Common Pet Cancers

What is lymphoma?

The lymphomas (malignant lymphoma or lymphosarcoma) are a diverse group of cancers that originate from a type of white blood cell called a lymphocyte. They are one of the most common cancers diagnosed in dogs and cats. This cancer usually arises in lymph tissues such as lymph nodes (lymph glands), spleen, and bone marrow; however, it can arise in almost any tissue in the body including the skin, the brain or spinal cord, bones, heart, or intestines. In the cat, it is most commonly found in the intestinal tract.

Lymphoma can occur in any breed. Middle-aged to older dogs and cats are most commonly affected; however, young cats that carry the feline leukemia virus may also develop lymphoma. In most cases, the cause of lymphoma is unknown; however, a weak association with herbicide (weed killers) is known in the dog and the feline leukemia and immunodeficiency virus can occasionally cause the disease in cats.

Signs

Because lymphoma can affect any tissue in the body, symptoms can be vague and may reflect the organ that is involved. Signs of lymphoma can include the following:

Generalized lymph gland swelling and enlargement (most easily felt under the jaw, in front of the shoulder and behind the knee)

Loss of appetite

Weight loss

Vomiting and/or diarrhea

Lethargy

Excessive drinking

Skin lumps

Neurologic signs

Most dogs with lymphoma still feel fine and the glandular enlargements are often noticed incidentally by the owner, the groomer, or the veterinarian during routine examination. In the cat, most cases involve the intestinal tract so diarrhea and weight loss are the most common symptoms in that species.

Diagnosis

In most cases a needle aspirate or biopsy of the affected organ(s) is required to make the diagnosis. This usually involves a biopsy or removal of a lymph gland in dogs as that is the most common organ involved. In cats and dogs with the intestinal form of the disease, biopsies may be obtained through the use of a fiberoptic scope or an abdominal exploratory surgery.

In addition to establishing a diagnosis based on a biopsy, several other tests are recommended prior to treatment. These tests are used to establish the extent of the cancer (i.e., clinical stage; how advanced the cancer is) and to more accurately determine the prognosis in each individual. Certain test results are known to predict whether a particular dog or cat is likely to respond to therapy. For example, determining if a pet has high blood calcium, bone marrow involvement, and/or tumors in the front part of the chest are all helpful in predicting outcome. Therefore your veterinarian will likely recommend blood tests, a bone marrow aspirate, and chest X-rays as part of the work-up. Also, a special type of analysis of the biopsy, called immunophenotyping, is recommended in dogs to determine whether the cancerous lymphocyte is a "B-cell" or a "T-cell" type. This is important because dogs with T-cell disease (approximately 20% to 25% of cases) tend to do poorly when compared with dogs with B-cell disease.

Your veterinarian will also ask a pathologist to determine if the lymphoma is a "high" or "low" grade subtype. This will help determine how aggressive the treatment will be. Additionally, blood tests to assess the overall health of the pet are important because the disease usually occurs in older animals that may have complicating diseases such as diabetes, kidney, heart, or liver disease.

Treatment

The type and extent of treatment varies greatly based on several factors including the extent (i.e., stage) of disease, the organs involved, the cell type involved and the caregiver/guardian's wishes with regard to cost, time commitment, and level of comfort regarding the potential to tolerate side effects.

In general, most pets with lymphoma have involvement of multiple glands and/or multiple organs, so called "multicentric" lymphoma. Therefore most treatment types available involve medications that go to all parts of the body (i.e., chemotherapy). The gold standard of care for multicentric lymphoma is a combination of four or five chemotherapy drugs given into the vein in an alternating fashion either weekly or every other week. This will usually result in a remission (absence of disease) in 90% of dogs and 65% of cats within 2 to 4 weeks. Treatment is normally continued for approximately 6 months depending on the protocol used. After the treatment is stopped, dogs and cats are checked monthly for recurrence of their cancer.

Alternatives to multi-drug chemotherapy protocols exist. These usually involve the use of just one of the drugs used in combination protocols. In general, these tend to be less effective but also require fewer visits to the veterinarian, are less costly, and have less chance for side effects. Clients who do not wish to pursue chemotherapy should consider a course of an oral steroid medication called prednisone. Although this may help some dogs, the response to prednisone tends to only last 1 or 2 months.

In rare cases where the lymphoma only involves a single site (e.g., one skin lesion, bone, nasal cavity), then whole-body chemotherapy may not be necessary and local treatment by surgery and/or radiation therapy may be the best treatment. It should be remembered, however, that the disease may eventually return in a "multicentric" form months or years down the road.

In the less common forms of low-grade lymphoma, a less aggressive form of oral chemotherapy is often recommended. This also applies to certain forms of lymphoma that involve the skin.

