Neuroendocrine tumors of the lung: recent developments in histopathology.

Lung tumors with neuroendocrine differentiation are made up of several neoplasms with particular epidemiologic, clinical, morphologic, and molecular characteristics. Typical and atypical carcinoid tumors represent low-grade and intermediate-grade carcinomas, respectively, whereas small-cell carcinoma and large-cell neuroendocrine carcinoma are considered high-grade carcinomas. Recent studies support the use of this four-tumor, three-tier classification scheme, but in practice, definitive diagnoses on small tissue samples remain a challenge for even the most experienced lung pathologists.