Potential of the signature index (S-index), a diffusion MRI Biomarker, to enhance diagnosis of clinically significant prostate cancer and decrease the rate of unnecessary biopsies

Abstract

Objective To evaluate the potential of the signature index (S-index) and the shifted apparent diffusion coefficient (sADC), both markers of non-Gaussian diffusion, for the diagnosis of prostate cancer. Methods This retrospective study included 180 patients with 264 target lesions (95 cancerous) who underwent MRI–US fusion-guided prostate biopsy. Multiparametric MRI, including diffusion-weighted imaging with three b values (0, 200, and 1500 s/mm²), was performed to calculate sADC and the S-index. The diagnostic performances of these biomarkers were assessed using ROC analyses and compared with PI-RADS. Results The global S-index and its 85th percentile values for clinically significant cancers (ISUP ≥ 2) in both the peripheral zone (PZ: 103.0 ± 30.2 and 139.4 ± 31.6) and the transition zone (TZ: 106.6 ± 32.3 and 142.2 ± 33.9) were significantly higher (p < 0.001) than those for all other lesions and normal tissue (PZ: 45.7 ± 27.9 and 71.1 ± 38.1; TZ: 67.6 ± 25.4 and 93.0 ± 30.5). In the PZ, the S-index (global and 85th percentile) and sADC showed higher diagnostic performance for classifying clinically significant cancers (AUC = 0.92 [0.87–0.95], 0.90 [0.86–0.94], and 0.92 [0.87–0.95], respectively) compared with PI-RADS alone (AUC = 0.70 [0.59–0.70]), whereas performance in the TZ was similar across markers (AUC ∼ 0.82). The reduction in unnecessary biopsies for PI-RADS > 3 lesions in the PZ was 22.4% with sADC and 21% with the S-index. In the TZ, the highest reduction rate was achieved with the S-index 85th percentile (9.5%). Conclusion The S-index and other diffusion MRI biomarkers demonstrate strong performance for the classification of clinically significant prostate cancer and may represent an important step toward enhancing the clinical value of MRI for improving prostate cancer management.