Prediction of rectal lymph node metastasis by pelvic computed tomography measurement

Rectal cancer staging represents a crucial step to select the best treatment for this tumour. Particularly after neo-adjuvant chemoradiotherapy (CRT), it may influence the surgical procedure (e.g. radical resection vs. local excision). The aim of this study was to determine the best lymph node size cut-off at computed tomography (CT) to predict nodal metastasis in rectal cancer patients with and without preoperative CRT.

A consecutive series of patients operated on for primary mid-low rectal adenocarcinoma, all staged with pelvic CT scan, were subdivided as follows: those who underwent surgery alone treatment without CRT (Group A) and those who underwent preoperative CRT (Group B). All CT scans were re-viewed by a single radiologist and, based on the lymph node size, findings were compared with pathologic lymph node status (pN). At each lymph node size cut-off value, the following were calculated: accuracy, sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV). The best cut-off value was defined as having an accuracy >or=70% with the highest NPV.

The study population consisted of 162 patients: Group A (n=52) and Group B (n=110). Patients classified as pN-positive (n=45) had a higher number of and larger sized lymph nodes by CT scan than patients classified as pN-negative (n=117). The cut-off values with an accuracy >or=70% ranged between 7 and 11 mm in Group A and between 9 and 14 mm in Group B. The cut-off with the best NPV was 7 mm for Group A and 10mm for Group B.

Acknowledging the limitations of the dimensional criterion, lymph node size cut-off values found in our study may be useful for planning rectal cancer treatment using CT scan.