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
Year: 2026
Type: article
Abstract: 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. 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. The pCR rate was 51.0%. MRI (n = 118), ERUS (n = 81), and PET/MRI (n = 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). 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. Doctors use imaging tests to check how well rectal cancer responds to treatment. If there is no residual cancer, some patients may be able to avoid surgery. However, imaging tests cannot always tell whether cancer is still present. This study included 143 patients with locally advanced rectal cancer who received short-course radiotherapy followed by chemotherapy plus immunotherapy (a drug that helps the immune system fight cancer). All patients later underwent surgery. No cancer was found in 73 patients (51%). We then compared three imaging tests with the results from tissue removed during the surgery to see how well these tests could detect remaining cancer. MRI scans use a special machine to take detailed pictures of the inside of the body. Endorectal ultrasound (ERUS) uses sound waves from a small probe placed inside the rectum to make pictures. MRI scans and ERUS often failed to identify which patients had no cancer left. A newer test called 18F-FAPI PET/MRI, combines MRI with a scan that uses a special tracer to help find any cancer left. Only 25 patients had PET/MRI. Of the ten patients whose surgical samples had no remaining cancer, PET/MRI correctly identified six but missed four. Overall, our findings suggest that imaging tests alone may not be enough to confirm whether cancer is still present. Doctors should still combine physical exams, a camera exam of the inside of the rectum (endoscopy), and imaging scans when considering whether surgery can be safely avoided.
Source: Figshare
Language: English
Subfield: Oncology
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W7225660938
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