Term in statistics used as a benchmark in clinical trials . PFS is usually defined as the time between the start of a clinical trial and the onset of disease progression (the progression of the disease) or the patient's date of death - whichever comes first. "Progression Free Survival" is a good measure of the effectiveness of a therapy. Clinically significant as an indirect measure of palliation and quality of life because control of tumor disease is thought to reduce or eliminate or postpone most complications e.g. pain, path. Fracture, hypercalcemia, anorexia, etc.