Bronchial carcinoid tumor mimicking acinic cell tumor

A rare case of carcinoid tumor mimicking an acinic cell tumor is presented. A bicameral tumor measuring 8 x 6 mm in size was recognized in the right lung (B5bi) upon gross examination. Microscopically, the tumor consisted of basophilic polygonal cells forming an acinar pattern. Ultrastructurally, the majority of tumor cells had large membrane-bound cytoplasmic granules, measuring about 600 nm in diameter, which were similar to secretary granules of serous acinar cells (zymogen granules). These findings suggested that the tumor might be an acinic cell tumor originating from the bronchial gland. However, tumor cells were shown to be negative for periodic and Schiff (PAS) stain or lactoferrin, lysozyme and amylase immunohistochemically. On the other hand, they were shown to be argyrophilic by Grimelius stain and showed immunohistochemically positive reaction for serotonin, suggesting that the granules were neurosecretory granules and not zymogen granules. Based on these findings, we concluded that this tumor was an unusual variant of carcinoid tumor mimicking acinic cell tumor. Although carcinoid tumor has a wide histological spectrum, there has been no reported case, to our knowledge, of acinic cell tumor-like carcinoid tumor.