OBJECTIVETo study the morphologic features and clinical significance of atypical small acinar proliferation (ASAP) suspicious but not diagnostic of cancer in prostatic biopsies.

METHODSThe slides of 11 cases of prostatic needle biopsies collected during a two-year period with the diagnosis of ASAP were reviewed. Immunohistochemical study for 34betaE12, p63 and P504S was performed on the archival paraffin sections.

RESULTSAll the 11 ASAP cases were characterized by the presence of a few compacted small acini in the prostatic stroma. Six cases had acini of less than three in number. The acini were round or slightly irregular in shape. The nuclei were enlarged, round or irregular, arranged in single layer and focally separated by broad interval. The nucleoli were usually prominent. Cytoplasm was amphophilic or pale and the lumen border was often well-defined. Basophilic mucus was also seen in some of the lumen. Immunohistochemical study for 34betaE12 and p63 was negative, while that for P504S was positive. In 4 of the 11 cases, the acini were more than three in number, round or slightly irregular, but without cytologic atypia. The nuclei were slightly enlarged with small or inconspicuous nucleoli. Immunohistochemical study for 34betaE12 and p63 was negative or at most focally positive. P504S staining was either negative or weakly positive. Second repeat biopsy was carried out in all cases, and 4 of them (36%) showed definite adenocarcinomatous changes. The positive cases were those with fewer acini but definite cytologic atypia in the initial biopsy.

CONCLUSIONSASAP is a morphologic interpretation closely associated with prostatic adenocarcinoma. The histologic features are suspicious of but not diagnostic of cancer, due to insufficient criteria in terms of acinar number, cytologic or architectural abnormalities. The positive rate in subsequent repeat biopsy is higher than that for cases with usual nodular hyperplasia.