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Prostate cancer is the most common form of cancer in men in western countries. The use of efficient focal therapies is currently hampered by limitations in early prostate cancer detection. With limited success, several quantitative ultrasound perfusion imaging methods have aimed at detection of microvascular changes associated to cancer growth. Alternatively, we recently introduced contrast ultrasound diffusion imaging, hypothesizing that these complex microvascular changes are better reflected by diffusion than by perfusion. In this paper we introduce the analysis of spatial similarity as an indirect estimation of diffusion. The passage of an intravenously injected contrast-agent bolus is recorded by transrectal ultrasound imaging, thereby measuring indicator dilution curves with a pixel resolution. The spatial similarity among these curves, within a kernel determined by the ultrasound scanner resolution, is estimated using coherence analysis. The coherence images generated from four patients were compared with histology data on a pixel basis. The results show a receiver operating characteristic curve area of 0.91, higher than that of any perfusion-related parameter. Although a method optimization and an extensive validation are required, these results confirm the promising value of contrast ultrasound diffusion imaging for prostate cancer detection.