Diffusion-weighted MR imaging in primary rectal cancer staging demonstrates but does not characterise lymph nodes

OBJECTIVES: To evaluate the performance of diffusion-weighted MRI (DWI) detection of lymph nodes and for differentiating between benign and nodes during primary rectal cancer staging. METHODS: Twenty-one patients underwent 1.5-T MRI followed by surgery (+/- preoperative 5 x 5 Gy). consisted of T2-weighted MRI, DWI (b0, 500, 1000), and 3DT1-weighted MRI 1-mm isotropic voxels. The latter was used for accurate detection and histological validation of nodes. Two independent readers analysed the intensity on DWI and measured the mean apparent diffusion coefficient each node (ADCnode) and the ADC of each node relative to the mean tumour (ADCrel). RESULTS: DWI detected 6 % more nodes than T2W-MRI. The signal was not accurate for the differentiation of metastatic nodes (AUC 0.45- Interobserver reproducibility for the nodal ADC measurements was 0.93). Mean ADCnode was higher for benign than for malignant nodes (1.15 vs. 1.04 +/- 0.22 *10-3 mm2/s), though not statistically significant (P Area under the ROC curve/sensitivity/specificity for the assessment of nodes were 0.64/67 %/60 % for ADCnode and 0.67/75 %/61 % for ADCrel. DWI can facilitate lymph node detection, but alone it is not reliable differentiating between benign and malignant lymph nodes. KEY POINTS: * Diffusion-weighted (DW) magnetic resonance imaging (MRI) offers new in rectal cancer. * DW MRI demonstrates more lymph nodes than standard T2-weighted MRI. * Visual DWI assessment does not discriminate between metastatic nodes. * Apparent diffusion coefficients do not discriminate benign and metastatic nodes.