Progression-Free Survival (PFS) measures the duration a patient lives with a disease, such as cancer, without the condition worsening after starting treatment. This measurement begins from treatment initiation and continues until the disease shows signs of progression or death occurs from any cause. It provides insight into how long a therapy can keep the disease under control.
Defining Disease Progression
In a medical context, disease “progression” is not based on how a patient feels, but rather on specific, measurable changes in the disease itself. Doctors often determine progression through objective methods like medical imaging, including CT scans, MRIs, and PET scans, which reveal changes in tumor size or the appearance of new tumors. For solid tumors, standardized guidelines like RECIST (Response Evaluation Criteria in Solid Tumors) are commonly used. According to RECIST, progression is defined as at least a 20% increase in the sum of the longest diameters of target lesions, using the smallest sum recorded since treatment began as a reference.
Progression also occurs with the unequivocal growth of existing non-target lesions or the emergence of one or more new lesions in other parts of the body. While imaging is the primary method, some definitions of progression can also include a significant increase in tumor markers, which are substances found in blood or urine that can indicate cancer activity. These criteria ensure consistency in assessing how a disease is responding to treatment across different patients and studies.
Distinguishing from Overall Survival
Progression-Free Survival (PFS) is distinct from Overall Survival (OS), which measures the total length of time a patient lives from diagnosis or the start of treatment, regardless of disease progression. OS is considered a direct measure of clinical benefit because it tracks how long patients are still alive. For instance, a patient might have a PFS of 10 months, meaning their cancer started growing again after 10 months on a particular therapy. Their OS could be three years, as they might have received subsequent treatments after the initial progression, extending their life further. While PFS assesses the effectiveness of a specific treatment in preventing disease worsening, OS captures the full impact of all treatments a patient receives over their lifetime. In some cancers, like advanced colorectal and ovarian cancers, there can be a strong correlation between PFS and OS. However, in others, such as sarcoma or certain breast and lung cancers, the link is less clear, indicating that extending the time without progression does not always directly translate into longer overall life.
Role in Clinical Trials and Drug Approval
Progression-Free Survival serves as an endpoint in clinical trials, helping researchers evaluate how effective a new treatment is. Regulatory bodies, such as the U.S. Food and Drug Administration (FDA), often consider PFS data when reviewing new drugs for approval. PFS is used in clinical trials because it can be measured sooner than Overall Survival. This allows researchers to get an earlier indication of a drug’s efficacy, potentially accelerating drug development and making new treatments available to patients more quickly, especially for serious conditions with limited options. The FDA’s Accelerated Approval pathway, for instance, often relies on surrogate endpoints like PFS to expedite access to new therapies. While OS remains the standard for demonstrating direct clinical benefit, a substantial improvement in PFS, particularly in areas of unmet medical need, can lead to drug approval. Such approvals may require confirmatory trials to later establish a direct link to improved Overall Survival.
Interpreting Progression Free Survival Data
When reviewing Progression-Free Survival data, the “median PFS” is a commonly reported statistic. This value represents the point at which half of the patients in a study group have experienced disease progression or death, and half have not. For example, a median PFS of 7.4 months means that 50% of patients in that group had their disease worsen or died within 7.4 months, while the other 50% experienced a longer period without progression. An improvement in PFS does not always guarantee a longer overall life or a better quality of life. A drug might delay cancer progression for several months, but if it comes with severe side effects, the patient’s daily experience might not improve. The length of “survival post-progression” (SPP)—the time a patient lives after their disease progresses—can influence the correlation between PFS and OS. If patients receive effective subsequent treatments after progression, a modest PFS benefit might not translate into a statistically significant OS advantage. The frequency and consistency of disease assessments in a clinical trial can also affect PFS measurements. If patients are evaluated less frequently, the detected PFS might appear longer than the actual time to progression.