Apparent Diffusion Coefficient (ADC) Ratio Versus Conventional ADC for Detecting Clinically Significant Prostate...

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.