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Prostate cancer is the form of cancer with the highest incidence in men. The invasiveness or low specificity of available diagnostics hampers a timely use of efficient focal therapies. New imaging techniques are therefore needed for an early prostate cancer localization. We propose a new ultrasound imaging method for prostate cancer localization based on quantification of the (intravascular) local diffusion of ultrasound contrast agents. Local diffusion is expected to correlate better than perfusion with cancer microvascular growth and, therefore, aggressiveness. Local diffusion is estimated by transrectal ultrasound imaging of an ultrasound contrast-agent bolus passing through the prostate circulation after a peripheral intravenous injection. A time-intensity curve (TIC) is measured at each pixel by acoustic quantification. The measured TICs are fitted by a modified Local Density Random Walk model, a solution of the convective diffusion equation that provides a physical representation of the diffusion process. The obtained parametric diffusion images were compared with the histology results after radical prostatectomy. The resulting receiver operating characteristics (curve area = 0.91) were superior to those obtained by estimation of perfusion related parameters. Although extensive validation is necessary, contrast ultrasound diffusion imaging is a promising method, with potential to assess cancer aggressiveness.