Diagnostic performance of MRI, endorectal ultrasound, and FAPI PET/MRI for identifying pathological complete response after short-course radiotherapy and chemoimmunotherapy in locally advanced rectal cancer: a retrospective study

BACKGROUND: Accurate imaging-based response assessment is essential for proficient mismatch repair (pMMR) locally advanced rectal cancer (LARC). This study aimed to evaluate the diagnostic performance of MRI, endorectal ultrasound (ERUS) and 18F-FAPI PET/MRI for identifying pathological complete response (pCR) after neoadjuvant short-course radiotherapy (SCRT) followed by chemoimmunotherapy. METHODS: This single-center retrospective study included 143 patients. Diagnostic performance was assessed using sensitivity, specificity, and accuracy with 95% confidence intervals. Cohen's κ assessed agreement between imaging-defined complete response (iCR) and pCR. Paired comparisons used Cochran's Q and Holm-adjusted exact McNemar tests. RESULTS: = 25) achieved sensitivity, specificity, and accuracy of 4.76%, 98.18%, and 48.31%; 30.95%, 87.18%, and 58.02%; and 60.0%, 100.0%, and 84.0%, respectively. PET/MRI-assessed imaging complete response showed substantial agreement with pCR (κ = 0.643). CONCLUSION: Neoadjuvant SCRT followed by chemoimmunotherapy achieved high pCR rates in pMMR LARC. Conventional MRI and ERUS showed limited sensitivity despite relatively high specificity. 18F-FAPI PET/MRI showed potential diagnostic utility in a small subgroup, but its moderate sensitivity and limited sample size warrant further validation. These findings highlight the limitations of imaging alone and the need for comprehensive response assessment.