MKI67 and lung cancer: An association of Ki-67 expression with age in 2506 lung cancer patients was identified using the fixed effects model in 19 studies, and higher Ki-67 expression was found to be more common in older patients (OR = 1.246, 95%CIs: 1.039–1.494; Z = 2.37, P = 0.018, I2 = 0.00%, P = 0.967, Table 5 and Additional file 1: Figure S1).