CT Colonography

Conventional colonoscopy requiring passage of an endoscope through the rectum to the large bowels is increasingly being supplemented by imaging the colon using computed tomography (CT) and this is called CT colonography (CTC). Other names are virtual colonoscopy or virtual colonography. CT colonography takes about 15 minutes. At the beginning of the procedure, the colon is filled with carbon dioxide or air with a tube introduced into the rectum. The patient is asked to roll over into several different positions. Then CT scans are acquired in both prone and supine positions and images of the colon are obtained which are interpreted by a trained radiologist. The procedure was first described in 1993-1994. The number currently being done is not known, but in a 2003 UK national survey, 36% of radiology departments offered it. Conventional endoscopy is the gold standard for sensitivity, but has problems in respect of its invasiveness and patient acceptability.


CT Colonography. A 3-dimensionally reconstructed image shows a growth protruding into the lumen of the colon.
1.What is the radiation dose from CT Colonography?
2.Are there any possible dose reduction strategies for CT Colonography?
3.What are the risks from CT Colonography?
4.Is CT Colonography suitable for health screening like mammography?
1. What is the radiation dose from CT Colonography?
As with other medical procedures involving radiation, there is a wide variation in doses reported for the same type of CT scan. Normally the procedure involves paired scan (one obtained in the prone position and one in the supine position). Thus, when an effective dose is quoted in the literature, it is important to know whether it is per CT scan or for the paired scans.

There are significant differences in technique and doses between single-detector and multi-detector CT scanners. Effective dose values reported in the literature for CT colonography range from 1 to18 mSv. Many factors can contribute to this including the volume scanned; the collimation; reconstruction filter; slice thickness and overlap of slices; the tube current, scan time, optimum patient centring and other technical factors like the use of dose-reduction technique like tube current modulation. The effective doses from MDCT scanners may sometimes be higher than from single detector scanners, in part, due to over scanning and use of narrower collimation which increases overlap, and hence effective dose. For most MDCT colonography protocols, effective doses are in the range of 2-6 mSv per scan or 4-12 mSv for the examination involving scan in supine and prone position. The effective dose can be roughly estimated from the mAs per slice. As a rule of thumb (which may not be universally applicable) MDCT colonography with 40 mAs per slice results in an effective dose of about 2 mSv; 80 mAs about 4 mSv; and 160 about 8 mSv. The effective dose from CT colonography is about 25% higher in females than males [IANNACCONE et al. 2003]  (due in a large part to the ovaries being within the direct radiation beam which increases the contribution of ovarian dose to effective dose).

The BSS and Safety Report No 39 recommend the use of formally established reference or guidance doses for medical procedures, to assist in the implementation of optimization programmes. The dose values cited here provide a valuable basis for comparison, and represent what has been achieved in some experienced centres. However, they are not guidance or reference levels, as these remain to be established.
The CTC doses cited here are consistent with the published values for pelvic CT dose in various surveys. It is instructive to compare the doses from CTC with that from standard radiographic contrast barium enema. Effective dose from a double contrast barium enema is typically about 7 mSv. Comparison to other sources/procedures is also shown in Table 1 below.
TABLE 1. EFFECTIVE RADIATION DOSE FROM CT COLONOGRAPY COMPARED TO OTHER COMMON PROCEDURES
Source
Approximate effective dose (mSv)*
CT colonography
8 (with a range of 1.8 – 15)
CT Pelvis
6 – 10
Barium enema
7
Annual natural background radiation
2.4
Chest X ray (single film)
0.02
Lumbar spine X ray
1.3
*From [RCR, 1998]
2. Are there any possible dose reduction strategies for CT Colonography?
Yes. There are clear opportunities for dose reduction with almost any type of CT scan. Of importance are using the highest pitch and lowest tube current (mAs) consistent with acceptable images. Increasing the pitch from 1.0 to 2.0 usually reduces the dose by half. The patient must be carefully centred at the isocentre of the scanner. Tube current should be set at the lowest level that allows adequate visualization of the colonic wall , or tube current modulation (TCM), a technique that adapts tube current to the morphology of the patient, can be used to achieve dose reductions of about 35%.Most protocols use kVp between 110-120. A number of authors have developed innovative colonic phantoms to help optimize CTC protocols.
Ultra-low dose protocols result in an effective dose of 0.5 to 2 mSv using 10 mAs and 120 kVp These have been shown to be capable of a sensitivity of over 80% for polyps > 5 mm and specificity in excess of 95%. Imaging of polyps with doses as low as 0.05 mSv has been shown to be feasible. When compared with results from 8-12 mSv scans, the lesion detectability did not change significantly although image noise increased substantially. The latter problem can be ameliorated through use of noise reduction filters and image smoothing.This adds to the widely shared opinion that there is much scope for dose reduction, with some loss of image quality, but without significant reduction in lesion detectability.
3. What are the risks from CT Colonography?
At the dose level from a CT scan, radiation risk primarily derives from the potential for radiogenic cancer induction. Individual radiation risk from a CT examination varies significantly depending upon many factors including the absorbed dose, age and sex of the patient and expected lifespan. Risk is higher in children and younger patients compared to adults. The risk is somewhat higher in females than males. In those persons over the age of 50, the most radiosensitive tissues relative to cancer induction are lung and bone marrow which receive relatively small doses from CT colonography. Notwithstanding this, there is considerable concern at the level of individual and population risk arising from newer CT applications, particularly screening programmes..
Excess cancer risk at doses below 100 mSv is a debatable issue as there is paucity of data demonstrating this effect in epidemiological studies. Since dose from a single CT colonography is lower than this, the potential risk can only be estimated by assuming a dose response relationship, but multiple CT scans performed on the same patient with no dose reduction techniques may result in doses above 100 mSv The ICRP has estimated that the radiogenic fatal cancer risk for an adult population is about 5%/Sv which, using the linear non-threshold dose-response hypothesis, is equivalent to 0.005% /mSv. With the protocols in use to date, the effective dose for a paired scan is about 8 mSv. This, using the ICRP estimates, gives an approximate risk for a fatal radiogenic cancer of 0.04% or 1:2500. This must be taken in the context of a spontaneous risk of cancer incidence (40%) and fatality of about 20%. The risk values are comparable with those that would prevail for a barium enema, which would be regarded as a moderately high dose procedure. However, in view of repeated CT examinations on the same patient, the total dose to that patient may exceed 100 mSv.


