Contrast-enhanced ultrasound parameters for predicting complete response after neoadjuvant chemoradiotherapy in rectal cancer

Background Accurate identification of complete responders before neoadjuvant chemoradiotherapy (nCRT) is essential for organ-preservation strategies for rectal cancer. However, current preoperative radiological assessment methods lack sufficient accuracy. The purpose of this study is to evaluate the diagnostic value of contrast-enhanced ultrasound (CEUS) in distinguishing complete response (CR) in patients with rectal cancer after nCRT. Methods In this prospective study, 100 patients with rectal cancer treated between January 2023 and February 2025 underwent endorectal ultrasound (ERUS) and CEUS examinations before and 6–8 weeks after nCRT. Quantitative perfusion parameters were derived using time–intensity curve analysis. Surgical histopathology served as the reference standard for pathological CR, and multimodal clinical evaluation defined complete CR. Diagnostic performance was assessed using logistic regression and receiver operating characteristic curve analysis. Result A total of 100 participants (mean age, 57 ± 11 years; 77 men) were included. Pathological CR occurred in 29 (29.0%) participants, and clinical CR in 13 (13.0%), yielding an overall CR rate of 42%. Post-nCRT CEUS parameters—relative area under the time–intensity curve (rAUC*), relative enhancement intensity, and enhancement intensity difference—were significant predictors of CR (all p < 0.05). The AUCs were 0.72 (95% CI: 0.617, 0.829), .76 (95% CI: 0.665, 0.858), and 0.76 (95% CI: 0.695, 0.875), respectively. Combined models achieved AUCs of 0.80 (95% CI: 0.708, 0.887) and 0.82 (95% CI: 0.741, 0.903), respectively. Conclusion Quantitative contrast-enhanced ultrasound (CEUS) parameters, including enhancement intensity (EI), relative AUC*, and their derived ratio values (rEI, rAUC*) and difference (ΔEI), demonstrated robust discriminative capacity in complete response participants after nCRT. The CEUS-based diagnostic model also demonstrated a high level of diagnostic efficacy. The clinical diagnostic model constructed from the above parameters demonstrated higher diagnostic value.