Metastatic involvement of the mesorectal lymph node is a major independent prognostic factor. It has been observed that the presence of >3 nodes is associated with a poor prognosis. Moreover, identification of a metastatic perirectal lymph node is important as these patients may benefit from pre-operative adjuvant radiotherapy and some of the early T1 or T2 lesions with mesorectal node involvement are not suitable for local excision.
Sonographic evaluation of lymph node metastases is somewhat less accurate than depth of invasion . Undetectable or benign appearing lymph nodes are classified as uN0. Malignant appearing lymph nodes are classified as uN1 (<3 lymph nodes) or uN2 (>3 lymph nodes). Normal, non-enlarged perirectal nodes are not usually seen on ERUS. The criteria used to identify metastatic lymph nodes in most of the studies are echogenicity, border demarcation and node diameter. Inflamed, enlarged lymph nodes appear hyperechoic, with ill defined borders. Much of the sound energy is reflected because the lymphatic tissue has not changed. In contrast, metastatic lymph nodes that have been replaced with tumour do not provide the normal tissue architecture and appear hypoechoic with an echogenicity similar to the primary tumour (Fig. 17). Malignant lymph nodes tend to be circular rather than oval, have discrete borders and are most commonly found adjacent to the primary tumour or in the mesorectum proximal to a tumour (Fig. 18). The sonographic features of lymph nodes generally can be distributed into four groups. If lymph nodes are not visible by ultrasound, the probability of lymph node metastases is low. Hyperechoic lymph nodes are often benign and result from non-specific inflammatory changes. Hypoechoic lymph nodes larger than 5 mm are highly suggestive for lymph node metastases. Lymph nodes larger than 5 mm that are visible with mixed echogenic patterns cannot be classified accurately but should be considered metastatic. On size characteristics alone, sonographically detected nodes in the mesorectum greater than 5 mm in diameter have a 50-70% chance of being involved, whereas those smaller than 4 mm have a less than 20% chance. However, up to 20% of patients have involved nodes of less than 3 mm, limiting the accuracy of the technique. Hulsmans et al.  studied several features by correlating pathologic and sono-graphic findings in the lymph nodes of specimens
Fig. 18a, b. Three-dimensional ERUS showing malignant lymph nodes
Fig. 18a, b. Three-dimensional ERUS showing malignant lymph nodes obtained from a series of 21 consecutive patients with resected rectal cancer. These features included ratio of long axis to short axis diameter, referred to as roundness index; lobulations (multiple notches); echogenicity; inhomogeneity (not uniform); border delineation; presence of an echo-poor rim (the outer rim being more hypoechoic than the rest of the node); presence of a peripheral halo; and presence of a hilar reflection. The Authors showed that 3 ultrasonographic features of a node significantly related to its being benign or malignant at histopathologic examination are short axis diameter, degree of inhomogeneity and presence or absence of hilar reflection.
Overstaging and understaging can occur during assessment of lymph node involvement. Oedematous lymph nodes transmit more sound energy and appear in echo patterns that are similar to metastases. The cross-sectional appearance of blood vessels in the perirectal fat may be commonly confused with positive lymph nodes. The sonographic continuity of hypoechoic vessels over a distance greater than the cross-sectional diameter is the criterion used to distinguish vessels from hypoechoic lymph nodes. With careful scanning, blood vessels appear to branch or extend longitudinally. In addition, it may be difficult to differentiate islands of tumour outside the bowel wall from involved nodes. With careful scanning, one can demonstrate continuity with the main tumour that may not have been recognised initially. Even with an improved understanding of the characteristic of malignant lymph node and utilising criteria of shape, echogenicity and border character, micrometastases and granulomatous inflammation likely will be difficult if not impossible to differentiate by ERUS. If a whole node is replaced by tumour or the node is enlarged secondary to it, detection is more likely. However, if only a small deposit or a micrometastasis is present in a node, the characteristics of the node are unlikely to be sufficiently altered to allow detection. This explains in part the lower accuracy rates for lymph nodal detection with current, conventional ultrasonography. Grossly malignant lymph nodes located a distance from the primary tumour also remain undetected if they exceed the depth of penetration of the transducer. This is particularly true of nodes in the proximal mesorec-tum above the reach of the rigid probe. To obtain high sensitivity and high specificity, the combination of a small cutoff value and ERUS-guided needle biopsy or ERUS-guided fine-needle aspiration biopsy may be helpful.
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