Chemotherapy and its use in small animal medicine

Chemotherapy is the use of drugs or chemicals to treat cancer. Chemotherapy may be used as the sole treatment for certain cancers or it may be used in combination with other treatments, such as surgery. It is often recommended for treatment if there is evidence that the cancer has spread (metastatic disease), or tumours are found in a number of sites (multicentric disease), or if tumours are unable to be removed by surgery.

Cytotoxic drugs attack cells in the process of growth and division. Individual drugs work in a number of different ways and certain cancers are sensitive to different drugs in the same way that different bacteria are sensitive to different antibiotics. Unfortunately, cytotoxic drugs cannot distinguish between cancer cells and other normal fast growing cells and it is the effect of the drugs on these cells that are often responsible for the side effects seen. Usually these normal cells are continuously growing and are able to repair any damage caused

The use of chemotherapy in small animals is slightly different to when used as a treatment in human medicine. In dogs and cats chemotherapy is used as a palliative treatment i.e. it is used where it can improve and maintain that animal’s life and a cure is rarely achieved. The ideal scenario is where we have managed to achieve a state of complete remission for the patient. This is where there is no sign of the cancer but there is a chance it could recur. Partial remission is where some of the cancer cells have been killed but not all but the cancer has stopped growing. Stable disease is where the cancer is at the same stage as it was prior to the start of treatment but it has not progressed.

How a chemotherapeutic agent is administered, how often it is given and how many treatments are given varies from case to case. The type of cancer, the extent of the disease and the general health of the animal help your vet derive a treatment protocol appropriate for your pet.

Some drugs are oral preparations (tablets/capsules) that you give at home. Others are injections that require an outpatient appointment. In some cases, slow infusions or repeated treatments throughout the day may require an animal to spend the day at the clinic. The treatments can be repeated from weekly to every third week. Blood tests may be required to monitor the effects of the chemotherapy.

The duration of the treatment depends on the type of cancer and the extent of disease. Some animals need to receive chemotherapy for the rest of their lives, while for other animals treatment may be discontinued after a period of weeks / months if the cancer has entered a remission state. Chemotherapy can be resumed when the cancer relapses. Unfortunately not all animals respond well to chemotherapy and so continuous assessment of the patient is essential, in order to ensure we maintain a good quality of life.

Compared to people who receive chemotherapy, pet animals experience fewer and less severe side effects because we use lower doses and do not combine as many drugs as is used in human medicine. Normal tissues, such as intestinal lining, bone marrow and hair follicles, are sensitive to chemotherapeutic drugs and so this results in the side effects seen.

Toxic effects to the gastrointestinal tract are responsible for decreased appetite, vomiting and diarrhoea. These effects can mild or severe but in most cases are mild and usually resolve on their own or with oral medication given at home. Rarely, severe side effects can occur and these can require hospitalisation and often occur 3 to 5 days after then chemotherapy is administered.

Suppression of the bone marrow by chemotherapy may cause a drop in the white blood cell count, often 7 to 10 days after the drug is given, leading to increased susceptibility to infection. The infection usually comes from the animals own body (such as bacteria normally found in the gut, mouth etc). Severe infections may require hospitalisation for intensive supportive care, including intravenous fluids and antibiotics. If the white blood cell count is low but your pet is feeling well than antibiotics are likely to be given as a preventative measure at home. Subsequent doses of chemotherapy are adjusted. Anaemia can be a complication of many cancers but is rarely caused by chemotherapy.

Hair follicle cells in animals that are non-shedding or wire-haired may be particularly susceptible to chemotherapy and a few breeds can show hair loss e.g. poodles and Old English Sheepdog. Hair loss can be more evident on the face and tail. Cats may lose their whiskers and long hairs over their eyes. Hair regrowth will occur but might have slightly different texture or colour.

There are many different types of chemotherapy agents and each has a different likelihood of causing side effects. If your pet is treated with drugs known to cause certain side effects, your vet will prescribe medications to help prevent these complications e.g. antiemetics to help prevent your pet from vomiting.

Please remember that any animal can have an unexpected reaction to any mediation.

Lastly, in order for your vet to decide on the appropriate treatment for your pet they must also consider your pet’s family life i.e. is there young children about, is there anyone who is pregnant or trying to become pregnant in the house, is there a member of the household receiving chemotherapy/radiotherapy treatment of their own. Answering yes to any of these questions does not mean that your pet would not be eligible for chemotherapy but it may mean that some protocols are more suitable than others.

Site by: CSS Web Design
© Clyde Veterinary Group 2006-2011