Diffusion-weighted MR Imaging for Prediction of Early Response of Locally Advanced Rectal Cancer to Chemoradiation Therapy

PURPOSE: To evaluate diffusion-weighted imaging (DWI) for assessment of early treatment response of locally advanced rectal cancer (LARC) 2 weeks after chemoradiotherapy (CRT). METHOD AND MATERIALS: Twenty-eight patients with LARC underwent magnetic resonance imaging prior to, 2 and 8 weeks after CRT, including T2-weighted images and echo-planar DWI with 6 b-values (0 to 1000 sec/mm2); apparent diffusion coefficient (ADC) values were measured. Imaging was correlated to post-surgical histopathologic tumor regression grade (TRG), according to Mandard’s classification. Two and 8 week post-treatment ADC changes respect to baseline (%ΔADC) in the group with complete response (CR) (TRG1) were compared with those in the non complete response group (non-CR) (TRG2-5). Moreover, 2 and 8 week post-treatment ADC changes were compared either in CR or in non-CR group. A p-value ≤ .05 was considered significant. RESULTS: At 8 weeks the median %ΔADC of CR group (86.91 [95% CI 2.49-138.94]) was significantly higher compared to that of non-CR group (37.66 [95% CI 5.59-66.78]) (p= .05). Similarly, at 2 weeks the median %ΔADC of CR group (47.5 % [95% CI .22 to 102.17]) was significantly higher than that of non-CR group (-0.84 % [95% CI -20.07 to 24.25]) (p=.024). The best %ΔADC cut-off to differentiate CR from non-CR at 2 weeks was 25% (p=.0001), which yielded a sensitivity of 91%, a specificity of 77%, and an Az value of 0.88; at 8 weeks it was 29.5% (p=.0001) with a sensitivity of 83%, a specificity of 90% and an Az value 0.91. When comparing median %ΔADC obtained at 2 and 8 weeks after CRT, no significant differences were found either for CR or for non-CR group (p=.13). CONCLUSION: The median %ΔADC at 2 weeks after CRT represents an early and reliable predictor of preoperative treatment outcome. CLINICAL RELEVANCE/APPLICATION: ADC-changes 2 weeks post-treatment compared to baseline seems to be a useful tool to evaluate early response to chemoradiation therapy in patients with locally advanced rectal cancer.