MRI after preoperative radiotherapy for rectal cancer; correlation with histopathology and the role of volumetry

The objective is to assess if tumor size after radiotherapy in patients with rectal cancer can be assessed by a second magnetic resonance imaging (MRI), after radiotherapy prior to surgery and to correlate changes observed on MRI with findings at histopathology at surgery. Twenty-five patients with MRI before and after radiotherapy were included. Variables studied were changes in tumor size, T-staging and distance to the circumferential resection margin (CRM). RVs was measured as tumor volume at surgery (Vs) divided by tumor volume at the initial MRI (Vi) in percent. RVm was defined as the tumor volume at the second MRI (Vm) divided by Vi in percent. The ypT-stage was the same or more favorable than the initial MRI T-stage in 24 of 25 patients. The second MRI was not more accurately predictive than the initial MRI for ypT-staging or distance to CRM (p > 0.05). Vm correlated significantly to Vs, as did RVs to RVm, although the former was always smaller than the latter. Vm and RVm correlated well with ypT-stage (p < 0.001). Volumetry seems to correlate with ypT-stage after preoperative radiotherapy for resectable rectal cancer. The value of a second MRI after radiotherapy for assessment of distance to CRM and ypT-staging is, however, not apparent.