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CT Colonography - An Alternative to Colonoscopy?
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Colorectal
cancer is the third leading cause of cancer deaths in both men and
women and the likelihood of an individual developing it some point in
his or her life is about 6%. However, early stage colorectal cancer can
be successfully treated and the mortality benefit of screening for
colorectal cancer is well established. Despite this, fewer than 40% of
those for whom screening is recommended comply and only about 37% of
colorectal cancers are detected while the disease is still localized.
Will CT colonography (CTC) be a way to improve the screening rate and
reduce the mortality from this disease? A recent large prospective
study has demonstrated that it is at least as effective as the current
gold standard, optical colonoscopy, for detecting adenomatous polyps
(Pickhardt, Choi et al. 2003). At this time, both procedures require
preparatory bowel cleansing. However, colonography is a better
tolerated procedure that does not require conscious sedation, which is
necessary during the optical colonoscopy examination. For this reason
alone, the availability of CT colonography may improve compliance. In
addition, in the cases in which colonoscopy cannot, be completed due to
stricture, obstruction, or the complex tortuosity of the colon, CT
colonography has been established as the imaging technique of choice.
Perhaps the most important disadvantage of CT colonography is that any
significant findings have to be followed up by optical colonoscopy for
polyp removal. Hence, one may ask what the value of the CTC is when any
positive lesion detected by the new exam will require further referral?
It is estimated that six to nine percent of non-symptomatic patients
over 50 years may be expected to have a polyp greater than 10 mm in
size; lesions of this size range include the "advanced adenomas" that
represent the primary target of screening. The currently practiced,
though more conservative approach is that lesions somewhat less than 10
mm should also be resected, adding approximately another 10% of
screening individuals who may be referred to colonoscopy. The value of
colonography is in permitting the large fraction of the at-risk
screening populace who do not have significant lesions to avoid the
additional risk, discomfort, and expense associated with conventional
colonoscopy.
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Coronal reconstruction CT colonography image
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air filled colon appears dark. Arrow indicates 1.5 cm adenoma. Other
soft tissue projections into the lumen of the colon are normal haustral
folds. |
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How Good is CT Colonography?
In
studies that included more than 100 patients, comparing the detection
rate colonography with that of colonoscopy, the reported per patient
performance for detection of polyps equal to or greater than 10 mm
ranged from 93-100% sensitivity and 80-100% specificity. The
sensitivity per polyp in these studies ranged from 62-90% for polyps
equal to or greater than 10 mm and 16-82% for 6-9 mm polyps. False
positive results may occur because of retained stool, diverticular
disease, misinterpretation of thick or complex haustral folds, and
artifacts due to motion or metal (e.g. hip prostheses). Colonography
does, however, have the potential advantage of identifying cancers that
may not be adequately assessed by endoscopy, such as those that are
located close to complex haustral folds.
In
comparison, studies of back-to-back colonoscopies performed on
individual patients have demonstrated miss rates of about 6% for
adenomatous polyps equal to or greater than 10 mm (the size above which
lesions are considered to have a significant risk of harboring
malignancy) and 13% for polyps in the 0.6-0.9 mm range. These miss
rates are comparable to results achieved with state of the art CT
colonography, as interpreted by experienced readers.
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Colonoscopy
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CT Colonography
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Patient preparation
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Purgative bowel cleansing
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Purgative bowel cleansing*
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Patient tolerance
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Need for conscious sedation
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Minimal discomfort
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Radiation
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None
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Low dose (approximately 20% less than barium enema)
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Complications
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0.3-1% perforation and bleeding
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None reported
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Visualization
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Optical. Lesions identified by both color and shape.
View lumenal surface of colon only. May be incomplete due to blockage or complex fold structure
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Tomographic
and 3D reconstruction with views from any angle. Lesion identified
primarily by shape, making it harder to identify flat adenomas.
Occasionally find non-colonic pathology
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Localization
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Inferred from length of colonoscope inserted
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Precise 3D localization within abdomen
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Lesion removal
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Can be done at time of procedure. Cancerous lesions are followed up by surgery.
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Need subsequent colonoscopy/ surgery
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Cost
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$900 – $2000; covered by most insurance
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$500 - $700; covered by insurance for symptomatic individuals
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*Current research is evaluating a procedure that does not require bowel
cleansing. Instead, the patient must ingest a contrast agent with meals
and snacks for two days. Computer image processing subtracts bowel
contents for radiologist to view apparent empty bowel. This technique
is currently in clinical trial.
