Progression-free survival

Progression-free survival (PFS) is a type of survival rate that measures the length of time during and after medication or treatment during which the disease being treated (usually cancer) does not get worse. It is sometimes used as a metric to study the health of a person with a disease to try to determine how well a new treatment is working.

It is often used as a clinical endpoint in randomized controlled trials for cancer therapies.

It is a metric frequently used by the UK National Institute for Health and Clinical Excellence and the U.S. Food and Drug Administration to evaluate the effectiveness of a cancer treatment. PFS has been postulated to be a better ("more pure") measure of efficacy in second-line clinical trials as it eliminates potential differential bias from prior or subsequent treatments.

However, PFS improvements do not always result in corresponding improvements in overall survival, and the control of the disease may come at the biological expense of side effects from the treatment itself. This has been described as an example of the McNamara fallacy.

Time to progression
Time to progression (TTP) may not count patients who die from other causes but is often used as equivalent to PFS. The FDA gives separate definitions and prefers PFS.