Accuracy

Most of the studies have reported the accuracy rates of ERUS in the evaluation of rectal tumour invasion to be 81-94% (Table 1) [15]. Overestimation occurred in 10% of cases and underestimation occurred in 5% of cases. The accuracy of ERUS in assessing the depth of rectal wall invasion varies with tumour stage. In the literature ERUS remains the most accurate technique for determining the depth of tumour invasion in early stage rectal cancer. Gar-cia-Aguilar et al. [17], from the Division of Colon and Rectal Surgery at the University of Minnesota, reported that ERUS correctly staged most villous adenomas (accuracy: 87%) but less than half of T1 tumours (accuracy: 47%). However in a systematic literature review Worrell et al. [18] reported that ERUS correctly established a cancer diagnosis in 81% of 62 biopsy-negative rectal adenomas which had focal carcinoma on histopathology. In another study

Table 1. Accuracy of endorectal ultrasonography (ERUS) in determining depth of rectal wall invasion of rectal tumours

Authors Sensitivity (%) Specificity (%) Positive Negative Overstaged (%) Understaged (%)

predictive predictive value (%) value (%)

Sentovich et al. [31] 79 uT2 uT3/T4

Holdsworth et al. [29] 96 50

Early pT1 99 74

Deep pT1 98 88

pT2 97 93

pT0/T1 81 98

17 4

81 88

90 100

72 93 18 13

97 87

94 88 25 11

from the Cleveland Clinic Florida [19] the final pathology results confirmed the pre-operative ERUS diagnosis of rectal villous tumours without evidence of malignancy in 26 of 27 patients (96%). Akasu et al. [12] reported the results of a study on 154 patients with early stage rectal cancer pre-operatively evaluated by ERUS. Sensitivity, specificity and overall accuracy rates for detection of slight or massive submucosal invasion were 99%/74%/96% and 98%/88%/ 97%, respectively. Konishi et al. [20] reported that the overall accuracy of ERUS-based evaluation of tumour invasion depth was 60% in villous lesions and 91% in non-villous lesions. In differentiating mucosal neoplasias (M)/submucosal cancers with slight invasion (SM-s) from non-M/SM-s the accuracy of ERUS in villous and non-villous lesions was 66% and 96%, respectively. Akahoshi et al. [21] improved the accuracy of ERUS by using a high-frequency (12 MHz) ultrasound catheter probe. The depth of invasion was correctly assessed in 87% (46/53) of pT1 tumours. Starck et al. [22] reported their experience with high multifrequency probes. The sensitivity of ERUS with regard to invasion was 89% (16/18), specificity 88% (37/42) and accuracy 88% (53/60). They concluded that rectal endosonog-raphy can distinguish between benign rectal lesions and early invasive rectal cancers. Similar results were recently reported by Hunerbein et al. [23] with a high-frequency (12.5 MHz) miniprobe ultrasonogra-phy in the staging of colonic tumours. The infiltra tion depth was correctly classified in 78 of 88 patients (accuracy, 87%). We conducted a prospective study to compare the accuracy of 3D-ERUS with high-frequency ultrasound probe to conventional 2D-ERUS in the pre-operative staging of early invasive rectal cancer [24]. Eighty-nine consecutive patients with rectal villous lesions were examined using both 3D-ERUS and conventional 2D-ERUS. All lesions were resected either endoscopically or surgically. Histo-logically malignant transformation was found in 35 rectal villous adenomas. 2D-ERUS correctly determined the depth of invasion of villous polyps in 6 of 7 M neoplasias (85.7%), 8 of 12 SM-s lesions (66.6%) and 12 of 16 SM-m lesions (75%), whereas the accuracy of 3D-ERUS was 85.7% for M neoplasias, 83.3% for SM-s and 87.5% for SM-m lesions. Overall accuracy of the 2D-ERUS-based evaluation of villous lesions was lower than that of 3D-ERUS-based evaluation (27/35, 77.1%, vs. 30/35, 85.7%), however there was no significant difference (p=0.5). In the evaluation of SM-s lesions the accuracy of 3D-ERUS was significantly superior to 2D-ERUS-based evaluation (p<0.029). Tumour location and gross morphology (sessile or pedunculate) did not correlate with accurate T-staging. Eight of 54 pT0 tumours (14.8%) were overstaged by 2D-ERUS, while 5 of 54 (9.2%) were overstaged by 3D-ERUS. Incidences of lymph node metastasis in M, SM-s and SM-m lesions were 0%, 0% and 12.5%, respectively. The findings showed 3D-ERUS to have a significant advantage over 2D-ERUS

