Publication:

Correlation between magnetic resonance imaging-based radiological response after 8 weeks of neoadjuvant chemoradiotherapy and postoperative histopathological response in rectal adenocarcinoma

Background Neoadjuvant chemoradiotherapy (NACRT) is the standard of care for locally advanced rectal adenocarcinoma, facilitating tumor downstaging and improving surgical outcomes. Magnetic resonance imaging (MRI) is widely used to assess treatment response after NACRT; however, its ability to accurately predict histopathological tumor regression remains variable. Reliable radiological prediction is essential for individualized treatment planning and consideration of organ-preserving strategies.

Methods This prospective observational study included 47 patients with mid or low rectal adenocarcinoma (≤12 cm from the anal verge) who received long-course NACRT followed by total mesorectal excision. Pelvic MRI performed 8 weeks post-NACRT assessed tumor response using the MRI tumor regression grade (mrTRG). Histopathological response was evaluated using the Rödel tumor regression grade (TRG) system. Correlation between mrTRG and pathological TRG was analyzed using Spearman’s correlation, and the diagnostic accuracy of MRI in predicting pathological complete response (pCR) was calculated.

Results MRI demonstrated mrTRG 1–2 in 22 patients (46.8%), mrTRG 3 in 16 (34.0%), and mrTRG 4–5 in 9 (19.1%). Histopathology revealed pCR (TRG 0) in 11 patients (23.4%). MRI correctly identified 8 of 11 pCR cases. MRI prediction of pCR showed a sensitivity of 72.7%, specificity of 85.3%, positive predictive value of 61.5%, and negative predictive value of 90.3%. A significant correlation was observed between mrTRG and pathological TRG (Spearman’s r=0.64, p<0.001).

Conclusions MRI-based assessment at 8 weeks post-NACRT demonstrates a significant correlation with histopathological response and provides reliable exclusion of complete response. Integration with clinical assessment is recommended for optimal treatment decision-making.