CT COLONOGRAPHY
CTC is an emerging CRC screening test that is also known as virtual colonoscopy. According to available studies, CTC and colonoscopy might be equivalent for diagnosing cancer.14
Procedure
The preparation for CTC requires the patient to consume a low-residue diet one day prior to the procedure, which is considered to be an advantage over colonoscopy due to the decreased bowel preparation.3 With this procedure, a small rectal catheter is inserted into the anus and advanced to the rectum to allow carbon dioxide to be instilled for bowel insufflation. The patient lies supine on the table for a CT scan of the abdomen with the resulting 2D images visualizing polyps and CRC, if present.3 If necessary, 3D images can be compiled by a specialized software program to obtain a 360-degree view of the colon. In fact, recent studies show that 3D CTC is preferred to 2D because 3D polyp measurements are more representative of the true polyp size found on optic colonoscopy or surgery than are 2D measurements.17
Patient Experience
In one study, CTC screening was described as uncomfortable but not painful and was reported to be the most impersonal of all three tests because of less direct interaction, reducing patient embarrassment. This study reviewed qualitative interviews with patients regarding the fairly new CTC procedure and found that patients received little visual or verbal feedback and were confused regarding their test outcome immediately after the procedure.6
CTC was preferred by 72% of patients compared to colonoscopy and by 97% of patients compared to DCBE.18 In a study that evaluated the performance characteristics of CTC among 1,233 asymptomatic patients, 68% deemed CTC to be more convenient than colonoscopy, and more patients indicated that they preferred CTC over colonoscopy for screening (49.8% vs 41.1%; 9.2% had no preference).19
Advantages and Disadvantages
Compared with colonoscopy, CTC is comparably sensitive but safer and more acceptable to patients.2,6,14 CTC has a sensitivity of 96% for detecting lesions > 10 mm in diameter, but sensitivity decreases to 89% for lesions 6 mm to 10 mm in diameter (see Table 1).2,3,14
The follow-up screening intervals for CTC parallel those of colonoscopy. In a recent audit of 1,011 screening participants with a negative baseline CTC, a single carcinoma occurred during an average follow-up period of 4.73 ± 1.15 years.1
The current Colonography Reporting and Data System (C-RADS) guidelines for CTC interpretation recommend 6 mm as the minimum size for polyp reporting.17 This reporting threshold will result in a 77% reduction rate in invasive endoscopic procedures since it minimizes the number of cases that are sent for colonoscopy after CTC screening.17
A study performed by the American College of Radiology Imaging Network found there would be an approximate 12% referral rate for colonoscopy when using the current 6-mm polyp size threshold, but the referral rate would increase to 17% if a 5-mm threshold were used.17 The American Gastroenterological Association has stated that diminutive lesions—those measuring ≤ 5 mm—are of little to no clinical significance because only a fraction of them are neoplastic. Of these, fewer than 1% are histologically advanced, and essentially none are malignant.6 By not reporting diminutive lesions, there would be an incremental gain in the cost-effectiveness of the CTC scan and only a 1.3% loss in clinical CRC prevention efficacy.
It is widely accepted that any polyp ≥ 10 mm detected with CTC screening indicates a need for polypectomy via colonoscopy or surgery.20 For lesions ranging from 6.1 mm to 9.9 mm in diameter, CTC is suggested as a surveillance tool; patients may receive repeat CTC every 1 to 3 years until resection via colonoscopy is warranted.17
Limitations
CTC screening has many limitations, and evidence is lacking for a reduction of CRC incidence or mortality after CTC. Research has revealed a discrepancy between the polyp size measured by CTC versus the true polyp size seen on optic colonoscopy; CTC measurement can underestimate the size of a polyp by nearly 1.2 mm. Considering that C-RADS level 2 includes polyps 6.1 to 9.9 mm and level 3 includes polyps ≥ 10 mm, which automatically requires further investigation with colonoscopy, such a 1.2-mm discrepancy in measurement can make all the difference.17
CTC requires some bowel preparation, special resources, and expertise. The cost-effectiveness and risk profiles will vary, depending on whether referral for colonoscopy is required. Also, treatment recommendations for patients with polyps < 6 mm in diameter are uncertain.
CTC screening exposes the patient to increased amounts of ionizing radiation, which raises concern regarding risk for radiation-induced cancers. The effective dose to the whole body during a CTC is 6 to 20 mSv, compared to 0.02 mSv for a chest x-ray.16 One study estimated that performing CTC screening every five years from ages 50 to 80 would prevent the development of 24 CRC cases for every one radiation-induced cancer.21 Thus, the risks of radiation must be weighed against the benefits of screening, and the decision is often made by the individual patient.
Finally, the greatest limitation of CTC is that further testing may be required based on the preliminary results. Therefore, the patient may undergo two procedures rather than just one all-inclusive procedure, such as colonoscopy.
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