Canine mast cell tumors, also known as mastocytomas, mast cell sarcomas or MCTs, are abnormal, cancerous (neoplastic) and unfortunately fairly common accumulations of mast cells that form nodular skin masses in dogs. About fifty percent of canine mast cell tumors are malignant. When MCTs metastasize, they typically go first to lymph nodes in the region of the primary mass. Thereafter, metastasis normally is to the spleen, liver, mesenteric lymph nodes, other cutaneous (skin) sites and to the bone marrow. MCTs are one of the most frequently diagnosed cancers in domestic dogs.
Causes of Canine Mast Cell Tumors
Mast cells normally are involved in inflammatory and allergic responses as part of the body's appropriate response to contact with allergens or other offending substances. Mast cells contain granules of various bioactive substances called cytokines, including histamine and heparin. When the cells degenerate or break apart (degranulate), those granules are released into circulation, causing a number of different bodily effects that can contribute to gastrointestinal ulceration, skin lesions, itchiness, bleeding disorders and systemic symptoms. Why mast cells accumulate into potentially malignant tumors in dogs is not known. The fact that certain breeds have a higher incidence of this type of cancer suggests a likely but complex genetic component.
Prevention of Canine Mast Cell Tumors
Because the causes of mast cell tumors are not known or even well-understood, there is no real way to prevent their development in domestic dogs. However, because more than 15% of dogs diagnosed with a mast cell tumor will develop more of them during their lifetime, affected animals should be rechecked by a veterinarian regularly, so that any new growths can be identified and treated as quickly as possible.
Special Note obn Mast Cell Tumor
The outlook for mast cell tumors depends on how progressed the cancer is. Mast cell tumors graded in the I – II stage usually have a good prognosis, while mast cell tumors in the III – IV stage have a guarded prognosis and are dependent on the response to chemotherapy and radiation treatments.
Mast cell tumors (MCTs) vary greatly in appearance and can occur in many different places on a dog's body. Unfortunately, it is difficult if not impossible to determine whether a mast cell tumor is malignant or benign based upon appearance alone.
Symptoms of Mast Cell Tumors
A mast cell tumor usually shows up as an isolated lump or mass, although they can appear in clusters or in multiple areas of the skin. Most affected dogs show no symptoms of irritation or illness. Owners of dogs with mast cell tumors may notice one or more of the following:
Lumps or bumps on or under the skin of the torso (trunk), underbelly (abdominal area) and hind legs, and around the anus and genital area
Lumps or bumps anywhere on or under the skin
Raised circular masses on or under the dog's skin that feel soft on the outside but solid on the inside
Skin mass that is multi-nodular (like cauliflower, with several bumps in a cluster)
Skin mass that is reddish
Skin mass that is hairless
Skin mass that is itchy (pruritic)
Skin mass that is ulcerated (like an open, weeping wound)
Skin mass that appears completely normal other than that it is a lump on or under the skin
Skin lump that looks the same for months or years, then suddenly changes in size or appearance
Skin lump that shows up suddenly and enlarges rapidly
Skin lump that fluctuates in size
Signs of gastric irritation – vomiting, diarrhea, weight loss, bloody stool
Many of these signs can be associated with a number of conditions other than mast cell neoplasia. Again, the key point for owners to remember is that any bump or lump on a dog may be abnormal, may be cancerous and should be examined by a veterinarian.
Dogs at Increased Risk
Dogs of either gender and of any age can develop mast cell tumors. Older dogs, with a mean age of 8 to 9 years, are most commonly affected, although very young dogs have been diagnosed with this form of cancer. While dogs of any breed or mixed breed can be affected, certain breeds do seem more prone to developing MCTs, including brachycephalic breeds such as English Bulldogs, Boston Terriers, Pugs and Boxers, among others. The reason for this association is not clear. Other predisposed breeds are the Golden Retriever, Labrador Retriever, Shar-Pei, Staffordshire Bull Terrier, Jack Russell Terrier and Rhodesian Ridgeback. Bernese Mountain Dogs reportedly have a genetic predisposition to developing mast cell tumors.
Any lumps or bumps felt on or under a dog's skin should be assessed by a veterinarian as soon as possible. Mast cell tumors (MCTs) are common in domestic dogs and are especially variable in appearance, size, location and growth pattern. They often are mistaken by owners as a minor insect bite or other unimportant skin reaction.
How Mast Cell Tumors are Diagnosed
Canine mast cell tumors can present in any number of ways. They can look like any other skin tumor, lesion or "bump," including insect bites, skin tags, lipomas (fatty tumors), hives, weeping sores, allergic reactions or other benign or malignant masses. Usually, MCTs are solitary, although they can occur in clusters or in multiple places. Most of these tumors are found incidentally during a routine veterinary physical examination, because most dogs show no clinical signs associated with mast cell tumors.
