A Pattern-Based Approach Combining Tumor Morphology on MRI With Distinct Signal Patterns on Diffusion-Weighted Imaging to Assess Response of Rectal Tumors After Chemoradiotherapy

Background: Diffusion-weighted imaging is increasingly used in rectal cancer MR imaging to assess response after chemoradiotherapy. Certain pitfalls (e.g. artefacts) may hamper diffusion- MRI assessment, leading to suboptimal diagnostic performance. Combining diffusion- weighted MRI with the underlying morphology on standard (T2-weighted) MRI may help overcome these pitfalls. Objective: To evaluate the diagnostic performance of a pattern-based approach combining tumor morphology on T2-weighted MRI with distinct diffusion-weighted imaging signal patterns to assess response after chemoradiotherapy in rectal cancer. Patients: n=222 locally advanced rectal cancer patients undergoing chemoradiotherapy. Design: Response to chemoradiotherapy was scored according to four patterns: A: cases with either a clear residual mass with corresponding high diffusion signal (A+) or completely normalized wall without diffusion signal (A-); B: cases with circular and/irregular fibrosis with (B+) or without (B-) small foci of diffusion signal scattered throughout the fibrosis; C: cases with semi-circular fibrosis with (C+) or without (C-) high diffusion signal at the inner margin of fibrosis; D: polypoid tumors showing regression of the polyps and fibrosis at the site of the stalk with (D+) or without (D-) focal high diffusion signal in the stalk. N=75 cases were re-scored by an indipendent second reader to studdy interobserver variations. Standard of reference was histopathology or long-term outcome. Main outome measures: Diagnostic performance to discriminate between a complete response and residual tumor. Results: The pattern-based approach resulted in a sensitivity of 94%, specificity 77%, PPV 88%, NPV 87%, and overall accuracy of 88% to differentiate between tumor vs. complete response. Accuracy per pattern were 100% (A), 73% (B), 86% (C) and 92% (D). Interobserver agreement was good (k0.75). 2 Limitations: No comparison with “routine” (non-pattern) diffusion-MRI assessment. Conclusions: A pattern-based approach combining tumor morphology with distinct diffusion-weighted imaging patterns results in good diagnotic performance to assess response.