Carcinoid tumors account for <1 % of all lung cancers. Carcinoid tumors are neuroendocrine epithelial malignancies and divided into two subcategories:

Typical carcinoids are carcinoid tumors with <2mitosis per 2 mm2 and lacking necrosis. They measure >0.5 cm in size. Atypical carcinoids are carcinoid tumors with 2-10 mitosis per 2 mm2 and /or foci of necrosis. They occur more in people who are aged <60 years, females or white than in people who are older, male or nonwhite. Typical carcinoids are not related to tobacco smoking, although atypical carcinoids have been reported more frequently in smokers. Carcinoid tumors can be found from the trachea to the bronchioles. Most central carcinoids are seen in the main stem or lobar bronchi. Metastatic disease may involve ipsilateral and contralateral hilar and mediastinal lymph nodes as well as the liver and bones. However, lymph node and distant metastasis are more frequently encountered with atypical carcinoid than typical carcinoid. No genetic segregation was observed between atypical and typical carcinoids which appear to derive from the same clonal proliferation. Distinction between typical and atypical carcinoids is the most important prognostic factor. Atypical carcinoids have a worse prognosis than typical carcinoids.

Criteria for diagnosis of neuroendocrine tumors (Travis et al)

Typical carcinoid: A tumor with carcinoid like morphology and <2 mitosis per 2 mm2, lacking necrosis and more than or equal to 0.5 cm.

Atypical Carcinoid: A tumor with carcinoid morphology and 2-10 mitosis per 2 mm2 and /or necrosis often punctate or both.

Large cell neuroendocrine carcinoma: A tumor with neuroendocrine morphology. High mitotic rate>10 mitosis per 2mm2, median of 70 mitosis per 2mm2.

Cytological features of non-small cell carcinoma, large cell size, low N/C ratio, vesicular coarse chromatin and or frequent nucleoli. Positive immunohistochemically staining for one or more neuroendocrine markers and /or neuroendocrine granules by electron microscopy.

Small Cell Carcinoma: Small size, (generally less than the diameter of 3 small resting lymphocytes).

Scant cytoplasm, nuclei with finely granular nuclear chromatin, absent or faint nucleoli, High mitotic rate>10 mitosis per 2mm2, median of 80 mitosis per 2mm2; Frequent necrosis.