The positive correlation between tumor mutation burden and the purity of tumor samples in non–small cell lung cancer and colorectal cancer.

Abstract

e13683 Background: Tumor mutation burden (TMB) is reported to be a biomarker for predicting immunotherapy responses in patients with non–small cell lung cancer (NSCLC). To provide a more accurate calculation of TMB, the correlation between tumor purity and TMB was investigated. Methods: 959 patients were enrolled in this study from July 2019 to January 2020, including 663 patients with primary NSCLC and 296 patients with primary colorectal cancer (CRC). Formalin-Fixed Paraffin-Embedded (FFPE) and paired white blood cells were collected from each patient. These FFPE samples were stained with HE for the evaluation of tumor purity. DNA was extracted from samples and sequenced using a 605-gene panel on the Illumina platform (approximately 3,000× coverage). TMB was calculated for each sample based on the sequencing data. Results: The median TMB of 663 NSCLC patients was 2.29mut/Mb, and the TMB is positively correlated with the purity of tumor samples. Among samples in which tumor cells accounted for ≥40% of the sample cells (n = 456), the median TMB reached 2.29 mut/Mb, and 90.35% (412/456) patients had a TMB higher than 0 mut/Mb. In samples that tumor cells accounted for 20% to 40% of sample cells (n = 120), the median TMB was 1.53mut/Mb and 72.5% (87/120) of patients had a TMB larger than 0 mut/Mb. The median TMB was 0.76 mut/Mb for samples contained less than 20% of tumor cells, and only 50.57% (44/87) patients had a TMB larger than 0 mut/Mb. Similar results were obtained in 296 CRC samples, which exhibited a median TMB of 3.55mut/Mb. Tumor cells accounted for 40% of the sample cells in 191 cases. The median TMB of those samples was 4 mut/Mb, and 96.34% (184/191) samples had a TMB larger than 0 mut/Mb. When tumor cells accounted for 20% to 40% of total sample cells, the median TMB was 2.94mut/Mb, and 94.44% (51/54) of patients had a TMB larger than 0 mut/Mb. In 50 samples that contained less than 20% of tumor cells, the median TMB was 2.29mut/Mb, and 80 % (40/50) samples had a TMB larger than 0 mut/Mb. Conclusions: TMB is positively correlated with the purity of tumor samples. Moreover, data suggest that the TMB is more reliable when tumor cells account for more than 40% of the sample population.