Thymic carcinoid. A histologic, immunohistochemical, and ultrastructural study of 12 cases.

Twelve cases of thymic carcinoid were studied by electron microscopy and immunohistochemistry. Clinically, all of them presented either as asymptomatic, radiographically detected mediastinal masses or as neoplasms producing symptoms of thoracic structural displacement or Cushing's syndrome. By light microscopic examination, the carcinoids manifested an organoid growth pattern, being composed of uniform polygonal cells with central, oval nuclei and finely dispersed chromatin. Immunohistochemical analysis disclosed the presence of cytoplasmic adrenocorticotropic hormone in four cases (three associated with Cushing's syndrome), of somatostatin in seven cases, and of serotonin in one case. Ultrastructurally, these tumors generally showed short, blunt, interdigitating cell processes; focal, scanty basement membrane material; infrequent but well-formed junctional complexes; moderate to large numbers of dense-core granules; and lipoautophagosomes. Six tumors contained small numbers of type I microfilaments, and three demonstrated perinuclear whorls of type II microfilaments. Rough and smooth endoplasmic reticulum was well developed in each tumor, the smooth form being prominent in cases with clinical Cushing's syndrome.