Locally Advanced Rectal Cancer: MR Imaging for Restaging after Neoadjuvant Radiation Therapy with Concomitant Chemotherapy Part II. What Are the Criteria to Predict Involved Lymph Nodes?1

Purpose: To prospectively determine diagnostic performance of predictive criteria for nodal restaging after radiation therapy with concomitant chemotherapy by using ultrasmall superparamagnetic iron oxide (USPIO)-enhanced magnetic resonance (MR) imaging in patients with rectal cancer. Materials and Methods: After institutional review board approval and informed consent were obtained, 39 patients (24 men, 15 women; mean age, 64 years) with rectal cancer underwent USPIO-enhanced two-dimensional (2D) T2-weighted fast spin-echo, three-dimensional (3D) T1-weighted gradient-echo, and 3D T2*-weighted MR for restaging. Two observers evaluated nodes for border irregularity, short- and long-axis diameters, and estimated percentage of white region (50%) within the node (3D T2*-weighted images). Ratio of the measured surface area of the white region within the black node to the measured surface area of the total node (Ratio A ) was calculated. Signal intensity (SI) in gluteus muscle (SI GM ) and in total node (SI TN ) were used to calculate SI TN /SI GM ratio. Histopathologic findings were reference standard. Receiver operating characteristic (ROC) curves were compared and interobserver agreement was determined. Results: Lesion-by-lesion analysis was feasible in 201 lymph nodes. Area under the ROC curve (AUC) of border and short- and long-axis diameters for observer 1 were 0.85, 0.87, and 0.88 and for observer 2 were 0.70, 0.89, and 0.87, respectively. AUC for estimated percentage of white region within the node, Ratio A , and SI TN /SI GM ratio for observer 1 were 0.98, 0.99, and 0.62 and for observer 2 were 0.97, 0.98, and 0.65, respectively. AUC for USPIO-enhanced MR criteria was significantly better than AUC for conventional MR criteria ( P < .01). All criteria except border irregularity and SI TN /SI GM ratio showed high interobserver agreement (κ > 0.79). Conclusion: The most reliable predictors for identifying benign nodes after radiation therapy with concomitant chemotherapy by using USPIO-enhanced MR imaging for restaging in patients with rectal cancer were estimated percentage of white region within the node and Ratio A . Measurements on standard 2D T2-weighted fast spin-echo images versus primary staging results offer reasonably good accuracy to identify benign lymph nodes after therapy. © RSNA, 2009