Diffusion-weighted MRI for selection of complete responders after chemoradiation for locally advanced rectal cancer: a multicenter study

Abstract

Purpose: In 10-24% of patients with rectal cancer who are treated with neoadjuvant chemoradiation, no residual tumor is found after surgery (ypT0). When accurately selected, these complete responders might be considered for less invasive treatments instead of standard surgery. So far, no imaging method has proven reliable. This study was designed to assess the accuracy of diffusion-weighted MRI (DWI) in addition to standard rectal MRI for selection of complete responders after chemoradiation.

Methods: A total of 120 patients with locally advanced rectal cancer from three university hospitals underwent chemoradiation followed by a restaging MRI (1.5T), consisting of standard T2W-MRI and DWI (b0-1000). Three independent readers first scored the standard MRI only for the likelihood of a complete response using a 5-point confidence score, after which the DWI images were added and the scoring was repeated. Histology (ypT0 vs. ypT1-4) was the standard reference. Diagnostic performance for selection of complete responders and interobserver agreement were compared for the two readings.

Results: Twenty-five of 120 patients had a complete response (ypT0). Areas under the ROC-curve for the three readers improved from 0.76, 0.68, and 0.58, using only standard MRI, to 0.8, 0.8, and 0.78 after addition of DWI (P = 0.39, 0.02, and 0.002). Sensitivity for selection of complete responders ranged from 0-40% on standard MRI versus 52-64% after addition of DWI. Specificity was equally high (89-98%) for both reading sessions. Interobserver agreement improved from κ 0.2-0.32 on standard MRI to 0.51-0.55 after addition of DWI.

Conclusions: Addition of DWI to standard rectal MRI improves the selection of complete responders after chemoradiation.

Fig. 1

Standard T2-weighted images of a female patient with a tumor (T) in the mid-rectum, before (a) and after (b) preoperative chemoradiation therapy. After chemoradiation, the tumor has completely disappeared and a normalized rectal wall can be visualized (arrowheads). This feature was considered strongly predictive for a complete tumor response

Fig. 2

Standard T2-weighted images of a male patient with a tumor (T) in the rectum, before (a) and after (b) preoperative chemoradiation therapy. After chemoradiation, a solid residual tumor mass is still visualized (arrow). This feature was considered strongly predictive for the presence of residual tumor

Fig. 3

Standard T2-weighted images of two patients with a tumor (T) in the rectum before (a, d) and after chemoradiation treatment (b, e). In both cases, the tumor bed has become fibrotic after chemoradiation (arrowheads), which makes it difficult to discriminate between residual tumor and a complete response. In the upper patient, there is still a clear high signal intensity area on DWI (arrow in c), which was confirmed to be a ypT2 residual tumor at histology. In the lower patient, no high signal is shown on DWI (f) and a complete tumor response (ypT0) was confirmed at histology

Fig. 4

Receiver operator characteristics curves and areas under the curve (AUC) of the three readers for identification of a complete tumor response after CRT using only standard MRI and standard MRI + DWI, respectively. Diagnostic performance improved significantly (*) for reader 2 (P = 0.02) and reader 3 (P = 0.002). For reader 1, there was no significant improvement (P = 0.39)