Prognosis

Without treatment, dogs and cats with multicentric lymphoma live an average of 6 weeks. This, of course, varies greatly with the type and extent of lymphoma. 90% of dogs and 65% of cats with multicentric lymphoma will respond to multi-drug chemotherapy. The average lifespan for a dog treated this way is 1 year, with approximately 10% living 2 years or longer. In the cat, the average survival with treatment is 7 months; however, if a complete response is achieved approximately one third will live 2 years or longer. This is an average, and some dogs and cats live much longer (even cures in 5% of cases), whereas others, unfortunately, have resistant cancers and respond for shorter periods of time or do not respond at all.

Individual outcomes can be better predicted if the cell type, grade, organ involved, and stage are known. Using single-agent chemotherapy can result in remissions; however, the length of the remission is shorter than in multi-drug protocols. For those clients who choose simple steroid therapy (e.g., oral prednisone), responses tend to be only a month or two in length.

What is a mast cell tumor?

Mast cell tumors (MCT) are one of the most common malignancies (cancers) found on and under the skin of dogs. Although certain breeds such as Golden Retrievers, Labrador Retrievers, Boxers, Boston Terriers, Pugs, and Shar Peis appear to be predisposed to this tumor, any breed or mixed breed can develop MCT. Some dogs appear to be genetically predisposed to developing MCTs, but the cause of this common cancer is not known. Although MCTs most commonly afflict middle-aged to older dogs, they are also found with surprising frequency in pets that are young. Approximately 10% to 15% of dogs with this cancer develop multiple MCTs throughout their lifetime. When this unique behavior occurs, the tumors may seem to erupt simultaneously or develop over months to years.

Signs

A wide range of signs are associated with MCT. Most common are variably sized skin swellings. Other signs that are commonly associated with MCT are related to the unique role that normal mast cells play in animals and humans. If you look at a normal mast cell under the microscope, you will often see a number of purple "granules" within the cell. Histamine and other substances that play important roles in inflammation are within these granules and are responsible for many of the signs that you might see in people with allergic reactions after a bee sting or asthma attack. Therefore, if the contents of these granules are released from a MCT, their symptoms may include the following:

Swelling, itching, redness, skin ulceration, or bruising at the tumor site

Changes in size and shape of mass, particularly after it has been manipulated

Abdominal discomfort and vomiting may indicate ulceration of the gastrointestinal (GI) tract, caused by histamine release from the granules

Anemia, caused by extensive bleeding into the stomach or intestines

Diagnosis

When making decisions regarding a dog with MCT, three questions should be addressed:

1. Is there evidence that the tumor has spread in my dog?

Mast cell tumors tend to spread first to lymph nodes that are near the tumor. For that reason, the most valuable diagnostic tests that may be recommended are the following:

Aspiration cytology (withdrawal of a small tumor sample via placement of a small needle into the tumor) of the "regional" lymph node

Biopsy (surgical removal of a sample of the tumor) of the tumor or "regional" lymph node.

After lymph nodes, MCT may spread to the spleen, liver, bone marrow and rarely lungs. Whether testing of these additional sites is indicated will be based upon the ease of obtaining a good sample from the lymph node, the apparent aggressiveness of the pet's MCT, and the clinical judgment of your veterinarian. In most circumstances, testing of blood, liver, spleen, and bone marrow will be associated with a lower yield of helpful information. This is explained by two facts: first, small numbers of normal mast cell tumors may be found in these particular organs. Therefore it may be difficult to differentiate an increased number of normal mast cells in these organs from similar- looking cells that represent tumor spread. Recall that normal mast cells are part of the response to allergic diseases. Secondly, the incidence of tumor spread to internal organs (spleen, liver, bone marrow) is quite uncommon with low-grade or well-differentiated mast cell tumors. Unfortunately, if evidence of MCT spread is documented in lymph nodes or internal organs, the chance for cure is reduced. Instead of common treatments, your veterinarian may discuss different types of therapy with you.

2. What is the grade of my dog's MCT?

After a biopsy or tumor removal, the pathologist will evaluate the tumor tissue and report which category (grade) it best fits into:

Grade I: A more well-differentiated or lower grade tumor

Grade II: A tumor with intermediate differentiation that extends more deeply into surrounding tissues

Grade III: A higher grade, poorly differentiated tumor that may replace the skin and underlying tissues

This report will be important in providing information that is useful for predicting how aggressively this tumor may spread and whether additional therapy such as chemotherapy should be added to the treatment plan. As the tumor grade increases, the chance for spread to internal organs is also higher. The pathologist will also determine whether the entire tumor was removed or if tumor cells remain behind. If tumor cells remain behind, you may be provided with additional options for treatment including a second surgery, radiation therapy, or chemotherapy.

