Arterial-phase mucosal linear enhancement as an indicator of pathological complete response after immunotherapy in pMMR/MSS locally advanced rectal cancer.

PURPOSE: Immunotherapy-based neoadjuvant chemoradiotherapy (iNCRT) has recently emerged for proficient mismatch repair/microsatellite stable (pMMR/MSS) locally advanced rectal cancer (LARC). Accurate identification of pathological complete response for primary tumor (ptPCR) post-treatment is critical for selecting patients eligible for watch-and-wait strategies. This study aimed to evaluate arterial-phase mucosal linear enhancement (AMLE) on contrast-enhanced T1-weighted imaging (CE-T1WI) for predicting ptPCR after iNCRT in pMMR/MSS LARC, compared to conventional T2-weighted/diffusion-weighted imaging (T2DWI) and rectal endoscopy.

METHODS

This retrospective study included patients with pMMR/MSS LARC who underwent total mesorectal excision after iNCRT between July 2022 and Oct 2024 at a tertiary referral academic center. Preoperative re-staging examinations were rectal endoscopy and MRI, included T2DWI and arterial-phase CE-T1WI for primary tumor assessment. Baseline and post-therapy features associated with ptPCR were identified using univariate and multivariable regression analysis. Diagnostic performance of endoscopy and different MRI protocols to identify ptPCR after iNCRT was evaluated using ROC curves.

RESULTS

In total, 75 patients (mean age, 57 years ± 10 [SD]; 54 male patients) were assessed. At histopathology, 29 patients achieved ptPCR. AMLE was more common in the ptPCR group than in the non-ptPCR group after iNCRT (75.9% vs 15.2%, respectively; P < 0.001). AMLE was associated with higher odds of ptPCR in the multivariable regression analysis (odds ratio, 19.14; 95% CI 4.03, 90.87; P = 0.001). And AMLE exhibited the best diagnostic performance in identifying ptPCR after iNCRT, with highest sensitivity, specificity, PPV, NPV, and AUC (0.80; 95% CI 0.70, 0.89).

CONCLUSION

AMLE at CE-TlWI of rectal MRI could be a potential indicator of ptPCR after a new iNCRT in pMMR/MSS LARC, suggesting a relatively credible preoperative evaluation strategy for this group of patients in clinical practice to accurately exclude residual tumors and select watch-and-wait approach, avoiding unnecessary surgery.