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| Journal of the New Zealand Medical Association,
08-June-2012, Vol 125 No 1356
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Computed tomographic colonography (CTC): a
retrospective analysis of a single site experience and a review of the
literature on the status of CTC
Marcus Ghuman, Ngaire Bates, Helen Moore
Colorectal cancer (CRC) is the second most common cause of
cancer death in New Zealand,1 and we have
amongst the highest age-standardised rates of the disease in the world. Barium
enema and colonoscopy have been the traditional investigations used in the work
up of patients presenting with symptoms suggestive of
CRC.2 Increasingly, computed tomographic
colonography (CTC) is displacing barium enema as a non-invasive rapid imaging
technique to investigate these patients, which can be cost effective, and as
accurate as colonoscopy for colorectal cancer
detection.3
While it is evident that the sensitivity of CTC for
detection of polyps over 10mm is generally equivalent to that of colonoscopy, as
seen in local4 and international clinical
trials,5–8 areas requiring consensus
remain. These areas particularly include the reporting, management, and follow
up smaller polyps; and also issues surrounding extracolonic findings. Radiation
exposure issues will also be discussed.
MethodsStudy design—This observational
study was conducted as a retrospective analysis using as a population all the
patients undergoing CT colonography (CTC) for the 3-year period from 9 August
2007–12 August 2010 conducted at a single site; Greenlane Hospital, which
is the outpatient hospital for Auckland District Health Board (ADHB), and
currently the only ADHB site performing CTC in public.
These patients were identified through Picture
Archiving and Communication System (PACS) coding, along with their corresponding
National Health Index (NHI) numbers. This dataset also included the date of
referral and performance of examination, and the referral source. Each
patient’s CTC report was obtained from the Auckland District Health Board
software package “Concerto”, where demographic data (sex, ethnicity,
age), symptoms leading to referral, findings of the examination (intra and
extracolonic), and recommended follow-up were extracted.
Each CTC study was performed on a Phillips 16-slice CT
scanner typically using standard full bowel preparation (LoSo Prep, E-Z-M),
although some more frail patients had reduced preparation or only fecal tagging.
All had fecal tagging (Tagitol) and iv Buscopan unless contraindicated. Supine
and prone scans, and occasional supplementary decubitus scans were performed at
120kvP or 90kvp, and mAs 50-150. Colonic distension was primarily using CO2
insufflation (ProtoCO2l, Bracco Diagnostics), or occasionally manual air
insufflation.
The CTC reports were coded according to the CT
Colonography Reporting and Data System (CRADS), as defined by the Working Group
for Virtual Colonoscopy, 2005.9 The vast
majority of the scans in this study were reviewed by two consultant radiologists
with considerable experience in CT colonography reporting, suggesting that the
reporting in this sample was robust.
Statistical analysis—Categorical
data were presented as frequency (percentage) and continuous data were presented
as mean (standard deviation). Categorical variables were compared using a
Chi-squared test or Fisher’s exact test as appropriate. Continuous data
were compared using the t-test and the one-way Anova test. All p values reported
were two-tailed and a p value <0.05 was considered significant. SAS (version
9.1) statistical software was used for statistical analysis.
ResultsThis study identified 302 patients undergoing CT
colonography (CTC), of whom 184 were female (61%). The mean age of patients in
the study population was 66 years (range 16–91 years). New Zealand
European (59%), Asian (17%), and Other Europeans (10%) made up the majority of
the study population. The demographic data of the study population are recorded
in Table 1.
Table 1. Demographic data of the study
population
Nearly all of the referrals for CTC came from four sources:
General Practice (121 referrals), Gastroenterology (103 referrals), General
Surgery (60 referrals), and General Medicine (9 referrals); 57 patients, which
accounted for 19% of the study population, were referred for CTC following a
failed optical colonoscopy (OC).
The average time from referral to performance of the scan
was 43 days, with patients referred from General Medicine and those referred
following a failed OC showing a trend toward shorter referral time, though these
findings did not reach significance.
During the study period there was no formal priority
criteria used to stratify the referrals, however they consisted of symptomatic
patients, mostly of low to medium risk category. These findings are summarised
in Table 2.
Table 2. Referral base and referral times by
referrer for CT colonography
NB: OC is
optical colonoscopy; referral time refers to the time from referral for CT
colonography to performance of the scan.
CT colonography identified 12 patients as having colorectal
cancer (4% of the total study population), 24 as having polyps over 5 mm (8%),
and 111 with diverticular disease (37%).
The majority of patients identified as having cancer were of
New Zealand European ethnicity, a statistically significant finding (p=0.017).
