Computed tomography (CT) colonography (CTC) was first developed in 1994 as a less invasive alternative to traditional optical colonoscopy to assess for colonic neoplasia. The diagnostic validity of CTC for clinically relevant lesions has since been demonstrated, with studies showing that the sensitivity of CTC compares favorably to that of optical colonoscopy, with acceptable specificity . However, in addition to this relative parity for the diagnosis of colonic neoplasia, a unique capability of CTC over other colorectal examinations is the provision of supplemental pericolonic and extracolonic diagnostic information regarding the entire abdomen and pelvis obtained through the cross-sectional volumetric CT scanning technique of the CTC. Thus, because colorectal screening is now recommended for all adults (average risk patients older than age 50; higher risk patients older than age 40), CTC presents a unique opportunity to synchronously evaluate the abdomen and pelvis for extracolonic pathologies that afflict this older population.
Several reports on extracolonic findings have been published in the past decade . Most of them presented cautious conclusions, and the objective benefits of these extracolonic findings on CTC were subsequently debated within the medical community. One point raised by many investigators was that extracolonic findings, although common, were infrequently of clinical importance. Some researchers argued that the ability to detect extracolonic abnormalities can be viewed as a double-edged sword . On the positive side, it is beneficial to have the potential to detect serious pathology at an early curable stage. On the negative side, CTC may lead to higher cost and extra radiation from superfluous additional examination of benign lesions, and it has the potential to increase patient anxiety, complications, and potentially morbidity.
In this issue, Wernli, Rutter, Dachman, and Zafar summarize investigations on extracolonic findings on CTC to date and provide a systematic review. They searched the US National Library of Medicine PubMed database for works issued between 1994 and 2010 and selected 24 relevant publications to conduct a meta-analysis. They categorized the study populations as screening (asymptomatic), nonscreening (symptomatic), and mixed. The meta-analysis included prevalence, false-positive rate, and positive predictive value. They focused their evaluation on indeterminate masses of the kidneys, lungs, liver, pancreas, and ovaries associated with high mortality. After conducting careful statistical analyses, the results were stratified by population types and the five targeted organs.
Their summary of existing reports provides insights into how extracolonic findings on CTC can be incorporated into current medical practice. The meta-analysis results will help clinicians better understand the likelihood of detecting extracolonic findings on CTC in different patient populations and the chances that the findings may correspond to false-positive diagnoses. Conclusions drawn from this analysis suggest that more priority should be given to follow-up of indeterminate renal masses findings on CTC, whereas less emphasis should be placed on cystic ovarian lesions, particularly for the asymptomatic screening population.
There are a few points of contention regarding currently used CTC scanning protocols that limit its acceptance as a viable modality for diagnosis of extracolonic findings. First, CTC studies are often performed with low radiation dose CT protocols. Although this is adequate for colonic lesion detection because of the inherent high contrast between intraluminal air and adjacent soft tissues, it is suboptimal for diagnosing extracolonic lesions compared to standard diagnostic CT protocols for the abdomen and pelvis. Another issue is that CTC is often acquired without intravenous contrast, which makes the sensitivity of CTC for identifying extracolonic lesions much lower than that of a contrast-enhanced CT scan. Some investigators have advocated the use of intravenous contrast in CTC to increase the conspicuity of lesions. However, adding intravenous contrast will invoke additional risks for and increase the cost of the examination, limiting its use for screening populations. Therefore, intravenous contrast is currently only recommended for symptomatic patients. Because of these limitations and the concern that most extracolonic findings are not clinically significant, members of the CTC community began work on guidelines to define clinical and radiological criteria under which further investigation of extracolonic findings is warranted, using as a model the reporting structures of other radiological investigations such as the BI-RADS schema of mammography. In 2005, an ad hoc group of investigators active in the field of CTC formed a working group and proposed formal guidelines for the interpretation of CTC results: the CT Colonography Reporting and Data System, or C-RADS . The C-RADS guidelines rate extracolonic findings using a scale of E1 to E4. E4 category findings are likely clinically significant, and recommendations are for following accepted practice guidelines for communication of results to the referring physician. Findings of E3 categorization are classified as indeterminate and unlikely of clinical significance, with further diagnostic characterization considered discretionary. The remaining categories (E2 and E1) are reserved for findings considered clinically unimportant and anatomic variant findings, respectively.
Advanced computing techniques can play a role in improving the efficacy of CTC for extracolonic findings. Preprocessing study data to remove artifacts, enhance image features such as texture and boundary, and provide for advanced three-dimensional visualization can increase the sensitivity for diagnosis of extracolonic pathology. Computer-aided detection (CAD) can also be developed to assist in the detection of extracolonic lesions. CAD systems for colonic polyps and masses have been under development in past decades, and commercial systems are also available . It is a natural extension to develop CAD systems that screen for extracolonic findings on the same CT data set. These CAD systems have the potential to find lesions that may be missed or ignored by clinicians. Developing CAD systems for extracolonic findings is a new and potentially a very important research area, particularly if CTC exams are read by nonradiologists or are prescreened by radiology assistants or technologists to reduce costs. Early efforts have been undertaken in CAD of extracolonic findings, such as renal calculi .
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