How often does alpha-methylacyl-CoA-racemase contribute to resolving an atypical diagnosis on prostate needle biopsy beyond that provided by basal cell markers?

Background: Alpha-methylacyl-CoA-racemase (AMACR), a recently discovered tumor marker for prostate cancer, is being used increasingly in conjunction with hematoxylin and eosin (H&E) histology and basal cell markers in the workup of difficult prostate needle biopsies. However, it is not known how often a positive AMACR staining is used merely to support a malignant diagnosis that could otherwise be established based on routine H&E histology and negative basal cell staining. Methods: This study included 307 prostate needle biopsies that were sent to us for consultation diagnosed as "atypical" by contributing pathologists. Immunohistochemistry for AMACR, high molecular weight cytokeratin, and p63 was performed. AMACR staining intensity was graded as negative, weak, moderate, and strong. Only staining that was significantly stronger than that of background benign glands was considered positive. An "expert review" diagnosis was first rendered as benign, atypical, or cancer based on the H&E-stained section and basal cell marker stains without the knowledge of AMACR expression. The AMACR stains were then reviewed and a final diagnosis was rendered. Results: A total of 215 cases had a final diagnosis of cancer following evaluation of the H&E-stained section, basal cell markers, and AMACR. Of these 215 cases, 176 (81.9%) were positive and 39 (18.1%) were negative for AMACR staining. Of 81 cases with a final diagnosis of atypical following review of all material, 42 (51.9%) were positive and 39 (48.1%) were negative for AMACR staining. When AMACR staining was negative, in no case was the initial cancer, atypical, or benign diagnosis (based on routine histology and negative basal cell markers) changed based on AMACR stain results. Of 115 cases called atypical after expert review, 76 were positive for AMACR: of these 76 cases, 34 (44.7%) were changed to a final diagnosis of cancer. AMACR expression was positive, with moderate and strong staining in 30 of the 34 cases (88.2%), for which the immunohistochemical result converted the expert review atypical diagnosis to a final cancer diagnosis. Of these 34 cases, 11 underwent radical prostatectomy, and cancer was found in all cases. Three additional patients underwent repeat biopsy, and cancer was present in the repeat biopsy in 2 patients. The cases whose diagnosis was changed from "atypical" on expert review to cancer were all highly suspicious for cancer based on H&E histology and negative basal cell markers, yet a definitive cancer diagnosis could not be established because of small size, insufficient cytologic atypia, or biopsy artifact. Conclusion: Interpretation and use of AMACR staining should be executed with caution. A negative AMACR stain can be seen in approximately 18% of cases considered to be cancer based on H&E stain combined with negative basal cell markers. A positive AMACR staining converted an atypical diagnosis, based on suspicious histology and negative basal cell marker stains, to cancer in approximately 10% (34 of 307) of cases thought to be atypical by contributing pathologists and in approximately 50% (34 of 76) of cases thought be atypical on expert review by a specialist in genitourinary pathology.