[Tumor-to-tumor metastasis: report of three cases].

Tumor-to-tumor metastasis is a very rare event. We report three cases of tumor metastasizing in another tumor: a clear cell renal cell carcinoma in a vesicular thyroid adenoma, a lung carcinoma in a meningioma and a neuroendocrine lung carcinoma in a clear cell renal cell carcinoma. According to the literature, clear cell renal cell carcinoma is the most common tumor recipient of metastasis while lung carcinoma is the most common donor tumor. Several physiopathological mechanisms can explain this phenomenon, but many of them are still unknown.