Prognostic Significance of Diffusion-Weighted Imaging in T-Staging of Rectal Cancer and Its Association with 18F-FDG PET/CT Parameters

Background: This study aimed to investigate the relationship between the apparent diffusion coefficient (ADC) derived from diffusion-weighted imaging (DWI) and metabolic parameters obtained from 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) across different T-stages in patients with rectal cancer (RC), and to evaluate the prognostic significance of these imaging biomarkers for survival outcomes.Methods: This retrospective study included patients with non-mucinous rectal adenocarcinoma who underwent both pelvic magnetic resonance imaging (MRI) and 18F-FDG PET/CT before treatment. T-staging was performed using T2-weighted magnetic resonance imaging images, and ADC measurements were obtained from DWI. Metabolic tumor parameters, in-cluding maximum standardized uptake value (SUVmax), metabolic tumor volume (MTV), and total lesion glycolysis (TLG), were obtained from PET/CT images. A correlation analysis was performed between ADC and 18F-FDG PET/CT parameters and the T-stage. ROC curve analysis was performed to evaluate the diagnostic performance of ADC and 18F-FDG PET/CT parameters in the early T stage. Overall survival (OS) and progression-free survival (PFS) were estimated and analyzed.Results: A moderate negative correlation was found between T-stage and ADC (r = -0.43, p = 0.003), and a low-moderate positive correlation was found between TLG and ADC (r = 0.33, p = 0.027). ADC showed good diagnostic performance in identifying early T-stage tumors (AUC = 0.86; cutoff = 864 × 10⁻⁶ mm²/s). The OS in the low ADC group was 58.1±6.5 (45.3-70.9) and in the high ADC group 97.4±10.9 (76.1-110.8). PFS was 53.7±5.9 (42.1-65.1) in the low ADC group and 93.3±10.4 (73.0-113.6) in the high ADC group. OS (p = 0.018) and PFS (p = 0.042) were longer in the high ADC group, indicating a statistically significant association.Conclusion: ADC showed a significant association with both tumor stage and survival outcomes and may serve as a nonin-vasive prognostic biomarker in non-mucinous rectal adenocarcinoma. Although TLG correlated with T stage, PET/CT-de-rived metabolic parameters did not predict survival. These findings support the potential role of DWI in pre-treatment risk stratification.