MKI67 and neuroendocrine carcinoma: Grading in the WHO system, which is also used by the ENETS, is based entirely on the proliferation rate of the tumor and categorizes PanNETs into grade 1 well-differentiated PanNETs (mitoses <2/10 HPF and Ki-67 index <3%), grade 2 well-differentiated PanNETs (mitoses 2/10–20/10 HPF and Ki-67 index 3–20%), and grade 3 neuroendocrine carcinoma (mitoses >20/10 HPF and Ki-67 index >20%).