Table 2. Accuracy of endorectal ultrasonography (ERUS) in determining lymph node metastasis from rectal tumours

Author Sensitivity (%) Specificity (%) Accuracy (%) Positive Negative predictive predictive value (%) value (%)

Table 2. Accuracy of endorectal ultrasonography (ERUS) in determining lymph node metastasis from rectal tumours

Author Sensitivity (%) Specificity (%) Accuracy (%) Positive Negative predictive predictive value (%) value (%)

Hildebrandt et al. [3]

72

83

78

-

-

Herzog et al. [33]

89.2

73.4

80.2

71.2

90.4

Sentovich et al. [31]

100

100

-

-

73

Glaser et al. [28]

78

80

79

76

82

Holdsworth et al. [29]

57

64

61

50

70

Garcia-Aguilar et al. [17]

33

64

64

52

68

Akasu et al. [12]

100

60

65

26

100

Kim et al. [14]

53.3

75

63.5

70.6

58.8

Santoro et al. [15]

70

79

74

72

84

for the accurate evaluation of superficial submucosal cancer invasion. Stereoscopic visualisation provided easier and more complete understanding of depth of submucosal invasion.

Overstaging is a particular problem with T2 tumours. In a prospective study Sailer et al. [25] examined the value of ERUS in the pre-operative staging of 160 rectal tumours. For T2 tumours, the sensitivity was only 41% and the specificity 92% as the majority of pT2 neoplasias were overstaged (uT3). Authors concluded that ERUS is of no help in the assessment of T2 carcinomas. Katsura et al. [26] reported that the predictive value of positive rate in the assessment of rectal wall invasion by ERUS was 96.2% in uTl and 87.5% in uT2. Three-dimensional ERUS offers a significant advantage over conventional bidimensional ERUS for the accurate evaluation of rectal cancer. In a preliminary study, Kim et al. [27] showed that the accuracy of 3D ERUS was 90.9% for pT2 whereas that of 2D-ERUS was 84.8%. Glaser et al. [28] reported that the sensitivity of ERUS for detection of perirectal fat infiltration (uT3) was 97%, specificity was 90%, negative predictive value was 98% and positive predictive value was 90%. The inability of ERUS to distinguish between malignant infiltration or peritumoral inflammation results in a somewhat lower staging accuracy with regard to T4 cancers.

The accuracy of ERUS in assessing lymph node involvement varies from 58% to 86% (Table 2) [15]. Holdsworth et al. [29] carried out endorectal sonography by means of a 5.5 MHz transducer. They identified lymph node metastases with a sensitivity of 57% and a specificity of 64% and concluded that the technique is not reliable to identify metastases. Beynon et al. [30], based on identification of circular or oval echo-poor lesions in the mesorectum, obtained a sensitivity of 86% and a specificity of 79% for detection of involved nodes. Kim et al. [27] reported that lymph node metastases were accurate ly predicted by 3D ERUS in 84.8% of patients, whereas 2D ERUS predicted the disorder in 66.7%. Although the findings did not show 3D ERUS to have a statistically significant advantage over 2D ERUS, stereoscopic visualisation provided easier and more complete understanding of lymph nodes.

