Non-timely clinically applicable ADC ratio in prostate mpMRI: a comparison with fusion biopsy results

Abstract

Purpose: The purpose of the study was to assess the diagnostic accuracy of ADC ratio and to evaluate its efficacy in reducing the number of false positives in prostatic mpMRI.

Materials and methods: All patients who underwent an mpMRI and a targeted fusion biopsy in our institution from 2016 to 2021 were retrospectively selected. Two experienced readers (R1 and R2) independently evaluated the images, blindly to biopsy results. The radiologists assessed the ADC ratios by tracing a circular 10 mm² ROI on the biopsied lesion and on the apparently benign contralateral parenchyma. Prostate cancers were divided into non-clinically significant (nsPC, Gleason score = 6) and clinically significant (sPC, Gleason score ≥ 7). ROC analyses were performed.

Results: 167 patients and188 lesions were included. Concordance was 0.62 according to Cohen's K. ADC ratio showed an AUC for PCAs of 0.78 in R1 and 0.8 in R2. The AUC for sPC was 0.85 in R1 and 0.84 in R2. The 100% sensitivity cut-off for sPCs was 0.65 (specificity 25.6%) in R1 and 0.66 (specificity 27.4%) in R2. Forty-three benign or not clinically significant lesions were above the 0.65 threshold in R1; 46 were above the 0.66 cut-off in R2. This would have allowed to avoid an equal number of unnecessary biopsies at the cost of 2 nsPCs in R1 and one nsPC in R2.

Conclusion: In our sample, the ADC ratio was a useful and accurate tool that could potentially reduce the number of false positives in mpMRI.

Keywords: Apparent diffusion coefficient; Gleason score; Multiparametric MRI; Prostate cancer.

Fig. 1

Flowchart of the study. FPB, fusion prostate biopsy; mpMRI, multiparametric prostate MRI

Fig. 2

a, b ADC ratio calculation in a suspected PZ lesion in a 71 years old man. ADC ratio is 0.32 (565/1756). FPB later confirmed a PCa Gleason score 7 (4 + 3) in the area. c, d ADC ratio calculation in a suspected TZ lesion in a 70 year old man. ADC ratio was 0.22 (338/1477). FPB afterward confirmed a PCa Gleason score 8 (4 + 4) in the mpMRI signaled lesion. ADC apparent diffusion coefficient, PZ peripheral zone, FPB fusion prostate biopsy, PCa prostate cancer, TZ transitional zone, mpMRI multiparametric prostate MRI

Fig. 3

Bland Altman plot showing agreement between the two readers. adcr1, apparent diffusion coefficient ratio—reader 1; adcr2, apparent diffusion coefficient ratio—reader 2

Fig. 4

Blue lines represent R1 and red lines represent R2. a ADC ratio ROC curves for all PCas. The AUCs are 0.78 in reader 1 and 0.8 in reader 2. b ADC ratio ROC curves for sPCs. c ADC ratio ROC curves for all PCas in TZ. d ADC ratio ROC curves for all PCas in PZ. e ADC ratio ROC curves for all PCas in PI-RADS 3 category. f ADC ratio ROC curves for all PCas in PI-RADS ≥ 4 categories. ADC apparent diffusion coefficient, ROC receiver operating characteristics, AUC area under the curve, PCa prostate cancer, sPC clinically significant prostate cancer, TZ transitional zone, PZ peripheral zone

Fig. 5

Two cases in which ADC ratio underperformed. a, b a 67 year old man with a FPB confirmed PCa Gleason 7 (3 + 4). The lesion was too small for a correct assessment with a 10 mm² ROI. A smaller 2.6 mm² ROI would have given a more correct assessment. c, d a 70 year old man with a SB confirmed PCa Gleason 7 (3 + 4), FPB was negative. In this case the lesion was so peripheral that both the measuring radiologist and the urologist missed it on the axial images. ADC apparent diffusion coefficient, FPB fusion prostate biopsy, PCa prostate cancer, ROI region of interest