Prostate cancer is the most prevalent malignancy in males. Gleason grading system is the most validated used pathological system that reflects prostate cancer aggressiveness and predicts the clinical course.¹ The treatment strategy of localized prostate cancer ranges from active surveillance to radical whole-gland therapy. Recently, as an individualized and optimized treatment option, focal treatment has received a great deal of attention in regard to its ability to selectively eradicate significant cancer focus while preserving uninvolved parenchyma.^2,3 Considering the biological heterogeneity and multiplicity of prostate cancer, precise evaluation of the biological character of each prostate cancer focus is essential in the selection of the tumor foci that need to be treated or untreated. Diffusion-weighted MRI (DWI) is a non-invasive imaging technique that quantifies the diffusion of water molecules in tissues without any contrast agents, tracers, or exposure to radiation. DWI may provide qualitative information regarding the pathophysiological character within the tissue, and the utility of DWI for characterizing prostrate cancer has been shown repeatedly.⁴ Apparent diffusion coefficient (ADC) is a quantitative marker that reflects the magnitude of water diffusion in the tissue, and significant correlation of ADC value with Gleason grade has been reported. Yet, ADC measurement has an intrinsic limitation in that ADC value depends on the MRI system and imaging protocols. To overcome this limitation, we developed a new semi-quantitative method to predict the aggressiveness of prostate cancer using a computed DWI (cDWI) technique.^5,6 cDWI is a computational technique that enables us to synthesize arbitrary b-value DWI from a set of b-value images. Here, we report the utility of cDWI in idenfifying aggressive prostate cancer, and propose an optimal b-value to aid in the discrimination of aggressive prostate cancer in clinical practice.Of the patients who underwent multi-sequence MRI, including DWI, prior to the radical prostatectomy, 51 patients whose index cancer was located in the peripheral zone shown in a radical prostatectomy specimen were evaluated in this study. Multi-sequence MRI, including T1- and T2-weighted imaging and DWI, was performed using a 1.5-Tesla imager (Intera Achieva; Philips, Best, Netherlands) with a 32-channel sensitivity encoding body coil under free breathing. cDWI was generated from DWI images with b-values of 0 and 1000 sec/mm². We evaluated the change in cDWI signal intensity that results from increasing the b-value in the index cancerous and normal peripheral zone regions. Regions of interest were placed on the index cancerous region and in an area of normal peripheral zone by reference to the pathological findings of radical prostatectomy specimen. The changes in the cDWI signal intensity of prostate cancer were compared according to the Gleason grade.The objective index cancers were Gleason grade 3/4/5: n=18/30/3. In all cancerous regions and all normal peripheral zone regions, the signal intensity decreased as b-value increased. In all patients, the signal intensities of cancerous regions were lower at b = 0 sec/mm² and higher at b = 1000 sec/mm² than those of normal peripheral zone regions. At one b-value, however, the signal intensities of the cancerous and normal peripheral zone regions were equal; we defined this b-value as the “iso-b-value”. The iso-b-value of Gleason grade 3 cancer was significantly higher than that of Gleason grade 4-5 cancer (median, 736 vs 561 sec/mm²; p < 0.001). The iso-b-value showed an AUC of 0.950 for discriminating Gleason grade 4-5 cancer from Gleason grade 3 cancer. When b-value was set at 700 sec/mm², signal intensity in Gleason grade 4-5 cancer was higher than that in normal peripheral zone regions, whereas in Gleason grade 3 cancer, it was equal or lower than that in normal peripheral zone regions, with a sensitivity of 97.0% and a specificity of 72.2%. The current analysis showed that the iso-b-value correlated inversely with Gleason grade of the prostate cancer. Furthermore, b = 700 sec/mm² was proposed to be an optimal b-value for easy discrimination of aggressive prostate cancer in clinical practice.Iso-b-value calculated from the cDWI might be a useful biomarker for discriminating aggressive prostate cancer. Semi-quantitative analysis using DWI at b = 700 sec/mm² might be a simple method for discriminating aggressive prostate cancer.