Understanding Veterinary Lingo: What is a 'Prognostic Factor'?
Prognostic factors are characteristics possessed by a patient, its tumor, or both. They predict the likely course of the cancer, and ultimately, your pet's prognosis, or final outcome.
Prognostic factors could help estimate a patient’s survival time, chance of success with a particular treatment plan, or risk for recurrence of disease following surgery, radiation, or chemotherapy.
Prognostic factors are designed to help owners and veterinary oncologists decide on the need for additional testing, potential treatment options, and also to provide a realistic expectation of outcome. Most studies investigating various cancers in pets include an analysis of specific prognostic factors in some capacity.
Much weight is given to the statistical significance of prognostic factors and they largely influence meaningful medical decisions, including those related to life and death. For example, immunophenotype is a prognostic factor for dogs with lymphoma. For dogs being treated with chemotherapy, those who have a B-cell phenotype tend to have a longer lifespan than dogs with a T-cell phenotype. Some owners will therefore base their decision to pursue treatment based solely on the result of the phenotype testing.
Unfortunately, many times prognostic factors fail to provide clinically relevant information. Dogs with nasal tumors who experience nosebleeds have a significantly shorter survival time than dogs without nosebleeds (88 days vs. 224 days). At first glance, one might assume dogs with nosebleeds have more aggressive tumors, or are sicker from their disease. Yet clinically, my observations tell me this is untrue.
I would argue that a bleeding nose is a negative prognostic factor for a dog with a nasal tumor primarily because the nosebleed is perceived as producing a negative impact on the pet’s quality of life. The nosebleed also negatively impacts the owner’s lifestyle, as these events can be dramatic, messy, and difficult to manage.
I still explain to owners of dogs with nasal tumors and nosebleeds that research tells me their dog’s expected lifespan is about three months. However, I am clear that most of those dogs are euthanized because of the physical issues caused by the nosebleed itself, rather than because of outward signs of pain, illness, or other concerns.
As another example, data tells me the tumor size is a prognostic factor for dogs with oral melanoma, with differences in outcome for dogs with tumors less than 2cm, those with tumors between 2-4cm, and those with tumors >4cm. Logically, we can make sense of the concept that the larger a tumor is, the more impacting it would be for the pet.
Does this mean I offer the same prognosis for a tiny Chihuahua as I would for a Great Dane if both were diagnosed with a 2cm oral melanoma tumor? Logic dictates that although tumor size would be important, so would the size of the mouth hosting the tumor. Veterinary patients exist on an enormous spectrum of shapes and dimensions, therefore tumor size must be interpreted in light of patient size.
A particular characteristic determined to be a statistically significant prognostic factor in one study can be refuted with additional study. For example, age was shown to be a prognostic factor for dogs with osteosarcoma in one research study, but had no impact on survival in another.
When we focus too much on specific prognostic factors, we lose sight of the bigger picture. My patients are more than a simple set of descriptive values or categorical characteristics. Generalizations are valuable to an extent, but they cannot predict individual response.
I always consider known prognostic factors when making recommendations about my patients' care. I’m also humble enough to remember that every animal is a uniquely created organism with unpredictable responses and outcomes, and that treating the individual is far more important than treatments based solely on statistics and probabilities.
Prognostic factors have value, but they certainly aren’t the bottom line. I urge owners to keep this in mind when considering pursuing treatment for their pet with cancer.