Carcinoid tumors account for 1-2 % of all lung tumors. Carcinoid tumors are neuroendocrine epithelial malignancies and divided into two subcategories:  

1) Typical carcinoid, consistent of tumors with <2 mitosis per 2 mm², lacking necrosis and measuring more than or equal to 0.5 cm in size.  Typical carcinoids tend to growth slowly and usually do not spread outside the lung.

2) Atypical carcinoid, consistent of tumors with 2-10 mitosis per 2 mm² and /or foci of necrosis. Atypical carcinoids are more prompt to metastasize.

Carcinoid tumors occur more commonly in white, middle age females. 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. Distinction between typical and atypical carcinoids is the most important prognostic factor. Atypical carcinoids have a worse prognosis than typical carcinoids.

Classification of neuroendocrine tumors

Well-differentiated (Typical and Atypical carcinoid): Tumors with an organoid growth pattern forming rosettes, trabeculae, ribbons, festoons, and nests. These tumors are positive for neuroendocrine markers, including chromogranin, synaptophysin, CD56, and insulinoma-associated protein 1 (INSM1).

• 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.

Poorly differentiated (Large cell neuroendocrine carcinoma and small cell neuroendocrine carcinoma): Tumors showing trabecular to solid to diffuse growth patterns, high mitotic count, and extensive/geographic necrosis. These tumor are unevenly positive for neuroendocrine markers, including chromogranin, synaptophysin, CD56, and insulinoma-associated protein 1 (INSM1). The differentiation between large and small cell neuroendocrine carcinoma is based on morphologic features.

• Large cell neuroendocrine carcinoma (LCNC): High mitotic rate>10 mitosis per 2mm², no upper limit of mitosis. Cytological features consist of large cell size, low N/C ratio, vesicular coarse chromatin and or frequent nucleoli.  
• Small Cell Carcinoma (SCNC): High mitotic rate>10 mitosis per 2mm², no upper limit of mitosis. Small nuclear size (generally less than the diameter of 3 small resting lymphocytes), scant cytoplasm, nuclei with finely granular nuclear chromatin, absent or faint nucleoli, and frequent necrosis.