In vivo PET imaging in male Copenhagen rats bearing the R-3327-AT-1 prostate cancer tumor showed accumulation of the tracer preferentially at the periphery rather than the center of the tumor, which could be attributed to the heterogeneity of GRPR expression in the different regions of the tumor or to limited accessibility of the drug to the center of the tumor. The gene discussed is GRPR; the disease is prostate carcinoma.