Colorectal Carcinoma

Adenocarcinomas are the most common colonic primary tumors. The peak incidence is between 50 and 70 years of age. Approximately 90% arise from benign adenomatous polyps. Most carcinomas show an exophytic, polypous type of growth with frequent central degeneration. Adenocarcinomas tend to infiltrate the bowel wall circumferentially and 50% are found in the rectum, and 25% in the sigmoid. In up to 5% of cases, a synchronous carcinoma is present (Fig. 13.18a,b).

The main indication for CT colonography in colorectal cancer is the evaluation of the pre-ste-notic colon to detect additional tumors or polyps (Genlon et al. 1999; Morin et al. 2000a; Neri et al. 2002) (Table 13.3). CT colonography also offers information about local tumor invasion, lymph nodes, and distant metastases (Filippone et al. 2004; Chung et al. 2005; Iannaccone et al. 2005). For this purpose, the i.v. administration of contrast media is indicated (Morrin et al. 2000b). The role of CT colonography for T staging of known colorectal cancer is still a matter of discussion because primary tumors are resected, even if metastases are present, to prevent bowel obstruction. Less invasive surgical approaches, such as local tumor resection (mucosal

Table 13.3. CTC features of colorectal carcinoma

Focal, asymmetric or circular wall thickening Annular stricture Wall irregularity CM enhancement

Pericolic invasion (pericolic soft tissue stranding) Local lymphadenopathy Metastases T Staging

T1: Invasion of the mucosa and submucosa T2: Infiltration of the muscularis propria T3: Infiltration of pericolic fat T4: Invasion of adjacent organs

Reliable differentiation between mucosal /submucosal invasion (T1) and infiltration of the muscularis propria (T2) with CT is still not possible resection) in stages below T3, may indicate the need for a re-evaluation of the role of CT colonography in T staging.

Unlike polypoid lesions, which are more easily detected on 3D endoluminal views, invasive mass lesions are better depicted on 2D images, which allow mural and extramural evaluation (Pickhardt 2004).

Colorectal cancer typically shows extensive focal polypoid, asymmetric, or circular wall thickening with short extension (<5 cm), especially with shoulder formation (Fenlon et al. 1998; Taylor et al. 2003a). Colorectal carcinomas show moderate enhancement with intravenous contrast (Oto et al. 2003; Sosna et al. 2003) (Fig. 13.13a,b). CT differentiation between stage T1 (invasion of mucosa and/or submucosa) and T2 (invasion of the muscularis propria) is not feasible, but tumor extension beyond the colon wall (T3), characterized by stranding, an indistinct boundary, and nodular protrusions into pericolic fat tissue, is readily appreciated by CT (Fig. 13.14a,b). Tumor infiltration to adjacent organs (T4) is most likely if the carcinoma shows a broad-based contact, no intervening fat planes, and indistinct boundaries

Fig. 13.13a,b. Polypoid rectal cancer (arrow): large polypoid, lobulated mass in the rectum (a,b). The lesion shows soft tissue attenuation and CM enhancement (b)
Fig. 13.14a,b. Semicircular sigmoid carcinoma (arrow): focal, asymmetric, semicircular wall thickening with shoulder formation in the sigmoid colon (a,b). The lesion shows CM enhancement and pericolic soft tissue stranding (b)

to other organs (Fig. 13.15a,b). Pericolonic lymph nodes and distant metastasis are signs of progression of the disease and can be evaluated with 2D planes.

The most common pitfalls are inflammatory stenosis and the segmental colonic spasm. Inflammatory and post-inflammatory stenosis more often show cone-shaped mild wall thickening with involvement of a long segment (>10 cm) and pericolonic fat stranding. Sometimes fluid is present at the root of the mesentery (Chintapalli et al. 1999) (compare Figs. 13.10b and 13.11a, vs Fig. 13.14 and 13.15).

Segmental colonic spasm is a physiological lumi-nal narrowing due to peristaltic muscular contraction of the colon. The administration of antispas-motic drugs, such as butylscopolamine (Buscopan) and glucagon may reduce the appearance of spasms and improve colonic distension (Taylor et al. 2003b). Often, these pseudostenoses disappear during the examination when changing the position from prone to supine or vice versa. In such cases, the evaluation of the second series can be diagnostic to see whether the pseudostenosis disappears when the spasm relaxes (Fig. 13.16a-d).

Fig. 13.15a,b. Circular sigmoid carcinoma T4 (arrow): focal, symmetric, circular wall thickening with shoulder formation and pericolic soft tissue stranding in the sigmoid colon (a,b). The lesion shows a broad-based contact, no intervening fat planes and indistinct boundaries to the psoas muscle, indicative of infiltration (b)

Fig. 13.15a,b. Circular sigmoid carcinoma T4 (arrow): focal, symmetric, circular wall thickening with shoulder formation and pericolic soft tissue stranding in the sigmoid colon (a,b). The lesion shows a broad-based contact, no intervening fat planes and indistinct boundaries to the psoas muscle, indicative of infiltration (b)

Fig. 13.16a-d. Segmental colonic spasm in the descending colon (arrow): focal, irregular circular wall thickening with shoulder formation in the supine position (a-c). The lesion shows soft tissue attenuation and CM enhancement (a). Normal colon wall without wall thickening or stenosis in the prone position (d). It is important to identify the same segment as in the supine position

Fig. 13.16a-d. Segmental colonic spasm in the descending colon (arrow): focal, irregular circular wall thickening with shoulder formation in the supine position (a-c). The lesion shows soft tissue attenuation and CM enhancement (a). Normal colon wall without wall thickening or stenosis in the prone position (d). It is important to identify the same segment as in the supine position a a

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