Tumor-to-Tumor Metastasis: Breast Cancer Metastasizing to EGFR Exon 19-Mutated Lung Adenocarcinoma with Long-Term Disease-Free Survival

Tumor-to-tumor metastasis (TTM) is a rare phenomenon characterized by the metastasis of one malignant tumor into another histologically different tumor. While breast and lung cancers are prevalent among women globally, TTM involving breast cancer metastasizing to lung adenocarcinoma is exceptionally uncommon. Herein, we report a rare case of Luminal B breast cancer metastasizing to EGFR exon 19 deletion-mutated lung adenocarcinoma. The patient achieved prolonged disease-free survival following comprehensive treatment, including surgical resection, chemotherapy, EGFR-tyrosine kinase inhibitor (TKI) therapy, and endocrine therapy. This case highlights the importance of molecular profiling in guiding personalized therapeutic strategies for TTM, particularly when actionable mutations are present.