Issues.
Cut-off size of polyp detection
As radiation dose is potentially harmful, it should be reduced to a minimum when screening asymptomatic people for CRC with VC.
A major advantage of VC is the high contrast difference between the colonic wall and the intra-luminal CO2.
This allows to decrease the radiation dose considerably without a loss in sensitivity/specificity for the detection of polyps.
Using a regular dose for VC (120 kV, 65 mA) the absolute life time risk for cancer in a 50 year-old patient is 0.14 %, with the risk of cancer death being much lower.
Reducing the mAs to 10, and technical innovations such as the automated tube current modulation, this risk may be decreased by a factor 5 to 10.
This is considerably lower than the incidence of colorectal cancer with a probability of developing cancer of 6 % and of dying from the disease of about 3 %.
Ultra-low dose VC with 10 mAs and 140 kV has successfully been used by Iannaccone et al.
In our personal experience, using a 64-slice scanner and an ultra-low dose of 10 mAs and 140 kV in combination with automated tube current modulation, the radiation dose is reduced to 2 + 0.2 mSv for dual positioning without loss in image resolution .
We can conclude that the advantage of detecting colorectal lesions at an early stage using ultra-low dose VC exceeds the potential harm caused by the radiation.
An important limitation of VC is of course the inability to remove the detected polyps.
As there is a positive correlation between the size of an adenomatous polyp and the possibility of villous or malignant transformation, advise should be given to the clinician whether or not the lesion should be removed.
This size related advise is also dictated by a second limitation of VC: its low sensitivity and specificity for diminutive lesions < 5 mm.
Is there a "safe" cut-off size for lesion detection ?
The target lesion when screening for CRC is the advanced adenoma being a lesion > 1 cm or having villous or dysplastic components.
With an average 1 % chance of malignant degeneration, all observers agree that lesions > 1 cm should prompt immediate work up with endoscopic or surgical removal.
There is no agreement for lesions < 1 cm. The possibility of malignant transformation in lesions < 5 mm is much less than 1% and less than, but close to 1 % in the intermediate 6-9 mm lesion.
In a recent publication, the working group on VC favored only immediate optical colonoscopy and removal of lesions > 1 cm or of > 3 lesions 6-9 mm in size.
The rationale is that, if ever malignant degeneration occurs, it will take 5-10 years for a lesion < 5 mm to develop in a cancer.
With a screening interval of 5-10 years, depending on the age, it is not necessary to remove lesions < 5 mm.
For patients with less than three 6-9 mm lesions a surveillance interval of 3 years is proposed.
Most gastroenterologists however do not agree with this statement and are in favor of performing colonoscopy whenever a polyp is detected at VC and this irrespective of size because patients benefit from polypectomy.
A less aggressive attitude is to only perform colonoscopy if > 3 polyps of 5 mm or a lesion > 6 mm are detected because of increased prevalence of advanced adenomas in these cases
The flat or non-polypoid adenoma has been defined by the working group on VC as a lesion with a plaque-like morphology with a height < 3 mm.
These lesions can be flat topped, completely flat or depressed. Controversy exists on their importance.
They have been described at frequencies varying between 8.5 and 42.7 % .
Their frequency of malignant degeneration is not known.
Optical colonoscopy misses a lot of these lesions unless specialized techniques as magnification colonoscopy or chromoscopy are used.
It is obvious that their detection on VC will be difficult too.
In the only large study to date reporting on the detection of flat lesions good results were obtained by Pickhardt et al.
In their study 55 flat lesions out of 344 polyps > 6 mm were detected. Of these flat lesions 26 were hyperplastic (7.5 %) and 29 were adenomatous (8.4 %) with 1 cancer.
This cancer was clearly higher than 3 mm. Overall sensitivity for these lesions was 79.6 %.
It is clear that as with optical colonoscopy detection of flat adenomas with VC needs a lot of experience and necessitates careful inspection of the colonic wall.
The use of multi-detector scanners (16 and 64 slices) has resulted in thinner slice thickness (collimation) and still a faster examination (less than 15 seconds). Using a thinner slice collimation should improve the accuracy of the technique.
As a screening procedure should be harmless, the radiation dose generated by computed tomography cold be a blocking issue when screening a population at average risk for colorectal cancer. However since the introduction of the multi-row detector scanners efforts to reduce the radiation dose have been performed. In fact Iannaccone et al. obtained a sensitivity of % and % for lesions 6-9 mm and > 1 cm respectively, using an ultra-low dose of 140 kV and 10 mAs. With the new 64-slice scanners the identical ultra-low dose can be used without deterioration of the image quality.
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