Dear Pet Docs: My dog feels fine, but while petting him, I noted some swelling behind his knees. My vet says they are lymph nodes and has me worried about cancer. Should I be?

A: Dogs and cats have lymph nodes just as we do. They are located all over the body, both on the inside and the outside. The easiest to find are external lymph nodes located on either side of the base of the jaw, in front of the shoulder, along the chest wall where the arm attaches (arm pit), inguinal area (between the back legs) and behind the knee. A normal lymph node is hard to find. The two reasons they will become large enough for us to feel are in response to inflammation (normal) and cancer (abnormal).

Cancer may spread from another location into the lymph node. I (Perry Jameson) see this more commonly with internal tumors. Examples would be a lung tumor that has spread to lymph nodes in the chest or an anal sac tumor that has spread to lymph nodes in the abdomen. Cancer can start in the lymph node itself and this is called lymphoma.

Lymphoma is a very common cancer of dogs and is one of the most common forms of cancer in dogs that we treat here at our hospital. It is not known why certain dogs develop lymphoma. Some dogs may have an underlying genetic component, but for most dogs, there is no identifiable predisposing cause.

Lymphoma usually arises in the lymphoid tissues of the body (lymph nodes, spleen and bone marrow), but because lymphocytes are circulating in the blood stream and located in most tissues, it can affect any part of the body. The enlarged lymph nodes are usually not painful.

Lymphoma is divided into five stages. These stages are based on what areas of the body are involved.

With stage I lymphoma, only one single lymph node is involved, or a single lymphoid organ.

When the enlarged lymph nodes are confined to one side of the body (either in front of or behind the diaphragm) this is called Stage II.

With stage III lymphoma, lymph nodes are enlarged on both sides of the diaphragm.

If the liver and/or spleen are involved this is classified as a stage IV.

Stage V means that the lymphoma is in the blood or bone marrow or organs other than lymphoid organs such as eye, skin, nervous system, gastrointestinal tract.

Stages III and IV are most common.

By staging the lymphoma, we can provide a better prognosis as to how the dog will respond. There are other factors that we use when giving a prognosis for lymphoma as well, called substaging.

Substage A is when there are no outward signs of illness, the dog feels fine, and substage B is when signs of illness are present. The less symptoms and lower stage usually the better the response to therapy.

The diagnosis of lymphoma is usually made by aspirating a small sample from a lymph node and submitting this to a pathologist for review. Since the needle is small, this is no more painful than obtaining a blood sample. In rare situations, the pathologist may not be able to provide us with an answer from this small sample. The next step would be to submit the same sample for genetic testing to confirm or remove an entire node for evaluation.

To fully stage the extent of disease in some patients’ abdominal ultrasound, thoracic X-rays, and even bone marrow samples may be recommended. We may also recommend genetic testing to determine if it is B or T cell lymphocytes that are involved.

We recommend most dogs be treated with systemic chemotherapy, for lymphoma is always considered a systemic disease. This means that even though we are only finding it in one location, there are high odds it is already elsewhere at an undetectable level. Rarely surgery or radiation may be recommended, but it is almost always combined with chemotherapy.

Dogs tolerate chemotherapy well with most having few to no side effects. However a small percentage will and they can be severe. Chemotherapy primarily works by killing rapidly growing cells. Most cells in the body are not rapidly growing so are spared. There are few areas that continue rapid cell division throughout life. The biggest concern with bone marrow damage is a low white blood cell count that can predispose the dog to infections and a low platelet count that can cause bleeding. Intestinal injury can result in anorexia, vomiting and diarrhea. Side effects can be mild to life threatening. Hair loss is rare in dogs.

Without any treatment, the average survival for dogs with lymphoma is four to six weeks. Approximately 50 percent of dogs with lymphoma will respond to prednisone (a steroid) alone, but the remission times are only two to four months.

Remission is defined as the disappearance of the signs and symptoms of lymphoma in response to treatment, but the disease is still present.

Most dogs with lymphoma respond very well to chemotherapy and go into a state of remission. Approximately 10 percent of dogs with lymphoma are cured, so a “cure” is rare.

Our goal with chemotherapy is to obtain a remission, meaning that the lymphoma is under control and not detectable. When lymphoma comes out of remission, we can try other chemotherapy drugs that the lymphoma has not been exposed to.

For Stage I-IV substage B cell lymphomas, the average remission time is approximately 12-14 months with 25 percent having a remission rate of two years.

According to the literature, T cell lymphomas have shorter remission times (usually of approximately 6 months), but we have certainly had T cell lymphomas and stage V lymphomas that live much longer than 6 months with a very good quality of life.

Approximately 92 percent of dogs with lymphoma will go into a state of remission after the first one to two treatments. When a dog responds well to our normal chemotherapy protocol and is in a state of remission at six months, then we would stop chemotherapy and monitor her closely for recurrence.

When the lymphoma comes out of remission, it will usually be the same manifestation as the original presentation (i.e. the lymph nodes will enlarge again). At that time we may either start the protocol again or try a “rescue” protocol, meaning that we would use chemotherapy that the patient had not previously received. If during the original treatments the patient does not respond to the drugs that are normally used for first-line therapy for lymphoma, then a “rescue” protocol would be instituted.

So to answer your question, yes, I do feel it is worth determining why your dog’s lymph nodes are enlarged. Hopefully, it is not a serious problem but if it is the sooner you start treatment the better the response.

Dr. Henri Bianucci and Dr. Perry Jameson are with Veterinary Specialty Care LLC. Send questions to petdocs@postandcourier.com.