Pulmonary Carcinoid vs Metastatic Carcinoid in Lung: Metastatic carcinoid from gastrointestinal tract should be excluded. Glandular structures are commonly seen in gastrointestinal carcinoids but are rare in pulmonary carcinoids. A panel consisting of TTF-1, CDX2, and NESP-55 is useful. Pulmonary carcinoids may show positivity for TTF-1 (especially the peripheral ones) but are negative for CDX2 (positive in gastrointestinal carcinoids) and NESP-55 (positive in pancreatic origin tumors). Pulmonary Carcinoid vs Small Cell Carcinoma: Pulmonary carcinoids may be difficult to distinguish from small cell carcinoma in small biopsies with crush artifact. Ki-67 labeling index is lower in carcinoids (<10-20%) as compared to small cell carcinoma and large cell neuroendocrine carcinoma (>50%, often as high as 80%). Other entities in the differential diagnosis include salivary gland-type tumors (positive for CK8/18 and AE1/AE3, pos. for p63, neg. for neuroendocrine markers), metastatic lobular breast carcinoma (pos. for CK8/18 and AE1/AE3, pos. for ER/PR, neg. for neuroendocrine markers), and paraganglioma (neg. for CK8/18 and AE1/AE3, pos. for neuroendocrine markers). The image shows tumor cells forming nests and anastomosing trabeculae separated by hyalinized stroma in a case of typical pulmonary carcinoid.