Whole‐lesion apparent diffusion coefficient metrics as a marker of percentage Gleason 4 component within Gleason 7 prostate cancer at radical prostatectomy

Abstract

Purpose To retrospectively assess the utility of whole‐lesion apparent diffusion coefficient (ADC) metrics in characterizing the Gleason 4 component of Gleason 7 prostate cancer (PCa) at radical prostatectomy. Materials and Methods Seventy patients underwent phased‐array coil 3T‐magnetic resonance imaging (MRI) before prostatectomy. A uropathologist mapped locations and Gleason 4 percentage (G4%) of Gleason 7 tumors. Two radiologists independently reviewed ADC maps, aware of tumor locations but not G4%, and placed a volume‐of‐interest (VOI) on all slices including each lesion on the ADC map to obtain whole‐lesion mean ADC and ADC entropy. Entropy reflects textural variation and increases with greater macroscopic heterogeneity. Performance for characterizing Gleason 7 tumors was assessed with mixed‐model analysis of variance (ANOVA) and logistic regression. Results Among 84 Gleason 7 tumors (G4% 5%–85%, median 30%; 59 Gleason 3+4, 25 Gleason 4+3), ADC entropy was significantly higher in Gleason 4+3 than Gleason 3+4 tumors (R1: 5.27 ± 0.61 vs. 4.62 ± 0.78, P = 0.001; R2: 5.91 ± 0.32 vs. 5.57 ± 0.56, P = 0.004); mean ADC was not significantly different between these groups (R1: 0.90 ± 0.15*10⁻³cm²/s vs. 0.98 ± 0.21*10⁻³cm²/s, P = 0.075; R2: 1.06 ± 0.19*10⁻³cm²/s vs. 1.14 ± 0.16*10⁻³cm²/s, P = 0.083). The area under the receiver operating characteristic (ROC) curve (AUC) for differentiating groups was significantly higher with ADC entropy than mean ADC for one observer (R1: 0.74 vs. 0.57, P = 0.027; R2: 0.69 vs. 0.61, P = 0.329). For R1, correlation with G4% was moderate for ADC entropy (r = 0.45) and weak for mean ADC (r = −0.25). For R2, correlation with G4% was moderate for ADC entropy (r = 0.41) and mean ADC (r = −0.32). For both readers, ADC entropy (P = 0.028–0.003), but not mean ADC (P = 0.384–0.854), was a significant independent predictor of G4%. Conclusion Whole‐lesion ADC entropy outperformed mean ADC in characterizing Gleason 7 tumors and may help refine prognosis for this heterogeneous PCa subset. J. Magn. Reson. Imaging 2015;41:708–714.