Impact of prior chemotherapy or radiation therapy on tumor mutation burden in NSCLC.
2627 Background: Higher non-synonymous tumor mutation burden (TMB) in non-small cell lung cancer (NSCLC) is associated with a higher likelihood of response to checkpoint inhibitors. Tissue samples subject to TMB analysis may be obtained after exposure to cytotoxic chemotherapy or radiation therapy – both of which introduce somatic mutations in DNA and can influence the number of identified mutations. The role of TMB as a potential predictive marker for immunotherapy is evolving, and the impact of prior therapy on TMB could influence interpretation. Methods: Eligible cases were from patients with confirmed NSCLC, available clinical annotation and tumor molecular profiling including TMB analysis at a CLIA-certified genomics laboratory (Caris Life Sciences, Phoenix, AZ) using the Illumina NextSeq platform. TMB was calculated using only missense mutations that had not been previously reported as germline alterations. Treatment history was obtained for each patient under an IRB approved protocol to determine whether patients had had received chemotherapy or radiation therapy in the year prior to collection of the tissue subject to TMB analysis. Data analysis was performed using the chi-square test of deviance to evaluate whether TMB was statistically significantly different between groups, correcting for smoking status. Results: Out of 1,118 patients identified, 459 cases met all eligibility criteria and were evaluated. 76 patients (17%) received either chemotherapy or radiation prior to tissue collection. Samples acquired prior to any therapy had a median TMB of 10 mut/Mb vs. 11 mut/Mb in samples acquired after any therapy. After adjusting for smoking, there was no significant difference in TMB between these cohorts (p = 0.41). Secondary pair wise analysis showed no statistically significant difference in TMB from chemotherapy-naïve and chemotherapy-treated samples (p = 0.28). The same was true for radiation (p = 0.75). Collection of clinical data is ongoing and further analysis, including additional cases will be presented. Conclusions: Though cytotoxic chemotherapy and radiation therapy can introduce somatic mutations, prior exposure to either was not associated with a significant difference in TMB.