In the WJOG8815L phase II clinical study, we monitored EGFR mutation fractions (MFs) in the circulating tumor DNA (ctDNA) of EGFR T790M mutation‐positive patients with non‐small cell lung cancer (NSCLC) that received treatment with the tyrosine kinase inhibitor (TKI) osimertinib. Here, EGFR is linked to neoplasm.