Polypoid Lesions of the Colon
Characterization of Polypoid Lesions at CT Colonography
If a polypoid filling defect is detected in the colon, it needs to be further analyzed by CT morphological imaging criteria to decide whether it is a true polyp or a pseudolesion. A feasible and systematic approach to characterizing every candidate polyp detected is based on the stepwise evaluation of three main imaging criteria: morphology, structure, and mobility.
Polypoid filling defects of the colon are generally characterized by three criteria:
First, the external morphology of the filling defect, i.e., its shape and surface, is evaluated (Fig. 4.26). This is done on 3D endoluminal views and on axial or multiplanar reformatted 2D images. Three-dimensional evaluation is generally easier, as the spatial aspect of the lesion is better depicted. The shape of a filling defect may be round and sessile or pedunculated or flat. The surface may appear smooth, lobulated, nodular, or irregular (Fig. 4.27).
Next, the internal structure of the filling defect in terms of attenuation and homogeneity is assessed, and also, if intravenous or oral contrast has been administered, the contrast uptake (Fig. 4.28). This is usually done on 2D sectional views.
Intracolonic pathologies usually demonstrate homogeneous soft-tissue attenuation. Only lipomas show homogeneous fat attenuation. The majority of intracolonic pathologies enhance after intravenous administration of an iodinated contrast medium; in polyps and carcinomas the attenuation increases by 50–60 HU. On the other hand, orally administered contrast intended to label residual stool (fecal tagging), which is important for the differential diagnosis, is of course not taken up by intrinsic colonic lesions (Fig. 4.29). Some software manufacturers offer what is known as a 3D translucent view, by means of which the inner structure of an intraluminal filling defect can be represented on 3D views using color coding (see Chapter 3, “Translucency Rendering,” p. 56).
Finally, the mobility of the colonic filling defect is evaluated. When the patient changes position from the supine to prone, filling defects that are not adherent to the wall will change their position under the influence of gravity, while lesions that are attached to the wall will remain in place. This difference in mobility allows lesions arising from the bowel wall to be distinguished from mobile residual stool with a relatively high degree of confidence (Fig. 4.30).
Pseudomobility. In clinical practice, however, pedunculated polyps may also appear to move because the polyp head, which is connected to the colonic wall by a stalk, can change position relative to the bowel wall, resulting in pseudomobility. Lesions in mobile segments without retroperitoneal fixation, such as the transverse colon, the sigmoid colon, and the cecum, can also demonstrate pseudomobility due to movement of the entire bowel segment (Laks et al. 2004). This can affect up to 27% of lesions and means that a lesion that appears to change position between the prone and supine scans should not automatically be interpreted as residual fecal material.
Correlation of lesions in the supine and prone positions.
To evaluate the mobility of a filling defect, the supine and prone scans should be compared. The following procedure is recommended for performing a manual correlation between the prone and supine positions:
Identify the colon segment of interest in the corresponding position (prone or supine).
Record the endoluminal position of the lesion in relation to anatomical landmarks in the colon.
To find a lesion on the corresponding scan, it is helpful first to identify the colonic segment in which it is located. An easy way to do this is to use global 3D views (3D maps) or coronal 2D views as these provide a better overview of the colonic segments. Once the segment has been identified in the corresponding scanning position, it is carefully evaluated and searched for the lesion. Correlation is readily accomplished with this approach.
If precise correlation of a lesion remains impossible, or if it is still not possible to locate it with confidence or at all, a search should be made at the segmental level for endoluminal landmarks, i.e., typical anatomical structures in the bowel whose location can be related to that of the lesion (Fig. 4.32). The spatial relationship between the lesion and a semilunar fold or taenia, to a diverticulum, or to another lesion can be helpful, for example. If the lesion has maintained its relationship to the endoluminal surroundings, mobility may be ruled out. If the same endoluminal region is found, but the lesion is not detected in it, it is a mobile finding and thus a pseudolesion.
