Time to progression (TTP) is a measure of time after a disease is diagnosed (or treated) until the disease starts to get worse.
Progression-free survival (PFS) is the length of time during and after treatment in which a patient is living with a disease that does not get worse.
In the Annals of Oncology, it states that clinical investigators seem to be frequently using PFS and TTP interchangeably in cancer. Such use of terms may lead to confusion when results of different trials are compared.
The survival gain induced by chemotherapy is considered to be the time the tumor requires to return to a size similar to what existed before chemotherapy. In calculating survival gained by tumor shrinkage, it is approximately 2 months in 50% reduction, 3 and a half months in 75% reduction and 1 year in 99% reduction. The data indicate that in cases evaluated with reduction rates more than 90%, there may be a significant correlation between response rate and survival.
Clinical trials virtually always have time to disease progression as a primary endpoint. Without imaging studies, one can't get accurate time to progression data. So tests are performed for the benefit of drug companies seeking new drug approval, for clinical investigators seeking contracts and publications, and for clinicians seeking an easy way to make clinical decisions.
The final arbiter of clinical approval should be overall survival. Progression-free survival does not address the patient's quality of life during what little additional months of some serious side effects a number of women experience.
New Targets in Breast Cancer
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Response rates (how much a tumor decreased in size) can be inflated when excluding patients during clinical trials (evaluable patients). Patients not considered "evaluable" are often those who do not get the benefit of an entire treatment plan. The response rate is calculated after removing patients, who die or have been excluded, from the calculation. This inflates the response rate.
But clinical oncologists want to publish their papers. They need to report on the outcomes of their experiments, but if they had to wait for survival data, it could take years until all the data was aggregated. That wouldn't bode well for them to participate in pharma-sponsored trials in the future.
Response rates give clinical oncologists the opportunity to take a more optimistic look at therapies that have limited success. They can describe results as being complete remission, partial remission or simply clinical improvement.
If they treat all patients for three weeks, they can fairly evaluate the efficacy of a compound, which takes that lone (on average) before it can be regarded as effective. If they disregard all patients who die or were excluded after onset of therapy, and include only those treated three weeks or more, they can improve their data.
To justify their existence, they have to publish papers. That's what they do.