During a CT colonographic examination, the entire abdomen and pelvis are scanned. This means that, along with the colon, extracolonic organs and structures are also examined, including the bases of the lungs. This does give CT colonography a potential advantage over conventional colonoscopy. The evaluation of extracolonic organs has the potential for detecting relevant but so far asymptomatic disease. For example, a clinically unsuspected aortic aneurysm may be shown, or a malignant tumor discovered at an early stage, and in such cases prompt initiation of therapy can improve the outcome for the patient. In fact, a recent study of Pickhardt et al. (2010) confirmed in a large series of 10 286 patients that screening CT colonography detects unsuspected presymptomatic extracolonic malignancy at a localized or early stage in one per 300 patients. As a group, the noteworthy extracolonic cancers outnumbered the colorectal cancers (one per 500 cases).
However, there is a disadvantage in that some extra-colonic findings may be indeterminate at CT colonography and may require further diagnostic work-up. This can be upsetting for the patient, involve further cost, and may even (if additional invasive diagnostic studies are needed) have a negative impact on the patient′s health. The costs for further diagnostic studies for extracolonic pathologies detected at noncontrast screening CT colonographic studies are about US $30 per patient in the United States (Pickhardt et al. 2008).
Extracolonic findings are frequently classified according to their clinical relevance, which is divided into high, moderate, and low (Hara et al. 2000). Table 4.7 lists a few standard examples from each group.
Extracolonic findings of high clinical relevance. These include findings that require prompt medical or surgical therapy or further investigation. Examples are solid masses on parenchymal organs, lymphadenopathy, aortic aneurysm, indeterminate solitary pulmonary nodules, pneumothorax, pneumonia, free intra-abdominal air, acute infections, and bone metastases (Figs. 4.148, 4.149, 4.150).
Extracolonic findings of moderate clinical relevance. These include probably benign findings that do not require immediate therapy or intervention, but do require further investigation, surveillance, or medical or surgical therapy at a later time. Examples include gallbladder and renal stones, cysts of uncertain clinical significance in various organs, uterine enlargement in postmenopausal women, coronary calcifications, splenomegaly, cardiomegaly, and pleural effusion (Fig. 4.151).
Extracolonic findings of low clinical relevance. Findings of low relevance are those that are classified as benign and hence do not require further therapy, diagnosis, or surveillance. Examples include simple liver or kidney cysts, typical liver hemangiomas, fatty liver, lung granulomas, small hiatal hernias, abdominal wall hernias containing only fat, lipomas, vascular calcifications, and vertebral body hemangiomas (Figs. 4.152 and 4.153).
CT Colonography Reporting and Data System
A classification for reporting extracolonic findings has been proposed in the CT Colonography Reporting and Data System (C-RADS), a consensus statement on a standardized reporting structure for CT colonographic findings published by Zalis et al. in 2005. In addition to categorizing colonic findings, the C-RADS classification also categorizes extracolonic findings with corresponding recommendations for their management. Findings are generally classified into five categories E0–E4. Briefly, E0 stands for a technically limited examination and E1 for a normal examination, E2 for clinically unimportant extracolonic findings, E3 for probably unimportant extra-colonic findings, and E4 for potentially important extra-colonic findings. The CT Colonography Reporting and Data System is described in detail in Chapter 5 (p. 173; see also Table 5.3).
Frequency and Distribution
The frequency of extracolonic findings detected at CT colonography ranges from 33% to 85% in different studies. The incidence of extracolonic findings with a high clinical relevance is comparatively low, between 10% and 23%. The proportion of irrelevant findings, at 40%–50%, and moderately relevant findings, at 32%–50%, is significantly larger. This shows that although extracolonic findings are common, it is rare that they are truly clinically relevant. The differences in results may be explained by differences in the patient populations and examination techniques used in the studies. About 6% of patients will have to undergo additional investigations because of unsuspected extracolonic findings at CT colonography.
Patient population. In terms of patient population, the presence of symptoms and patient age both play an important role. The number of clinically relevant extracolonic findings is likely to be lower in asymptomatic screening patients than in symptomatic patients. Extra-colonic findings have been reported to be significantly more common (74%) in seniors (age ≥ 65 years) than in nonseniors (55.4%), although the rate of recommendations for additional imaging was nearly equally low in both groups (6% and 4.4%, respectively) (Macari et al. 2011).
