|Home | About | Journals | Submit | Contact Us | Français|
Conventional colonoscopy is considered the optimal investigation for rectal bleeding; however, this article explores the use of virtual colonoscopy as a new investigative technique
An 83 year old man was referred to a surgical outpatient clinic by his general practitioner via the “two week wait” bowel cancer pathway for patients with persistent rectal bleeding (painless dark red blood with clots per rectum for one month with blood mixed in the stool) but no anal symptoms. He had had no weight loss or change in bowel habit, and except for hypertension, he was a fit non-smoker with no family history of colorectal cancer. Abdominal examination showed no masses, and rigid sigmoidoscopy to 15 cm was unremarkable.
The patient’s symptoms suggested a possible underlying colorectal cancer, which was therefore the primary target of investigation. Flexible sigmoidoscopy was deemed inappropriate in this patient as (regardless of whether the sigmoidoscopy result was negative or positive) the proximal colon would have required examination to detect a proximal cancer or to exclude a synchronous cancer (or advanced polyp), which could be included in the subsequent surgical resection.
Currently three options are available for whole colon investigation of rectal bleeding to look for colorectal cancer: conventional colonoscopy, barium enema, and virtual colonoscopy (or computed tomography colonography).
Conventional colonoscopy is the only option that enables tissue biopsy or polyp removal. It is often used as the reference standard by which new techniques for detecting colorectal cancer and polyps are judged.1,2,3 Yet conventional colonoscopy is imperfect. For example, the “miss” rate for large polyps (10 mm or larger in maximal diameter) is 2% (95% confidence interval 1%-7%) with tandem colonoscopy.4 Furthermore the probability of conventional colonoscopy finding an important abnormality of the colon is low; the positive predictive value of rectal bleeding for colorectal cancer is very small in the general population, estimated to be 2-3% when reported to primary care and 5-7% when referred for specialist investigation.5 Even though dark blood mixed with stool is a classically unfavourable symptom (positive predictive value of 10-13% in referred patients), about 90% will have a normal colon.5 With this in mind, less invasive alternative tests may be preferred by patients and their referring doctors.
Barium enemas are offered by most radiology centres in the United Kingdom and are less expensive than colonoscopy. However, the demand for barium enemas has decreased in favour of colonoscopy as the sensitivity of barium enemas for detecting cancer is relatively poor (about 85%6) and patients find the procedure less tolerable owing to greater discomfort7 (probably related to retention of a liquid enema and frequent repositioning on the examination table).
After a prospective multicentre, intraindividual comparison study of 1233 patients published in 20038 and favourable meta-analysis of diagnostic performance data (sensitivity about 90% for large polyps and 96% for cancer),1,2,3 virtual colonoscopy received approval in 2005 by NICE (National Institute for Health and Clinical Excellence) for use in the NHS in England and Wales as an alternative first line investigation for patients with symptoms of colorectal cancer or high risk asymptomatic patients.9
Virtual colonoscopy uses computed tomography to examine the prepared, distended colon (either air or carbon dioxide colonic insufflation via a thin, flexible rectal catheter) enabling a rapid examination of the whole colon with excellent completion rates of 99%.8 Subsequent interpretation of the scan data combines traditional two dimensional methods of review with three dimensional “endoscopic fly-through” simulations, hence “virtual” colonoscopy (fig 11).).
Virtual colonoscopy does not require sedation, is well tolerated by patients,7 and is relatively safe, with no reported deaths worldwide since its introduction in 1994. As with endoscopy and barium enema, retrospective survey data has shown that perforation of the colonic wall does occur but is rare (occurring in 0.009% to 0.08% of cases).10 Furthermore, because computed tomography is extremely sensitive for detecting extraluminal gas, asymptomatic perforations (4 of 9 (44%) in patients with perforation related to virtual colonoscopy in a UK survey11) are readily detected and these patients can be successfully managed conservatively.
