Colorectal tumors are basically visualized as a homogenously hypoechoic mass in which the echo level is intermediate between the hyperechoic level of the third and fifth layers and the hypoechoic levels of the sixth and eighth layers. It is impossible to distinguish adenomas from Tis by EUS because there are no marked differences in the EUS images. By comparing EUS images with histological findings, the tumor invasion depth observed by EUS is as shown in Fig. 5.2. Adenoma and Tis, both of which can be confirmed histopathologically, are visualized as a hypoechoic mass localized within the first three layers, while the fourth thin hypoechoic layer (muscularis mucosae) is intact (Fig. 5.2a). In contrast, T1 cancer is imaged as a hypoechoic mass destroying the first three layers and extending into the fifth layer (T1a, focal extension to the fifth layer [Fig. 5.2b, c]; T1b [SM2], moderate extension to the fifth layer [Fig. 5.2d]; T1b [SM3], massive extension to the fifth layer with the sixth layer intact [Fig. 5.2e]). Furthermore, cancer extending into the muscularis propria is imaged as a hypoechoic mass penetrating the first five layers and spreading into the sixth layer (MP1, focal extension to the sixth layer [Fig. 5.2f]). Thus, EUS is useful for not only obtaining a detailed invasion depth diagnosis in early colorectal carcinomas, especially T1 carcinomas, but also for discriminating between early carcinomas and advanced ones.

Over the past 7 years, a total of 274 lesions that subsequently underwent endoscopic resection (EMR/ESD) or surgical resection were subjected to an EUS diagnosis. In the 274 lesions examined, 123 were polypoid type and 151 F&D type macroscopically. The accuracy rate is shown in Fig. 5.3. The overall accuracy rate was 77.3% (211/274) for the choice of therapy (discrimination between Tis-T1a lesions, which are indications for endoscopic resection, and T1b ones, which are indications for surgical resection) (Fig. 5.3a). As shown in Fig. 5.3a, the accuracy rate was 69.2% (108/156) in Tis-T1a carcinomas and 87.3% (103/118) in T1b carcinomas; the values were significantly higher in T1b carcinomas than in Tis-T1a carcinomas (p < 0.001; chi-square test). Regarding the macroscopic type, the accuracy rate was significantly higher for F&D-type carcinomas (82.8%; 125/151) than for polypoid-type ones (69.9%; 86/123) (p < 0.001) and significantly higher for T1b carcinomas than for Tis-T1a carcinomas in both macroscopic types (p < 0.001, respectively), as shown in Fig. 5.3b. These findings suggest that lesions suspected of being T1b carcinomas by conventional or (NBI) magnifying colonoscopy are a good indication for EUS.

Representative cases of T1 carcinomas subjected to EUS are shown in Figs. 5.4, 5.5, and 5.6.