on tumor grade, location, and response to treatment.
 
References
1. Blackwood L, Murphy S, Buracco P, et al. European consensus document on mast cell tumours in dogs and cats. Vet Comp Oncol. 2012;10(3):e1-e29. doi:10.1111/j.1476-5829.2012.00341.x
2. Couto CG. Mast cell tumors: to cut or not to cut. Paper presented at: 2018 Michigan Veterinary Conference; January 26-28, 2018; Lansing, Michigan. Accessed February 14, 2020. https://www.michvma.org/resources/Documents/MVC/2018%20Proceedings/couto_04.pdf
3. Henry C, Herrera C. Mast cell tumors in cats: clinical update and possible new treatment avenues. J Feline Med Surg. 2013;15(1):41-47.
 
Photo by JC Gellidon

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