If a tumour arises in a polyp it is important to determine whether the stalk is invaded. Differences in classification are reported between Western and Japanese pathologists. In 1985 Haggitt et al.  divid
ed the depth of invasion into four levels: Level 0, carcinoma in situ or intramucosal carcinoma; Level 1, carcinoma invading through the muscolaris mucosa into the submucosa but limited to the head of the polyp; Level 2, carcinoma invading the level of the neck of the adenoma; Level 3, carcinoma invading any part of the stalk; Level 4, carcinoma invading into the submucosa of the bowel wall below the stalk of the polyp. By definition all sessile polyps with invasive adenocarcinoma are Level 4 (Fig. 10). They studied 129 patients with pTis to pT1 colorectal tumours and found that Level 4 invasion was a statistically significant factor (p<0.001) predicting positive nodes. Similar results were reported by Nivatvongs et al.  on 151 patients with pT1 colorectal tumours undergoing bowel resection in which invasion into the submucosa of the bowel wall at the base of the stalk (Level 4) was the single most significant risk factor for positive nodes. For sessile polyps the risk was 10% and for pedunculate polyps 27%. Suzuki et al.  determined the risk of lymph node metastases in 65 patients having Haggitt's Level 4 invasion into the submucosa. Lymph node metastasis was noted in 11 (16.9%) of the 65 patients, however the width of submucosal invasion was significantly greater in node-positive than in node-negative patients (p=0.001). When 5-mm-wide submucosal invasion was used as an indicator for intestinal resection, 37 patients were found to have indications for bowel resection and 11 (29.7%) of the 37 had lymph node metastases. The positive predictive value increased from 17 to 30% when the width of submucosal invasion was added to Haggitt's Level 4 as an indicator for bowel resection. Seitz et al.  suggested that Hag-gitt's classification applies well for pedunculate polyps, however it should not be used for malignant sessile polyps.
Kudo et al.  were the first to differentiate three different types of early invasive cancers: (1) SM-1 tumour, invading the superior third of the submu-cosa, (2) SM-2 tumour, invading the superficial two thirds of the submucosa, and (3) SM-3 tumour, invading the deep third of the submucosa. Within the group type SM-1, there are three subtypes: type SM-1a (indicates that invasion is <1/4 of the submucosa), type SM-1b (indicates that invasion is <1/2 of the submucosa) and type SM-1c (indicates that invasion is >1/2 of the submucosa). Kikuchi et al.  found that the risk of lymph node metastasis was 0% for SM-1 lesion, 10% for SM-2 lesions and 25% for SM-3 lesions (p<0.001). In their study the SM-3 was the only independent risk of lymph node metastasis.
Akasu et al.  recently proposed a classification of the depth of submucosal cancer into two groups: (1) SM-slight (SM-s), extent limited to the upper third of the submucosa; and SM-massive (SM-m), tumour invasion extended to the middle or lower third of the submucosa (Fig. 11). In their series, incidences of lymph node metastasis in pTis, pT1-slight and pT1-massive were 0%, 0% and 22%, respectively. Thus massive submucosal invasion can be considered a risk factor for lymph node metastasis. They suggested that patients with massive submucosal invasion are best treated by radical surgery. A recent study from the Mayo Clinic confirms these data . Among patients with T1 carcinoma in the middle or lower third of the rectum the multivariate risk factors for long-term, cancer-free survival was invasion into the lower third of the submucosa. For lesions with SM3 invasion, the oncologic resection group had lower rates of distant metastasis and better survival compared with patients who underwent local excision. Therefore a decision whether to perform radical surgery or local excision or polypectomy should be
based principally on assessment of invasion depth.
Our ERUS criteria to determine the depth of tumour invasion are as follows: (a) benign lesions (uT0), the mucosal layer is expanded but the submucosal layer remains intact around the entire breadth of the tumour; (b) mucosal or intramucosal neoplasia (M) (uTis), presence of echo-poor spots within the homogeneously echo-rich pattern of villous adenoma. The third hypoechoic layer representing the submucosal interface is intact; and (c) submucosal cancer (uTl), the hyperechoic submucosal layer is irregular or interrupted, consistent with tumour invasion (Fig. 12). The depth of submucosal cancer invasion is classified into two subtypes: slight (SM-s: extent limited to the upper third of the third layer.
The fourth hypoechoic layer of the muscolaris propria is intact) and massive (SM-m: tumour invasion extended to the middle or lower third of the third layer. The fourth hypoechoic layer is thickened consistent with peritumoral inflammation and desmo-plastic reaction). If a distinct break is seen in the submucosal layer, the muscolaris propria has been invaded (uT2 lesion).
Over- and understaging of rectal tumours continues to be a problem in staging with ERUS due to a variety of well documented causes as reported by Adams and Wong  and Kim et al. . A source of error can be due to the compression of the rectal wall by the water-filled balloon. To prevent any distortion of the lesion or separation of the balloon from the rectal wall with the interposition of non-conductive air between the probe and the rectum, a sufficient quantity of water can be instilled to fill the entire rectum. In this case the transducer is covered with a sonolucent plastic cap that does not cause compression of the rectal wall as with the balloon. A source of errors in the evaluation of early rectal cancer by ERUS can also frequently be caused by examiner confusion or a tendency to overestimate a malignant lesion because of concern for undertreatment despite clear ERUS imaging.
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