ERBB2 and non-small cell lung carcinoma: Moreover, T-DXd introduces a “bystander effect” (Bartsch, 2020), wherein the released payload diffuses into neighboring tumor cells, irrespective of HER2 expression, and this is particularly advantageous in heterogeneous tumors, such as HER2-mutant NSCLC, where intratumoral heterogeneity and varying levels of HER2 expression often leads to treatment failure (Hinohara and Polyak, 2019).