Tumor-to-tumor metastasis: case report of a neuroendocrine tumor of the lung metastasizing to a benign oncocytoma of the kidney.

Tumor-to-tumor metastasis (TTM) is a rare phenomenon in which a secondary tumor colonizes within a primary tumor of a different histogenesis. It is hypothesized that TTM is encouraged by conditions that promote increased cell growth and division in the primary tumor, such as hypervascularity and expression of oncogenic cytokines. However, the exact causes of TTM likely vary on a case-by-case basis and are dependent on the microenvironment of both the primary and secondary tumors. Herein, we present the first reported example of TTM in which a pulmonary neuroendocrine tumor (NET) metastasizes to a renal oncocytoma.