Diffusion-weighted MRI in locally advanced rectal cancer. Pathological response prediction after neo-adjuvant radiochemotherapy

Background and purpose: The aim of this study was to assess the predictive potential of diffusion-weighted magnetic resonance imaging (MRI) for the selection of favorable pathological responders after radiochemotherapy for locally advanced rectal cancer. Patients and methods: In 59 patients with locally advanced rectal cancer, the apparent diffusion coefficient (ADC) in the tumor was obtained at 3 Tesla before radiochemotherapy and surgery. The predictive potential for pathological complete response (pCR) and good response (GR) was assessed. GR was defined as pCR and near-pCR based on the tumor regression grade. Results: The GR group consisted of 13 patients (22%) with 9 complete responders. Both the preradiochemotherapy ADC values and relative change in ADC (ΔADC) were predictive for pathological response. Preradiochemotherapy ADC values showed a positive predictive value of 42% for pCR and 67% for GR using a similar cut-off value of 0.9710-3 mm2/s. For ΔADC, the optimal threshold for predicting GR or pCR was a 41% increase of the ADC. With this threshold, positive predictive values of 64% and 91% were found for pCR and GR, respectively. Conclusion: Low preradiochemotherapy ADC values and high ΔADC correspond to pathological good response. Diffusion-weighted MRI may be used as an additional tool for selecting good treatment responders after radiochemotherapy. (orig.)