Small acinar lesions of the prostate may mimic prostate cancer. In the central and transition zone of the prostate, atypical adenomatous hyperplasia (AAH) must be differentiated from low grade carcinoma (Gleason score 2-5). In the dorso-peripheral zone, high grade prostatic intraepithelial neoplasia (PIN) and atypical small acinar proliferations (ASAP) are the most important lesions mimicking carcinoma. Further differentiation is necessary between high grade PIN and intraductal carcinoma. ASAP, on the other hand, may mimic low grade carcinoma. The significance of basal cell type cytokeratin immunohistochemistry (IHC) in the differentiation between ASAP and low grade carcinoma of the prostate was substantiated by additional MIB-1 IHC. The status of the basal cell layer in ASAP was found to be variable (complete, fragmented and absent). Independent of the status of the basal cell layer, the mean MIB-1 proliferation index of ASAP was significantly higher than that of clearly benign lesions and did not differ from that of low grade carcinoma. As carcinoma is frequently detected in rebiopsies, close clinical follow up of patients with ASAP is advisable.
