A comparison of the diagnostic ability of 1-mm computed tomography and 3-mm magnetic resonance imaging for detecting lateral pelvic lymph node metastases from rectal cancer.

Aim: The best modality for evaluating lateral pelvic lymph node (LPLN) metastases from rectal cancer remains unclear. This study compared the diagnostic ability of 1-mm computed tomography (CT) and 3-mm magnetic resonance imaging (MRI) in identifying LPLN metastases based on size.

Methods: This observational study analyzed not individual patients but 191 sides from 100 rectal cancer patients without preoperative treatment for whom preoperative CT and MRI and corresponding pathological results for LPLNs were available. A swollen LPLN was defined as an LN with a short-axis size of ≥5 mm on 1-mm CT.

Results: LPLNs were detected significantly more frequently with 1-mm CT than with 3-mm MRI (p < 0.001). Among the 117 sides without swollen LPLNs, metastasis was observed in 1.7% of patients. In contrast, LPLN metastasis was confirmed in 28.4% of 74 sides with swollen LPLNs. In the evaluation of swollen LPLNs, 3-mm MRI yielded a 34% improvement in the diagnostic performance of LPLN metastasis over 1-mm CT (categorical net reclassification improvement: 0.341, p = 0.045).

Conclusions: In conclusion, 1-mm CT should be performed preoperatively to evaluate LPLNs. Further evaluations can be omitted in the absence of swollen LPLNs. In patients with swollen LPLNs, a careful assessment of LPLN metastasis should be conducted via additional MRI, even in early T-stage tumors.