Rectal cancer: mesorectal lymph nodes at MR imaging with USPIO versus histopathologic findings--initial observations

Abstract

Purpose: To compare histopathologic findings with appearances of mesorectal lymph nodes at magnetic resonance (MR) imaging with ultrasmall particles of iron oxide (USPIO) in rectal cancer.

Materials and methods: Mesorectal lymph nodes in 12 patients with adenocarcinoma of the rectum were evaluated with USPIO and high-spatial-resolution MR imaging. Appearance and signal intensity of lymph nodes at T2- and T2*-weighted imaging were recorded before and after USPIO administration. Two radiologists visually assessed pattern of enhancement; interobserver agreement was tested with the kappa statistic. After total mesorectal excision, MR imaging of surgical specimens was performed, and it enabled node-by-node correlation with histopathologic findings.

Results: Appearances of 74 nodes at in vivo MR imaging were compared with histopathologic findings. Sixty-eight nodes were nonmalignant (34 were normal, 34 showed reactive changes); six nodes were malignant. Four patterns of USPIO uptake were demonstrated at T2*-weighted imaging: uniform low signal intensity, central low signal intensity, eccentric high signal intensity, and uniform high signal intensity. Two radiologists showed good interobserver agreement (kappa = 0.88, P <.01) in classification of nodes into these four categories. Sixty-five (96%) of 68 nonmalignant nodes showed uniform or central low-signal-intensity patterns; 16 (47%) of 34 reactive nodes showed central low-signal-intensity patterns. Compared with uniform low-signal-intensity pattern, central low-signal-intensity pattern was more commonly observed in reactive nodes (P <.01, chi(2) test; positive predictive value, 67%; 95% CI: 47%, 87%). Eccentric and uniform high-signal-intensity patterns were observed in lymph nodes that contained metastases larger than 1 mm in diameter.

Conclusion: Mesorectal lymph nodes can be characterized by using USPIO and T2*-weighted MR imaging. Uniform and central low-signal-intensity patterns are features of nonmalignant nodes. Reactive nodes frequently show central low signal intensity at T2*-weighted imaging.