Tumor-to-tumor metastasis: a rare case of clear cell renal cell carcinoma metastasizing into lung adenocarcinoma

Tumor-to-tumor metastasis (TTM) is a very rare condition in which a malignant tumor (donor) metastasizes into another primary neoplasm, either benign or malignant (recipient). We report the case of a 74-year-old man previously surgically treated for a clear cell renal cell carcinoma(ccRCC) who developped a new subsolid nodule in the right upper pulmonary lobe discovered at a four-year follow-up CT scan. The patient underwent a right uniportal thoracoscopic S1 segmentectomy and mediastinal lymph node dissection. Histopathological examination revealed a lung adenocarcinoma (LUAD) containing a focus of metastatic ccRCC without lymph node involvement or evidence of additional metastatic disease. To date, the patient remains disease-free. Metastasis of renal tumor cells into a primary lung tumor is exceptionally rare. To our knowledge, this is the first reported case of ccRCC metastasizing into LUAD without evidence of systemic dissemination.