Apparent Diffusion Coefficient (ADC) Ratio Versus Conventional ADC for Detecting Clinically Significant Prostate Cancer With 3-T MRI.

OBJECTIVE. The purpose of this study is to evaluate the performance of the apparent diffusion coefficient ratio (ADC _ratio ; the ADC of the suspected prostate cancer [PCa] focus on MRI divided by the ADC in a noncancerous reference area) with that of conventional ADC for detection of high-grade PCa (Gleason score [GS] ≥ 3 + 4) versus low-grade PCa (GS = 3 + 3) with whole-mount (WM) histopathologic analysis used as a reference. MATERIALS AND METHODS. The cohort of this retrospective study included 218 men with 240 unilateral PCa lesions assessed by both 3-T multiparametric MRI and whole-mount histopathologic analysis. ROIs were placed on individual lesions verified by WM histopathologic analysis, to calculate the mean ADC (ADC _{tumor_mean} ) and lowest ADC within each lesion (ADC _{tumor_min} ), and within non-tumor-containing regions of the same tumor zone but on the contralateral side (ADC _benign ), to calculate the background ADC. The ADC _{ratio_mean} (the ADC _{tumor_mean} divided by the ADC _benign ) was calculated. The performance of individual ADC _tumor and ADC _{ratio_mean} values in discriminating PCa with a GS of 3 + 3 from PCa with a GS of 3 + 4 or greater was assessed using the AUC value. RESULTS. The ADC _{ratio_mean} had a higher AUC value for discriminating PCa lesions with a GS of 3 + 3 from those with a GS of 3 + 4 or greater (the AUC value increased from 0.70 using the ADC _{tumor_mean} and 0.67 using the ADC _{tumor_min} [the minimum ADC of the PCa lesion] to 0.80 for the ADC _{ratio_mean} and 0.72 for the ADC _{ratio_min} [the ADC _{tumor_min} divided by the ADC _benign ]; p = 0.043). When stratified by PCa zonal location, the ADC _{ratio_mean} had significantly more robust accuracy in the transition zone (the AUC value increased from 0.63 for ADC _{tumor_mean} to 0.87 for ADC _{ratio_mean} ; p = 0.019) compared with the peripheral zone (the AUC value increased from 0.74 for ADC _{tumor_mean} to 0.78 for ADC _{ratio_mean} ; p = 0.44). When analyzed on the basis of endorectal coil use, the ADC _{ratio_mean} performed nonsignificantly better in both the endorectal coil and non-endorectal coil subcohorts, although it performed better in the former. CONCLUSION. As an intrapatient-normalized diagnostic tool, the ADC ratio provided the best AUC value for discrimination of low-grade from high-grade PCa on 3-T MRI.