Accuracy of preoperative MRI in predicting pathology stage in rectal cancers: node-for-node matched histopathology validation of MRI features.

BACKGROUND: Few studies that meticulously match individual lymph nodes seen on MRI with their precise histologic counterparts after total mesorectal excision have been reported. OBJECTIVE: The objective of this study was to determine whether preoperative MRI could detect lymph node metastases accurately in the node-by-node analysis. DESIGN: This was a prospective, observational cohort study. SETTINGS: The study was conducted at a tertiary-care hospital. PATIENTS: Forty patients with rectal cancer were enrolled. MAIN OUTCOME MEASURES: Specimens were assessed using MRI for clinical staging. After surgical resection of the tumor, the specimens were again imaged with ex vivo ultrasound scan to localize the perirectal node. The locations of each lymph node were precisely matched with its corresponding magnetic resonance image to enable a node-for-node comparison of magnetic resonance images and histologic findings. RESULTS: Agreement between MRI and histologic assessment of T stage was 82.5%. Of the 341 nodes harvested, 120 were too small (<3 mm) to be depicted on magnetic resonance images, and 18 of these contained metastasis (15%). A correlation between the results of MRI and histopathology was feasible for 205 lymph nodes, and the overall success rate of matching between the 2 techniques was 91.1% (205 of 221). Preoperative MRI revealed a node-by-node sensitivity and positive predictive value of 58.0%, and 61.7%. There was no difference in the diagnostic accuracy between the primary surgery subgroup and preoperative radiation subgroups. LIMITATIONS: The study is limited by its heterogeneity of cohorts including the subgroup with preoperative chemoradiation and the lack of preoperative ultrasound assessment. CONCLUSIONS: Preoperative MRI was moderately accurate for the prediction of mesorectal lymph node metastasis. Moreover, preoperative MRI was insufficient for detecting small lymph nodes (<3 mm) with metastasis.