Although approximately half of patients with EGFR-mutated NSCLC have been suggested to develop PD during treatment with EGFR-TKIs, and this has been exclusively attributed to existing lesions, it remains unclear whether the pattern of disease progression during EGFR-TKI treatment varies according to the number of metastatic lesions (Figure 3). Here, EGFR is linked to non-small cell lung carcinoma.