Whether tumour site (in terms of height) and position (with respect to rectal circumference) have an influence on the reliability of endoluminal ultrasound staging is not settled as yet. Sentovich et al. [31] and Senesse et al. [32] reported a significantly better result for tumours within 6 cm of the anal verge. This is in contradiction to the study conducted by Herzog et al. [33] who found a significantly poorer accuracy rate for tumours of the distal third. The reason for the less accurate staging in the lower rectum is a technical one, that is the difficulty in reaching all sites of the ampulla recti with a rigid probe. This consideration prompted us to make a new dedicated rectoscope to allow easy passage of the probe above the rectal lesion. We performed a prospective study to determine if tumor site and tumor position have an influence on the accuracy of three-dimensional ERUS (3D-ERUS) staging [34]. Endorectal ultrasonography was performed on 173 consecutive patients with primary rectal cancer. In 65 patients the tumour was located 0.1-6 cm from the anal verge (lower rectal tumour), 77 patients had tumours 7-12 cm from the anal verge (middle rectal tumour) and 31 tumours were 13-18 cm from the anal verge (upper rectal tumour). With regard to position, 46 tumours were situated anteriorly, 30 in the left lateral rectal wall, 43 posteriorly and 42 in the right lateral rectal wall. In 12 patients the tumour occupied two-thirds of the rectal circumference. All lesions were resected either endoscopically or surgically. ERUS determined the depth of invasion in 62/65 (95.3%) lower rectal tumours, 74/77 (96.1%) middle rectal tumours and 28/31 (90.3%) upper rectal tumours. With regard to position, accuracy was

93.4% for tumours situated anteriorly, 90.4% for tumours in the right lateral position, 90.6% for tumours situated posteriorly and 86.6% for tumours in the left lateral position. The accuracy of 3D-ERUS for lymph node metastases, evaluated in 142 patients, was 44/46 (95.6%) for lower rectal tumours, 61/65 (93.8%) for middle rectal tumours and 28/31 (90.3%) for upper rectal tumours. Analysis showed that there was no difference between the various locations and positions, which means that all tumours are equally amenable to ultrasound staging if they are within reach of the scanner.

A number of comparative studies have been performed to assess the efficacy of endorectal ultra-sonography, CT, MRI and digital examination in the pre-operative staging of rectal cancer. Some studies have shown a clear superiority for endorectal ultra-sonography whereas other studies have shown little difference. CT and MR imaging are accurate in assessing spread beyond the rectal wall, invasion of contiguous structures, spread to regional nodes or distant metastases. The lateral pelvic lymph nodes, such as the obturator nodes, are located too far from the rectum to be imaged effectively with rectal probes. Therefore, possible advantages of MRI and CT can be considered in assessing the lateral pelvic lymph nodes, pelvic wall invasion and involvement of levator ani muscle. However, CT and MR imaging currently lack the accuracy in determining depth of wall invasion required by the surgeon. Overall accuracy of CT for the staging of rectal tumours is approximately 50-75%. Goldman et al. [35] compared CT with ERUS and found accuracy rates of 52% and 81% respectively, for perirectal fat invasion and 64% and 68% respectively, for lymph node involvement. Similarly, Beynon et al. [30] showed that ERUS was significantly more accurate than CT for both depth of tissue invasion and lymph node involvement. Accuracy rates were 68%, 82% and 91% for 44 patients evaluated with digital examination, CT and ERUS respectively. Civelli et al. [36] reported in the identification of neoplastic infiltration of perirectal fat (T3) endorectal balloon CT had 100% sensitivity, 78.7% specificity and 86.8% accuracy. The CT sensitivity for detecting lymph node metastases was 52.6%, specificity 85.3% and accuracy 73.6%. MRI with endorectal coils has been studied in a number of small studies for the evaluation and staging of rectal tumours [37]. With the addition of endorectal surface coils to conventional MR imaging, spatial resolution is increased and anatomic definition is improved. T2-weighted turbo spin-echo sequences allow the five layers of the rectal wall to be distinguished. Rectal carcinoma in T2-weighted turbo spin-echo sequences gives medium-to-low signal intensity, higher than the muscular layer. MRI and

ERUS demonstrate equivalent efficacy in the pre-operative staging of rectal tumours. Overall accuracy rates of 70-90% have been reported for staging of rectal tumours using MRI with endorectal coils. In the evaluation of lymph nodes, MRI does not offer significant improvement in accuracy rates compared with ERUS. It is unlikely, however, that MRI will gain widespread usage because of lack of widespread availability and significantly increased financial costs.

In conclusion, ERUS is currently the best modality available in the pre-operative staging of rectal cancer. Future improvements may include integration with other modalities such as MRI or PET. However, until further improvements are made, the speediest and best tolerated modality is ERUS.

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