event-free survival

Event-free survival is the length of time after primary treatment for a cancer ends that the patient remains free of the complications or events that the treatment was intended to prevent or delay. What counts as an event is set out in advance in the trial protocol, and can include the return of the cancer, the cancer growing, or the onset of certain symptoms, such as bone pain from cancer that has spread to the bone. Death is normally counted as an event too. It is often shortened to EFS.

In a clinical trial, measuring event-free survival is one way to see how well a new treatment works. It registers setbacks before deaths have accumulated, so a difference between two treatments can appear in the figures well before overall survival can settle the question. That is also its limit. The measure combines outcomes of very different weight, and the definition of an event varies from trial to trial, so two figures are comparable only when the protocols match. Results are usually quoted as the share of patients still event free at a fixed point, at two years for example, which is why a single trial can report several event-free survival numbers.