Lung Adenocarcinoma with Ipsilateral Breast Metastasis: A Simple Coincidence?

Introduction: Lung cancer with breast metastasis is rare. However, differentiating between primary breast cancer and metastatic lung adenocarcinoma is of clinical importance. The metastasis cascade of how cancer cells migrate from the primary lung tumor to the breast is not clear yet. Methods: Pathology and cytology databases were searched for patients diagnosed to have lung adenocarcinoma with breast metastasis. Their medical records, chest computed tomography images, and pathology slides were reviewed independently. Results: We identified six lung adenocarcinoma patients with breast metastases in a 10-year period from a tertiary medical center. Interestingly, all breast metastases affected the same side as the primary lung cancers. In addition, all our cases shared other clinical manifestations, namely, ipsilateral pleural effusion/thickness and axillary lymph node enlargement. Conclusion: Because this distinctive feature could not be explained by simple coincidence, we consider that lung adenocarcinoma may preferentially metastasize to the ipsilateral breast through a stepwise mechanism, involving pleural seeding, axillary lymph node metastasis, and retrograde lymphatic spreading into the breast.