Primary renal carcinoid. Case report and literature review.

We report a case of primary renal carcinoid, which is a very rare neoplasm: to our knowledge, only 19 cases have been previously reported. The tumor displayed histologic features typical of carcinoid tumors from other sites, including growth in nests and ribbons, uniform cells with finely granular, eosinophilic cytoplasm, and stippled chromatin. Electron microscopy confirmed the presence of membrane-bound dense-core granules. Immunohistochemical analysis revealed staining for chromogranin A, neuron-specific enolase, Leu-7, and synaptophysin, as well as pancreatic polypeptide. An interesting finding was the positive staining for prostatic acid phosphatase, while staining for prostate-specific antigen was negative. Although prostatic acid phosphatase is commonly seen in primary gastrointestinal hindgut carcinoids, in this case a primary hindgut carcinoid was ruled out by clinical examination and endoscopy. The patient developed metastases to the liver, but was well and without symptoms 15 months after diagnosis.