Eus

During the last decade EUS has been increasingly used to evaluate lesions lying in the last 10-12 cm of rectum from the anal verge. Basically it is a morphologic study of the rectal wall with its mucosal, submu-

cosal and muscular layers, visualised as a classic 5-layer echoic pattern (or a 7-layer pattern if a 10-MHz probe is used). Cancer usually appears as a hypoe-choic lesion invading one or more echoic layers through the rectal wall. Accuracy for T-stage has been reported in the literature from 67 to 95%, with a sensitivity of 83-98%, specificity 75-87%, ppv 89 and npv 95 [10,16,17,19]. Pitfalls involve the operator's experience, the degree of tumour infiltration and some technical issues such as artefacts, peritumoral inflammation and post-biopsy alteration of the echoic pattern. Artefacts are generated by the presence of air bubbles between the rectal wall and the probe, causing a complete loss of signal or by the position of the probe with respect to the bowel wall or lesion. If not visualised at right angles, layers of different echogenic property appear thicker at their interface until a mirror image is generated. Peritumoral inflammation appears as a hypoechoic band at the infiltration margin that can be easily confused with the true level of infiltration, negatively impacting T-stage. Peritu-moral inflammation is the main cause of overstaging, especially in T2 cancers [20]. EUS is rather inaccurate for N-staging mainly because morphologic and echogenic characteristics of lymph nodes alone are not sufficient to clearly assess possible lymph node positivity and also because lymph nodes other than those located in the mesorectum are out of the reach of the rigid probe [16]. As a matter of fact accuracy figures varying from 61 to 83% [10,17] are reported in the literature, with a sensitivity as low as 33%, and a specificity of 82% [3]. N-stage accuracy can be increased by echo-guided lymph node needle biopsy performed during the examination [21], but further works are necessary to address the benefits and pitfalls of this promising technique.

EUS is at present the most accurate technique for pre-operative local staging of rectal cancer, as confirmed in a recent meta-analysis [22] that compared EUS with MRI and TC (Table 1).

The increasing interest in the circumferential resection margin (CRM) (Fig. 2), whose prognostic value is considered by some Authors to be superior to T-stage [16,23], and the introduction of more powerful coils up to 1.5 Tesla, have drawn new attention to MRI.

Average figures of accuracy are around 66-82% for T-stage and 60-72% for N-stage and do not differ much from those obtained with EUS, but as evidenced by Bipat et al. [22], MRI allows an accurate evaluation of the CRM. For when N-stage is concerned, the recent introduction of new contrast agents such as utrasmall superparamagnetic iron oxide (USPIO) seems promising for the diagnosis of positive lymph nodes. This agent is captured by macrophage cells in the reticulo-endothelial system of normal lymph nodes, while metastatic lymph nodes are incapable of taking USPIO as the reticulo-

Fig. 2. The circumferential resection margin (CRM) (with permission from [23])
Table 1. EUS, CT and MRI: pre-operative staging accuracy for rectal cancer (Modified from [22])

Stage

Imaging modality

Sensitivity (%)

Specificity (%)

Muscularis propria invasion

EUS

94

86

CT

NA

NA

MR

94

69

Perirectal tissue invasion

EUS

90

75

CT

79

78

MR

82

76

Adjacent organ invasion

EUS

70

97

CT

72

96

MR

74

96

Lymph node involvement

EUS

67

78

CT

55

74

MR

66

76

endothelial system is altered to some extent [23]. Although promising, this new technique needs further validation.

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