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Diffusion-weighted MR volumetry for assessing the response of rectal cancer to combined radiation therapy with chemotherapy.

Editor: In the study by Curvo-Semedo and colleagues (1), which appeared in the September 2011 issue of Radiology, the utility of tumor volumetry derived from diffusion-weighted (DW) magnetic resonance (MR) images was evaluated in the assessment of patients with rectal cancer after preoperative combined radiation therapy with chemotherapy (CRT). They showed that post-CRT DW volumetry and volume shrinkage at both T2-weighted and DW MR imaging were highly accurate in the determination of complete response (areas under the receiver operating characteristic curve [AUC] of 0.93, 0.84, and 0.92, respectively). The apparent diffusion coefficient (ADC) and ADC modification rate were not accurate (AUC, 0.54 and 0.51, respectively). The intraclass correlation coefficient (ICC) between T2-weighted and DW MR volumetry was good for pre-CRT measurements but unsatisfactory for post-CRT measurements. This last observation might be attributable to post-CRT fibrosis, which affects the reliability of segmentation of tumor volume on T2-weighted images with respect to post-CRT DW volumetry based on ADC maps (1). In their study, Curvo-Semedo and colleagues defined complete response as tumor regression grade 1 (2) at pathologic examination or a relapsefree follow-up of 17 months. The preliminary results of our study (3), performed with DW imaging using a b value of 800 sec/mm2, are consistent with those reported by Curvo-Semedo and colleagues. In our study of 21 patients who underwent surgical resection after CRT, the interobserver agreement was good (ICC = 0.948 for T2-weighted MR volumetry and 0.916 for DW MR volumetry), whereas the preand post-CRT DW and T2-weighted volumetry concordance consisted of ICC values of 0.828 and 0.151, respectively. The volume segmented from the DW data set on the basis of hyperintensity of hypercellular tissue on DW images showed a good correlation with tumor regression grade (r = 0.556, P , .05). However, DW MR volumetry has some drawbacks, including the difficulty in detecting small neoplastic residual tumors on the post-CRT ADC maps and T2 shinethrough artifacts, owing to necrosis or mucin, which can produce overestimation of residual tumor at the only qualitative eval uation of DW images. These limitations, in our experience, make it difficult to differentiate between patients with tumor regression grades of 1 or 2 (Mandard five-point scale). The relationship to DW MR volumetry of the nodal stage evaluation could increase its prognostic relevance because good prognosis has been demonstrated for low tumor regression grades ypN2 with respect to ypN+ (4). DW MR volumetry seems to be a promising tool for assessing response after preoperative CRT and deserves further study.