To investigate the efficacy of afatinib treatment for EGFR T790M-positive NSCLC patients showing resistance to osimertinib and alterations in somatic mutations and tumor mutation burden (TMB) in plasma circulating tumor DNA (ctDNA) during afatinib treatment, we conducted a prospective study using Cancer Personalized Profiling by deep Sequencing (CAPP-Seq). The gene discussed is EGFR; the disease is cancer.