Locally Advanced Rectal Cancer: Added Value of Diffusion-weighted MR Imaging in the Evaluation of Tumor Response to Neoadjuvant Chemo- and Radiation Therapy

Purpose: To investigate the added value of diffusion-weighted (DW) magnetic resonance (MR) imaging in the evaluation of complete response (CR) to neoadjuvant combined chemotherapy and radiation therapy (CRT) in patients with locally advanced rectal cancer. Materials and Methods: Institutional review board approval was obtained for this retrospective study, and the patient informed consent requirement was waived. Forty consecutive patients with locally advanced rectal cancer (>= T3 or lymph node positive) who underwent CRT and subsequent surgery were enrolled in this study. All patients underwent pre- and post-CRT 1.5-T rectal MR imaging and post-CRT DW MR imaging. For qualitative analysis, two radiologists who were blinded to pathologic staging and prior imaging data retrospectively and independently reviewed conventional MR images and the combined set of MR images and DW MR images over a 2-week interval and recorded their confidence level with respect to the CR to neoadjuvant CRT. Diagnostic accuracy was calculated for each reviewer with receiver operating characteristic (ROC) curve analysis. For quantitative analysis, a third radiologist measured the apparent diffusion coefficient (ADC) of the region of interest three times. Mean ADCs in the CR group were compared with those in the non-CR group. Pathology reports served as the reference standard. Results: Diagnostic accuracy (area under the ROC curve [Az]) in the evaluation of CR was significantly improved after additional review of DW MR images for both reviewers: For reviewer 1, Az improved from 0.676 to 0.876 (P = .005), whereas for reviewer 2, Az improved from 0.658 to 0.815 (P = .036). Mean ADC ([1.62 +/- 0.36] x 10(-3) mm(2)/sec) (standard deviation) of the CR group (n = 11) was significantly higher than that ([1.04 +/- 0.24] x 10(-3) mm(2)/sec) of the non-CR group (n = 29) ( P < .0001). Conclusion: Adding DW MR imaging to conventional MR imaging yields better diagnostic accuracy than use of conventional MR imaging alone in the evaluation of CR to neoadjuvant CRT in patients with locally advanced rectal cancer. (C) RSNA, 2009