Typical and Atypical Carcinoids
Primarily occurring in the airways, carcinoids tumors may also arise peripherally within the lung. Their histopathologic hallmark is a growth pattern that suggests neuroendocrine differentiation (organoid, trabecular, insular, etc.). Two major variants of carcinoid tumors are recognized: typical and atypical. Some carcinoid tumors, particularly the ones that are peripherally located, may be clinically silent. Fever, cough, expectoration, chest pain, and hemoptysis occur in a substantial number of centrally located cases. The most common radiologic manifestation, seen in about 80–85% of cases, is evidence of bronchial obstruction, owing to the majority of these tumors occupying a central endobronchial location. Bronchial obstruction is revealed as atelectasis or obstructive pneumonitis. Peripheral carcinoids, often the spindle cell variant, occur as solitary pulmonary nodules manifested as well-defined homogenous densities, usually up to 3 cm in diameter.