Chemotherapy – a realistic route for pets with cancer?

04 February 2020


The true incidence of cancer in companion animals is not currently known but cancer remains a major cause of death, particularly in older pets. For some years, chemotherapy has been one of the treatment options offered by the veterinary profession but what does this involve and how can we decide whether it is the right choice for any particular pet?

What is chemotherapy?

Chemotherapy refers to the use of drugs that target rapidly dividing cells and, as such, can kill or damage cancer cells. These drugs may be given as a single agent or in combination protocols, depending on the type of cancer and general health of the pet. Chemotherapy protocols are tailored to individual pets to provide the most effective treatment, while reducing side effects, and the protocols may be adjusted after treatment has begun.

How is chemotherapy given?

Most chemotherapy drugs are administered as a short injection or an infusion, via a cannula placed into a vein. Some drugs are given orally as a tablet form. Less commonly, chemotherapy drugs are given as an injection under the skin or into a body cavity such as the chest or the abdomen. The procedure can take up to several hours depending on the drug(s) used but treatment is typically administered as an outpatient, meaning that the pet returns home after the treatment. Before each session, blood needs to be taken and analysed to ensure that the patient can cope with the treatment.

What are the potential side effects of chemotherapy?

The basic principle of chemotherapy is that the higher the dose the more effective the treatment but also the more likely the side effects. In people, the main goal is to prolong survival for as long as possible and it is commonly accepted that up to 50% of patients experience side effects.

In pets, the aim is more to maintain or improve quality of life and therefore chemotherapy protocols are less aggressive than in people. No more than 20% to 30% experience side effects, and serious problems are seen in less than 5% of the cases.

Because of the way these drugs act (ie against dividing cells), healthy areas of the body can be affected adversely. The cells that line the gut and the cells in the bone marrow are the main areas that may be involved. Consequently, side effects can include inappetence, nausea, vomiting or diarrhoea and myelosuppression (a reduced function of the bone marrow), leading to decreased white blood cell and platelet counts. If the white blood cell count becomes very low, there is a risk of infection. With a very low platelet count, there is a risk of bleeding. Whiskers can be commonly lost (especially in cats) but substantial hair loss is not typically experienced by animals on chemotherapy for cancer. Dogs that have ‘synchronous’ hair follicle activity such as the poodle, Old English Sheepdog, Lhasa Apso and Shit Tzu are notable exceptions and can develop baldness with chemotherapy.

When do we use chemotherapy and how long is a course of treatment?

Chemotherapy may be used as a neoadjuvant treatment, prior to surgical intervention, ie it is used to reduce the size and invasiveness of the tumour and thus increase the surgical success rate. Typically, one to 3 doses of chemotherapy may be used prior to surgery, depending on the response seen.

Chemotherapy may be used as an adjuvant treatment after surgery, to eradicate hidden cancer cells that remain but are undetectable. The chemotherapy protocol depends on the type of cancer and general health of the pet but is typically given for a set period of time varying between 3 and 6 months.

When surgery is not possible because the disease is too advanced, chemotherapy can be given as a palliative treatment. The treatment is typically maintained for as long as a response is seen and as long as the pet maintains a good quality of life.

Finally, chemotherapy is the primary treatment for some cancers affecting the whole body such as lymphoma or leukaemia. There are different protocols used, some being continuous while others are discontinued after a period of a few months, providing that the pet is in remission.

Regular monitoring is performed throughout the treatment but also after completion of the chemotherapy protocol. This often requires repeated imaging procedures and other diagnostic tests.

Are any special precautions required during chemotherapy?

It is sometimes necessary to administer cytotoxic tablets or capsules at home. In this event, it is important to remember that they can be harmful to people and special precautions should be taken:

1) Women that are pregnant, attempting to conceive or lactating and small children should never be in contact with cytotoxic drugs or waste.

2) Disposable gloves should be worn at all times when handling cytotoxic medications;

3) Tablets should never be split or crushed, and capsules should not be opened;

4) Any unused medication should be returned to the veterinary practice for safe disposal;

5) If there are concerns with administering the tablets, or if the pet has received an overdose, veterinary advice should be sought immediately.

6) Saliva, vomit, urine and faeces should be considered as contaminated for at least the first 3 days after treatment (this can be up to 7 days or longer with some medications). Accidents within the household should be absorbed with paper towels (to contain the exposure) and thoroughly cleaned with bleach, as it is able to inactivate most cytotoxic medications. Gloves should be used for this purpose.

7) Water/food bowls should be washed separately

8) Soiled bedding should be washed twice, separately from other laundry, adding bleach to the normal detergent

What do ‘remission’ and ‘relapse’ mean?

Remission means that the treatment has been effective and the number of cancer cells has decreased to an undetectable level. However, this does not mean the pet is cured because even a few surviving cells can cause the tumour to relapse at some point in the future.

Relapse or recurrence is when a tumour comes back after treatment. Depending on the type of cancer, further treatment may be available using different drugs; this is commonly called ‘rescue’ or ‘second-line’ treatment

The Oncology Team at DWR Veterinary Specialists is run by Dr Davide Berlato, supported by a team of clinicians and dedicated oncology nurses.