[Assessment of lymph node status in rectal cancer by imaging]

Assessment of lymph node status in patients with rectal cancer is important before operation and neoadjuvant treatment because it has an influence on definition of operative plan, strategy of chemoradiotherapy and prognosis of the patients with rectal cancer. The imaging modalities that can be used to assess lymph node status include magnetic resonance imaging (MRI), endorectal ultrasonography (ERUS), multi-row detector computer tomography (MDCT), and PET-CT. The existing imaging criteria for metastatic lymph nodes in rectal cancer have not been reached. Combined with morphologic and functional imaging characteristics can be accurately predict metastatic lymph nodes. Based on the size of lymph node, the accuracy of rectal cancer N stage defined by imaging modality is poor. If the border and signal intensity of lymph nodes are used to predict the metastasis, the sensitivity and specificity can be significantly improved. However, the reproducibility needs an improvement because of limitation in imaging quality and difference in ability of imaging interpretation. In comparison, ultra small superparamagnetic iron oxide MRI (USPIO-MRI) has relatively higher reproducibility in definition of the metastatic lymph nodes while it is not permitted as an imaging tool used in clinical practice. In addition, USPIO-MRI has been used to evaluate the lymph node response to chemotherapy. As functional imaging tool, DW-MRI has high sensitivity in detection of lymph nodes, while the specificity is low. In contrast, PET-CT has lower sensitivity because of the limitation in imaging resolution, which can not find the small metastatic lymph nodes. ERUS and high resolution MRI have similar ability in distinguishing metastatic and benign nodes within mesorectum. While MRI and DW-MRI have greater diagnostic view than ERUS, thus they can effectively screen and diagnose the pelvic lateral lymph nodes and rectal upper arteriovenous lymph nodes. MDCT is the better choice in screening chest and abdominal metastatic lymph nodes compared with other modalities. The proper selection of imaging modality or combination of multi-modalities is necessary for special purpose in clinical practice.