Based on the value-added index (Ki-67) and mitotic count of tumor cells, neuroendocrine tumors are classified into G1, G2, and G3 levels. According to WHO 2010 criteria, neuroendocrine tumors are currently classified into 5 categories: 1. neuroendocrine tumor grade 1 (NET G1) 2. neuroendocrine tumor grade 2 (NET G2) 3. neuroendocrine carcinoma (NEC G3 small cell carcinoma, large cell carcinoma) 4. mixed glandular/neuroendocrine carcinoma (MANEC) 5. proliferative and preneoplastic lesions