This method is especially important for findings in which true mobility is not easily distinguishable from pseudomobility, as for sessile polyps in mobile colonic segments. Various software solutions are now available which automatically identify the corresponding bowel segments in the two scans. This software primarily relies on a longitudinal comparison of a central pathway through the colon. Automatic correlation only works with a thoroughly distended colon, however.
Definition. A polyp is any macroscopically visible growth of tissue that is elevated above the level of the colonic mucosa and thus protrudes into the intestinal lumen. The majority of polypoid colonic lesions originate in the mucosa. Submucosal lesions, or organs and masses outside of the colon, may also cause an elevation of the colonic mucosa, or of the entire colonic wall, that results in an intraluminal “filling defect.” A distinction is made between the following:
Polypoid filling defects of extracolonic origin
For colonic polyps, a general distinction is made between adenomatous and nonadenomatous polyps, although it must be noted that these cannot be differentiated at CT colonography.
Adenoma–carcinoma sequence. Adenomatous polyps represent the most common benign neoplasm affecting the colonic mucosa. However, 80%–90% of all colorectal carcinomas develop from initially small adenomatous polyps via the adenoma–carcinoma sequence (Fig. 4.33). Over the course of 10–15 years, several genetic mutations occur that cause small adenomas (<5 mm) to develop into large advanced adenomas (>10 mm) and finally to invasive carcinomas. The risk of malignant transformation increases with the size of the lesion. Invasive carcinoma is found in less than 1% of all adenomas with a diameter smaller than 1 cm, less than 5% of all adenomas with a diameter greater than 1 cm, and in 30%–50% of adenomas with a diameter greater than 2 cm.
Advanced adenoma is defined by a lesion size of at least 10 mm and/or the presence of a substantial villous component (> 25%) and/or the presence of high-grade dysplasia. This benign lesion is considered to be associated with a relatively high risk of progression to cancer. It therefore represents the ideal target lesion for prevention of colorectal cancer.
The detection and correct measurement of adenomatous polyps is thus of central importance in CT colonography, because the size of a lesion has a high prognostic value for the risk of malignancy. Painstaking endoscopic removal of all adenomas can therefore help to reduce the incidence of colorectal carcinoma.
Histology. Based on their histological structure, adenomas may be divided into tubular, villous, and tubulovillous adenomas. Tubular adenomas are the most common, accounting for 80%–85% of colorectal adenomas. They usually have a smooth surface, and are usually smaller than 1 cm. They comprise 30%–40% of all polyps smaller than 5 mm. They are well differentiated and have a lower rate of malignant transformation than the other sub-types. Tubulovillous adenomas make up 10%–15% of all colorectal adenomas, are often larger than 1 cm, and undergo malignant transformation more often than tubular adenomas. Pure villous adenomas are rare, accounting for only 5% of colorectal adenomas. They often have a lobulated surface, usually a broad base, and are often larger than 2 cm. These lesions have the highest rate of malignant transformation of all adenoma types. Villous adenomas are grouped together with adenomas with a villous component of more than 25% and invasive carcinomas as advanced neoplasms of the colon. Serrated adenomas also exist; it is assumed that there is a separate pathway of malignant transformation for these lesions which allows smaller lesions to transform into invasive carcinomas more rapidly than through the classic adenoma–carcinoma sequence.
Nonadenomatous polyps are a mixed group. Histologically, a general distinction is made between hyperplastic polyps, mucosal polyps (elevations of normal epithelium), juvenile polyps, inflammatory polyps, and hamartomas. Hyperplastic polyps are the most common nonadenomatous polypoid lesion, accounting for 75% of nonadenomatous polyps. The majority of hyperplastic polyps are smaller than 6 mm. Lesions larger than 10 mm may, however, often have a flat or atypical morphology, which can make their identification at CT colonography difficult. In general, all nonneoplastic polyps share the common feature that they have no malignant potential. Nonadenomatous polyps comprise the majority (up to about 80%) of polyps smaller than 6 mm and around 40% of all polyps 6 mm in size or larger.
