Evaluating the Prognostic Impact of Apparent Diffusion Coefficient in Definitive Radiotherapy for Gleason Score 7 Prostate Cancer Patients

Background: To investigate the utility of diffusion-weighted magnetic resonance imaging (DW-MRI) in evaluating Gleason score (GS) 7 tumors before definitive radiotherapy (RT) and to explore its association with clinicopathological factors and treatment outcomes. Materials and Methods: Clinical data of 266 prostate cancer (PCa) patients with biopsy-confirmed GS 7 who underwent RT were retrospectively analyzed. Pretreatment DW-MRI was utilized to measure apparent diffusion coefficient (ADC) values of primary tumors. Treatment outcomes, including biochemical disease-free survival (bDFS) and prostate cancer-specific survival (PCSS), were assessed. Statistical analyses were conducted to determine the correlation between tumor ADC values, clinicopathological factors, and treatment outcomes. Results: Tumors with a GS of 3 + 4 had significantly higher ADC values than those with a GS of 4 + 3 (0.746 ± 0.150 vs. 0.702 ± 0.157 × 10⁻³ mm²/s; p < 0.001). Median follow-up time was 8.6 years, and the 7-year rates for bDFS and PCSS were 89.1% and 95.3%, respectively. Lower tumor ADC values were significantly correlated with higher GS and increased risk of disease progression. A primary tumor ADC cutoff value of 0.682 × 10⁻³ mm²/s was identified for predicting disease progression. Patients with higher ADC values exhibited significantly better 7-year bDFS rates (92.8% vs. 83.2%; p = 0.02). However, GS 4 + 3 tumors independently predicted poorer bDFS and PCSS outcomes. In the multivariable analysis, only GS 4 + 3 tumor was predictive for worse bDFS and PCSS. Conclusions: Tumor ADC values are a reliable biomarker for differentiating between GS 3 + 4 and 4 + 3 tumors in the GS 7 category. Tumors exhibiting lower ADC values have been associated to higher risk factors and an increased likelihood of disease progression, particularly in GS 3 + 4 tumors where GS upgrading could happen.