Understand Common Pet Cancers

Mast Cells and Tumors

(Adapted from an article on www.veterinarypartner.com by Wendy C. Brooks, DVM, DABVP)

What is a Mast Cell?

Mast cells are a normal part of the immune system and are designed to participate in the destruction of parasites. They are bound within tissues that ‘interact’ with the external world, such as the skin, respiratory system, or intestinal tract. They do not circulate throughout the body.

The mast cell contains granules of inflammatory biochemicals meant for use against invading parasites. When signals from the immune system indicate that a parasite is near, the mast cell degranulates, releasing its toxic biochemicals. These chemicals are harmful to the parasite and also initiate a general inflammatory response.

At least this is what is supposed to happen. Unfortunately, what tends to happen with this common pet cancer is that other substances that are similar in shape or size to parasite antigens stimulate the mast cell to degranulate. These substances are commonly pollen proteins, and the result is an allergy. Instead of killing the invading parasite, the mast cell biochemicals produce local redness, itching, swelling, and other symptoms that we associate with allergic reactions.

Mast Cell Tumors

The mast cell can form a tumor made of many mast cells. When this happens, the cells of the tumor are unstable. They release their toxic granules with simple contact or even at random, creating allergic symptoms that do not correlate with exposure to any particular allergen.

Most mast cell tumors arise in the skin, but technically they can arise anywhere that mast cells are found. The mast cell tumor doesn’t have a characteristic appearance because of the tumor’s ability to cause swelling through the release of its granules. It is not unusual for the owner to notice a sudden change in the size of the growth or that the growth is itchy or bothersome to the pet.

Mast Cell Tumors in Dogs

Mast cell tumors are common in dogs, accounting for roughly one skin tumor in every five. The Boxer is at especially high risk, as are related breeds such as the English bulldog and Boston terrier. Also at higher than average risk are the Shar Pei, Labrador Retriever, Golden Retriever, Schnauzer, and Cocker Spaniel.

Are some mast cell tumors worse than others?

Anatomic location — Mast cell tumors arising in the nail bed, genital areas, muzzle, and oral cavity tend to be the most malignant (or cancerous). Mast cell tumors that originate in deeper tissues such as the liver or spleen carry a particularly poor prognosis.

After biopsy, mast cell tumors can be classified as Grade I, II, or III. See below for more information.

How are mast cell tumors diagnosed?

Diagnosis is usually made with a needle aspirate, which collects some cells of the tumor with a needle. The cells are then examined under a microscope, where the granules have distinct staining characteristics.

What happens after diagnosis?

Depending on the location and size of the tumor, the next step is to surgically remove it and have it biopsied. Mast cell tumors can be very extensive, with cancerous cells spreading beyond the visible portion of the tumor. This may require a wide area of tissue be removed in order to get all of the cancerous cells. The pathologist will confirm the diagnosis as well as determine if the tumor was completely excised.

The pathologist will also grade the mast cell tumor. The grade is a reflection of the malignant characteristics of the cells under the microscope, which generally correlates to the behavior of the tumor. Grade I is benign, grade III is malignant, and grade II is somewhere in between, meaning a grade II tumor can be unpredictable in its behavior. The majority of mast cell tumors are classified as grade II tumors.

In many cases, mast cell tumors can be cured with surgery. If the original biopsy shows that the tumor has only narrowly been removed of that the tumor extends to the margins of the sample, a second surgery may be done to get the rest of the tumor. If the mast cell tumor is not completely excised it will grow back eventually; it is best to get it all and be done with it as quickly as possible.

What if surgery alone doesn’t cure my pet?

If surgery did not result in complete excision of the tumor, if the location of the tumor makes it difficult to remove, or if your pet has evidence of more extensive spread, additional therapy is required, such as radiation therapy or chemotherapy. Before any further treatment is started, your veterinarian may recommend the following tests:

  • Blood work — A basic blood panel will uncover any problems that limit kidney or liver function and thus determine what drugs can or cannot be used. Bloodwork will also show important red blood cell, white blood cell and platelet counts, which are monitored closely during chemotherapy.
  • Lymph node aspiration — The lymph nodes near the location of the tumor may be aspirated to see if there is any evidence of tumor spread.
  • Radiographs/Ultrasound — These tests are used to detect the presence of tumor in deeper organs such as the spleen and abdominal lymph nodes. The size of the spleen can be evaluated with radiographs, but ultrasound guidance is generally needed to withdraw some cells for testing. While the mast cell tumor does not spread to lungs the way other tumors do, there are many lymph nodes in the chest; it is helpful to radiograph the chest to assess the size of these lymph nodes and thus determine the extent of tumor spread.

Treatments

Radiation Therapy

While radiation therapy tends to be expensive, the potential to permanently cure a grade I or II mast cell tumor is likely worth it for most pet owners. Radiation therapy is most appropriate for localized disease. If there is evidence of more distant spread, radiation becomes less helpful and medications, which can be delivered to the tumor through the patient’s own blood system, become necessary.

Chemotherapy

Chemotherapy simply means treatment with medication instead of treatment with surgery or radiation. Some of these medications are oral (usually a pill), some are injectable (or intravenous).

