Tumor-to-tumor metastasis of colon cancer metastasizing to a pancreatic neuroendocrine tumor associated with von Hippel-Lindau disease: a case report.

Von Hippel-Lindau disease (VHL) is frequently associated with pancreatic neuroendocrine tumors (PNETs). Here, we report a case of tumor-to-tumor metastasis in a VHL patient in whom colon cancer metastasized to the interior of a PNET. A 65-year-old man had undergone bilateral adrenalectomy for pheochromocytomas in both adrenal glands in his 50 s. Genetic screening was performed considering his family history of pheochromocytoma, and he was diagnosed with VHL. PNET was detected, for which the patient was regularly monitored by follow-up imaging. One year ago, the patient underwent right hemicolectomy to remove a tumor in the ascending colon (pT3N0M0, pStage IIA). He was admitted to our department for detailed examination because the pancreatic tumor had grown, and thus, pancreaticoduodenectomy was performed. Diagnostic imaging and histological findings indicated tumor-to-tumor metastasis, in which the patient's previous colon cancer had metastasized to and proliferated within the PNET. Colon cancer metastasizing to a PNET is extraordinarily rare and has never been reported in the literature. Thus, practitioners should be vigilant for tumor-to-tumor metastasis when performing imaging surveillance of PNETs.