Though there was a trend toward more females than males having cancer, and a
higher average age for the cancer cohort as compared to the study population,
these findings did not reach significance.
Of the 24 patients identified as having polyps, 13 were male
and 11 female. The majority of the polyp patients were also of New Zealand
European ethnicity (p=0.0002).
More females than males were found to have diverticular
disease (p=0.013), and again patients of New Zealand European ethnicity
dominated this cohort (p=0.0001). Table 3 summarises the cancer, polyp, and
diverticular disease findings.
Follow-up recommendations in the formal CTC reports of
patients discovered to have colorectal cancer varied from no follow-up advice,
to recommendation for referral to colorectal multidisciplinary meeting (MDM) and
direct visualisation and biopsy via optical colonoscopy.
Follow-up recommendations for patients with polyps varied,
but included repeat CTC (with interval duration ranging from 1–5 years)
and/or optical colonoscopy.
In total 21 patients (7% of the study population) were
referred for optical colonoscopy due to the detection of malignancy, polyps, or
for other reasons including poor quality of study, following their CT
colonography. In total, 3 patients (1% of the study population) had inadequate
bowel imaging at their CTC, as indicated by the C0 notation.
Table 3. Cancer, polyp, and diverticular
disease findings
NB: One of
the patients identified as having cancer did not have ethnicity recorded.
Of the 57 patients who were referred for CTC following a
failed optical colonoscopy, two were found to have cancer, two had polyps over 5
mm, and 25 had diverticular disease.
Although approximately 50% of patients in the study
population had some mention of extracolonic findings in their formal CTC report,
most were benign incidentals, and only 34 patients (11% of the study population)
had a recommendation made in their CTC report for further imaging as a result of
the extra-colonic findings. The number of patients who actually went on for
further imaging may well be less than this.
Nine patients (3%) were coded as E4 (potentially important
extracolonic finding) and were made up of three abdominal aortic aneurysms,
three pulmonary lesions suggestive of malignancy, two patients with evidence of
disseminated metastatic malignancy, and one porcelain gallbladder.
A further 54 patients (17.8% of the study population) were
coded as E3 (likely unimportant extracolonic finding, incompletely
characterised, therefore possibly requiring further investigation). These were a
heterogeneous collection of pathologies, with some of the more common being
renal stones and cysts, small pulmonary nodules, and pelvic/gynaecological
cysts.
Of the follow-up recommendations, 3 were for GP follow-up, 7
for specialist follow-up, and 24 for further imaging (comprising 16 USS, 7 CT
scans, and 1 MRI scan).
DiscussionThis observational study suggests that, for the patient
population undergoing CT colonography, Europeans and Māori were
proportionately under-represented, while Asians were over-represented, as
compared to Auckland population demographics.
The average time from referral for CT colonography to
performance of the exam was just over 6 weeks. This is within acceptable
guidelines for the current Northern Cancer Network Regional Prioritisation
criteria for priority 2, 3, or surveillance
patients.10
This study found that patients of New Zealand European
ethnicity had higher rates of colorectal cancer (p=0.0117) and polyp detection
(p=0.0002) as compared to other ethnic groups, both significant findings. This
finding is consistent with data from the New Zealand Cancer
Registry.11
Diverticular disease was shown to be more common in females
than males (p=0.0132), and in patients of New Zealand European ethnicity
(0.0001). These findings are well established in the
literature.8 The prevalence of colorectal
cancer in this symptomatic group, at around 4%, is similar to reported rates
from other CTC studies in New Zealand.12
A limitation of this study is the lack of formal comparison
of CTC findings with those who had colonoscopic assessment; and further work is
being undertaken in this regard. Interestingly there is quite limited data
available on the verified performance characteristics of both CTC and Optical
Colonoscopy for detection of colorectal cancer in NZ. Two studies from
Canterbury, assessing the miss rate of Colonoscopy and CTC using the NZ National
Cancer Registry as a reference, showed equivalent miss rates of around
5-6%.2,12 The follow-up advice in patients with
intermediate sized polyps (6mm–9mm) varied, with the most common advice
being repeat CT colonography or direct visualisation via optical colonoscopy.
The recommended interval length varied from 1 to 5 years, with a tendency to
shorter intervals for larger polyps.
