Background: TMB is routinely reported in cancer patients tested with broad-panel next generation sequencing and has become a predictive biomarker associated with response to checkpoint inhibitor (CPI) therapy. Sequencing of paired tumor and normal specimens allows correction of TMB estimates with patient-specific germline variants. When a paired normal specimen is unavailable, TMB estimates are corrected using germline variant annotations derived from population-scale germline variant surveys. Germline variants do not generate neoantigens, which is the putative target of the immune response in CPI treated patients. To evaluate TMB differences in paired sequencing (PS) and tumor-only sequencing (TOS), we compared TMB assessments—stratified by race—in two common malignancies.

Methods: Using de identified records from the Tempus clinico-genomic database, cohorts of patients with non-small cell lung cancer (NSCLC) and breast cancer sequenced using the Tempus xT NGS platform (DNA-seq of 595-648 genes at 500x coverage, whole exome capture RNA-seq) and noted to not have microsatellite instability, were identified for analyses. The Kruskall Wallis test was used to compare TMB distributions.

Results: Among 4,817 NSCLC patients with race information (13% Black (B), 5% Asian (A), 82% White (W), 3,052 had PS, and 1,765 had TOS performed. Median TMB for B, A, and W patients was 5.8, 2.6, and 4.7 (within group p<0.0001), respectively in patients with PS, and 9.5, 6, and 7.4 (within group p<0.0001), in patients with TOS. Comparisons across PS and TOS were highly significant (p<0.0001). The absolute difference in median TMB was 3.7, 3.4, and 2.5, respectively. Among 3,191 patients with breast cancer (17% B, 4% A, 78% W), 2,220 had PS, and 971 had TOS. Median TMB for B, A, and W patients was 2.6, 2.1, and 2.6 (within group p=0.11), respectively, in patients with PS, and 6.3, 5.8, and 4.7 (within group p<0.0001) in patients with TOS. Comparisons across PS and TOS were highly significant (p<0.0001). The absolute difference in median TMB was 3.7, 3.7, and 2.1, respectively.

Conclusions: PS reduces estimated TMB compared to TOS across all racial groups with a pronounced difference in Black and Asian racial groups. This is expected as population databases of germline variation are based on cohorts predominantly from individuals of European ancestry, leading to artifactually high TMB in minorities tested with TOS. As a result, artifactually elevated TMB estimates from TOS may promote treatment with CPI in patients with a low probability of response which could exacerbate known race-based outcome disparities. PS provides a more accurate estimate of TMB regardless of race and could reduce the use of CPI in patients with a low likelihood of response.

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