TMB has emerged as a significant biomarker in modern oncology, offering a new way to understand the underlying biology of a patient’s cancer. TMB measures the total number of genetic errors accumulated within a tumor’s DNA, reflecting the degree of genetic instability. This metric is important for treatment planning because a high mutation count signals a specific vulnerability in the tumor.
Defining Tumor Mutational Burden and the “High” Threshold
Tumor Mutational Burden is a quantifiable measure of somatic mutations found within the coding region of a tumor’s genome. It is calculated by counting the number of non-synonymous mutations—genetic changes that alter the resulting protein sequence—per megabase (Mb) of sequenced DNA. This measurement is reported as mutations per megabase (mut/Mb) and provides a standardized score for genetic instability.
The accumulation of these genetic errors often results from defects in the cell’s DNA repair machinery. Cancers with impaired DNA damage repair (DDR) pathways, such as those with deficiencies in mismatch repair, cannot fix genetic mistakes during cell division. This failure leads to an uncontrolled proliferation of mutations, increasing the tumor’s mutation count.
A tumor is designated as “TMB-High” (TMB-H) when its mutation count exceeds a specific numerical threshold. While the precise cutoff varies depending on the testing platform and cancer type, a widely recognized threshold is 10 mutations/Mb. Tumors that meet or exceed this score are considered hypermutated.
Methods for Quantifying TMB in Cancer Patients
Determining a patient’s TMB status relies on advanced laboratory techniques, primarily Next-Generation Sequencing (NGS). DNA is extracted from tumor biopsy tissue and sequenced to identify somatic mutations—non-inherited changes that developed within the tumor itself.
The gold standard for a complete TMB assessment is Whole Exome Sequencing (WES), which analyzes all protein-coding regions of the genome. However, WES is often costly and time-consuming for routine clinical use. In clinical oncology, TMB is most commonly measured using targeted NGS panels, which analyze a select group of several hundred cancer-related genes.
These smaller panels provide a score that accurately correlates with the WES result, making testing faster and more accessible. The total number of non-synonymous mutations identified is divided by the size of the sequenced region in megabases to yield the final TMB score (mut/Mb). This score then stratifies the cancer into low, intermediate, or high mutational burden categories.
Why High TMB Makes Tumors Immunogenic
The high number of somatic mutations in TMB-H tumors makes the cancer highly “immunogenic,” or visible to the immune system. Each genetic error can lead to the creation of an abnormal protein sequence not found in healthy cells. These novel protein fragments are known as neoantigens.
The immune system, specifically T-cells, monitors the body for foreign material. When a T-cell encounters a cell displaying neoantigens, it recognizes the fragments as foreign invaders. A tumor with high TMB produces a much larger library of different neoantigens than a low-TMB tumor, significantly increasing the probability that one will be strongly recognized by the patient’s T-cells.
This increased visibility acts as a powerful “danger signal,” prompting a robust immune response against the cancer cells. The resulting immune-inflamed tumor microenvironment is poised for an attack, though the cancer often employs cloaking mechanisms to survive. The high concentration of neoantigens provides targets for the immune system to exploit when those cloaking mechanisms are therapeutically disabled.
TMB-H as a Predictor for Immunotherapy Success
The most significant clinical application of TMB-H status is its ability to predict a favorable response to immune checkpoint inhibitors (ICIs). These therapies, such as PD-1/PD-L1 inhibitors, disable the tumor’s immune-evading checkpoints, releasing the brakes on the patient’s existing immune response. Because the immune system in TMB-H tumors is already activated due to the high neoantigen load, these inhibitors are highly effective in unleashing a pre-existing attack.
Patients whose tumors are classified as TMB-H are more likely to achieve durable clinical benefit and higher response rates with these targeted immunotherapies. This predictive value is strong enough that the U.S. Food and Drug Administration (FDA) granted “tumor-agnostic” approval to the PD-1 inhibitor pembrolizumab for use in any advanced solid tumor with a TMB of 10 mutations/Mb or greater. This decision validated TMB as a biomarker that transcends the traditional classification of cancer by its site of origin.
While TMB-H status is a powerful predictor, it is not the sole determinant for selecting immunotherapy. Physicians also consider other factors, such as the expression of the PD-L1 protein on tumor cells, to create a complete picture of the tumor’s immune landscape. Nevertheless, a high mutational burden offers a strong indication that the cancer is vulnerable to immune manipulation, improving the chances of a successful therapeutic response to checkpoint blockade.