Atypical endocrine granules in atypical endocrine tumor (AET) of the lung. An immunoelectron microscopic study

Highly dense granules are a hallmark for recognizing atypical endocrine tumor (AET) of the lung. We report a case of AET with many atypical neurosecretory-type granules: moderately dense granules (mean size 373.7 nm) and "target" granules with a central dense core (425.1 nm), both apparently larger than the highly dense granules (223.3 nm). Immunoelectron microscopical studies demonstrated that all three types of granule were positive for gastrin-releasing peptide (GRP), human chorionic gonadotropin alpha-subunit (hCG alpha), calcitonin or serotonin. Although the size profiles of positive granules were similar for calcitonin and hCG alpha, they were different from those of GRP or serotonin granules. The presence of atypical granules and the different size profiles of hormonal products in AET indicate that caution is required in ultrastructural evaluation of granules in lung carcinomas.