Examination technique. Important factors in examination technique are radiation exposure and use of intravenous contrast agent. In screening CT colonography, examinations are routinely performed without intravenous contrast and with a low-dose CT protocol. It has been shown that, because of the high contrast between air and tissue, polyp detection is not impaired in unenhanced low-dose scans. Because of the high level of image noise, however, the assessment of solid organs is poorer, especially when, in addition, thin-slice protocols are used. That aside, the evaluation of solid organs without administration of an intravenous contrast agent is limited anyway.
Extracolonic pathologies are easier to detect and may be better classified in an examination in which at least one series is performed at a standard dose and after intravenous administration of a contrast agent. Such protocols are recommended for diagnostic CT colonography in patients with symptoms, those with known colorectal cancer, patients with known multimorbidities, and also for preoperative evaluation. However, they are generally not used in asymptomatic patients with an average risk for colorectal cancer.
Incidental findings, beyond the targeted organ or region, can occur in any kind of CT examination and are generally evaluated and reported accordingly. The controversy over whether the extracolonic findings in CT colonography need to be reported or not may appear to be exaggerated, especially when the focus is on diagnostic CT colonography in patients with clinical indications. The controversy about the clinical relevance of incidental extracolonic findings, however, focuses mainly on the socioeconomic and ethical aspects of screening studies. In screening cohorts, most extracolonic findings are not clinically significant. It may be argued that because of the intrinsic technique-related limitations of screening CT colonography (i.e., low-dose scans without intravenous contrast) and because of the low prevalence of clinically relevant findings in a screening cohort, it may be neither necessary nor beneficial to fully evaluate the extracolonic organis and structures in screening patients. In particular, the documentation of irrelevant or only moderately relevant findings may lead to further diagnostic work-up and unnecessary additional costs; in the worst case, it may even be harmful to the patient. Potential harms include anxiety, inconvenience, and complications related to additional diagnostic work-up for findings that ultimately prove to be clinically unimportant.
On the other hand, CT colonography provides a low-dose CT examination of the entire abdomen and pelvis, and some of the relevant extracolonic findings, such as large masses and aortic aneurysms, are likely to be visible even on unenhanced low-dose scans. Therefore, entirely ignoring the extracolonic organs and structures, or not mentioning extracolonic findings in the diagnostic report, cannot be justified, and not only because of the medicolegal and ethical issues involved. An important point is to ensure before the start of any screening CT colonographic examination that both the patient and the referring physician understand the limitations on the assessment of parenchymal organs.
Strategies for Improving Detection and Differentiation of Extracolonic Findings
In the opinion of the authors, good patient care requires that extracolonic structures should be evaluated in every CT colonography examination and should be documented in the radiologist′s report (Table 4.8).
The screening patient. For evaluation of extracolonic structures, the CT dataset should also be reconstructed in thicker slices (3–5 mm). This reduces image noise, decreases the number of images to be reviewed, and thus improves interpretation (Fig. 4.154). The use of different window settings, such as soft-tissue, liver, lung, and bone windows, especially for noncontrast studies, will optimize parenchymal contrast within certain limits. Very narrow window settings, such as liver windows, improve the evaluation of solid organs. Generally, as with any other examination, comparison with prior examinations or imaging reports is helpful.
The symptomatic patient. In general, extracolonic findings in patients with symptoms or clinical indications have a higher clinical relevance than those in asymptomatic patients, and for this reason—in addition to optimizing slice thickness and window settings—performing one scan series (ideally the supine one) at a standard dose (e.g., 120 kVp, 150–250 mAs, dose modulation) with intravenous administration of a contrast medium can be justified. Under these circumstances, the evaluation of the extracolonic organs does not differ from that of a standard abdominal CT in the venous phase.
Recommendations on Best Practice for Reporting
Both the practice guideline for CT colonography of the American College of Radiology (ACR) (2009) and the recent consensus statement put forth by the European Society of Gastrointestinal and Abdominal Radiology (Neri et al. 2012) recommend fully evaluating the extracolonic organs and structures and reporting and clearly communicating any abnormalities, especially those of potential clinical significance. If an unenhanced and or low-dose technique was used, the limitations on the evaluation of extracolonic structures should be noted. As stated in the ACR practice guideline, caution should be used in the interpretation and reporting of extracolonic findings likely to be of low clinical significance, in order to avoid unnecessary subsequent or even serial diagnostic examinations and associated patient anxiety.
Additional information on reporting may be found in Chapter 5, “How to Generate a Useful Report.”