Signet-ring cell carcinoma of the stomach metastasizing to renal cell carcinoma: a case report and review of the literature.

Metastasis of one neoplasm to another is also known as metastasis of "tumor to tumor", "cancer to cancer", "tumor in tumor", and "one to another". We describe the case of a 75-year-old woman with the metastasis of signet-ring cell carcinoma of the stomach to clear cell renal cell carcinoma. Six years earlier, the patient had undergone gastrectomy for early gastric cancer. She complained of lumbago, and a mass was found in her left kidney. Examination of a partial nephrectomy revealed clear cell renal cell carcinoma containing signet-ring cells. Morphological and immunohistochemical findings of the signet-ring cells found in kidney cancer and gastric cancer were identical. Subsequent examinations revealed the patient had multiple bone metastases. The diagnosis of carcinoma metastasizing to renal cell carcinoma can be challenging; therefore, additional clinical information and immunohistochemical panel studies are requisite.