Neuroendocrine tumors are cancers that begin in specialized cells called neuroendocrine cells. These cells have traits similar to those of nerve cells and hormone-producing cells. Neuroendocrine tumors, while rare, can occur anywhere in the body. Most affect the cardiothoracic region, eg lungs, appendix, small intestine, pancreas as well as the rectum. There are many types of neuroendocrine tumors: some grow slowly while others develop very rapidly.