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 a superficial layer of the skin. In these cases, they often look like a small raised pink "button-like" nodule on the skin surface, which is hairless. 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, they feel exactly the same as 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. If treated properly, even dogs with aggressive MCT can live for years.

A few things that we (oncologists) take into consideration when deciding on the optimal treatment for an individual patient 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)?

*For tiny MCT there is evidence that less skin needs to be removed with the tumor; this should be discussed with an oncologist or surgeon that has experience with surgical oncology.

(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 of the skin mass 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 (different types of cancers will have a preference for spreading to different organs).

(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. The x-rays should be reviewed by a board certified radiologist. 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. If we suspect a dog has a high grade MCT we typically will aspirate (sample) the liver and spleen prior to surgery even if they appear normal. If MCT infiltrates one of these organs (liver/spleen) it is not uncommon for the organ to look normal on ultrasound.

What the biopsy (histopathology) 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 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.

Note that there are many cases when the pathology report will describe "complete" margins but the margins will be 1 mm or 0.1 mm. These margins are not acceptable and are although they technically might be complete, they are very narrow. The doctor needs to read the fine print to see exactly what the margins are. A tumor that has a 1 mm margin is at risk for recurrence and additional intervention (second surgery, radiation, electrochemotherapy) should be considered. Consultation with an oncologist is recommended.

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 in mitosis, 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 occurred, there is no reason to continue these medications.

What if surgery isn't performed?

There are certain cases in which a family elects not to pursue surgery for a mast cell tumor (or tumors). In these cases, there are other options. The most appropriate options for an individual patient will depend on things like (1) has metastasis occurred, (2) the size and location of the tumor, (3) if the tumor is recurrent disease from a previously removed mast cell tumor vs a "new" tumor, (4) the overall health of the patient, etc.

Discussing the best fit option for your dog should be done with an oncologist. There are many possible scenarios and oncologist are the best to advise you.

The most conservative options include prednisone alone or (better) prednisone plus chlorambucil (low dose oral chemo). For smaller tumors, this can achieve a partial or complete remission for some patients. One study showed that dogs with inoperable tumors treated with chlorambucil and prednisone had an median survival of 4 months; dogs that had a great response had a median survival of 17.8 months. Chlorambucil is an inexpensive drug (when compounded) and is well tolerated. The duration of control with these option is not expected to be as long for proper surgical removal, but these are alternative treatment options when the first-line options have been declined.

A second option (more aggressive than prednisone or prednisone plus chlorambucil) includes using a combination of radiation therapy plus oral Palladia and prednisone (tumors were an average of 4.3 cm, 59% had a complete response in a CSU paper, with the median survival time not reached/longer than 12.4 months). In other cases, Palladia alone can be pursued.

Recently, Stelfonta (tigilanol tiglate) became available. This is meant for small tumors (less than 10 cm3) which have not metastasized and are easily accessible (external). Treatment involves 1 to 2 injections of Stelfonta into the tumor, followed by a few weeks of expected dying and sloughing of the tumor. After a few weeks the wound is expected to heal. After 1 injection, 75% of patients are expected to have a complete response (tumor completely resolves); this increases to 88% with two injections. One study showed that 88% of patients were still in a complete remission 12 months after treatment, 11% in partial remission. Eighteen months post treatment, 70% of patients maintained their remission, 30% had local recurrence.

Initial work shows that Stelfonta also seems helpful for dogs with high grade mast cell tumors. As we begin to work with this new drug we will learn more about the 'best candidates'.

If my dog has one MCT, will he get another?

A dog that has had once mast cell tumor has about a 44% chance of developing a second mast cell tumor, just because he is genetically predisposed to them. If your dog has had a MCT tumor, monitor him for additional lumps and bumps. Have your vet (or oncologist) aspirate any new skin masses while they are still small and can have a good surgical outcome.

What is the prognosis?

In most cases, if the MCT is treated appropriately, dogs can live for years. The exception is for dogs that have very aggressive MCT - thankfully these are the minority of cases. Although dogs can live for years with appropriate treatment for high grade MCT, these tumors are unpredictable and treatment is not always successful.

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.

Dr. Lori Cesario

Board Certified Veterinary Oncologist

PS: I'm happy to now offer online oncology consultations. Learn more about how this service can help by visiting the Vet Cancer Consultants site.

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(2) The MDR1 Mutation - What it is and why you need to know!

(3) Cannabinoids and Your Canine

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