MCTs normally are not difficult to diagnose. They tend to be readily identifiable by veterinarians when observed on a stained glass slide under a microscope. Specifically, the granules inside mast cells usually stain a characteristic blue-ish purple, which can be seen microscopically on a smear sample taken from the mass. Most veterinarians will perform a complete blood count, serum chemistry panel and urinalysis of dogs suspected of having a mast cell tumor. A fine needle aspirate from the lymph nodes nearest to the mass is also typically taken. The results of each of these tests can increase the relative certainty that the mass is a mast cell tumor. However, the only way to absolutely confirm the diagnosis is by taking and assessing an actual tissue sample of the affected area.
Once a primary mast cell tumor has been diagnosed, the veterinarian will want to determine whether it has spread, or metastasized, to other locations. She also will want to stage the severity of the disease. Further diagnostics may include abdominal ultrasound, bone marrow aspiration and assessment, regional lymph node biopsy and/or tissue biopsy of the primary tumor. If other organs are enlarged or otherwise appear affected, they probably will be assessed by aspiration, biopsy and/or ultrasound as well.
Once they are diagnosed, mast cell tumors normally are staged, or graded, based upon a number of different parameters. The staging can range from I to III or IV. Grade/Stage I tumors are well-differentiated (they are easily recognizable by veterinarians and veterinary pathologists as being distinctly separated from surrounding normal tissue); they carry a good to excellent prognosis with appropriate treatment. Grade/Stage II tumors are moderately differentiated but still carry a very good prognosis with appropriate treatment. Grade/Stage III or IV tumors are considered very late-stage, poorly differentiated tumors, and they carry a guarded to grave prognosis due to the presence of malignancy and metastasis.
Mast cell tumors (MCTs) can be benign. However, they have the potential to become malignant and often metastasize to sites other than the skin. Therefore, any skin lump should always be assessed by a veterinarian and, if found to be a mast cell tumor, treated immediately. The goals of treating mast cell tumors are to control the local disease or other conditions associated with the tumors, and most importantly to prevent metastasis if at all possible. The ultimate goal, even with metastasis, is to improve, maintain or at least manage the affected dog's quality of life.
Treating Mast Cell Tumors in Dogs
Treatment options include surgery, radiation therapy, chemotherapy and immunosuppressive steroid administration. Veterinarians will stage mast cell tumors based on their size, number, degree of local involvement and presence or absence of metastasis. The appropriate treatment protocol will depend upon the diagnostic stage of the tumor(s) and the attending veterinarian's particular preferences and recommendations. Mast cell tumors of the prepuce, scrotum, perineal area (around the anus), toes (digits), ear flaps (pinnae), ear canal, oral mucosa (gums) and muzzle tend to be the most aggressive. These should always be assessed for potential metastasis.
Aggressive surgical resection is the primary treatment for MCTs in dogs. Wide surgical margins are recommended, and the resected tissue will be submitted for examination by a veterinary pathologist to determine whether sufficient surgical margins were achieved (that is, whether enough normal tissue around the tumor was removed, to be certain that all cancerous tissue at that site was removed). Nearby lymph nodes are also normally removed surgically, because they typically are the first place where the mast cell neoplasia will spread.
For early-stage tumors, surgical excision of the entire mass with normal margins of healthy tissue around the mass is the treatment of choice and is usually highly successful. Larger tumors, or tumors that are located in areas where removal with good margins is not possible, typically are treated by surgical resection of as much of the mass and margins as possible, followed by radiation, chemotherapy and/or immunotherapy.
Chemotherapy can be helpful for many dogs treated surgically for mast cell cancer, to prevent or reduce the chances of further metastasis. There are a number of chemotherapeutic drugs that can be prescribed and administered by skilled veterinarians to help manage mast cell cancer post-operatively. Radiation therapy can also be very helpful to affected dogs. All new lumps or bumps should be assessed quickly in dogs that previously have been diagnosed with mast cell neoplasia.
Prognosis for Dogs with Mast Cell Tumors
The outlook for dogs with mast cell tumors depends on how advanced the disease is at the time it is diagnosed and treated. Dogs with mast cell tumors graded in the early stages usually have a very good to excellent prognosis with appropriate treatment, while mast cell tumors in the later stage have a guarded to grave prognosis, depending upon the nature and extent of metastasis and the dog's response to chemotherapy and radiation treatments. Owners of dogs that have been diagnosed with mast cell tumors must be vigilant in the future and should regularly inspect the skin on their dog's entire body for any signs of new tumors. This should be done for the rest of the dog's life.