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Preparation and Procedure
The
present CT colonographic procedure requires purgative bowel cleansing
with phospho-soda and Bisacodyl (Fleet Prep Kit 1) or polyethylene
glycol electrolyte solution (NuLytely® or Go-Lytely®). Prior to the CT
scan, an enema tip or catheter is placed in the rectum and the colon
filled with air until the patient reports a full feeling. Few patients
report any more than minimal discomfort. Both supine and prone CT
images are acquired, which means that parts of the colon that are
compressed in one position are open in the other. Current multi
detector CT scanners can acquire all the image data in two breath
holds, minimizing movement artifacts.
The whole
procedure is complete within 15-20 minutes and, since there is no
sedation, the patient is able to resume normal activities immediately,
before the images are reviewed. The radiologist examines planar images
of the entire length of the colon and computer reconstruction of 3D
images are made to view any part of the bowel from an angle that the
radiologist selects. The radiology report is sent to the referring
physician the same day of the procedure.
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Patient Scheduling at MGH
Please
note that CT colonography for cancer screening is not covered by most
insurance at this time. CT colonography of asymptomatic patients can be
performed at MassGeneral West in Waltham or at MassGeneral Imaging in
Chelsea.
( Internal Access Only )
If you have any patients who are interested in being part of the
clinical study of the minimally prepped CTC, which requires both CTC
with ingested contrast agent and colonoscopy, please contact Dr. Michael Zalis , or Dr. Cordula Magee.
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3D reconstruction image of normal colon.
Follow Up and Therapy
Unlike
colonoscopy, CT colonography is not therapeutic. Therefore, all
positive colonography scans must be followed up by optical colonoscopy
for polyp removal or by surgery to remove larger masses. It should be
noted that the large majority of individuals presenting for colon
screening exam do not have significant polyps; CT colonography may
permit these individuals to avoid endoscopy while correctly and more
easily identifying those that require endoscopic or surgical resection.
If open surgery is necessary, colonography has the advantage of
indicating the position of the lesion within the abdomen, making
surgery easier.
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3D reconstruction image of colon.
Arrow indicates 1.5 cm adenoma.
Further Information
For further information about CT colonography, please contact Dr. Michael Zalis, 617-726-8396
This article provided useful information about the appropriate use of imaging studies:
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References
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Fidler, JL, Johnson, CD, MacCarty, RL, Welch, TJ, Hara, AK and Harmsen, WS (2002). Detection of flat lesions in the colon with CT colonography. Abdom Imaging 27: 292-300.
Gluecker, TM and Fletcher, JG (2002). CT
colonography (virtual colonoscopy) for the detection of colorectal
polyps and neoplasms. current status and future developments. Eur J Cancer 38: 2070-8.
Gollub, MJ (2002). Virtual colonoscopy. Lancet 360: 964.
Hardcastle, JD, Chamberlain, JO, Robinson, MH, Moss, SM, Amar, SS, Balfour, TW, James, PD and Mangham, CM (1996). Randomised controlled trial of faecal-occult-blood screening for colorectal cancer, Lancet 348:1472-7.
Johnson, CD and Dachman, AH (2000). CT colonography: the next colon screening examination? Radiology 216: 331-41.
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Levin,
B, Brooks, D, Smith, RA and Stone, A (2003). Emerging technologies in
screening for colorectal cancer: CT colonography, immunochemical fecal
occult blood tests, and stool screening using molecular markers. CA
Cancer J Clin 53: 44-55.
Pickhardt, PJ, Choi, JR,
Hwang, I, Butler, JA, et al. (2003). Computed tomographic virtual
colonoscopy to screen for colorectal neoplasia in asymptomatic adults.
N Engl J Med 349: 2191-200
Taylor, SA, Halligan, S and Bartram, CI (2003). CT Colonography:
Methods, Pathology and Pitfalls. Clin Radiol 58: 179-90.
Zalis, ME, Perumpillichira, J, Del Frate, C and Hahn, PF (2003). CT
colonography: digital subtraction bowel cleansing with mucosal
reconstruction initial observations. Radiology 226: 911-7.
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