Journal of the New Zealand Medical Association, 08-June-2012, Vol 125 No 1356
Computed tomographic colonography: colonic and extracolonic findings in an Auckland population
Helen Moore, Nicholas Dodd
Computed tomographic (CT) colonography is used to identify individuals with polyps or cancers, in order to triage the appropriate patients to colonoscopy and/or surgery. CT colonography is an established screening technique for colorectal cancer, and in many centres has effectively replaced barium enema as a first-line investigation in the patient with symptoms suggestive of bowel cancer.
It may also detect other causes for the patient’s symptoms, either related to the bowel (such as appendicitis or diverticulitis), or non-bowel-related “extracolonic” problems such as lymphoma. Up to 10% of extracolonic findings (ECF) are potentially the cause of the symptoms that lead to the CTC investigation.1
There is a large quantity of data available internationally regarding the distribution of colonic and extracolonic findings, but limited data as yet from New Zealand. This information is important in relation to quality assurance and workflow planning in our local environment.
All patients who underwent CTC in the 72-month period from 1 January 2004 to 1 January 2010 were included in the analysis. CTC was performed on either a 16 slice or 64 slice CT (GE Lightspeed series), after standard bowel preparation with LoSo Prep (E-Z-EM).
A supine and prone (and/or occasionally a supplementary decubitus scan) was performed for each patient. Standard technique factors were 40–50 mAs, 120 kVp or 100 kVp (latter if less than 75 kg weight), and either manual air insufflation prior to 2009, or subsequently CO2 insufflation (ProtoCO2l, Bracco Diagnostics).
Demographic data and CTC findings were recorded, according to the CT Colonography Reporting and Data System (CRADS).2 The CRADS code was prospectively reported from mid 2008, and coding was retrospectively applied to the other reports from 2004–2008 for the purpose of this study; after assessment of the report by two experienced CTC radiologists. A group of six CTC accredited radiologists reported the studies.
2152 consecutive CTC patients were available for study. The group was 52.6% female, with an average age of 59.4 years; range 19–87 years. Indication data was unable to retrieved in 605 of the earliest patients; (28%). Of the remainder, approximately 84% were symptomatic. The most common indication was “change of bowel habit”, usually not otherwise specified. 16% were asymptomatic (which included some higher risk patients also; comprising 6 failed colonoscopy patients, 5 FOBT positive patients and 17 surveillance patients following colorectal cancer resection). Diverticular disease was mentioned in 880/2152 reports; a prevalence of 41%. Of these approximately 360 (also 41%) were reported as mild-trivial.
Table 1. Colonic findings, CRADS: CT colonography reporting and data system
Colonic findings: The C2–C4 group comprises a total potential referral rate to surveillance, colonoscopy or surgery, of 10.7%. For extracolonic findings, the majority were normal or findings of no major importance. The E3 and E4 group together comprise the total of potentially significant extracolonic findings; of 8.3%.
Figure 1. Virtual dissection mode—view of a cancer indrawing the wall of the ascending colon
Figure 2. Volume-rendered “barium enema” type view—the image demonstrates multiple sigmoid colon diverticula, as well as a rectal polyp
Table 2. Extracolonic findings
Figure 3. Coronal view of abdomen—common extracolonic findings of a tortuous but not aneurysmal aorta with calcified atheroma, and a hiatus hernia
Table 3. Description of potentially significant extracolonic findings
There has been much debate in the literature as to whether detecting extracolonic abnormalities is an “asset or liability”;3 and there are valid issues on both sides. The anxiety and possible physical complications provoked by undergoing work-up of an incidental finding is not to be underestimated, and it is difficult to measure this effectively; particularly in economic terms. The potential financial and patient harm implications of extra testing generated from CTC are a concern.
The impact of these issues can be tempered by having good quality information regarding the prevalence and type of ECFs, and by having guidelines/agreements in place as to how to categorise them and how to deal with them. There is heterogeneity in how different studies have reported ECFs, and particularly in relation to the definition of “significant”, or “major” or “important” findings. These fall into two main groups; findings of direct clinical importance e.g. a probable cancer, or any finding that generates an extra investigation.
