MKI67 and primary central nervous system lymphoma: The observation in this study that PCNSL patients in the high-expression group showed more atypical enhancement features, i.e., ring enhancement, was due to the fact that the high Ki-67 index represents excessive tumor growth and high invasiveness, which may lead to cystic degeneration and necrosis due to insufficient blood supply within the tumors; and hemorrhage due to destruction of vascular endothelial cells (42).