Preoperative lymph node staging in rectal cancer: a difficult challenge

The specimens of 59 rectal cancers that had been scanned by preoperative endorectal ultrasound (EUS) were analysed by the pathologist in order to draw a map of the pararectal lymph nodes that should be detected by preoperative staging. 389 lymph nodes (LNs) were detected in the mesorectum, close to the tumour. Malignant LNs were larger than the non invaded: 17% of the LNs less than 6 mm in diameter were invaded whereas 23% of the LNs 6 mm or more in diameter were free of metastatic invasion. The non invaded LNs displayed three main patterns: follicle, sinusoidal and mixed types. Metastatic LNs were partially (n = 25) or totally (n = 76) invaded by tumoural cells. Diffuse involvement includes 4 different patterns: cellular proliferation, fibrosis, necrosis and cyst formation. Accuracy of EUS evaluated by a "patient by patient" comparison was 61%, with a sensitivity of 84% and a specificity of 39%. However, a comparison "lymph node by lymph node" showed a detection rate of 21% of the lymph nodes of 3 mm and more. It is concluded that a low percentage of LNs are detected by EUS in our experience. Metastatic and non metastatic LNs exhibit a great variety of morphological features and it seems difficult to reliably correlate metastatic invasion with a specific endosonic appearance. LN size remains the most reliable parameter.