Prostatic biopsies containing small glandular formations suspicious of, but not diagnostic for, carcinoma represent a diagnostic dilemma, as they cannot be definitely identified as either benign or malignant. The term 'atypical small acinar proliferation' (ASAP) in the differential diagnosis of carcinoma has recently evoked considerable discussion. This study has tried to assess the biological potential of ASAP by further immunohistochemical (IHC) analysis. Biopsy-proven cases of ASAP (n=114) were analysed, in which consecutive sections still contained the suspicious lesion. IHC studies were undertaken with anti-cytokeratin 34betaE12 and the proliferation marker MIB-1. Staining with 34betaE12 revealed a complete basal cell layer in 25 cases (21.9%), a fragmented layer in 58 cases (50.9%), and absence of basal cells in 31 cases (27.2%). MIB-1 labelling indices (LIs) in these three groups were significantly higher than in benign prostatic tissue (p0.05). The suspicious morphology of ASAP on haematoxylin and eosin-stained slides was supported by the finding of elevated proliferative activity. Subgroups were revealed by immunohistochemical assessment of basal cell status and cases without basal cells were diagnosed as carcinoma. Nevertheless, rebiopsy is recommended if radical surgery is planned, to exclude insignificant cancer. Cases with a complete or fragmented basal cell layer were regarded as non-malignant. Whether a fragmented basal cell layer reflects a technical artefact or transition to carcinoma is unknown, but the proliferative activity of both lesions was increased and corresponded to that of low-grade prostatic carcinoma. In these cases, therefore, at least clinical follow-up is strongly recommended and repeat biopsies are encouraged.
