Background: Neuroendocrine tumor/carcinoid tumor features

What are the pathological features of neuroendocrine tumors?

Neuroendocrine tumors appear as solid masses that often have a yellowish appearance. Under the microscope, neuroendocrine tumors are composed of round or ovoid cells with a granular cytoplasm and nuclei that have a "salt and pepper" appearance. The cells often form nests or may form small follicles or gland-like structures. By electron microscopy, the tumor cells have dense secretory granules, which store substances that are often secreted by the tumor. Neuroendocrine tumors can be further evaluated to determine if they make certain proteins or "markers" using a technique called immunohistochemistry. Common markers used include synaptophysin and chromogranin A. Neuron-specific enolase, keratin and Ki-67 are also useful in certain situations. Staining for substances that might be secreted such as insulin, glucagon or serotonin is applied in certain categories of neuroendocrine tumors.

The World Health Organization recently proposed a new pathologic categorization for neuroendocrine tumors based on tumor size, the presence of blood vessel or local tissue invasion, proliferative index, hormone secretion, and the presence of metastases in lymph nodes or distant organs such as the liver. The classification categorizes neuroendocrine tumors as either 1) well-differentiated benign neuroendocrine tumor or neuroendocrine tumors of uncertain malignant potential; 2) well-differentiated neuroendocrine carcinoma (NEC) with low-grade malignant potential and 3) poorly-differentiated NEC with high-grade malignant potential.