Tumor mutation burden (TMB)

This page was reviewed under our medical and editorial policy by

Priya Vishnubhotla, MD, Chief, Medical Oncology, City of Hope Atlanta

This page was updated on May 30, 2023.

For some cancer patients, doctors may use immunotherapy as a treatment option. Immunotherapy uses the body’s own immune system to identify and destroy cancer cells. By understanding a patient’s tumor mutation burden (TMB), doctors may be able to target immunotherapy more successfully and plan for other treatments.

This guide to tumor mutation burden is designed to help patients and their families learn more.

What is tumor mutation burden?

The tumor mutation burden is the number of changes (called genetic mutations) found in the DNA of cancer cells. Also known as tumor mutational burden, TMBs are biomarkers that may help doctors see how well a patient responds to medical treatment.

Patients undergoing immunotherapy may have tumor mutation burden testing. This allows doctors to know which therapy may be appropriate, since tumors with high numbers of mutations may be more responsive to different types of immunotherapy medications to help the body find and destroy cancer cells.

Tumors with a lower TMB have a lower number of mutations, which means the body’s immune system is less likely to identify cancer cells. Research has found that TMB numbers are a useful biomarker for immunotherapy treatments for advanced cancer patients. While studies suggest that TMB testing may be more useful for some cancers than for others, more studies are needed.

TMB testing

During the TMB testing procedure, a biopsy is taken to obtain a tissue sample from the tumor.

A doctor measures the tissue sample in the laboratory. The TMB is measured by mutations per megabase (mut/Mb).

In the future, it may also be possible to test TMB from a blood sample.

Tumor mutation burden range

After a TMB test, the doctor will explain the results and what they mean for the patient’s cancer treatment plan.

If a cancer has a high tumor mutation burden level of 10 mut/Mb or greater, it’s more likely to respond to immunotherapy medication. This means that a specific drug type—immune checkpoint inhibitors—may activate the immune system to help the body recognize cancer cells. With more mutations, the higher the chance that one of the mutations will be able to identify and target cancer cells.

Cancers with low tumor mutation burden levels of 10 mut/Mb or lower may be less successful in activating the immune system to identify and target cancer cells. However, each patient is different, so the care team will discuss whether or not immunotherapy will be part of the treatment plan.

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