These notes are provided to help you understand the diagnosis or possible diagnosis of cancer in your pet. For general information on cancer in pets ask for our handout "What is Cancer". Your veterinarian may suggest certain tests to help confirm or eliminate diagnosis, and to help assess treatment options and likely outcomes. Because individual situations and responses vary, and because cancers often behave unpredictably, science can only give us a guide. However, information and understanding about tumors and their treatment in animals is improving all the time.
We understand that this can be a very worrying time. If you have any questions please do not hesitate to ask us.
What is the lymphoid system?
"Lymph is a fluid that circulates in the body, transporting cells of the immune system."
Lymph is a fluid that circulates in the body, transporting cells of the immune system (macrophages and lymphocytes) to sites where they are needed and draining areas where excess fluid or debris has accumulated, such as occurs with inflammation. Macrophages (literally "big eaters") are cells that "eat" (phagocytose) and digest other cells, infections and debris. They collect antigens from various sites and carry them to lymphocytes, other specialized cells that are concentrated in the lymph nodes (glands). Macrophages are filtered out of the lymph in the lymph nodes. In the lymph nodes, 'B' type lymphocytes, through their derivative 'plasma cells', make antibodies to the antigens brought by the macrophages (humoral immunity); and 'T' type lymphocytes prepare themselves to attack the foreign antigens (cellular immunity).
Before birth, T-lymphocytes develop in the thymus in the chest and B-lymphocytes in the bone marrow. After birth, lymphocytes are found in large numbers in the thymus and lymph nodes, and accumulate in collections of lymphoid tissue in areas where foreign antigens are likely to enter the body. These include the tonsils in the throat, and throughout the respiratory and intestinal tracts. The spleen, which acts as a filter for the blood, also contains a large amount of lymphoid tissue.
What are the different types of tumor?
The lymph nodes can become swollen or enlarged for several different reasons, including hyperplasia, inflammation and cancer.
Swelling of the lymph nodes may be due to increased activity (hyperplasia) caused by generalized or regional infection or other antigenic stimulus. It can also be due to inflammation (lymphadenitis) within the lymph node. Sometimes the node is simply caught up in surrounding inflammation but some infectious organisms protect themselves by living inside lymphocytes and macrophages. These infections include immunodeficiency viruses, tuberculosis and a protozoan parasite called Leishmania. Sometimes, cancer cells from other tissues may travel through the lymph and collect in the regional lymph nodes (lymph nodes that drain a specific area or region of the body) where they continue to multiply.
Cancer of the cells of the lymph nodes (lymphoma, lymphosarcoma) has to be distinguished from other causes of lymph node swelling by histopathology. Some types of cancer are slower growing than others but all are potentially life-threatening. Cancer can originate in any lymphoid tissue.
What do we know about the cause?
"Infections are important causes of lymphoid cancers."
Infections are important causes of lymphoid cancers. Feline leukemia virus (FeLV) causes a variety of cancers of the blood and lymph system in cats. Different strains of the virus cause cancers at different times. If a cat is also infected with Feline immunodeficiency virus (FIV), the risk of developing cancer increases. Recently, a virus infection has been demonstrated in one form of canine leukemia (cancerous lymphocytes in the blood). Lymphoid tumors starting in the intestine may, as in people, be promoted by Helicobacter bacterial infection. However, not all lymphoid cancers are associated with infections, so other genetic and environmental factors are also important.
Why has my pet developed this cancer?
Your pet may have a genetic tendency to cancer and have had an infection or contact with chemicals in the environment that have initiated or promoted the cancer. Your cat may currently be infected with FeLV or FIV or have been exposed to viral infection.
Are these common tumors?
These are common cancers. In dogs, the risk of developing these tumors is high, with estimates of 13-24 cases of lymphosarcoma (lymphoma) per 100,000 dogs. Pups as young as four months may have these cancers but 80% of cases occur between the ages of 5 and 11 years. Boxers have a higher incidence than other breeds.
Lymphosarcoma is the most common cancer in cats making up approximately one in three cancer cases. In surveys, the incidence is 50-200 per 100,000 cats. The age of tumor onset has two peaks, one in early adulthood at about two years of age and a second in mature cats aged 6-12 years. Cats as young as six months may be affected and purebred cats such as Siamese are predisposed to these cancers. Male cats may have more cancers than females.
"Tumor incidence varies from region to region and in different countries."
With FeLV testing and vaccination, there has been a marked decline in the number of FeLV positive cats but not in the incidence of tumors. However, now these tumors occur more often in older cats and less often in younger cats than in the past. Tumors of the thymus still occur in young cats but tumors in lymph nodes, multiple organs and the intestine are more common in older cats. Tumor incidence varies from region to region and in different countries.
How will these cancers affect my pet?
Four out of five dogs with lymphoma or lymphosarcoma have tumors that start in multiple places (multicentric). There is bilateral and symmetrical swelling of the lymph nodes without pain. Other signs depend on the organs involved and include lethargy, fever, weight loss, diarrhea, increased urination, anemia and/or the appearance of small areas of hemorrhage or bleeding on body surfaces. Some dogs have abnormal lymphocytes in the blood (leukemia).
