Exceptional Response of HER2-Positive Breast Cancer to Osimertinib: A Case Report

Osimertinib is standard therapy for EGFR-mutant lung cancer, and HER2 amplification is a known resistance mechanism. We report HER2-positive breast cancer showing durable regression during osimertinib monotherapy for synchronous EGFR-mutated lung adenocarcinoma. Post-treatment analysis showed marked regression, HER2 amplification, no detectable EGFR, HER2, or PIK3CA mutations, ER/PgR negativity, preserved PTEN, and reduced proliferative activity. These findings suggest ERBB pathway inhibition may have contributed, although the mechanism remains speculative without functional validation. This hypothesis-generating observation describes unexpected breast cancer regression during osimertinib treatment.