Abstract: Background and purpose: With the widespread use of screening of prostate-specific antigen (PSA) levels, prostate cancers at organ-confined stage are increasing in newly diagnosed cases. However, some defects remain in conventional monoexponential diffusion-weighted imaging (DWI) for differentiating organ-confined prostate cancer from benign lesions. Therefore, the aim of this study was to obtain biexponential apparent diffusion parameters of prostate organ-confined cancer, chronic prostatitis in peripheral zone (PZ) and normal PZ tissue, and to compare with monoexponential apparent diffusion coefficient (ADC) for differentiating prostate cancer from prostatitis lesions. Methods: Sixteen patients with pathologically confirmed prostate organ-confined cancer in PZ, 14 with prostatitis underwent conventional (b-factors 0, 1 000 s/mm²) and 10 b-factors (0-3 000 s/mm²) diffusion-weighted imaging (DWI). The monoexponential ADC value and biexponential parameters fast ADC (ADCf), fraction of ADCf (f), slow ADC (ADCs) value for prostate cancer, prostatitis and normal tissues were calculated and compared. Receiver operating characteristic analysis was performed for those parameters. Results: Biexponential and monoexponential parameters were obtained for 18 prostate cancers, 18 prostatitis and 37 normal PZ tissues. The ADC value of prostate cancer tissues was remarkably lower [(0.83±0.11)×10^-3 mm²/s] than that of other tissues (P<0.01), while the ADC value of prostatitis [(1.45±0.19)×10^-3 mm²/s] was lower than that of PZ [(1.67±0.31)×10^-3 mm²/s] (^P<0.01). Prostate cancer tissues had lower ADCf [(1.54±0.23)×10^-3 mm²/s], f [(45.8±5.4)%] and ADCs [(0.52±0.15)×10^-3mm²/s] than the other tissues (P<0.01). The ADCf, f and ADCs were higher in PZ [(3.90±0.40)×10^-3, (67.3±8.2)% and (1.51±0.36)×10^-3 mm²/s] than prostatitis [(3.06±0.49)×10^-3, (47.9±3.9)% and (0.91±0.29)×10^-3 mm²/s) (P<0.01). The area under the curve (AUC) of ADCf and ADC were similar in differentiating cancer and prostatitis (0.96 vs 0.94) (P>0.01), but the AUC of f and ADCs in differentiating cancer from prostatitis (0.83 and 0.80) were significantly lower than that of ADC (P<0.01). Conclusion: The biexponential DWI provided additional tissue characterization parameters for different prostate tissues. ADCf yielded comparable accuracy with ADC in identification of prostate organ-confined cancer. The biexponential parameter could  further improve the diagnostic efficacy.

Key words: Diffusion-weighted imaging, Prostate, Biexponential decay, Apparent diffusion coefficients