OBJECTIVES: To characterize the clonality of TMPRSS2-ERG fusion in multifocal prostate cancer.

METHODS: From 80 consecutive radical prostatectomy specimens, we identified 32 cases with multiple spatially separate tumors. In each case, we assessed two to three tumor foci for TMPRSS2-ERG fusion using an ERG break-apart interphase fluorescence in situ hybridization assay.

RESULTS: Individual tumor foci showed homogeneity for fusion status (intrafocal clonal homogeneity). In 19 (59%) of the 32 cases, all foci within a case had the same fusion status (interfocal homogeneity). In 15 (80%) of the 19 cases, no foci had fusion, and in 4 (20%), all foci had fusion. Of the 32 cases, 13 (41%) demonstrated heterogeneity for fusion status within a case (interfocal clonal heterogeneity).

CONCLUSIONS: In this study, we have demonstrated interfocal heterogeneity and intrafocal homogeneity for TMPRSS2-ERG fusion in prostate cancer with multiple tumors. These findings support the multiclonal nature of prostate cancer with clinical implications for needle biopsy strategies and the development of urine-based screening tests.