Cancer Treatment in Dogs and Cats

In this article, cancer specialist Gerry Polton follows on from his previous article ‘What is Cancer?’ and gives an overview of the ways we can treat cancer in dogs and cats.

Cancer Treatment in Dogs and Cats – Three Treatment Options

In essence there are three broad strategies for the treatment of cancer in cats and dogs. These are the same three conventional therapies also applied to human patients. They are:

  • Surgery
  • Chemotherapy
  • Radiotherapy

Thanks to very rapidly progressing scientific research that allows us to understand cancer better, new therapeutic strategies have also reached the bedside, so to speak in the past few years. We will look at some of these later.

A Simplified View of Cancer

Cancer can be simplified by thinking of it in two parts:

  1. The primary cancer or “primary” – This is the lump from which it all started.
  2. The secondary cancer “secondaries”, cancer which has spread to other sites within the body. There could be a single secondary cancer, but more commonly secondary cancer is widely spread. Remember, not all cancers will spread and have secondaries.

This is a useful simplification because it allows us to think logically about what general forms of treatment are best used in a particular situation.

Surgery, Radiotherapy and Chemotherapy – when?

drug delivery system

Chemotherapy is the most effective way of killing cancer cells that have spread around the body

Surgery, at best, can hope to achieve complete removal of the solid cancer mass being operated on. It cannot cure a cancer which is already affecting multiple sites. Like surgery, radiotherapy is also applied to a single anatomical region. Although one can use radiation over a broader area than can be removed by surgery, it remains a treatment for visible lumps rather than a treatment for cancer that has spread around the body and has ‘secondaries’. The only member of this group which can successfully reach disseminated, secondary cancers, is one that is also disseminated all over the body via the bloodstream and that is chemotherapy i.e the treament of cancer with anticancer drugs.

Therefore, one of the first decisions that has to be made when considering the treatment options for a cancer, is whether that cancer is likely to have spread or not. Some cancers are invariably disseminated before the diagnosis is made; a good example of this is lymphoma. Some cancers never spread; a good example of this is the benign fatty lump called a lipoma. There are unfortunately many shades of grey between these two opposite extremes and the cancer specialist needs to understand what is going on in each case to make the best treatment decisions for the individual.

Other considerations when weighing up treatment options include:

  • the risk of therapy
  • the availability of alternatives
  • the skill and experience of the individual performing the treatment
  • treatment costs
  • the outlook or prognosis for the patient even with treatment. (In other words, how likely is a cure?)

If a tumour can be cured by appropriate surgery, one would be more likely to consider that the risks associated with surgery are easier to accept.

Combining Treatments For Increased Effectiveness

Sometimes treatment strategies are combined. For example, osteosarcoma is a painful cancer affecting bones. In dogs, 90% of cases will have secondary spread at the time of diagnosis. The bone cancer is consistently very painful and surgical removal of the affected limb, if it is something that the patient will cope well with, achieves immediate absolute relief from that pain and suffering. However, if no action is taken to manage the secondary cancer spread which is so often present, then the survival time is typically only four months from the date of amputation. In these cases life expectancy is significantly prolonged and quality of life can remain excellent if chemotherapy is administered starting a suitable time after surgery. In this example, neither treatment alone achieves a really satisfactory outcome. However, in combination, they are stronger than the sum of their parts.

New Cancer Treatments For Pets

There are new treatment strategies reaching clinical practice. These include small molecule inhibitor drugs. I like to call these ‘designer drugs’. The term ‘designer’ captures two ideas which resonate with most pet owners: they sound clever and they sound expensive and this would be correct on both counts. Designer drugs work by specifically interfering in a process which occurs at the most minute level, at the level of the individual molecule. A drug molecule is designed by a computer to specifically interfere with a biological interaction between two molecules, the end result of which would otherwise be progression of a cancer. By interfering with this molecular interaction, the drive to grow or survive is removed, and the cancer will either stop growing or will reduce and even go away altogether. Unfortunately, there must be a near infinite number of molecular interactions that need to be interrupted for us to cure all cancers but exciting progress is currently being made.

antibodies attacking cancer cells

Antibodies attacking a cancer cell

The other major development in veterinary cancer is the development of a vaccine for canine melanoma. Malignant melanoma frequently occurs as a disseminated cancer and therefore patients typically die of the secondary cancer, regardless of how well the primary tumour is treated. Unfortunately, malignant melanoma is rarely responsive to chemotherapy so there was previously no treatment for this disseminated secondary disease. The melanoma vaccine works by tricking the body’s immune system into recognising a protein on the surface of melanoma cells and treating it like a foreign invader, a germ. The vaccine works best in the context of the patient whose primary tumour has been removed and whose secondary cancers are microscopic, we describe this as being ‘optimally-controlled’. The addition of the vaccine to our cancer armamentarium has changed life expectancy for these patients from approximately six months to over four years.