Time to progression

Progression-free survival (PFS) is a measure of the activity of a treatment on a disease. It is the time that passes from a certain date (generally the first day of treatment, or the day in which a patient is enrolled in a clinical trial) and the date on which disease "progresses" or the date on which the patient dies, from any cause. In Oncology, PFS usually refers to situations in which a tumor is present, as demonstrated by laboratory testing, radiologic testing, or clinically. A similar term that applies to patients that have had operations and are left with no detectable disease, is Disease-Free Survival.

Time to progression (TTP) does not count patients who die from other causes but is otherwise a close equivalent to PFS (unless there are a large number of such events). The FDA gives separate definitions and prefers PFS.

PFS is widely used in oncology. Since (as already said) it only applies to patients with inoperable disease that are generally treated with drugs (chemotherapy, target therapies, etc.) it will mostly be considered in relation to drug treatment of cancer.

A very important aspect is the definition of "progression" since this generally involves imaging techniques (plain radiograms, CT scans, MRI, PET scans, ultrasounds) or other aspects: biochemical progression may be defined on the basis of an increase in a tumor marker (such as CA125 for epithelial ovarian cancer or PSA for prostate cancer). At present any change in the radiological aspect of a lesion is defined according to RECIST criteria. But progression may also be due to the appearance of a new lesion originating from the same tumor or to the appearance of new cancer in the same organ or in a different organ, or due to unequivocal progression in 'non-target' lesions - such as pleural effusions, ascites, leptomeningeal disease etc.

Progression-free survival is often used as an alternative to overall survival (OS): this is the most reliable endpoint in clinical studies, but it will only be available after a longer time than PFS. For this reason, especially when new drugs are tested, there is a pressure (that in some cases may be absolutely acceptable while in other cases may hide economical interests) to approve new drugs on the basis of PFS data rather than waiting for OS data.
PFS is considered as a "surrogate" of OS: in some cancers the two elements are strictly related, but in others they are not. Several agents that may prolong PFS do not prolong OS. PFS may be considered as an endpoint in itself (the FDA and EMEA consider it such) in situations where overall survival endpoints may be not feasible, and where progression is likely or very likely to be related to symptomatology.

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