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 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 cancer treatment is.
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 cancer 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.
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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 may 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.
MST Is a Statistic, Not a Prediction
It’s important to understand that the median survival time never applies to a specific dog. Looking 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.
Let’s take a look at an image that might help illustrate this.
Notice the following in this illustration:
- In the group of 100 dogs, 50 had passed at the four-month mark. Forty of those passed at two months, and the rest at 4 months.
- The other 50 dogs passed over the next eight months. Note that no dogs in this hypothetical group passed away until the one-year mark, and some lived much longer, to 18 months.
- Even though all 100 dogs had passed within one and a half years, the median survival time is four months, not nine months.
- The “halfway point” is measured in the number of dogs, not the number of months.
The emotional experience of being in the first half versus the second half is very different, of course. The people whose dogs died by month two probably get little comfort knowing many more dogs in the study lived longer than their own dog.
Your dog does not have an expiration date. Don’t take the median survival time as a prediction.
From your point of view as a dog lover, it’s important not to take the median survival time as a prediction of how long your dog has to live. It’s not.
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.
Instead, the median survival time is a mathematical understanding of an approximate expectation.
Some dogs will live beyond this estimate, and some will not reach the median survival time.
So Why Use MSTs at All?
Each patient is an individual, and there is no reliable way to predict where a dog will fall with respect to the MST. So why do we even bother to think about these numbers? Why is it important to hear them? Why are they included in reports, and why do veterinarians mention them?
Some veterinarians do not mention median survival times, because they do not want to put a date in your head. They will give them if asked, however, if there is one to share. (Not every cancer diagnosis comes with numbers attached.)
Each patient is an individual. So is each dog lover.
Some veterinarians mention median survival times because they can give you a “heads up” about what to expect with and without treatment. This can help you make decisions about how to treat your dog’s cancer.
You Can Use Median Survival Times to Measure Life Quality
It is also important to understand that this estimated number measures quantity, not quality.
The length of time is an important factor in considering treatment, but it is not the only thing that guides decision-making when given a challenging diagnosis.
An equally important question is asking what that time will feel like to your dog. How will your dog feel physically? Mentally?
It’s also important to think about yourself in this process. Your own ability to handle the “four budgets” of dog cancer is just as important for processing, planning, and preparing.
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- 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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