The Working Group for Virtual Colonoscopy has provided a sound base for addressing this with the publication of the CT Colonography Reporting and Data System (CRADS) in 2005,2 which although developed for a screening population, have been applied to and adapted for use in symptomatic patients also. 4,5 The more recent “CT Colonography Standards, an International Collaboration,” 6 published in 2010, has also contributed significantly to this process.
Our study is aligned along the CRADs definitions, although there may well be debate regarding specific categoratisation within each group. In the future, clearer guidelines may be available but this is a tricky area; because local practices may differ, availability of certain tests may differ, and patient “culture” in regard to acceptance of surveillance or active investigation may differ. Development of local guidelines may be a useful endeavour.
Data from multiple centres around the World has shown that extracolonic findings are present in at least half of the patients, and that there is an increased frequency in older patients,7 and in studies using intravenous contrast and higher radiation dose techniques.8 Symptomatic patients and females have also reported to have more ECFs.9 Despite the heterogeneity of study designs, the rates of “significant” extracolonic findings that require further work up or alter management are generally in the range of 6-16%, although it has been reported at up to 25%, in a study using intravenous contrast. 10
A recent Australian study of 258 symptomatic patients found significant ECFs in 8.9% . 11 An asymptomatic Australian cohort reported a rate of significant ECF in 7.4% . 12 These are both similar to our rate of 8.3%.
The majority of our E3 Group (probably benign but needs further work-up) were renal or hepatic or ovarian cysts that could not be clearly categorized as benign on the low dose CT, requiring only ultrasound follow up. However, a limitation of this study is that the cost estimates of these subsequent investigations has not been performed.
A recent USA study in a screening population of 2277 patients found a significant ECF rate of 11%, which generated extra cost of approximately $50 per patient. They detected 6 cases of colorectal cancer, and 6 cases of extracolonic malignancy. 13
Our extracolonic potential malignancy detection rate of 1.8% (39/2152) is close to our colonic potential malignancy detection (C4) rate of 2.36%; particularly as we know that some of these are false positive due to pathology such as diverticular strictures, or the occasional polypoid mass that is benign. Further follow up work will be required to ascertain the true rate of extracolonic malignancy and the false positive C4 colon cancer rate in our cohort.
A study from the Netherlands looked a group of 398 symptomatic patients, and applied the CRADs classification.5 They reported a rate of 7.5% of patients with C3-4 classification; i.e. with suspected colorectal cancer, polyps >10 mm or >2 polyps 6-9 mm. 8.3% were C2 (1-2 polyps 6–9 mm); for a total potential referral rate to OC of 15.8%. This may be due to the increased prevalence of disease in their entirely symptomatic cohort. They had a significant ECF rate of 15.6%.
An American study in a screening population reported 62/454 patients or 13.6% with at least one polyp >5 mm.7
In comparison to these studies, our polyp frequency is lower. This is of uncertain significance; potentially due to different population characteristics, or our detection rate may be lower. Comparison with other national data is awaited, and reassuringly our miss rate for colorectal cancer is well within the reported range, at 5.1%.14
This study is limited by its retrospective, descriptive nature; but it provides useful local data.
Our CTC population is largely symptomatic, and there is an acceptable referral rate from CTC to colonoscopy, surgery or surveillance of 10.7%. The 8.3% rate of potentially significant extracolonic findings is at the lower end of the reported international ranges, although in line with Australian data. Ongoing work will be required to assess the performance of CTC in our population.
Competing interests: None declared.
Author information: Helen Moore, Consultant Radiologist; Nicholas Dodd, Consultant Radiologist; Auckland Radiology Group, Auckland City Hospital, Auckland
Acknowledgements: We thank National Screening Unit, Ministry of Heath, for their funding contribution; NZ Branch office, Royal Australian and New Zealand College of Radiologists for their assistance; and Rachel Dodd for data collection.
Correspondence: Dr Helen Moore, PO Box 9889, Newmarket, Auckland, New Zealand. Email: Helen.email@example.com
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