Most cats present with masses in the chest or abdomen and therefore their symptoms include respiratory difficulty or weight loss, diarrhoea or constipation and vomiting. Kidney failure and anemia are also common. The thymic (chest) type is most common in young cats.
About 10% of these tumors induce signs that are not readily explained by spread of the tumors. These are known as paraneoplastic syndromes. Some are due to abnormal hormone production by the cancer. Examples include increased blood calcium levels and increased blood gamma globulin (immune system related protein). Both these adversely affect kidney function with increased thirst and urination.
How are these cancers diagnosed?
Your veterinarian may suspect the presence of this cancer based on the pet's clinical signs and the results of a physical examination. X-rays and ultrasound may be useful in detecting internal tumors. There are no consistent blood abnormalities for these types of tumors. Checking for FeLV antigen in the blood can sometimes be helpful in cats.
In order to identify the tumor definitively, it is necessary to obtain a sample of the tumor. Various sampling procedures may be needed, and may include needle aspiration, punch biopsy, full excision biopsy or exploratory surgery (for internal tumors). Once obtained, the tissue samples will be examined under the microscope. Cytology is the microscopic examination of cell samples, and is often useful for rapid or preliminary testing with this type of tumor. Histopathology is the microscopic examination of specially prepared and stained tissue sections from the tumor biopsy sample. Your veterinarian will send biopsy samples to a specialized laboratory where a veterinary pathologist will examine the prepared slides. The information from this examination is more detailed and reliable than cytology.
The histopathology report typically includes additional information that helps to predict how the cancer is likely to behave. Diagnosis of lymph node cancers can be difficult and not all enlarged lymph nodes are cancerous. Some types of hyperplasia may eventually progress to neoplasia (cancer), so repeat sampling may be needed for certain types.
What types of treatment are available?
A few tumors (e.g. in the intestine) are solitary, but most have already spread before diagnosis. It is currently believed that once cancer is present, there are always a few cancer cells circulating in the blood. The potential for spread of the cancer is determined by the 'homing' patterns of the tumor cells, which permits them to attach in some sites. Removal of a solitary enlarged lymph node, tonsil or other mass does not cure the disease although it does slow the progress of some types.
"Removal of a solitary enlarged lymph node, tonsil or other mass does not cure the disease although it does slow the progress of some types."
In some countries, chemotherapy is used to induce remission of disease and prolong life. It rarely cures the disease. Significant remission is more likely for smaller and more rapidly dividing tumors. The drugs used are toxic to organs with dividing cells such as the intestine, bone marrow and skin. Some are also toxic to other organs such as the liver and nerves so may induce malaise. The optimal chemotherapy protocol is still uncertain.
Steroid drugs such as prednisolone or prednisone will give short-term palliation for up to a few months. However, their use may promote resistance to other chemotherapy drugs and may shorten remission of subsequent multi-drug chemotherapy.
Can these cancers disappear without treatment?
In people, early intestinal cancers due to Helicobacter infection can disappear if the infection is cured. We do not know if this happens in animals. As all of these cancers have ready access to the lymph and blood transport systems, they are often widespread before diagnosis so even loss of blood supply to one tumor does not cause the cancer to disappear. Poor blood supply and degeneration of internal tumors is relatively common but does not eliminate them.
How can I nurse my pet?
After biopsy or surgery, your pet must not interfere with the surgical site, which needs to be kept clean. Any loss of stitches or significant swelling or bleeding should be reported to your veterinarian. You may be asked to check that your pet can pass urine and feces or to give treatment to aid this. He or she may also require a special diet. If you require additional advice on post-surgical care, please ask.
If your pet is to have chemotherapy, you need to understand the risks involved in the use of these unlicensed and potentially toxic drugs. The safety precautions required to protect yourself, other people and the environment when handling and disposing of the drugs will be explained if you consent to their use.
How will I know how the cancer will behave?
The histopathology report will give your veterinarian the diagnosis of the specific type of tumor, and that helps to indicate how it is likely to behave. The veterinary pathologist usually adds a prognosis that describes the probability of local recurrence or metastasis (distant spread).
When will I know if the cancer is permanently cured?
"As indicated above, these tumors can rarely be cured."
As indicated above, these tumors can rarely be cured. Without treatment, dogs have an average life expectancy of ten weeks, but a few live for six to twelve months. Survival with the intestinal form is only eight weeks. Older dogs tend to survive longer than younger dogs. Remissions induced by chemotherapy for up to a year are not uncommon in dogs but depend on the type and stage of the cancer at diagnosis.
Without treatment 40% of cats are dead within four weeks and 75% eight weeks following diagnosis. The average survival time with chemotherapy is 3.5 months for virus positive cats and 5 months for non-viral infected cats.
Lymphoid cancer in the chest, tonsil or bone marrow, blood or multiple organs often progress more rapidly than those only present in lymph nodes or a single organ.
Are there any risks to my family or other pets?
Feline leukemia virus can cause cancers of both the blood and lymphoid system. The virus is occasionally transmitted from an infected queen to her kittens before birth but is more commonly acquired from close contact with infected cats, which shed the virus in saliva, urine and feces. If your cat is infected, he or she can pass the infection to other cats. The infection is not transmissible to people. Similarly feline immunodeficiency virus, which is similar to HIV in people only affects cats and cannot infect people or other animals such as dogs.