Term used in statistics as a yardstick in clinical trials. Time To Progression refers to the time until treatment failure, either because a therapy is no longer sufficiently effective (usually until PD), or because the therapy can no longer be given due to toxicity, refusal, co-morbidity or death. TTF is usually longer than PFS because, in addition to lack of effect, it also includes all other reasons for treatment failure, mainly treatment failure due to side effects. Good measure for assessing the benefit of treatment because it is a hard and comprehensive endpoint.