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EUS errors in N stage can occur for similar reasons. A malignant node [23,24]can appear to have benign characteristics (Table 3) because micrometastasis may not produce any changes in the appearance of lymph nodes until disease becomes more advanced. In contrast, inflammation can cause changes in borders, echogenicity, shape, and size characteristic of a metastatic node. Overlap of criteria to differentiate neoplastic from inflammatory lymph nodes appears to be less for assessment of boundaries and echogenicity than for shape and size. CT relies primarily on size to evaluate N stage. Because a majority of metastatic lymph nodes are less than 10 mm, CT has also been insensitive to neoplastic regional lymph nodes. In contrast, because most lymph nodes larger than 10 mm are usually neoplastic, there is good specificity when CT is positive. For EUS, size resolution is not a limitation. Lymph nodes more than 3 mm are easily found [23]; however, even though EUS is superior to other imaging methods to differentiate malignant from benign inflammatory lymph nodes, criteria (see Table 3) overlap and still require improvement. Because high-frequency sound waves have a short penetration depth, a limitation of EUS is that the optimal focal range is relatively short, only 2 to 4 cm from the probe. Therefore, optimal focus of vessels or structure around a tumor larger than 5 cm may not always be achieved, and vessel or organ involvement may be missed. Also, because only portions of the liver can be visualized clearly, metastatic lesions may not be detected. However, in the part of the left lobe of the liver that can be visualized clearly, metastatic lesions less than 1 cm - missed by other imaging techniques - can be picked up by EUS [50]. EUS is very accurate in evaluation of M stage when it documents small distant malignant lymph nodes, providing that a stenotic lesion does not prevent the transducer from reaching a site such as the celiac axis. EUS can also detect small amounts of ascites not seen with other methods. An advantage of CT is that it can evaluate sites that are not accessible to EUS where tumor may spread, such as distant organs and distant malignant lymph nodes, which tend to be larger than 1 cm when associated with advanced disease. Table 3
a Criteria are useful only for frequency of 7.0 MHz or greater. |