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Cancer Study Headline, Translated: Progression-Free Survival
A plain-language research explainer for progression-free survival headlines and what patients should ask before applying the result.
Original commentary from the Cancer Explained editorial team.
Please note: this page is educational only — it is not medical advice, and it does not speculate about anyone’s health beyond reliable public reporting. For questions about your own health, talk with your healthcare team.
The two clocks a trial can measure
Cancer headlines run on numbers with months attached. Two of those numbers look alike and mean different things.
FDA's guidance on cancer drug endpoints defines overall survival as the time from randomization until death from any cause. It calls survival the most reliable cancer endpoint. Where a study can measure it properly, it is usually the preferred one. The reasons are practical. The endpoint is precise. It is easy to measure. It is fixed by a date of death. And the assessment is objective and quantitative.
Progression-free survival, or PFS, is the time from randomization until the tumor visibly grows or the person dies, whichever comes first. It is a different clock. It stops at the first bad thing, not the last one.
A third endpoint, time to progression, stops only at tumor growth and does not count deaths. FDA prefers PFS to it, because PFS includes deaths and so tracks overall survival more closely.
Why anyone uses PFS at all
If overall survival is the better endpoint, why is PFS everywhere?
FDA's guidance sets out the reasons plainly. PFS can be read before a survival benefit can be judged, so results arrive sooner. It is not muddied by whatever treatment someone gets after the trial drug stops working. And for a given number of participants, the effect on PFS can be larger than the effect on overall survival. That makes a difference easier to spot in a trial of realistic size.
The last point is worth sitting with. A bigger measured effect is not a bigger benefit. It partly reflects that PFS is measured over a shorter, cleaner stretch of time.
Where the reasoning gets thin
FDA is candid about the weak link. Its guidance says the data are usually not enough to judge how closely effects on PFS track effects on overall survival. Cancer trials are often small, and the proven survival benefits of existing drugs are generally modest.
In short: a PFS gain is sometimes a reliable stand-in for living longer and sometimes it is not. The evidence for which is which is often missing.
FDA treats PFS three ways, depending on the disease and the size of the effect. It can be a stand-in endpoint supporting accelerated approval. It can be a stand-in supporting full approval. Or for some diseases it can count as a real benefit on its own, because delaying tumor growth means delaying symptoms.
There is also a measurement problem. "Progression" is decided by comparing scans, and the exact definition has to be written into the protocol in advance. FDA says careful planning can limit bias in measuring PFS, which admits that bias is possible. Someone has to decide whether a shadow has grown, and how often to look.
Two worked examples
The abstract idea is easier with numbers from real trials.
In advanced pancreatic cancer, NCI records a trial in which FOLFIRINOX gave a median overall survival of 11.1 months against 6.8 months for gemcitabine, and a median PFS of 6.4 months against 3.3 months. Both clocks moved, in the same direction, which is the reassuring pattern.
Now the other pattern. Atezolizumab got accelerated approval for bladder cancer in 2016 on the strength of tumor shrinkage in a single-arm study, with no comparison group at all. The confirmatory trial did not meet its main endpoint of improved overall survival. FDA's register of withdrawn cancer accelerated approvals lists that use as withdrawn in 2021. An early signal is not a promise.
What to check before applying a headline
Four questions cover most of it.
Was PFS the main endpoint the trial was built around, or something reported alongside it? Did overall survival move too, and if not, was the trial even large enough to tell? Who was in the trial: what stage, what biomarkers, what prior treatments, what age? And what did the gain cost, in side effects and in time spent in treatment?
Our page on what trial endpoints are covers the vocabulary. Our page on reading clinical trial results covers what the summary numbers can support.
When to get checked
Reading trial results is not the same as watching your own situation. If you are in treatment or in follow-up, these are the changes to report rather than wait out:
- New or worsening pain that is present at night or does not shift with position
- Shortness of breath, a new cough, or coughing blood
- Unexplained weight loss, or losing appetite over more than two weeks
- A new lump, or swelling of a limb, the abdomen, or the face
- New headaches with nausea, vision changes, or weakness on one side, which needs same-day assessment
- A fever of 38 degrees Celsius, or 100.4 Fahrenheit, during chemotherapy, which is an emergency
Scans are scheduled in advance, but symptoms do not wait for the next appointment.
What this does not mean
- A longer PFS does not show that people live longer. FDA's own guidance says the link between the two is usually not well established.
- It does not replace the trial paper, the drug label, a guideline, or a talk with a care team.
- A result from one cancer type, stage, and biomarker profile does not transfer to another.
- "Median PFS" is the middle of a spread. Half the people in the trial did worse than that number and half did better.
- An endpoint is a research tool, not a forecast. No trial number describes one person.
Sources
- FDA, Clinical Trial Endpoints for the Approval of Cancer Drugs and Biologics: Guidance for Industry (December 2018) — https://www.fda.gov/media/71195/download
- NCI, How Do Clinical Trials Work? — https://www.cancer.gov/research/participate/clinical-trials/how-trials-work
- NCI PDQ, Pancreatic Cancer Treatment (Health Professional Version) — https://www.cancer.gov/types/pancreatic/hp/pancreatic-treatment-pdq
- FDA, Withdrawn: Cancer Accelerated Approvals — https://www.fda.gov/drugs/resources-information-approved-drugs/withdrawn-cancer-accelerated-approvals
How this article was prepared
An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed.
The National Cancer Information Foundation publishes Cancer Explained. This page is for learning. It is not medical advice and does not suggest a test or treatment.
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Put the story in context
Prevention, possible warning signs, screening, and diagnosis
This story relates to progression-free survival. The information below is general: it does not reveal anything else about a public person’s health, and not every point applies to every cancer. Personal advice depends on age, symptoms, family history, exposures, and medical history.
Prevention and risk reduction
Not every cancer can be prevented. Avoiding tobacco, protecting skin from ultraviolet radiation, limiting alcohol, staying active, and receiving recommended HPV or hepatitis B vaccination can lower the risk of certain cancers. A risk factor is not a prediction or a cause in one individual.
Symptoms and possible early signs
Possible signs vary and are often caused by conditions other than cancer. Changes worth discussing include a new lump, unexplained bleeding or weight loss, a persistent cough, lasting bowel or bladder changes, a changing skin spot, or symptoms that persist or worsen. Some early cancers cause no symptoms.
Screening and early detection
Screening looks for certain cancers before symptoms begin. Recommended tests exist only for some cancers and depend on age and risk. Screening can have benefits and harms; it is not the same as evaluating a new symptom, and there is no single routine scan or blood test that reliably screens for every cancer.
How cancer is diagnosed
Diagnosis may involve a history and exam, imaging, laboratory tests, and often a biopsy. Pathology can identify the cancer type and may test biomarkers that guide treatment. Symptoms, screening results, tumor markers, or online stories alone cannot confirm cancer.
A public story may encourage questions, but it should not be used to estimate your risk or choose testing. Contact a healthcare professional about a persistent or concerning change. Seek urgent care for severe or rapidly worsening symptoms.