Comparative analysis of the nuclear proliferative index (Ki-67) in benign prostate, prostatic intraepithelial neoplasia, and prostatic carcinoma.

High-grade prostatic intraepithelial neoplasia (HG-PIN) lies in the morphologic continuum between benign and carcinomatous prostate, but its status as a neoplastic precursor remains only putative. We measured nuclear proliferative activity using MIB-1 antibody to further characterize the cell kinetics of HG-PIN and to assess its relationship to prostatic adenocarcinoma. We studied 36 specimens from randomly selected patients who underwent radical prostatectomies for prostatic adenocarcinoma. Sections of formalin-fixed, paraffin-embedded tissue pretreated by a citric acid monohydrate antigen retrieval method were immunostained with the mouse monoclonal antibody MIB-1, which detects the Ki-67 antigen in formalin-fixed tissue. The Ki-67 antigen is expressed by non-G0 proliferating cells and has been used to assess cellular proliferative activity. A maximum of either 20,400 x fields or 100 positively stained nuclei in benign glands, areas of HG-PIN, and adenocarcinoma were counted to obtain an immunohistologic proliferation index for each case. For benign prostate, HG-PIN, and adenocarcinoma, the mean positivity was 0.4 +/- 0.42 cells per field (range, 0-2), 2.5 +/- 3.79 cells per field (range, 0-16.6), and 13.8 +/- 15.05 cells per field (range, 0.25-73.66), respectively. Using a Kruskall-Wallis analysis of variance (chi 2 = 58, P < 0.05) and the t test for dependent samples, we found that the mean Ki-67 antigen expression significantly differs between histologic categories (P < 0.01, all three comparisons). In addition, the proliferative index consistently increased along the continuum from benign to malignant. We conclude that the MIB-1 proliferative index of HG-PIN lies between that of benign and carcinomatous prostate, supporting the assertion that HG-PIN is a biologic intermediate in the multistep process of transformation into carcinoma.