TABLE 2. COMPARISON OF RISK FROM SEVERAL TYPES OF COLON IMAGING PROCEDURE PERFORMED ON AN ADULT

Approximate effective dose (mSv)
Approximate risk per scan of fatal radiogenic cancer*
Approximate spontaneous risk of fatal cancers
CT colonography
8 mSv
0.04 %
20%
Double contrast barium enema
7 mSv
0.035%
20%
MRI
0
0
20%
Conventional colonography
0
0
20%
*The risk calculation is based on ICRP 60. Radiogenic and spontaneous cancer incidence is approximately twice the fatal risk.
The ICRP estimates do not take into account age and gender. Hence, they are not well suited to application for risk calculations in this context, where there is a strong age bias in the population of interest. The U.S. National Academy of Sciences committee has recently provided radiogenic cancer risk estimates by age and sex. Table 3 lists calculated age and sex dependent risks based on these, using simple linear extrapolations. The values are for fatal radiogenic cancers following a paired CT colonography procedure giving an effective dose of 8 mSv. Clearly, the values presented are less than those from ICRP (Table 2), reflecting the older age profile of the colonoscopy group. The risk is higher for women than men, and declines with age. The risk at 70 is about half that at 50 for both men and women.
TABLE 3. POTENTIAL LIFETIME RADIOGENIC FATAL CANCER RISK FOR CT COLONOGRAPHY AT VARIOUS AGE
*Adapted from BEIR VII table 12 D-2, calculated for a scan with 8 mSv effective dose. Risks for varying doses may be estimated using simple proportionality.
The values for risk estimates reached by different authors vary somewhat depending on underlying assumptions.
4. Is CT Colonography suitable for health screening like mammography?
The goals of screening are cancer prevention and cancer detection. CTC is one of the screening methods that can detect polyps or cancer whereas Colonoscopy is the only method that also can remove polyps, thus effectively preventing cancer. If CTC were to become the screening method of choice for colorectal cancer in persons over the age of 50, the potentially exposed population could run to hundreds of millions of persons worldwide.Practices throughout the world vary greatly, as do the recommendations of professional bodies. However screening progress, particularly in the US has demonstrated that mortality can be significantly reduced. 
The American Cancer Society, the US multi-Society task force on colorectal cancer (American College of Gastroenterology, American Gastroenterological Association Institute and American Society for Gastrointestinal Endoscopy) and the American College of Radiology recommend one of the following approaches for colorectal cancer screening of those over 50.
1.Flexible sigmoidoscopy every 5 years, or
2.Colonoscopy every 10 years, or
3.Double contrast barium enema every 5 years, or
4.CT colonography (CTC) every 5 years, or
5.Annual g-FOBT with high test sensitivity for cancer, or
6.Annual FIT with high test sensitivity for cancer, or
7.Stool DNA test, with high sensitivity for cancer, interval uncertain.
References
1.FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS, INTERNATIONAL ATOMIC ENERGY AGENCY, INTERNATIONAL LABOUR ORGANIZATION, OECD NUCLEAR ENERGY AGENCY, PAN AMERICAN HEALTH ORGANIZATION, WORLD HEALTH ORGANIZATION, International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources, Safety Series No.115, IAEA, Vienna (1996).
2.INTERNATIONAL ATOMIC ENERGY AGENCY, INTERNATIONAL LABOUR OFFICE, INTERNATIONAL ORGANIZATION FOR MEDICAL PHYSICS, INTERNATIONAL SOCIETY OF RADIOLOGY, PAN AMERICAN HEALTH ORGANIZATION, WORLD HEALTH ORGANIZATION, Applying Radiation Safety Standards in Diagnostic Radiology and Interventional Procedures Using X Rays, IAEA Safety Report Series No. 39, IAEA, Vienna (2006).
3.ROYAL COLLEGE OF RADIOLOGY, Making the best use of a Department of Clinical Radiology, 4th Edition, RCR, London (1998).
4.INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Recommendations of the International Commission on Radiological Protection, Publication 60, Annals of the ICRP, Elsevier Publications, Oxford, UK (1990).
5.NATIONAL ACADEMY OF SCIENCES (U.S.), Health risks from exposure to low levels of ionizing radiation, Report of the BEIR VII committee, National Academy Press, Washington, DC (2005).
6.IANNACCONE, R., LAGHI, A., CATALANO, C., et al., Detection of colorectal lesions: Lower-dose multi-detector row helical CT colonography compared with conventional colonoscopy, Radiology, 229 3 (2003) 775-781.
7.AMERICAN CANCER SOCIETY, Screening and Surveillance for the Early Detection of Colorectal Cancer and Adenomatous Polyps, 2008: A Joint Guideline from the American Cancer Society, the US Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology, Consulted 31 March 2008.