Diffusion-weighted MRI and MR- volumetry--in the evaluation of tumor response after preoperative chemoradiotherapy in patients with locally advanced rectal cancer.

PURPOSE: To determine the diagnostic performance of diffusion-weighted MRI and MR volumetry for the assessment of tumor response after preoperative chemoradiotherapy (CRT) in patients with locally advanced rectal cancer.

MATERIALS AND METHODS: Forty-three patients with rectal cancer who underwent preoperative CRT were prospectively examined for the study. This prospective study was approved by our institutional review board. DW- and high resolution T2-weighted imaging were performed before and after therapy. Two different diffusion gradients (b = 0 and b = 600, then separately b = 0 and b = 1000) were applied. The mean tumor volume and mean ADC values were measured before and after therapy. To evaluate the responders and nonresponders to neoadjuvant CRT, two criteria, ypT stage determined in the pathologic examination after treatment and histopathologic tumor regression grade (Ryan), were used as reference standards. The patients with a lower ypT stage than T stage in the first MRI before neoadjuvant CRT were evaluated as the responder group, while the patients with a higher or the same ypT stage relative to the first MRI T stage were evaluated as the nonresponder group. According to Ryan tumor regression grade, grade 1 was evaluated as the responders, whereas grades 2 and 3 were evaluated as the nonresponder group. The percentage ADC increase and percentage tumor volume regression were compared between the responders and nonresponders using two reference standards: T downstaging and tumor regression grade (TRG).

RESULTS: Before CRT, the mean tumor ADC in the responder group was significantly lower than that in the nonresponder group (p < 0.001). At the end of CRT, the mean percentage of tumor ADC change in the responder group was significantly higher than that in the nonresponder group. The percentage tumor volume regression of the responders was significantly higher than that of the nonresponders (p = 0.001). The cut-off ADC value for discriminating between the responders and nonresponders after treatment was determined to be (b = 600) 1.03 × 10(-3)mm(2)/s and the sensitivity, 71%; specificity, 79%; accuracy, 74%; positive predictive value, 81%; negative predictive value, 68% respectively. The cut-off value for discriminating between the responders and the nonresponders after treatment was determined for b = 1000 as 1.20 × 10(-3)mm(2)/s and the sensitivity, 42%; specificity, 84%; accuracy, 60%; positive predictive value, 77%; negative predictive value, 53%.

CONCLUSION: The increase in the mean tumor ADC and percentage tumor volume regression in patients with rectal cancer treated with preoperative CRT was correlated with good response. DW MR imaging is a promising non-invasive technique that can help predict and monitor early therapeutic response in patients with rectal cancer who undergo CRT.