Twenty-two cases of neuroendocrine tumor comprising neurosecretory granules were identified ultrastructurally in 124 cases of resected pulmonary carcinoma. Among these, 10 cases of atypical carcinoid of the lung were examined by LM, EM and immunohistochemical staining for NSE. The diagnosis of atypical carcinoid is difficult by LM alone. Therefore light microscopic observation is required in combination with electron microscopic findings for obtaining an accurate diagnosis of atypical carcinoid of the lung. Based on the data observed in these 10 cases, the diagnostic criteria of atypical carcinoid of lung are as follows: Microscopically: Increased cellularity with organoid architecture and rosette formation. Patchy necrosis at the center of cancer nests. Pleomorphism and irregularity of nuclei with hyperchromatism. Presence of abundant mitotic figures. NSE is positive immunohistochemically. Ultrastructurally: Various amount of neurosecretory granules found in cancer cells. Rich organelles, such as mitochondria, RER and ribosomes found in cytoplasm. Presence of basal lamina and desmosomes.
Carcinoid Tumor Diagnosis, Differential