3. Are there other MCTs or clinical factors that may affect decision-making?

It is certainly possible to remove more than one mast cell tumor from a dog. However, some dogs develop dozens of tumors almost simultaneously or develop new tumors with such rapid frequency that local treatment options such as surgery or radiation become less effective. Other medical conditions may impact decisions in such dogs, particularly if your pet is so sick that anesthesia itself becomes life threatening.

Treatment

The treatment options that may be prescribed for a MCT include the following:

Surgery

Radiation therapy

Chemotherapy

Supportive medical care

Appropriate treatment choices for each individual pet will be made based upon your preferences and answers to the three questions discussed above. The unpredictable behavior of MCTs must always be kept in mind when making therapeutic decisions. For Grade I or II MCTs, complete surgical resection is typically the treatment of choice. With these more well differentiated tumors, the chance of spread to other organs is lower and the primary focus is to effectively treat the tumor locally.

Because MCTs are often more extensive than they would initially appear, your veterinarian may remove more tissue and leave a longer incision than you would have anticipated. This is often necessary if the surgery is done with curative intent. Sometimes, the tumor is located in an area that would be difficult to completely remove. In this case, a large portion of the tumor may be removed with the option of following with another type of therapy, such as radiation, after the surgical incision has healed. The most common reason to recommend radiation therapy would be in cases where all of the MCT could not be removed, tumor cells were left behind, and further surgery is not deemed possible.

For dogs with high-grade, poorly differentiated tumors, surgery and radiation may be used as local therapies, but these treatments will not address the high risk of cancer spread. For Grade III tumors, which have a high likelihood of spreading to other organs, and in cases with documented metastasis, regardless of the grade, chemotherapy may be offered as a palliative treatment alternative. Response to chemotherapy is somewhat unpredictable, but in most reports, it is suggested that approximately 30% to 40% of cases will have some response to the drugs that are currently most commonly used. Supportive medical care is appropriate for all patients and can include antihistamines, gastrointestinal protectants, and in some circumstances, corticosteroids.

Prognosis

The clinical course of MCTs is somewhat unpredictable, but all tumors are considered potentially malignant because of their ability to metastasize, or spread. Generally the spread potential for well-differentiated tumors is low (<10%) and that of intermediate grade tumors is low to moderate. Poorly differentiated tumors are associated with the greatest risk of metastasis and the shortest survival times.

What is a brain tumor?

Brain tumors are relatively common in older dogs and cats. Some tumors are primary brain tumors, meaning that they originate from the tissue in the brain cavity, and some are secondary brain tumors, meaning that they originate from outside the brain cavity but then invade the brain by extension (e.g., from the nose) or via the blood (metastasis).

Most brain tumors are diagnosed in dogs and cats older than 5 years and mainly in pets 9 years of age and older. However, younger animals can also be affected. Any pet can be affected but some breeds seem to be more predisposed (e.g., Golden Retrievers, Boxers, male domestic short-hair cats).

Signs

Clinical signs of brain tumor depend on the location of the tumor within the brain and its size. The most common symptoms of a brain tumor are circling, confusion, change in behavior, head tilt, loss of balance and seizures. Seizures are present in almost 50% of the cases. Of course, not all pets with those clinical signs have a brain tumor! Any other disease affecting the brain can also cause those clinical signs. For example, a pet that starts having seizures between 6 months and 5 years of age is more likely to have epilepsy whereas an older animal with a sudden onset of seizures probably does not have epilepsy and is more likely to have a brain tumor.

Depending on the rapidity of growth and the other problems that a tumor can cause (bleeding, swelling, compression), the symptoms can develop quickly (some people can swear that their pet was normal the night before) or slowly (sometimes the symptoms are subtle and progress over a period of weeks to months). Most cats with a brain tumor will be brought to the veterinarian because of a change in behavior. Sometimes, the only observation is that they are "not doing right." As you can see, no specific symptom can confirm a brain tumor.

Diagnosis

Because the brain is well protected in the skull and because most brain tumors won't cause changes in blood tests, advanced imaging of the brain via CT-scan or by magnetic resonance imaging (MRI) is necessary to diagnose a brain tumor. CT-scan is a technique using x-rays that allows "slices" of brain to be imaged. MRI is a technique using an extremely strong magnet (magnetic field) to produce good quality, detailed images of the brain. Both techniques are pain free and noninvasive, but because your pet would need to lie absolutely still, these procedures must be performed with your pet under anesthesia.

Before advanced imaging procedures are performed, blood tests, radiographs, and other diagnostic procedures might be recommended to rule out other diseases and to make sure that your pet can tolerate anesthesia. After the CT-scan or the MRI, a spinal tap analysis might be recommended to rule out other diseases that can mimic a brain tumor. Sometimes the type of tumor can be determined based on the appearance on imaging alone; however, it might be necessary to obtain a biopsy to know exactly what type of tumor it is.

