In cancer research, you will encounter terms that describe clinical trial outcomes, such as Progression-Free Survival (PFS) and Event-Free Survival (EFS). Researchers use these metrics to measure how effective new treatments are. While they sound similar, PFS and EFS measure different results, and understanding them is useful for interpreting medical news and discussing treatment options.
Defining Progression-Free Survival
Progression-Free Survival (PFS) is the length of time a patient lives with a disease without it getting worse. This period begins with treatment and ends when the cancer shows measurable growth or the patient passes away from any cause. Doctors define “progression” with specific criteria, using imaging scans to measure tumor size. For instance, the Response Evaluation Criteria in Solid Tumors (RECIST) is a standard used to determine if a tumor has grown by a certain percentage.
PFS measures a treatment’s ability to control cancer, reflecting stability rather than a cure. The goal is to pause the disease’s advancement, extending the period where the patient lives with the cancer without it worsening. This endpoint is used in trials for advanced cancers where the objective is to manage the disease as a chronic condition.
Data for PFS can be collected sooner than for other long-term outcomes. This allows researchers an earlier signal about whether a new therapy is working. A drug showing a significantly longer PFS compared to a standard treatment suggests it is more effective at holding the cancer at bay.
Defining Event-Free Survival
Event-Free Survival (EFS) is a broader measure used in clinical studies. Like PFS, it measures a period from the start of treatment, but it ends when a wider range of negative outcomes, called “events,” occurs. The specific events are defined by researchers before a trial begins and vary based on the disease and treatment.
An “event” in an EFS measurement includes disease progression and death from any cause. It also includes other significant medical issues, such as the cancer returning after remission, the development of a second primary cancer, or the need to start a new line of treatment because the current one is no longer effective. For certain diseases, an event might be defined as the disease progressing to a point where a planned surgery can no longer be performed.
The flexible definition of an “event” makes EFS applicable where controlling tumor growth is not the only goal. For example, in studies for blood cancers like acute myeloid leukemia (AML), preventing relapse is a primary objective, making EFS a relevant endpoint. EFS provides a comprehensive picture of a treatment’s impact on a patient’s disease course.
Primary Distinctions and Clinical Relevance
The primary difference between the two metrics is scope. PFS has a narrow focus on disease progression and death, while EFS casts a wider net by including other predefined negative health events. This distinction has practical implications for how clinical trials are designed and interpreted.
Researchers choose between PFS and EFS based on the specific cancer and the goals of the therapy. For many advanced solid tumors where the immediate goal is to slow tumor growth, PFS is a direct and logical endpoint. It measures the treatment’s ability to control the known, measurable disease.
In other contexts, EFS provides more meaningful information. Consider neoadjuvant therapy, which is treatment given before surgery. In this setting, an “event” might be defined as the tumor growing to a size that makes the planned surgery impossible. EFS is also relevant in studies of adjuvant therapy—treatment given after the main treatment to prevent recurrence, making “relapse” an important event to capture. This is common in trials for breast cancer and leukemias, where long-term disease control is a primary goal.
Connection to Overall Survival
The ultimate goal of cancer treatment is helping patients live longer, which is measured by Overall Survival (OS). OS is the length of time from the start of treatment that a patient is still alive. It is considered the most definitive endpoint in clinical trials because it shows if a new treatment extends life.
Collecting OS data can take many years, especially for cancers with a longer prognosis. This is why PFS and EFS are used as surrogate endpoints, meaning they stand in for Overall Survival to provide an earlier indication of a treatment’s effectiveness. Changes in tumor size (PFS) or other disease-related events (EFS) can be measured more quickly than final survival statistics.
Using surrogate endpoints allows researchers to assess a drug’s benefit sooner. A significant improvement in PFS or EFS suggests a likely improvement in Overall Survival. This correlation allows promising new treatments to move through the regulatory approval process and become available to patients more quickly. The validity of EFS as a surrogate for OS has been studied in various cancers to ensure it is a reliable predictor.