Progression-free survival is a trial endpoint measuring the time it takes for a patient to either progress or die. It is often thought to be a good substitute for overall survival, the “gold standard” endpoint. But with some forms of cancer, this can take an exceedingly long time to measure. So the FDA will often allow drugs to be approved if they can demonstrate that the treatment helps patients by lengthening the time it takes to progress or die from any cause (this could be due to cancer or due to the treatment).
PFS is different from time-to-progression (see “Time-to-event analysis“) because it factors in patient deaths. Therefore, it is generally considered the more robust endpoint in clinical trial design, although it can be difficult to interpret sometimes because of the inclusion of patient death.