DOI: 10.1111/1759-7714.70379 ISSN: 1759-7706
Exceptional Response of
HER2
‐Positive Breast Cancer to Osimertinib: A Case Report
Hirotoshi Yasui, Misaki Hatasa, Ko Ohara, Kaito Sano, Yasutaka Mori, Tomofumi Shibata, Yasutaka Fukui, Mitsuru Odate, Yoshifumi Arai, Masahiro Fujii, Yasushi Makino ABSTRACT
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.