Colorectal polyps may be divided by macroscopic morphology into broad-based sessile, flat, and pedunculated polyps. Differentiation on computed tomography between neoplastic and nonneoplastic polyps, or between individual histological subtypes of polyps, is generally impossible for technical reasons. The same CT morphological imaging criteria apply therefore to various histological subtypes of mucosal polyps. At the present state of CT colonography, only classification by the main macroscopic characteristics (sessile–flat–pedunculated) and by size is possible.
However, detection rates have been reported in the recent literature to be higher for adenomatous polyps than for nonadenomatous polyps. This effect might be related to the lower conspicuity of nonadenomatous polyps. Most hyperplastic polyps are small (<6 mm) and they have been reported often to have a flat or elongated shape, to flatten out or even become effaced with air distension of the colon.
The Paris endoscopic classification
The Paris classification (2002) is a classification for endoscopic assessment of superficial neoplastic lesions (Type 0) of the esophagus, stomach and colon. Based on this classification, neoplastic lesions found at optical colonoscopy are divided in polypoid colorectal neoplasms (type 0-I) and nonpolypoid colorectal neoplasms (type 0-II). Polypoid lesions are further divided into sessile (type 0-Is) and pedunculated lesions (type 0-Ip). Intermediate lesions—“subpedunculated” polyps (type 0-Isp)—are mentioned additionally and should be treated as sessile lesions. Nonpolypoid neoplastic lesions—so-called flat lesions—are further divided into slightly elevated (type 0-IIa), completely flat (type 0-IIb), and depressed (type 0-IIc) in relation to the normal adjacent mucosa. Although this endoscopic classification is increasingly referred to in connection with CT colonography, it has not yet been established whether adopting it for CT colonography makes sense.
Current Status of CT Colonography
Study results. Preliminary studies on symptomatic patients have reported promising detection rates for colorectal polyps. It is especially worth noting that some of these studies used single-slice CT scanners (Table 4.2). An early meta-analysis by Halligan and colleagues (2005) that included 24 studies published between 1999 and 2003 showed a sensitivity of more than 90% for the detection of patients with polyps measuring 10 mm or more. Despite much initial enthusiasm, however, some of the subsequent large prospective studies with more modern technical equipment have only partially confirmed these results.
In 2003, a prospective landmark study by Pickhardt and colleagues reported that CT colonography had a 93.8% sensitivity for patients with adenomatous polyps 10 mm in size or larger in an asymptomatic study population. However, in subsequent studies, other authors such as Cotton and colleagues (2004) and Rockey and colleagues (2005) reported sensitivities between 34% and 53% for lesions of the same size. These mixed results are largely due to methodological differences between the studies, and may be explained by differences in examination technique, in data analysis, and in the expertise of the individual examiners in CT colonography. Large prospective single- and multicenter studies have recently reported high sensitivity rates. The results of large prospective studies (ACRIN 6664 Trial, Munich Screening Trial, IMPACT Trial) demonstrate that, when well performed, i.e., with an adequate examination technique and with the necessary expertise on the part of the radiologist, CT colonography sensitivity rates of 90% and over can be achieved for detection of clinically significant polyps 10 mm in size or larger in asymptomatic patients (Table 4.3). With regard to detection of the clinically more relevant advanced neoplasia, Kim et al. (2007) and Stoop et al. (2012) reported in two studies that CT colonography is virtually equal to conventional colonoscopy. Most recently, further prospective trials such as the Madeira Teleradiology Study (Belgium, Madeira–Portugal) and the SIGGAR Trial (United Kingdom) have been completed, with preliminary results indicating that CT colonography performs as well as colonoscopy in the detection of relevant colorectal adenomas or colorectal cancer.