A well differentiated neuroendocrine neoplasm of low grade arising from the digestive system. It is composed of islands of rounded, oxyphilic, or spindle-shaped cells of medium size, with small vesicular nuclei. The mitotic count is less than 2 per 10 HPF and/or the Ki67 index is equal to or less than 2 percent. The tumor can occur anywhere in the digestive system; approximately 90% arise in the appendix. [from
NCI]