Prediction of N0 Irradiated Rectal Cancer Comparing MRI Before and After Preoperative Chemoradiotherapy

BACKGROUND: The prediction of lymph node status using MRI has an impact on the management of rectal cancer, both before and after preoperative chemoradiotherapy. OBJECTIVE: To maximize the negative predictive value and sensitivity of mesorectal lymph node imaging after chemoradiotherapy because post-chemoradiation nodenegative patients may be treated with rectum-sparing approaches. DESIGN: Retrospective study. SETTINGS: The study was conducted at a tertiary-care hospital. PATIENTS: Sixty-four patients with locally advanced rectal cancer who underwent preoperative chemoradiotherapy and MRI for staging and the assessment of response were evaluated. MAIN OUTCOME MEASURES: The sums of the sizes of all mesorectal lymph nodes in each patient on both pre- and post-chemoradiotherapy imaging datasets were calculated to determine the lymph node global size reduction rates, taking these to be the outcomes of the histopathologic findings. Other included measures were interobserver agreement regarding the prediction of node status based on morphologic criteria and the diagnostic performance of contrast-enhanced images. RESULTS: Using a cutoff value of a 70% lymph node global size reduction rate with only 15 node-positive patients on histopathology, the sensitivity in the prediction of nodal status and negative predictive value were 93% (95%CI: 70.2-98.8) and 97% (95%CI: 82.9-99.8) for Observer 1, and 100% (95%CI: 79.6-100) and 100 % (95%CI:62.9-100) for Observer 2. The areas under the receiver operating characteristic curves for the two observers were 0.90 (95%CI: 0.82-0.98, p<0.0001) for Observer 1 and 0.65 (95%CI: 0.50-0.79, p=0.08) for Observer 2. The efficacy of the morphologic criteria and contrast-enhanced images in predicting node status was limited after chemoradiotherapy. LIMITATIONS: This study is limited by its small sample size and retrospective nature. CONCLUSIONS: Assessing the lymph node global size reduction rate value reduces the risk of undetected nodal metastases and may be helpful in better identifying suitable candidates for the local excision of early-stage rectal cancer.