Locally Advanced Rectal Carcinoma Treated with Preoperative Chemotherapy and Radiation Therapy: Preliminary Analysis of Diffusion-weighted MR Imaging for Early Detection of Tumor Histopathologic Downstaging

Purpose To determine whether changes in apparent diffusion coefficients (ADCs) of rectal carcinoma obtained 1 week after the beginning of chemotherapy and radiation therapy (CRT) correlate with tumor histopathologic downstaging after preoperative CRT. Materials and Methods This prospective study was approved by an institutional review board; informed consent was obtained from all patients. Thirty-seven patients (mean age, 54.7 years; 13 women, 24 men) with primary rectal carcinoma who were undergoing preoperative CRT were recruited for the study. Diffusion-weighted (DW) magnetic resonance (MR) imaging was performed with a 1.5-T MR imager in all patients before therapy, at the end of the 1st and 2nd week of therapy, and before surgery. Tumor ADCs were calculated. Linear mixed-effects modeling was applied to analyze change in ADCs and volumes following treatment. Results Patients were assigned to the tumor downstaged group (n = 17) or the tumor nondownstaged group (n = 20) on the basis of histopathologic examination results following surgery. Before CRT, the mean tumor ADC in the downstaged group was lower than that in the nondownstaged group (1.07 × 10−3mm2/sec ± 0.13 [standard deviation] vs 1.19 × 10−3mm2/sec ± 0.15, F = 6.91, P = .013). At the end of the 1st week of CRT, the mean tumor ADC increased significantly from 1.07 × 10−3mm2/sec ± 0.13 to 1.32 × 10−3mm2/sec ± 0.16 (F = 37.63, P <.001) in the downstaged group, but there was no significant ADC increase in the nondownstaged group (F = 1.18, P = .291). The mean percentage of tumor ADC change in the downstaged group was significantly higher than that in the nondownstaged group at each time point (F = 18.39, P < .001). Conclusion Early increase of mean tumor ADC and low pretherapy mean ADC in rectal carcinoma correlate with good response to CRT. DW MR imaging is a promising noninvasive technique for helping predict and monitor early therapeutic response in patients with rectal carcinoma who are undergoing CRT. © RSNA, 2009