Patients having virtual colonoscopy are exposed to ionising radiation, but with dose modulation techniques the radiation dose for virtual colonoscopy can be minimised to the equivalent of background radiation for one year (about 2-3 mSv). This radiation level is substantially less than for barium enema and particularly relevant when considering virtual colonoscopy for asymptomatic patients.
As with barium enema, virtual colonoscopy provides extra information for therapeutic endoscopy in patients with a positive finding: it accurately locates the polyp or cancer and reports its size and morphology, as well as any anatomical factors that may help to predict difficult colonoscopy.12 Such information may improve both the quality of patient consent and the planning of subsequent cancer biopsy or polyp removal. Moreover, if radiologists and endoscopists agree complementary management strategies, patients with cancer found at virtual colonoscopy can potentially benefit from immediate computed tomography body staging (with intravenous contrast if this is not already part of the standard protocol for virtual colonoscopy) and same day endoscopic biopsy; this would avoid repeat bowel preparation and imaging delays. In addition, virtual colonoscopy shows major extracolonic disease in 10% of patients overall13; 30% of these patients have extracolonic cancers (which may be the cause of their presenting symptoms) and 10% have aortic aneurysms.
Virtual colonoscopy is available in about a third of NHS centres and is not currently included in core specialist registrar radiology training. In contrast, training in barium enemas is a core requirement, but falling demand is likely to decrease the opportunities for such training.
In our patient, virtual colonoscopy was requested and showed multiple polyps throughout the colon, including two measuring >2 cm in maximal diameter: one at the hepatic flexure (30 mm) and one in the rectum (23 mm). Virtual colonoscopy was performed using a standard protocol without intravenous contrast or use of faecal tagging agents (orally administered iodine or barium products used to label residual faeces or fluid, enabling their rapid discrimination from true pathology). There was no clear evidence of local invasion or metastases but the virtual colonoscopy raised the likelihood of difficult colonoscopy owing to a very long, tortuous sigmoid colon.
The patient agreed to have endoscopy later that morning. As predicted, conventional colonoscopy was technically challenging. However, with additional examination time, use of a magnetic endoscopic imager (to aid scope orientation), and appropriate operator expertise, a complete examination was possible, and this confirmed the findings of the virtual colonoscopy. The rectal polyp was on a short stalk and had an area of ulceration with bleeding on one side. The polyp stalk was injected with 3 ml dilute adrenaline to reduce acute bleeding risk before snare resection (fig 22).). The hepatic flexure polyp appeared macroscopically benign and a biopsy was taken (histology showed moderate dysplasia). The patient drove himself home later that afternoon.
On return to the clinic, the patient reported that his rectal bleeding had resolved. Histology assessment of the rectal polyp showed cancer with high risk features (fig 33).). On the basis of these findings, the patient had a laparoscopic anterior resection with covering colostomy. Histology of the resection specimen showed that the prior polypectomy had removed all of the primary cancer but there was one involved pericolic lymph node.
The hepatic flexure polyp was subsequently removed by local mucosal resection at the time of stoma reversal, with histology confirming an early synchronous cancer with minimal submucosal invasion. The remaining polyps were all removed endoscopically. The patient is currently fit and well.
This series provides an update on the best use of different imaging methods for common or important clinical presentations. The series advisers are Fergus Gleeson, consultant radiologist, Churchill Hospital, Oxford, and Kamini Patel, consultant radiologist, Homerton University Hospital, London.
We thank Brian Saunders, Robin Kennedy, and Thomas Guenther for their help with the details of the management of this case.
Contributors: DB and JEE did the clinical examinations and, along with SAT, contributed to the planning of this article. DB and JEE did a literature search and jointly wrote the original draft manuscript. SAT and DB edited the manuscript further before submission. DB is guarantor.
Competing interests: DB and SAT are remunerated consultants and hold share options for Medicsight (London), which develops computed tomography software. DB has received remuneration for consultant advice to Barco (Edinburgh), which also develops such software.
Provenance and peer review: Commissioned; externally peer reviewed.