When discussing cancer treatment, median survival time (MST) is a way of communicating how well a treatment may work to extend life. One way to think of it is as your dog’s “50/50” date - 50% of dogs are expected to live less than the MST, and 50% of dogs are expected to live longer than the MST. There may be a very wide range of survival times on either side of the MST.
What Median Survival Time Means
The median survival time (MST), or median overall survival, is the length of time from either the date of diagnosis or the start of treatment for a particular disease that half of the patients in a group with the disease remain living.1
The median survival time value may be used to compare dogs that receive treatment and those that do not. It may also be used to compare different treatments and how effective they are.
Where Median Survival Times Come From
In cancer research, the term median survival time comes from clinical trials that track how long patients survive once a diagnosis is provided or after a specific treatment is administered. It is a way of trying to understand how effective a treatment is or how aggressive a cancer is.
When a study is completed, the researchers will compile the data from all of the subjects and chart it out. The median survival time is the point at which half of the subjects have died, and half are still alive.
What It Means for Your Dog
There is no simple way to answer how long your dog will live with cancer, but median survival time gives you a way to understand how a diagnosis will impact your dog’s length of life and how potential treatment options compare to one another. This means being able to have detailed information to make the best decision for both you and your dog.
If the median survival time provided is for a specific cancer, that means that is the midway survival of dogs with that cancer who have been studied. Half of the dogs did worse; half did better.
If the median survival time provided is for a treatment, that means that it is the midway survival for dogs who received that treatment. Half of the dogs did worse, half did better.
It’s important to understand that the median survival time never applies to a specific dog. When you look at an entire group of dogs, you may see clusters of dogs who pass away all along the timeline, with no rhyme or reason. The median survival time is not “halfway” to the farthest point any dog in that group lived. It’s not an average.
Many dogs might die very close to the median survival time, or very far away from it. Once a dog has lived past that date, there is no telling whether it will die very soon after that date or well after it.
For example, in the following image, you will see that in a group of 100 dogs, 50 had passed at the four month mark. The other 50 dogs passed over the next eight months. Even though all 100 dogs had passed within one year, the median survival time is 4 months, not 6 months, because half of the group had passed by month four.
Median Survival Time is Not an Expiration Date
The median survival time as months, years, weeks, etc. does not provide an exact date for your dog’s end of life. It is a way of understanding an approximate expectation. There are dogs that will live beyond this estimate and dogs that will not reach the median survival time. It is important to remember that while MSTs can provide approximations of survival with and without treatment, which may assist in the decision-making process with respect to treatment, each patient is an individual and there is no reliable way to predict where a dog will fall with respect to the MST.
It is also important to understand that this estimated number is a measure of quantity, not quality. While the length of time is an important factor in considering treatment and diagnosis, it is not the only thing that guides decision-making when given a challenging diagnosis. An equally important question involves asking what that time looks like in terms of how you and your dog feel both mentally and physically. This is just as important for processing, planning, and preparing.
- NCI Dictionary of Cancer terms. National Cancer Institute. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/median-survival. Accessed November 15, 2022.
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