Early validation trials of CTC in enriched patient cohorts demonstrated sensitivity of 90 % detection of 1 cm and greater polyps and 98 % detection of colorectal cancer [8, 9]. Initial clinical uses of CTC began as an adjunct diagnostic examination following incomplete optical colonoscopy, largely in patients with redundant colons or obstructive cancers. Since 2007, CTC has had Medicare coverage in 47 states for diagnostic indications, predominantly in patients following incomplete colonoscopy or in patients at risk for colonoscopy [10]. This coverage decision, also supported by private insurance, was largely based on the diagnostic accuracy of the exam to detect colorectal polyps and cancer, along with its low-risk profile.

By early 2000, validation trials in asymptomatic cohorts began to show evidence that CTC could be used for colorectal screening. In 2003, the multicenter Navy trial of 1233 asymptomatic patients reported results of per-polyp sensitivities for detection of ≥ 6 and ≥ 10 mm polyps of 89 and 94 %, respectively [11]. In 2008, the multicenter trial of American College of Radiology Imaging Network (ACRIN) published similar results in a more diverse trial of 15 centers, including academic and private sectors [1]. In this trial, the per-polyp sensitivities for detection of ≥ 6 and ≥ 10 mm polyps were 78 and 90 %, respectively. In 2008, the American Cancer Society jointly with the US Multisociety Task Force on Colorectal Cancer and the American College of Radiology (ACR) endorsed CTC to be used for colorectal screening [12]. In that same time period, the US Preventative Task Force gave CTC an “I” rating (indeterminate), not able to include the ACRIN trial in their meta-analysis, largely due to stated risks of radiation dose, burden of extra-colonic findings, and small polyps left behind in the Medicare population [13].

After the initial publication of ACR practice guidelines for use of CTC in 2005, the document was revised in 2009, with the most recent update completed in 2014 [3]. These standards define aspects of indications and contraindications of CTC, qualifications of interpreting physicians, specifications of exam techniques, and documentation and reporting of findings.

Consistency and standardization in reporting of findings at CTC have been aided by a reporting structure, called CTC reporting and data system (C-RADS), developed in 2005 [30]. This reporting structure was modeled after the successful development of breast imaging RADS (Bi-RADS) used in mammography. C-RADS describes how to report the individual colorectal findings based on lesion size, morphology, attenuation (density), and location. Namely, lesion size is defined as the linear long axis measurement of the polyp, with exclusion of the stalk if pedunculated, using a polyp window-level setting (e.g., image display that evaluates the polyp with surrounding air interface). For sessile or superficially elevated lesions, the long axis at the base of the lesion is measured (Fig. 5.1). Both 2D multiplanar (axial, sagittal, and coronal views) as well as 3D endoscopic (virtual endoscopic views) can be utilized to best portray maximal lesion size. Typically, 3D can give the best overall view of the polyp, but when the colon is tortuous, partially collapsed, or fluid filled with incomplete visualization of the lesion at 3D, the 2D views can play an important complimentary role and provide accurate measurement of lesion size. Morphological criteria include sessile (lesion base broader than lesion height by two times or more), pedunculated, flat (less than 3 mm of vertical height), and advanced mural lesion of cancer. Lesion attenuation or density describes whether the lesion is soft tissue or fatty. Polyps or masses have soft tissue attenuation, compared to the fat attenuation of lipomas or lipomatous ileocecal valves. With the use of high-density stool tagging agents, stool can be completely or predominantly tagged with barium or iodine agents. In contrast, the surface of polyps or cancers can have a linear or nodular high-density tagging in more than 40 % of lesions [31]. Lesion location uses the standardized six segments of rectum, sigmoid, descending colon, transverse colon, ascending colon, and cecum. Flexures are referred to generically, recognizing that correlation between flexures described at colonoscopy or based on anatomic definitions may defer from what is directly visualized at CTC. A key attribute of CTC is the ability to provide very accurate localization of a polyp or cancer within the colon, using the 3D transparency view (i.e., barium enema like visualization) as an overview of the colon with the specific finding(s) marked within it (Fig. 5.2). This can be very helpful to orient the gastroenterologist or colorectal surgeon to understand the colonic anatomy and location of lesions found at CTC.