Mast Cell Tumors - The Great Imitator in Canine Cancer

How common are mast cell tumors?
It is important for a dog owner to have a good understanding of mast cell tumors (MCT) because they are the most common cutaneous (skin) tumor in dogs (up to 20% of all cutaneous tumors). The average age at diagnosis is approximately 8-9 years, but we see plenty of younger dogs affected as well. 

Which dogs get mast cell tumors?
Although most MCT occur in mixed breed dogs, certain breeds are at increased risk for developing MCT such as dogs of bulldog descent (pugs, Boston terriers, Boxers, English bulldogs, Frenchies), Labradors, golden retrievers, cocker spaniels, beagles, Staffordshire terriers, Rhodesian ridgebacks, schnauzers, Weimaraners, and Shar-Peis.

In general, it is felt that MCT in dogs of bulldog descent tend to behave less aggressively (note I said 'tend to' - this is not a hard and fast rule) and MCT in Shar-peis have a tendency to behave more aggressively.

What are mast cells anyway?
Mast cells are part of the normal immune system. Normal mast cells are found in organs such as lymph nodes. It's only when a mast cell mutates and becomes cancerous that we have a problem.

What do these tumors look like?
Most mast cell tumors occur in the dermis, which is the most superficial layer of the skin. In this case, they often look like a small raised pink "button-like" nodule on the skin surface. The difficulty is that MCT can also look like a skin tag, can sometimes appear pigmented (black or purple), and can even occur in the subcutis (under the skin surface). When MCT occur in the subcutis, it will feel exactly like a fatty lipoma (benign skin tumor). This is why they are considered the great imitator - they can look and feel like anything.

How do we diagnose mast cell tumors?
In the majority of cases, it's very easy to diagnose MCT. A simple needle aspirate (place a small needle into the tumor and smear the cells on a glass slide) will yield a diagnosis. Since MCT are the great imitator - a needle aspirate should be done for ALL skin masses. This is the only way to make sure that a mass that looks and feels like a lipoma, is not a MCT.

Some MCT are very small (a few millimeters) - too small to obtain a diagnosis with a needle. If I have a patient with a history of MCT, the best way to make a diagnosis for these small tumors is to remove the tumor with a small biopsy.

Why is it so important to make a diagnosis?
With proper treatment, most dogs that have MCT can live long healthy lives. Without treatment, a dog with an aggressive MCT might succumb to their tumor within a few months (thankfully aggressive MCT are less common). The proper treatment (for the best prognosis) will vary depending on each individual situation.

A few things that we (oncologists) take into consideration when deciding on the optimal treatment include:
(1) Is there 2 cm of normal skin around the tumor so we can achieve "clean margins" (100% of tumor cells removed from the skin)?
(2) Has the cancer spread to a local lymph node or elsewhere?
(3) What is the grade of the MCT and the mitotic index?

The best practices BEFORE removing a skin tumor:
(1) Perform an aspirate to establish a diagnosis. This allows the surgeon to know how much skin to remove around the tumor to prevent cancer from returning, AND which additional tests are needed to make sure the tumor has not spread (metastasized) before surgery.

(2) For a MCT, we typically aspirate the "draining lymph node" before removing the tumor. If the lymph node is affected, it should be removed at the same time the tumor is removed. A patient that has a MCT that has metastasized (spread) to a lymph node can still live for years, but only if the lymph node AND tumor is removed, and the patient receives follow-up chemotherapy.

(3) All patients undergoing surgery should have 3-view chest x-rays and full blood work. If an older patient is undergoing a mass removal, it's always a good idea to have an abdominal ultrasound performed. This is not necessarily because we think that their cancer might spread to their abdomen (typically, only high grade MCT spread to abdominal organs), but more to rule out other disease processes (a tumor of the spleen, for example) that might make the family change their plans for surgery.

What the biopsy report tells us...
Once the MCT is removed, we read the biopsy report to find out (1) if the surgeon removed all of the cancer cells in the skin, (2) the grade of the MCT.  

If the surgery was successful it means that "adequate margins" were achieved and there shouldn't be any mast cells remaining in the skin. This also means that the tumor should not grow back. We still recommend monitoring the surgery site every 3 months, just in case.

If the pathology report tells us that the surgical margins were "incomplete" or "narrow", there is a concern that cancer cells were left behind at the surgery site and a new MCT might regrow. Many times, the tumor that regrows is very aggressive and will lead to the death of the patient. In order to avoid this, there are 3 options: (1) a second surgery to remove extra skin (sometimes this is possible), (2) radiation therapy, (3) electrochemotherapy. 

The pathology report will also tell us the grade of the tumor. MCT are graded using two grading systems. They should be given a grade of I-III as well as high vs low. An oncologist will take this information as well as something called the mitotic index (an average of the number of cells actively dividing, which the pathologist counts as they look under the microscope) to determine how likely the MCT is to spread (metastasize), whether chemotherapy is indicated, and how long the patient might live with various treatment options.

Should I give Benadryl and Prilosec?
Mast cells contain histamine, and can feel itchy. They often have a history of flaring up, going away, then flaring up again. If your dog has a MCT, Benadryl (generic, diphenhydramine) is recommended twice daily until surgical removal. 

Similarly, mast cells cause the stomach to produce more acid and predispose dogs to stomach ulcers. Oncologists typically recommend that patients also receive Prilosec OTC (generic, omeprazole) until their tumor is removed to decrease the chance of stomach ulceration.

After surgery, provided that metastasis has not occured, there is no reason to continue these medications.

If my dog has one MCT, will he get another?
A dog that has had once mast cell tumor has a 44% chance of developing a second mast cell tumor, just because he is predisposed to them. If your dog has had this tumor, monitor him for additional lumps and bumps. Have your vet (or oncologist) aspirate any new skin masses.

What is the prognosis?
In most cases, if the MCT is treated appropriately, dogs can live for years with MCT. The exception is for dogs that have very aggressive MCT - thankfully these are the minority of cases.

MCT are incredibly common. These are tumors that oncologists treat every day. They are also tumors for which there are many treatment options (if the gold standard is not possible for a client). If you're interested in learning more, reach out to your local oncologist so that they can give you advice tailored to your dog and your dog's tumor.


Have questions about this article? Reach out!
Dr. Lori Cesario
Board Certified Veterinary Oncologist 

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