Reproducibility of a quantitative body diffusion score derived from whole-body diffusion-weighted imaging (DWIBS) for assessment of bone metastases in prostate cancer.

PURPOSE: To evaluate whether changes in diffusion-based total lesion volume (ΔtDV) on whole-body MRI correlate with changes in serum prostate-specific antigen (ΔPSA) after therapy, and to assess reproducibility of ΔtDV measurements. MATERIALS AND METHODS: This retrospective study included 20 prostate cancer patients who underwent whole-body MRI before and after treatment. Changes in total lesion volume (ΔtDV), mean apparent diffusion coefficient (ΔmADC), and PSA (ΔPSA) were calculated. Correlations between ΔtDV and ΔPSA, as well as between ΔmADC and ΔPSA, were evaluated using Spearman's rank correlation coefficient. Image interpretation was independently performed by two readers blinded to clinical information. One reader conducted two separate reading sessions with an approximately one-year washout period. Inter-observer and intra-observer agreement for ΔtDV measurements was assessed using Bland-Altman analysis and intraclass correlation coefficients (ICCs). RESULTS: ΔtDV demonstrated a strong positive correlation with ΔPSA across all three reader sessions, with Spearman correlation coefficients exceeding 0.7 and significant p values (p < 0.05). In contrast, ΔmADC showed a weak negative correlation with ΔPSA, which did not reach statistical significance. Bland-Altman analysis showed mean biases of -3.1 mL for inter-observer measurements and 1.4 mL for intra-observer measurements, with 95% limits of agreement of -22.6 to 16.5 mL and -19.0 to 21.9 mL, respectively. The inter- and intra-observer ICCs were 0.916 and 0.911, respectively, indicating excellent agreement. CONCLUSION: Changes in diffusion-based total lesion volume on whole-body MRI show a strong and reproducible association with PSA changes after therapy, suggesting that ΔtDV may serve as a robust quantitative imaging biomarker for treatment response in prostate cancer bone metastases.