As dogs get older, most dogs will develop a lump or a whole range of dogs. It can be tempting to ignore them or write them off as a different thing with old dogs. While they are often the case, they should always be checked by your veterinarian. Mast cell tumors in dogs are one of the five cancers that are most commonly diagnosed in dogs.

What is it?

As part of the immune system, the job of mast cells is to respond to inflammation and allergic reactions. Mast cells contain, among other things, histamine granules, which are released in the event of allergic / hypersensitivity reactions (think hay fever). They occur in connective tissues, especially those close to the surface. When mast cells multiply in a tissue site, a tumor can develop.

Mast cell tumors make up about 20 percent of all skin tumors and can be locally invasive, making them sometimes difficult to remove completely.

Signs and symptoms.

Any lump that can be seen or felt, or both, should be checked. When a mast cell tumor affects other organs (generally the spleen, lymph nodes, or liver), vomiting, diarrhea, and loss of appetite may also occur.

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How is it diagnosed?

To diagnose a mast cell tumor – specifically, to differentiate it from a lipoma (a benign fat tumor that grows just below the surface of the skin) – a veterinarian uses a thin injection needle to take a sample from the lump. This technique is known as fine needle aspiration and is a quick, well-tolerated, and inexpensive way to determine the type of lump, although it is not necessarily definitive. The sample is then examined under a microscope. If the sample does not provide enough cells or is otherwise inconclusive, the veterinarian may recommend biopsy of the lump.

Two systems are used to classify a mast cell tumor: the three-tier Patnaik and the newer two-tier Kiupel. Dr. Janet Patterson-Kane, Scientific Director of the Morris Animal Foundation, believes the Kiupel system is superior. “The two-tier scoring system is a significant improvement in predicting the behavior of mast cell tumors and a better match among pathologists … should increase the confidence of veterinarians making decisions about the treatment of these tumors.”

To determine if the mast cell disease is local or systemic, x-ray, ultrasound, and bone marrow cytology (which uses a needle to collect a small sample of liquid bone marrow material) may be recommended.

How is it treated?

Mast cell metabolism is inhibited by corticosteroids such as prednisone, which is why steroids are sometimes used to treat allergic reactions. Orally administered prednisone can help shrink mast cell tumors. Another approach is to inject a related steroid such as triamcinolone directly into the mast cell tumor at multiple sites to reduce the size of the tumor before surgical removal.

For non-metastatic skin-based mast cell tumors, a newer therapy uses tigilanol tiglate, a drug derived from the Australian blushwood plant. It stimulates the rapid destruction of mast cells within seven days of injection into a tumor. This can avoid the need for surgery, but it creates a wound that needs to be treated during the healing process.

Grade 1 and II tumors are usually removed surgically; If caught early and removed with a large margin of healthy tissue, the prognosis is good. However, Grade II tumors can present a dilemma. They can act like grade I tumors or grade III malignant tumors. The latter are always removed surgically.

In cases where removal has to be followed by another therapy, radiation is used more often than chemotherapy. Chemotherapy is usually reserved for Grade III tumors, although its response to it is unpredictable. Some dogs need a second, more aggressive, surgery. A dog that has had one mast cell tumor will likely develop another. These are new tumors rather than indications that the original tumor has metastasized.

Mast cell tumors also affect humans, and drugs developed for use in humans have shown promise for use in canine patients. Toceranib phosphate (Palladia), the first tyrosine kinase inhibitor approved by the Food and Drug Administration for the treatment of mast cell tumors in dogs, and masitinib (Masivet) appear to be well tolerated in treating these tumors and slowing their progression.

Are Certain Races Predisposed?

Although any breed or mix can develop mast cell tumors, flat-faced breeds such as Boston terriers, boxers, pugs, bulldogs, and retriever breeds appear to be predisposed. Both men and women are equally likely to develop it.