Primary carcinoid tumor in a polycystic kidney

A case of a primary carcinoid tumor within a polycystic kidney is reported. A 51‐year‐old woman with autosomal dominant polycystic kidney disease (ADPKD) underwent a bilateral nephrectomy because of an infection in her polycystic kidneys. A tumor (1.5 cm in diameter) was incidentally found close to the hilum of the left kidney. Histologically, the tumor exhibited a predominantly trabecular pattern of cuboidal or columnar cells. Grimelius staining showed numerous silver‐stained neurosecretory granules in most of the tumor cells; the tumor cells were also positive for chromogranin, synaptophysin, prostatic acid phosphatase (PAP) and neuron‐specific enolase (NSE). The tumor was diagnosed as a primary carcinoid tumor of the kidney. Primary renal carcinoid is a very rare neoplasm, and this is the first report of such a lesion arising in a polycystic kidney.