Malignant Diseases of the Colon and Rectum
By far the most common colonic malignancy is adenocarcinoma. Rare tumors include primary lymphoma, carcinoids, gastrointestinal stromal tumors, melanomas, mesenchymal malignancies (such as leiomyosarcoma), and metastases. At present, only limited data are available in the literature on the detection and characterization of these rare malignancies at CT colonography. It may be assumed, however, that in many cases it is not possible to distinguish with certainty between different types at CT colonography on the basis of morphological characteristics, and thus histopathological analysis is needed. Colorectal carcinomas and lymphomas are discussed in greater detail below.
The most common primary tumor of the colon is adenocarcinoma. In about 90% of cases the tumor develops from an adenomatous polyp. The incidence of disease increases sharply after the age of 50 years. The majority of carcinomas exhibit an exophytic growth pattern, often with ulcerations. Adenocarcinomas have a tendency to infiltrate the bowel wall circumferentially. As to location, 35% of adenocarcinomas occur in the rectum, 25% in the sigmoid colon, and about 10% in each of the ascending colon, descending colon, transverse colon, and cecum (Fig. 4.63). As many as 5% of affected patients may have a synchronous carcinoma.
Role of CT Colonography
CT colonography has rapidly established itself as a powerful imaging technique for the detection of advanced colorectal neoplasia. In particular there is strong evidence on the usefulness of CT colonography in detection as well as preoperative evaluation of colorectal carcinoma. Based on a recent meta-analysis by Pickhardt and colleagues (2011), the overall sensitivity in published trials for detection of colorectal carcinomas is 96.1%. Compared with optical colonoscopy, CT colonography is methodologically superior in terms of the identification of colonic masses, precise definition of the colonic segment in which the cancer is located, and completion of colonic evaluation (Neri et al. 2010). This enables accurate assessment of significant synchronous lesions in patients with colorectal cancer. In fact the working group of Park and colleagues recently reported CT colonography to be highly sensitive in detecting synchronous cancers proximal to a stenosing colorectal cancer. In a cohort of 284 patients, 8/8 synchronous cancers were detected, giving a negative predictive value of 100% (Park et al. 2011).
In addition, CT colonography could combine local and distant staging of disease in patients with colorectal cancer. With multiplanar 2D imaging and 3D endoluminal views of the colon, this technique enables simultaneous evaluation of the colon wall, the depth of tumor infiltration, lymph node status, and abdominal organs (Table 4.4). For this reason, intravenous administration of iodinated contrast agent is indicated when investigating patients with colorectal carcinomas.
The clinical value of CT colonography for presurgical local tumor staging is not clearly defined. In rectal cancer, MRI of the pelvis is often performed. In colon cancer, except when there is advanced-stage disease with infiltration of adjacent organs (T4), surgical resection (e.g. hemicolectomy or segmental resection) is the treatment of choice anyway. Less invasive surgical techniques such as local tumor resection (mucosal resection) for T1 tumors could make it necessary to re-evaluate the role of imaging techniques in staging primary tumors.
Main indications. The main indications for CT colonography in patients with stenosing colorectal carcinoma are to correctly identify the tumor, to evaluate the prestenotic colonic segments for the presence of any synchronous tumors or polyps which technically cannot be visualized on endoscopy, and to provide information about the local and distant extent of the disease. CT colonography also provides precise information about the location of the tumor which can aid surgical planning.
Limitations. CT colonography does not depict the individual layers of the colonic wall, so distinguishing between a T1 (limited to the mucosa/submucosa) and a T2 tumor (infiltration of the muscularis propria) is not possible. It also has limitations in the identification of invasive cancer in colonic polyps, since no robust CT colonography criteria exist for the differentiation of polyp histology. One further limitation is in the accurate assessment of lymph node status: No reliable conclusions can be drawn about the malignant status or otherwise of lymph nodes on the basis of criteria such as the size, number, and shape of the nodes or their enhancement. Differentiation from inflammatory diseases such as chronic diverticulitis may also sometimes be difficult and requires further histological evaluation.
CT Colonography Criteria
Unlike colonic polyps, which are often more conspicuous on endoluminal 3D views, invasive masses are better evaluated on 2D views, because these allow both the mural and the extramural extent of the lesion to be assessed. A combined interpretation of the axial and multiplanar reformatted 2D images has been recommended as best practice, especially for local tumor staging. In terms of morphology, colorectal carcinomas present as a focal polypoid mass or asymmetric semiannular or annular thickening of the colonic wall leading to a stenosis.
Polypoid tumors. The majority of polypoid carcinomas are broad-based, sessile masses of variable size. Polypoid tumors may have an irregular, lobulated, or sometimes smooth surface. Sometimes there is central necrosis with ulceration (Fig. 4.64).
Semiannular tumors. Semiannular lesions cause thickening of a variable portion of the colonic wall circumference. Generally, only a short segment of the colon (<5 cm) is affected, usually with an abrupt transition to normal bowel wall with the formation of overhanging edges, also referred to as “shoulder formation.” Depending on how much of the bowel wall circumference is involved, 3D views may show a saddle-shaped (Fig. 4.65) or semicircular appearance (Fig. 4.66).
Annular tumors. In circumferentially stenosing tumors, endoluminal 3D views usually show an annular, symmetrically or asymmetrically constricting mass with an irregular surface that may be nodular, bizarrely shaped, or even smooth. There is a variable degree of stenosis. In some instances the remainder of the lumen can be visualized with virtual endoscopy. Typically, thickening of the wall affects only a short segment of the colon (<5 cm). There is an abrupt transition to the unaffected bowel wall, often with shoulder formation. In annular carcinomas this corresponds to the “apple-core appearance” seen on barium enema studies (Fig. 4.67).
On 2D views the internal structure of colorectal carcinomas demonstrates soft-tissue attenuation. Average CT attenuation values on unenhanced 2D images are 43 ± 15 HU. Especially in larger masses, central hypodense areas are also often found, which are a sign of necrosis (Fig. 4.68). If an intravenous contrast agent has been administered, colorectal carcinomas demonstrate enhancement with an increase in CT attenuation to 90–120 HU, depending on contrast phase (Fig. 4.67b, c). So far as is known at present, the degree of enhancement and histological tumor differentiation are not related. Like all other colonic lesions, colorectal carcinomas do not take up contrast agent in fecal tagging (Fig. 4.69); this makes them easy to distinguish from tagged residual stool or surrounding fluid (Fig. 4.70).
Colorectal carcinomas do not move when the patient changes position: In both the prone and the supine position, they are found at the same location.
Deep rectal lesions are often better visualized with the patient in the prone position than in the supine position, because the colon is better distended.
The presence of a rectal catheter balloon may obscure the presence of a deep rectal pathology (Fig. 4.71). To avoid this, some investigators therefore remove the rectal catheter or deflate the catheter balloon in one scanning position.