Treatment

Treatment can be supportive and/or specific. Supportive therapies include, among others, treatment of seizures with an antiepileptic drug and treatment of the swelling around the tumor with steroids.

Specific treatments include surgical excision, radiation therapy, and chemotherapy. The most common types of brain tumors in dogs are meningiomas and gliomas. In cats, meningiomas are the most common. A meningioma is a tumor of the "envelope" of the brain (meninges) and, depending on its location, it may be readily accessible for surgical removal (mainly in cats where those tumors tend to be on the top of the brain and pushing, but not invading the brain). Gliomas are tumors of the brain cells and are most of the time deeper in the brain and more difficult to reach for a surgical removal. For hard-to-reach tumors, radiation therapy and/or chemotherapy are recommended.

Prognosis

The prognosis following a diagnosis of a brain tumor in a pet is often difficult to predict. With surgery and/or radiation therapy, a median survival time of 10 to 24 months is reported. For meningioma in cats, it can be as long as 2 years and sometimes longer. Median survival time means that 50% of the patients will live longer and 50% will live for a shorter period of time. With supportive therapy only, the survival time is usually a matter of a few weeks to a few months. The severity of the clinical signs and the response to the treatment chosen will influence the quality of life. As a companion, you are the best person to evaluate the quality of life of your pet.

What is a mammary tumor?

Mammary gland tumors are common in dogs and cats that are not spayed or in animals that were spayed later in life. In dogs, early spaying provides a strong protective effect; dogs spayed before their third heat cycle are much less likely to develop a mammary gland tumor than dogs spayed after their third heat cycle. Although the protective effect is not as large in cats, it is clear that early spaying is also of benefit to this species. Treatment with megestrol acetate is also linked to the development of mammary tumors in animals.

Signs

Mammary tumors are similar to breast tumors in people. Usually no symptoms occur other than finding a "lump" when petting your dog or cat. Uncommonly, mammary tumors can bleed or become infected. Because some of these tumors are malignant, the can spread to other parts of the body and cause illness, loss of appetite, weight loss, or lethargy

Diagnosis

The diagnosis of a mammary gland tumor relies primarily on evaluation of a biopsy. A safe rule of thumb regarding mammary gland tumors is that about 50% of these tumors are benign and 50% are malignant. Further, 50% of the malignant tumors in dogs can be cured with just surgery. Unfortunately, mammary gland tumors in cats are almost always malignant. In dogs and cats, the smaller the tumor is at the time of diagnosis the better the outcome.

Tests that may help your veterinarian define your pet's prognosis by checking for metastasis (spread of cancer) include the following:

X-rays of the thorax [and abdomen]

Abdominal ultrasound

Biopsy of lymph nodes that drain the site of the tumor

Fine needle aspirate of lymph nodes that drain the site of the tumor

Blood work to evaluate overall health of pet

Treatment

All dogs with benign mammary tumors can be cured with surgery. As mentioned above, 50% of dogs with malignant mammary gland tumors can be cured with surgical removal of the mass. However, even if the tumor is completely removed with surgery, your dog may develop another mammary tumor in the future. For the 50% of dogs with malignant tumors that cannot be cured with surgery, chemotherapy or radiation therapy may also be offered (see discussion below).

Factors that help your veterinarian decide whether your dog should be treated with chemotherapy rather than surgery alone include the following:

The size of the tumor at the time of diagnosis

Whether or not the tumor site is ulcerated

If evidence exists of spread of the tumor to the lymph nodes, lungs, or other sites of the body

Characteristics of the tumor on microscopic examination

Some drugs that are commonly used to treat dogs with malignant mammary gland tumors are piroxicam, doxorubicin, and carboplatin. Piroxicam is an anti-inflammatory drug that also has anticancer activities. The main side effects of piroxicam include loss of appetite and vomiting secondary to stomach irritation. The side effects may be minimized by giving the medication with food. If your pet has a diminished appetite, vomiting, or a change in the consistency of stool while on piroxicam, discontinue the medication and contact your veterinarian. The other drugs listed are more traditional anticancer drugs (i.e., chemotherapy drugs).

Two equally important goals should be kept in mind when choosing chemotherapy to treat your pet:

Control the tumor for as long as possible

Maintain a good to excellent quality of life throughout the duration of treatment

The chemotherapy drugs used for dogs and cats are the same drugs used in people. Because maintaining a normal quality of life is a main goal, chemotherapy is used somewhat less aggressively than in human medicine. Also, dogs and cats seem to be more tolerant of chemotherapy than people. They do not usually have side effects as severe as those in people. However, because chemotherapy drugs affect rapidly dividing cells, side effects can occur in dogs and cats.