MKI67 and prostate sarcoma: However, the expression of Ki-67 in these tumors is significantly increased, indicating a higher degree of malignancy and a more rapid growth and progression.[12] Prostate carcinoma used to appear mostly in the aged people with obviously increased prostate specific antigen (PSA), while primary prostate sarcoma mostly in younger age with normal PSA, simply because PSA is produced by prostate epithelial cells, while sarcoma originates from stromal cells.