How best to combine DWI and T2WI to predict pathologic complete response: a multi-center study on interpreting MRI following chemoradiotherapy of rectal cancer

Abstract

Objectives To explore the different criteria of integrating diffusion-weighted imaging (DWI) for predicting pathologic complete response (pCR) of rectal cancer on post-chemoradiotherapy (CRT) MRI. Materials and methods In this multi-center retrospective study, five radiologists reviewed pre- and post-CRT MRIs of patients with rectal cancer diagnosed in 2017-2021. In addition to mrTRG, three criteria were assessed: "AND" criterion (mrTRG 1-2 and absence of DWI restriction considered as CR), "OR" criterion (mrTRG 1-2 or absence of restriction), and a modified MR tumor regression grade (modMR-TRG). A crossed random effects model was used to pool sensitivity and specificity across five radiologists. F1 score and positive predictive value (PPV) were analyzed across varying pCR rates. Results In 146 patients (median age [IQR], 63 [57-70] years; 87 men), the AND criterion yielded higher specificity (77.4% [63.3-80.0%] vs 75.3% [60.5-79.0%], p = 0.001) without significant difference in sensitivity (63.9% [42.8-75.3%] vs 67.5% [45.3-76.0%], p = 0.063) compared with mrTRG. OR criterion yielded higher sensitivity (86.1% [65.3-89.3%]; p < 0.001) but lower specificity (49.5% [36.2-62.6%]; p < 0.001). The modMR-TRG demonstrated similar effects to the OR criterion. Assuming a 20% pCR rate, PPV and F1 score of the AND criterion (point estimate of 41.4% and 50.3%, respectively) were higher than those of the OR criterion (PPV, 29.9%; F1 score, 44.4%), although the difference diminished with increasing pCR rate. Conclusion The AND criterion-which utilizes DWI complementarily to further exclude patients with residual tumors after initial screening on T2WI-should be preferred over other criteria giving greater emphasis on DWI.