Primary pulmonary cancer colliding with metastatic breast carcinoma: hitherto unreported cases of cancer-to-cancer metastasis focusing on clinical implications.

Lung is one of the main sites of metastatic tumors, but collision neoplasms consisting of a primary lung cancer and metastatic breast carcinoma have never been so far reported. We describe here 2 cases of primary non-small cell lung cancers (squamous cell and adenocarcinoma, respectively) colliding with metastatic breast carcinomas (ductal and lobular carcinomas, respectively). Clinico-pathologic features characterizing this challenging diagnosis and the important therapeutic implications are discussed.