Chemotherapy in pets is different from chemotherapy in people. Because pets can’t understand that they have a serious disease they need to be treated for, we do not tolerate the same degree of side effects that are often seen with human chemotherapy treatments. Side effects can occur, but we do our best to minimize them as much as possible. The most common side effects include vomiting, diarrhea, and secondary infections. It is rare for animals to lose any hair during chemotherapy.

There are many different protocols (or combinations of drugs) suggested for the treatment of cancer. The specific drugs your pet receives will depend on your pet’s overall health status, any underlying medical conditions, and the financial commitment you are able to make.

Other Medications

One of the chemicals contained in mast cell granules is histamine, which can lead to inflammation and increased stomach acid secretion. These unpleasant symptoms may be alleviated with the use of certain anti-histamines (such as Benadryl) as well as drugs called H2 blockers (such as Pepcid AC).

Prognosis/Outlook

Many pets with a single tumor can be cured with surgical removal. However, these pets may be predisposed to additional mast cell tumors in the future, so any additional ‘lumps or bumps’ that come up should be immediately examined and a cytology performed.

In pets who require radiation or chemotherapy, a cure is somewhat more uncertain. Some pets are cured while others go into remission, meaning there is no sign of the tumors for a certain period of time, but eventually the cancer returns. It is impossible to predict which animals will be cured and which animals will go into remission, so our goal in treatment is to keep the pet free of any sign of cancer for as long a period as possible.

Lymphoma in Dogs

What is lymphoma?

Lymphoma is a common pet cancer of lymphocytes. To better understand what this means, it helps to know more about the lymphatic system (where lymphocytes live) and about cancer.

The lymphatic system is an important part of the immune system and involves a network of lymph vessels and lymph nodes throughout the body. The lymph nodes are connected by lymph vessels and serve as immune system centers where foreign proteins or disease organisms can be presented to the cells of the immune system. Lymphocytes are the primary cells of the lymph system. Lymph vessels also communicate with the blood stream at several areas.

How does lymphoma start?

Cancer starts with one cell or a small group of cells that have “gone wrong.” Such cells arise in our bodies all the time, and we have several natural mechanisms to destroy these cells before they get out of hand. Sometimes these abnormal cells escape our natural defenses and cancer develops. When this happens, the cells begin to divide quickly and without control.

When lymphocytes become cancerous, the lymph node swells. Malignant lymphocytes travel through lymph vessels to affect nearby lymph nodes. Soon all the nodes of the body are enlarged. Ultimately, the bone marrow (where cells of the immune system are formed) is affected and the immune system is destroyed, resulting in decreased appetite, weakness, vomiting, diarrhea, and/or secondary infections claim the victim’s life.

How did my dog get lymphoma?

In most cases, we do not know how dogs (or people for that matter) get cancer. There are many types of cancer and many possible causes of cancer, such as chemicals in our environment, sun exposure, assorted viruses, etc. There are also important genetic (or inherited) factors involved. At this time, there is no way to know what caused Lymphoma to develop in a given patient.

What is the treatment for lymphoma?

The treatment for lymphoma involves chemotherapy, which simply means treatment with medication (instead of treatment with surgery or radiation). Some of these medications are oral (usually a pill), some are injectable (intravenous).

Chemotherapy in pets is different from chemotherapy in people. Because pets can’t understand that they have a serious disease they need to be treated for, we do not tolerate the same degree of side effects that are often seen with human chemotherapy treatments. Side effects can occur, but we do our best to minimize them as much as possible. The most common side effects include vomiting, diarrhea, and secondary infections.

There are many different protocols (or combinations of drugs) suggested for the treatment of lymphoma. The specific drugs your pet receives will depend on your pet’s overall health status, any underlying medical conditions, and the financial commitment you are able to make.

My pet doesn’t seem sick. Can I wait to start treatment until they start to act ill?

Most dogs are not feeling particularly sick at the time of diagnosis, which makes the diagnosis of cancer so surprising. It may be tempting to hold off on treatment until your pet seems more ill. However, waiting can drastically reduce the chance for long-term survival. Better results are obtained if the patient is treated while he/she still feels healthy.

Can lymphoma be cured?

In some cases, lymphoma can be cured, meaning permanent removal of all traces of cancer such that no further treatment is needed for the rest of the dog’s life. However, it is much more common for lymphoma to go into remission, meaning after treatment with chemotherapy the pet has a period of time where there is no sign of cancer, but eventually the signs of cancer return and additional treatment is required.

For any patient, there is an approximately 75% chance of achieving remission. This means that there is an excellent chance of reducing the cancer to undetectable levels. The length of the remission (weeks, months, or years) is difficult to predict and depends on the drugs used and a number of other factors.

How will I know when my pet is in remission?

A patient in remission is indistinguishable from a completely cancer-free patient. The lymph nodes go down to a normal size and any signs of illness related to the cancer resolve.

How will I know when we have lost remission?

The most obvious sign will be that the lymph node enlargement has returned. This may mean that the cancer is now resistant to the drugs being used and new drugs may be chosen.

What are the chances of achieving a second remission?

Each time a cancer comes back, it becomes more difficult to treat. We are often able to achieve a second remission using different types of drugs, but generally, each subsequent remission is shorter and shorter in length.

What if I decide not to treat my pet?

Without treatment, animals with lymphoma are expected to live only 4-8 weeks from the time of diagnosis.

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