When reading about new treatments for metastatic breast cancer (MBC) or advanced breast cancer, you’ll likely come across terms you may not be familiar with. One is “progression-free survival (PFS),” which refers to how long it takes for cancer to progress (get worse or come back) after treatment. PFS is often used to measure how well a treatment works. Learning what this term means and how to interpret it in studies can help you better understand how different MBC treatments may affect your prognosis (outlook).

What Is Progression-Free Survival?

Current treatments for MBC can help control cancer, but a cure is unlikely. Instead, doctors and researchers focus on helping people live longer with stable disease. The success of MBC treatments isn’t measured by how many people are cured — instead, success is based on how well a treatment keeps cancer from progressing. In clinical trials for MBC and other cancers that are harder to treat, PFS is the preferred measurement.

The National Cancer Institute defines PFS as “the length of time during and after the treatment of a disease, such as cancer, that a patient lives with the disease but it does not get worse.” Doctors and researchers report PFS as the length of time (in months or years) from when a person starts treatment until their cancer begins progressing. The longer the PFS, the better a treatment is at preventing worsening disease.

How Does a Metastatic Breast Cancer Study Measure Progression?

Cancer progression in MBC studies is measured with imaging tests that monitor any changes in metastases around the body. At the beginning of a study, participants have scans taken to establish a baseline with the location and size of their lesions (tumors). Once treatment has started, they’ll undergo follow-up scans throughout the study to see if their lesions have changed in size. New lesions automatically indicate cancer progression.

Some studies mark cancer progression as a percentage change in lesion size. In a 2022 study, for example, researchers noted that a 20 percent increase in the size of lesions was considered progressive disease.

Progression-Free Survival in Clinical Trials

Doctors and researchers continue to look for new ways to treat MBC and extend the life span of those who have this disease. To be approved by the U.S. Food and Drug Administration (FDA), new medications must first go through clinical trials. These studies follow participants who have been given either the typical treatment (called standard of care) for MBC or a new medication, typically for months to years. The new treatment must prove to be safe and effective to receive approval.

Before a clinical trial begins, the investigators (doctors and researchers running the study) must tell the FDA how they’ll measure a drug’s efficacy (how well it works). These measurements are known as endpoints. In MBC trials, PFS is a common primary endpoint — the most important result of the study.

The investigators may also have secondary endpoints, which measure results that may be related to the primary endpoint. One example is overall survival, which indicates how long participants stay alive after beginning treatment.

The overall goal of most MBC treatments is to lengthen a person’s life rather than cure their cancer. As a result, clinical trials focus more on learning the PFS than the overall surviva.

Progression-Free Survival and Metastatic Breast Cancer Studies

The easiest way to understand PFS is to break down an example of an MBC clinical trial. These studies present lots of complicated information — knowing what numbers to look for and what they mean can help make reading the findings seem less overwhelming.

For example, the 2022 study was a phase 3 clinical trial that compared standard chemotherapy to the breast cancer drug trastuzumab deruxtecan (fam-trastuzumab deruxtecan-nxki, sold as Enhertu).

Published in the New England Journal of Medicine, the study looked at people who had MBC with low levels of HER2 (a cancer protein) and were previously treated with chemotherapy. Some participants also had hormone receptor-positive (HR-positive) breast cancer (positive for either estrogen or progesterone receptor), meaning that their cancer cells use estrogen or progesterone to grow. These participants had been treated before with one form of endocrine therapy (hormone therapy).

This was a randomized controlled trial — the investigators randomly assigned each participant to receive one of these two treatments:

• Trastuzumab deruxtecan
• Chemotherapy, such as capecitabine (Xeloda), gemcitabine (Gemzar), or paclitaxel protein-bound (Abraxane)

The study’s primary endpoint was PFS in participants with HR-positive MBC. This means that the investigators wanted to see how long each treatment option helped keep this type of breast cancer under control. The secondary endpoints were progression-free and overall survival in all participants. These results would show how well each option controlled all types of breast cancer in the study and how long participants using that treatment survived.

The investigators found that the median PFS in participants with HR-positive MBC was:

• 10.1 months for people treated with trastuzumab deruxtecan
• 5.4 months for people treated with chemotherapy

This means that it took 10.1 months for 50 percent of participants who received fam-trastuzumab to begin progressing, compared with 5.4 months for 50 percent of the chemotherapy group. Trastuzumab deruxtecan kept participants’ cancer stable for a longer period of time.

What Does Progression-Free Survival Mean for You?

Here’s what all these details about PFS mean for you: When weighing your treatment options, your oncology specialist will take into account data from MBC studies showing which therapies have the best PFS. They’ll also consider your specific MBC case, including whether you have HR-positive cancer and if it’s HER2-positive or HER2-negative, as well as any treatments you’ve already had.

The overall goals of treating MBC are to improve your quality of life and keep disease progression at bay. Your oncologist may have to balance some unwanted treatment effects (known as toxicity) to help you live as long as possible. Be sure to advocate for yourself during discussions with your health care provider to make sure you’re weighing all the risks and benefits of your cancer treatment regimen.

As new therapies are discovered, the survival rate for MBC continues to rise. These improvements are due to the increase in PFS from these treatments, allowing more people to live with stable disease for longer. If you have questions about PFS and how it relates to your treatment, talk to your oncologist.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 61,000 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Are you living with metastatic breast cancer? Have you discussed MBC study findings, such as progression-free survival, with your doctor? Start a conversation by posting on your Activities page.