Clinically, it is critical to select the population with the EGFR-T790M mutation when we consider obtaining re-biopsy samples from tumors and/or blood to decide on treatment with osimertinib. Previous reports have described various potential factors regarding the emergence of the EGFR-T790M mutation, such as the duration of initial EGFR-TKI treatment, EGFR gene status at baseline, and surgical history [6,7,8]. Our findings were inconsistent with these factors as predictors of the emergence of the T790 M mutation, while these other predictive factors have showed controversial results for the detection of the T790M mutation [6]. Therefore, a novel powerful biomarker is needed to predict the emergence of the T790M mutation using re-biopsy samples.

Although our observations showed that the ORR to EGFR-TKIs is one of the significant predictive biomarkers, it will be difficult to closely evaluate the response of EGFR-TKI using RECIST categorized into only 4 groups: complete response (CR), partial response (PR), stable disease (SD), and progression disease (PD). To further assess for the response to EGFR-TKIs, we validated the MTS rate as a novel evaluation method. A previous report showed that the MTS rate was not a good prognostic factor in EGFR mutation-positive NSCLC [9]. We have clearly shown here that the increased MTS rate in response to initial EGFR-TKIs was significantly associated with the emergence of the EGFR-T790M mutation among patients with NSCLC tumors with acquired resistance. Further, the optimal cutoff for the MTS rate in relation to the emergency of the EGFR-T790M mutation was 30%, which is the same as the RECIST-PR level. Crucially, multivariate analysis indicated that the MTS rate was an independent predictive factor for the emergence of the T790M mutation using re-biopsy; this showed a greater association than the duration of initial EGFR-TKI treatment, EGFR gene status, and tumor histology, as previously described.

Tumors with the EGFR-T790M mutation showed a relatively low tumor mutation burden (TMB) in next generation sequencing analysis [10], whereas tumors without the EGFR-T790 M mutation with EGFR-TKI acquired resistance showed molecular heterogeneity [11]. In fact, TMB was reported to be a promising predictor of the detection of the EGFR-T790M mutation [12]. These observations suggest that the original de novo tumors that ultimately acquire resistance with the T790M mutation are more likely to retain EGFR signal directivity than those without the T790M mutation. Similarly, our retrospective findings showed that de novo EGFR mutated tumors with good responses to initial EGFR-TKIs have a higher dependency on EGFR signaling in cases of acquired T790M mutation than those without the T790M mutation, suggesting that the MTS rate with initial EGFR-TKIs may be a useful predictor of the emergence of the T790M mutation using re-biopsy.

This study has several limitations. Firstly, it comprised a small retrospective sample. However, previous retrospective observations have used similar sample sizes, with occasional exceptions [6,7,8]. Second, we only considered a Japanese cohort. Third, there may have been bias considering when EGFR-TKI was started, even though treatment was administered in multiple centers, and in the timing of evaluation using CT scanning, even though it was performed every 1–3 months after treatment. Therefore, a further prospective study is warranted to identify the role of the MTS rate for detecting the emergence of the EGFR-T790 mutation in NSCLC following initial EGFR-TKI treatment.