This highlights one of the discussion topics with CT
colonography—specifically what size polyp should be referred for OC and
polypectomy and/or followed with repeat CTC. Although polyp natural history is
complex and not perfectly understood, there is a large amount of evidence from
pathological and colonoscopy-based studies showing that apart from actual
histology showing the presence of significant dysplasia or villous change, the
most dominant predictor of behaviour is polyp size.12
A recent colonoscopic study of 1468 patients found that of
414 polyps smaller than 10mm, only 41 (9.9%) were advanced adenomas, and 1.7%
were high-grade neoplasia. None were frankly malignant. Polyp size was the only
identified risk factor for the presence of advanced adenoma.13 There is general
consensus that patients with polyps greater than 10 mm should be referred for
colonoscopy, while the vast majority of diminutive polyps (those 5 mm and
smaller) are hyperplastic and do not require compulsory removal. The rate of
advanced histology in the diminutive polyp group is particularly low, with high
grade neoplasia or malignancy being extremely rare reported from zero to
0.06%.15
Thus the most controversy remains regarding what should be
done with small to medium sized (6–9 mm) polyps and how they should be
followed up, as there is a lack of consensus across the expert groups involved
(gastroenterological, surgical and radiological) regarding decision making in
this area.16 The radiologist authors utilised
the CRADS consensus, which gives the option of endoscopic polypectomy or
follow-up CTC for patients with these polyps of indeterminate size. It also
recommends consultation and adaptation in regard to local standards of practice
and patient preference.
In our opinion, this approach allows common sense to prevail
for individual patient circumstances, and existing data suggest this risk is
manageable.15,17 For example an 8mm polyp in a
younger or fit person would be actively dealt with compared to a similar polyp
in an elderly or frail individual with other comorbidities. Unnecessary
polypectomies have repercussions in economic terms and on patient morbidity and
mortality.17
Only 7% of patients undergoing CT colonography were referred
on for optical colonoscopy in the study, which is similar to rates found
elsewhere.3,15 It is important to note that the
higher the rate of on-referral to colonoscopy, then the higher the cost of a CT
colonography diagnostic approach. Conversely the cost of a primary optical
colonoscopy approach is increased by its “fail” rate, with
subsequent CTC necessary.
With reference to incidental extracolonic findings
discovered at CTC, a key consideration is the need to weigh the benefit of an
earlier diagnosis of a potentially important finding against the increased
patient anxiety and possible morbidity that necessarily results from a finding,
and the cost of further workup.
Although there was some mention of extracolonic findings in
close to 50% of the reports, only a minority of patients had potentially
important extracolonic findings, and only 34 patients (11% of the study
population) had recommendation for some form of follow up for their extracolonic
findings, including 24 follow-up scans. Of those 24 recommended follow-up scans,
two-thirds of these were for USS, a safe and comparatively cheap investigation.
The number of patients who actually went on for further imaging may well be less
than the number for whom it was recommended. These results compare favourably
with those found elsewhere, with rates of suggested follow-up for extracolonic
findings of CT colonography ranging from
11–20%.18,19
Although this study has not generated any data on radiation
exposure from CT colonography, this remains an important consideration when
considering this investigation as an alternative to optical colonoscopy. Due to
technical factors, individual radiation exposure could not be determined in this
audit, with only generic data being supplied by the particular CT machine.
Studies attempting to find a correlation between radiation
exposure and subsequent cancer risk in later life have relied on modelling
rather than direct observation,6 and thus there
is no direct data relevant to CT colonography in this area. However, most CT
colonography protocols deliver about 3–8 millisieverts of
radiation—which is a relatively low dose, and usually less than half that
used in a standard CT abdomen.8 The risk from
this radiation dose is negligible in a symptomatic adult, relative to their
background risk of cancer which is at least 30% lifetime
risk.20
The safety profile of CTC is excellent, with no perforations
or other complications in this cohort.
This observational study has reported on the experience of
CT colonography at Greenlane Hospital over a 3-year period. Though no attempt at
direct comparison with optical colonoscopy in regards to efficacy and safety
have been made, it has provided important local data on rates of detection of
colonic pathology. Key issues such as follow-up advice for small–medium
sized polyps, and investigation of extracolonic findings have been discussed.
One of the most important findings for primary care
practitioners is the under-representation of Māori and Pacific Island
patients referred for bowel symptoms.
Author information: Marcus Ghuman,
Radiology Registrar, Radiology Department,
Auckland Hospital, Auckland; Ngaire
Bates, Consultant Radiologist, Radiology Department, Auckland Hospital,
Auckland; Helen Moore, Consultant Radiologist, Radiology Department, Auckland
Hospital, Auckland
Correspondence: Dr Marcus Ghuman, 93 Tiki
Road, RD2, Te Awamutu 3872, New Zealand. Email: marcusghuman@hotmail.com
References:
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