Introduction to Chemotherapy for Osteosarcoma
The term “chemotherapy” describes a group of medications used in the treatment of cancer. It is usually given intravenously but certain medications can be given by mouth. The medications travel in the body and seek out cancer cells to destroy them. These medications are best given by veterinarians and technicians with experience in handling and administration, as well as the potential side effects and monitoring that should be done. To find an oncologist in your area, ask your primary veterinarian, call the nearest veterinary teaching hospital in your area, or contact www.acvim.org or www.vetcancersociety.org.
Many people are afraid of giving chemotherapy to their pets because of the potential for side effects. There are effective medications that can minimize the most common side effects. In addition, the doses of chemotherapy given to our dogs for the management of bone cancer are not high enough to cause severe toxicity.
In the treatment of bone cancer, chemotherapy is usually prescribed in addition to other treatments, namely amputation surgery. The traditional approach is to perform amputation surgery first, then follow up with the chemotherapy. This approach has the highest survival rate of all treatment options for managing bone cancer, based on currently accepted statistics. Depending on the oncologist and the overall health of the dog, the chemotherapy may be administered immediately after surgery, or within two weeks of the surgery.
The purpose of giving the chemotherapy after the primary tumor has been removed is to catch any stray cancer cells that may have already traveled through the blood to other areas. In bone cancer, the most common place for the cells to travel is the lungs. When cells travel, this is called “metastases”. These metastatic deposits can be microscopic, not visible on xray. Since the medications also travel in the blood, they can find the metastatic cells and kill them.
The most commonly used chemotherapy medications used for management of bone cancer are cisplatin, carboplatin, and adriamycin. Some oncologists use a single agent in multiple doses several weeks apart, others suggest using combination therapy, rotating two drugs several weeks apart. The chemotherapy is given in “cycles” to provide the most exposure to different cancer cells as they are multiplying. Combination chemotherapy works in different ways and allows for cells to be destroyed at different stages in their cycle.
There is also another type of chemotherapy protocol, called the metronomic protocol. This protocol combines three medications: Piroxicam, deramaxx, Rimadyl (NSAID), doxycycline (an antibiotic) and cyclophosphamide, an oral chemotherapy drug. These three medications work on slowing down the blood supply to cancer cells, and also slow down their growth. The principle behind this is called “anti-angiogenesis” (angiogenesis means “creation of blood”). The metronomic protocol is usually used once the metastases are visible either on xray or exam, or in some cases where amputation is not an option. There are a lot of anectodal reports that support the use of these medications. The type of chemotherapy protocol for your dog should be discussed in detail with your oncologist. Most dogs handle chemotherapy very well. The most common side effects are nausea, vomiting, change in appetite and diarrhea. These side effects usually last for several days and can be minimized or avoided altogether with medications prescribed by your oncologist. Most doctors will also advise not to feed your pet on the morning of the treatment. The chemotherapy sessions can last several hours or possibly all day. Blood work and urine tests are usually done before the chemotherapy is administered, and should be checked in between cycles as well. Occasionally the chemotherapy can affect the white blood cell count or liver/kidney functions, which is why monitoring is so important.