3031 - Assessing Tumor Regression Grade Accuracy via MRI in Rectal Cancer Patients Following Neoadjuvant Chemoradiotherapy: Impact on Clinical Complete Remission Rate

G. Liu¹, X. Chen², B. Song³, H. Hu³, B. Liu⁴, R. Li⁵, and Q. Peng⁶; ¹School of Medicine, University of Electronic Science and Technology of China, Chengdu, Sichuan, China, ²Department of Radiology, Sichuan Cancer Hospital &Institute, Sichuan Cancer Center, University of Electronic Science and Technology of China, Chengdu, Sichuan, China, ³Department of Colorectal Surgery, Sichuan Cancer Hospital &Institute, Sichuan Cancer Center, University of Electronic Science and Technology of China, Chengdu, Sichuan, China, ⁴Department of Radiotherapy, Sichuan Cancer Hospital &Institute, Sichuan Cancer Center, University of Electronic Science and Technology of China, Chengdu, Sichuan, China, ⁵Department of Radiation Oncology, Sichuan Cancer Hospital, Chengdu, 610041, China, Chengdu, China, ⁶Sichuan Cancer Hospital & Institute, School of Medicine, University of Electronic Science and Technology of China, Chengdu, China

Purpose/Objective(s): This study aims to determine the accuracy of MRI-based Tumor Regression Grade (mrTRG) assessment performed by radiologists for patients with locally advanced rectal cancer (LARC) after neoadjuvant chemoradiotherapy (nCRT), emphasizing its significance in achieving clinical complete remission (cCR). Materials/

Methods: This retrospective analysis examined clinical data from patients diagnosed with LARC who received nCRT at one institution. Long-course radiotherapy entailed sessions of 1.8-2 Gy administered 5 times weekly, totaling 45-50.4 Gy, along with capecitabine (825 mg/m2, bid) on radiotherapy days. The short-course radiotherapy consisted of 5 Gy per session, administered 5 times weekly, totaling 25 Gy. Chemotherapy regimens included XELOX and mFOLFOX6. Radiologists completed the questionnaire titled "Information for Determining mrTRG Grading". Assessment of mrTRG was conducted by the radiologists using rectal high-definition MRI within 8-12 weeks after the completion of radiotherapy. In cases where no residual tumor was discernible via rectal fingerprinting, colorectal endoscopy, blood tumor markers, thoracic and abdominal imaging, as well as rectal ultrasonography, the mrTRG score designated a score of 1 to denote cCR, while a score ranging from 1-2 or 2 indicated near-cCR.
Results: A total of 172 patients diagnosed with LARC who underwent nCRT followed by TME were enrolled in this study. 30 radiologists participated, all adhering to the NCCN guidelines. 26 radiologists (86.67%) opined that both T2W MRI and DW MRI could effectively delineate tumor and fiber signals. 12 radiologists (40.00%) considered mrTRG1 as indicative of cCR, while mrTRG1-2 or 2 suggested near-cCR. 17 radiologists (56.67%) believed that mrTRG1 represented cCR, mrTRG1-2 denoted near-cCR, and 1 radiologist (3.33%) suggested that when the tumor signal was less than 10%, mrTRG2 indicated near-cCR. mrTRG scores were distributed as follows: 1 in 1 patient (0.6%), 1-2 in 2 patients (1.2%), and 2 in 77 patients (44.8%). cCR was observed in only 1 patient (0.6%), while near-cCR was noted in 24 patients (14.0%). Among those achieving cCR and near-cCR, 23 (92%) attained pCR, while 2 (8%) achieved near-pCR. The results of the chi-square test revealed a significant difference between pCR and cCR (?2=109.960, p<0.001).
Conclusion: The study highlights a significant disparity between cCR and pCR after evaluating mrTRG by multiple radiologists. Concerns were raised by each radiologist regarding the potential impact of a low mrTRG grade on patient outcome assessment. Consensus among physicians emphasizes the urgent need to standardize the delineation of tumor and fibrous signals within the 2-point scale. Clinicians should reevaluate the reliability of mrTRG in predicting cCR post-neoadjuvant radiotherapy for LARC. Additionally, they should contemplate adopting a personalized approach to ascertain whether a "Watch and wait" strategy is suitable for patients with mrTRG scores of 1-2 or 2.