Mast Cell Tumors & Skin Cancer in Dogs
Mast cells are cells that occur in most body tissues but that are found most often in the skin and linings of the digestive tract and the lungs. They play an important role in immunity and in allergic reactions. Mast cell tumors of the skin account for about a fifth of all skin tumors in dogs.
What Are Mast Cell Tumors?
Mast cell tumors are formed when mast cells proliferate or grow abnormally. These tumors can be benign, which means they are non-cancerous and will not do much harm to the body, or they can be malignant, or cancerous. It’s possible for malignant mast cells to spread, or metastasize, to other organs of the body, like the lymph nodes or the liver. Mast cells are able to release chemicals like histamine and heparin, and when they do so excessively as in cases of abnormal growth, they can damage the body and alter important body functions as well. Since mast cells are active during allergic reactions, a proliferation of these cells can even affect the heart rate and blood pressure.
Mast cell tumors appear most commonly in the dog’s trunk or limbs. It’s easy to misdiagnose them as lipomas, or benign fatty tumors, and there is no way to tell for certain what they are without a biopsy and subsequent pathology report. So if you see a lump on your dog, it’s important to take him to the vet right away to find out what’s wrong.
Grades and Stages
Mast cell tumors in dogs are classified to indicate their severity according to grades and stages. The grade of the tumor tells us how likely it is that the tumor is malignant, while the stage indicates how far the tumor has spread into the dog’s body.
Tumor Grades
In this classification, a higher grade number means that the mast cell tumor is more likely to be malignant.
• Grade I – This is the most common type of mast cell tumor, and it is benign, which means it won’t spread. Most mast cell tumors are Grade 1.
• Grade II – This type of tumor goes deeper, into the lower layer of skin or subcutaneous tissue. It may or may not become malignant.
• Grade III – This tumor is more aggressive and goes below the skin into other body areas. It’s more likely to be malignant.
Tumor Stages
Mast cell tumor stages define how far the tumor has spread. The higher the stage number, the farther it has spread.
• Stage I – This indicates the presence of a single tumor with no metastasis.
• Stage II – This indicates a single tumor with metastasis into the nearby lymph nodes.
• Stage III – This means there are multiple tumors which may be deep in the skin, and there may or may not be involvement of the lymph nodes.
• Stage IV – This indicates one or more tumors with metastasis to an organ or to the blood. Lymph nodes are probably involved as well.
Causes
The actual causes of mast cell tumors in dogs are unknown, but it is known that cell mutation is the cause of the uncontrolled abnormal cell growth. Environmental factors such as local pollution may be involved, as vets who work near chemical plants often treat many dogs with cancer.
Excessive exposure to sun may also be a factor, and whether it’s a cause of mast cell tumors or not, too much sun can burn a dog’s skin, especially if it is a breed with a thin coat. It’s not crazy to put some sun screen on your dog before letting him spend a lot of time outside.
It is also known that certain dog breeds tend to get mast cell tumors more often than others. Boxers, Bulldogs, Boston Terriers, Pit Bull Terriers and Labrador Retrievers are some of the dogs that commonly get these tumors. Older dogs are also more susceptible to these tumors.
Signs and Symptoms
You should inspect your dog at least weekly for any changes in the appearance of his skin. Skin tumors can most commonly be found on the trunk or the legs, but can be found anywhere. Any lump under the skin should be reported to your vet, especially one that changes rapidly in size. Fluid buildup or redness surrounding the area of the lump is another common symptom of mast cell tumors.
If you stroke the skin of a tumor and it begins to change, you are seeing a positive Darier’s sign. This means that the tumor is swelling or becoming reddened as a result of the pressure on the mast cells and their subsequent response.
If the tumor has metastasized or spread, there may be additional, more systemic symptoms, like vomiting and diarrhea, loss of appetite, lethargy, enlarged lymph nodes, or coughing and difficulty breathing. All of these mean it’s crucial to get to the vet as soon as possible.
Diagnosis and Treatment
To diagnose a mast cell tumor, the vet will usually do a fine needle aspiration, which means he’ll take a sample of the tumor cells using a needle. These cells will then be sent to a lab to be observed for microscopic changes that indicate abnormal cell growth.
The treatment options for mast cell tumors will depend largely upon the severity.
• Surgery – Simply removing the tumor and any surrounding tissue that is likely to be involved is the most straightforward way to deal with mast cell tumors. Tissue samples are sent to a lab for pathology reports to determine the nature of the tumor and surrounding tissues.
• Radiation – Radiation therapy is expensive and has side effects, but is often very effective in treating mast cell tumors.
• Chemotherapy – If a tumor is malignant, chemotherapy is usually recommended. This can include traditional chemotherapy drugs like vinblastine, or the more modern tyrosine kinase inhibitors, like Palladia or Kinavet. Tyrosine kinase inhibitors, or TKI’s, work by inhibiting a receptor on mast cells that often mutates and causes abnormal growth.
• Medications – Drugs like prednisone, a steroid, may be given because they are known to kill cancerous mast cells. Other drugs such as anti-nausea medications may be given for the dog’s comfort during treatment.
• Palliative Care – If a dog has a cancerous tumor that has spread so far it cannot be cured, then treatment is aimed at comfort, nutrition and control of pain and other symptoms.In this type of care, the dog’s comfort is the priority.
Other Forms of Skin Cancer
Malignant melanoma and squamous cell carcinoma are two other forms of skin cancer that commonly occur in dogs.
Squamous Cell Carcinoma
The outer layer of the epidermis, or skin, of dogs is made up of cells called squamous epithelium. This type of epithelium covers the skin as well as provides lining for the body cavities. Squamous cell carcinoma occurs in this layer of cells. These tumors tend to grow quickly and may present as raised lumps or masses, or as bleeding, ulcerated sores. Dogs who spend a lot of time in the sun are more likely to get this type of skin cancer.
Malignant Melanoma
Malignant melanoma involves the melanocytes, which are pigment-producing cells. These tumors are often found in the mouth area, especially the gums, lips, tongue, and hard palate. Malignant melanomas are more common in certain dog breeds, such as Boston Terriers, Cocker Spaniels, Scottish Terriers, and Dobermans. These tumors can be benign or malignant and are diagnosed by biopsy.
Prognosis of Skin Cancer
The prognosis of skin cancer in dogs depends largely on the severity, but it also depends on how quickly the tumor is diagnosed and treated. The lower the grade and stage, the better the prognosis. Tumors that are located on the limbs have the best prognosis because a limb can be surgically amputated, preventing further spread, and because a tumor on a limb does not affect other organs as easily. Tumors that are found in the mouth or on the genitals or other areas closer to internal organs are harder to treat and have a generally poorer prognosis. It’s always crucial to take your canine best friend to the vet as soon as possible if you notice any abnormal growth on the skin.
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