About clinical trial results
Researchers look at different factors to see how well a treatment works. They call these the trial end points. Trials usually have both primary and secondary end points.
The primary end point is the main aim of the trial. It is the main question that the trial is designed to answer. It may be to see:
if people who have the trial treatment live longer
if the cancer comes back in fewer people after the trial treatment
if the trial treatment causes fewer side effects
how well a new test to diagnose cancer works
which dose of a new treatment is best to use
Secondary end points are other things that the researchers are also interested in, but they are not the main aim of the trial.
Below are some of the terms researchers use when they are looking at trial results.
Overall survival (OS) | How long people live after they join a trial or start treatment |
Median overall survival | The time at which half of the people in the trial have died and half are living |
Progression free survival (PFS) | The time between starting treatment aimed at shrinking or controlling the cancer, and signs that it has started to grow again |
Median progression free survival | The time from starting treatment aimed at shrinking or controlling the cancer, to signs that the cancer has started to grow again in half the people taking part |
Event free survival (EFS) | The time between starting treatment and having a specific medical ‘event’. For example, this could be a specific symptom, type of pain or a bone fracture. |
Median event free survival | The time from starting treatment to when half the people taking part have had a specific medical ‘event’ |
Disease free survival (DFS) | The time between starting treatment aimed at curing cancer, and signs that the cancer has come back |
Median disease free survival | The time from starting treatment aimed at curing cancer, to signs that the cancer has come back in half the people taking part |
5 year survival | The number of people who live for at least 5 years. It does not mean the number of people who only live for 5 years. |
10 year survival | The number of people who live for at least 10 years. It does not mean the number of people that only live for 10 years. |
Many researchers use guidelines called the Response Evaluation Criteria in Solid Tumours (RECIST) to analyse how well trial treatments work. They measure the area of cancer on a scan before and after treatment, to see how much it’s changed.
Complete response (CR) | There are no signs of cancer on scans or tests |
Partial response (PR) | The cancer has shrunk by at least one third (30%) and there are no signs the cancer has grown anywhere else in the body |
Stable disease (SD) | The cancer has stayed the same size, it hasn’t got better or worse |
Progressive disease (PD) | The cancer has grown by at least a fifth (20%) or there are new areas of cancer |
Overall response rate (ORR) | The total number of people whose cancer has either gone away (a complete response) or shrunk (a partial response) |
Statistically significant | The difference in the results of the groups is probably caused by the different treatments. It is unlikely to have happened by chance. |
Not statistically significant | The difference in the results of the groups is small and may not be caused by the different treatments. It could have happened by chance. |
Clinically meaningful response | An improvement that is important to the patient. It varies depending on the cancer and the trial, and could relate to survival or quality of life. |
Remission | There are no signs of cancer on tests or scans |
Recurrence | The cancer has started to grow again after it had gone away |
Relapse | The cancer has started to grow again after it had got smaller or stopped growing |
An adverse event (AE) is a medical problem that may or may not be caused by the treatment in the trial. It could be a coincidence.
A side effect is a medical problem which doctors think is probably caused by the treatment in the trial. You may see side effects called treatment related adverse events.
Side effects are given a grade depending on how bad they are. The grades go from 1 to 5:
Grade 1 - no symptoms or only mild symptoms that don’t affect day to day life and don’t need treatment
Grade 2 - slightly worse symptoms that may affect parts of day to day life and probably need treatment
Grade 3 - significant symptoms that do affect day to day life and will need treatment
Grade 4 - severe symptoms that can be life threatening and need urgent treatment
Grade 5 - so severe that the person dies. It is very rare for this to happen.
A serious adverse event (SAE) is a medical problem that causes the person in the trial to be seriously ill. They may be admitted to hospital for treatment or have long term medical problems.
Pharmacokinetics (PK) means how the drug gets to different parts of the body, and how the body breaks down and gets rid of the drug.
Pharmacodynamics (PD) means finding out how a drug works and the effect it has on the body.
Quality of Life (QoL) is also important. Researchers measure any positive or negative effects that treatment has on people and their day to day lives. You may hear the terms PROM (patient reported outcome measures) or PREM (patient reported experience measures).
Last reviewed: 26 Jun 2025
Next review due: 25 Jun 2028
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