MRI and Diffusion-weighted MRI Volumetry for Identification of Complete Tumor Responders After Preoperative Chemoradiotherapy in Patients With Rectal Cancer: A Bi-institutional Validation Study.

BACKGROUND: Retrospective single-center studies have shown that diffusion-weighted magnetic resonance imaging (DWI) is promising for identification of patients with rectal cancer with a complete tumor response after neoadjuvant chemoradiotherapy (CRT), using certain volumetric thresholds. OBJECTIVE: This study aims to validate the diagnostic value of these volume thresholds in a larger, independent, and bi-institutional patient cohort. METHODS: A total of 112 patients with locally advanced rectal cancer (2 centers) treated with a long course of CRT were enrolled. Patients underwent standard T2W-magnetic resonance imaging and DWI, both pre- and post-CRT. Two experienced readers independently determined pre-CRT and post-CRT tumor volumes (cm) on T2W-magnetic resonance image and diffusion-weighted magnetic resonance image by means of freehand tumor delineation. Tumor volume reduction rates (Δvolume) were calculated. Previously determined T2W and DWI threshold values for prevolume, postvolume, and Δvolume were tested to "prospectively" assess their respective diagnostic value in discriminating patients with a complete tumor response from patients with residual tumor. RESULTS: Twenty patients had a complete response. Using the average measurements between the 2 readers, areas under the curve for the pre-/post-/Δvolumes was 0.73/0.82/0.78 for T2W-magnetic resonance imaging and 0.77/0.92/0.86 for DWI, respectively. For T2W-volumetry, sensitivity and specificity using the predefined volume thresholds were 55% and 74% for pre-, 60% and 89% for post-, and 60% and 86% for Δvolume. For DWI volumetry, sensitivity and specificity were 65% and 76% for pre-, 70% and 98% for post-, and 70% and 93% for Δvolume. CONCLUSIONS: Previously established DWI volume thresholds can be reproduced with good results. Post-CRT DWI volumetry offers the best results for the detection of patients with a complete response after CRT with an area under the curve of 0.92, sensitivity of 70%, and specificity of 98%.