CT colonography method reduces radiation risk

May 07, 2003

Results of a recent study reconfirm previously published data, suggesting that CT colonography (CTC) can be performed with decreased radiography exposure to patients, says Michael Zalis, MD, of Massachusetts General Hospital and lead author of the study.

"Standard CTC is performed with approximately 100 mA, and the dose in our study was approximately 50% of this," says Dr. Zalis.

To evaluate the performance of this low-dose CTC technique as a way to reduce radiation risk for non-invasive colon cancer screening, Dr. Zalis and colleagues compared the studies of 100 patients at moderate risk for development of colorectal cancer who underwent CTC and video assisted conventional colonoscopy. Dr. Zalis says, "Moderate risk usually refers to ages 50-70, without history of inflammatory bowel disease, prior colon carcinoma, or familiar polyposis syndrome."

Reduced-dose CTC examinations were performed following polyethylene-glycol electrolyte preparation (the traditional bowel prep used for colonoscopy) and patient-controlled self-insufflation (the administration of air into the colon) on a multidetector CT. The size of lesions and the evaluation times of two readers were compared on a per-polyp basis to conventional colonoscopy, which was performed following each CTC, the same day.

Reader 1 demonstrated sensitivity of 90% for lesions 10 mm or larger, 67% for 5-10 mm lesions, and 50% for lesions that were 0-5 mm. Reader 2 also demonstrated sensitivity of 90% for lesions 10 mm or larger, but sensitivity of 58% for 5-10 mm lesions and 44% for 0-5 mm lesions. Dr Zalis says both readers took an average of 9-10 minutes to read the scans.

Dr. Zalis says, "We found no statistically significant difference in terms of detecting polyps using the low-dose technique. However, the low-dose technique will make subtle differentiation of extracolonic soft tissues more limited." He notes that the main point of CTC is to screen the colon, and even with low-dose technique, it is still feasible to detect gross extracolonic findings such as masses and lymphadenopathy.

Dr. Zalis recommends using the low-dose technique for asymptomatic screening, but says, "For patients with active symptoms, it may be useful for them to receive a more standard dose, as this might help characterize subtle extracolonic lesions." However, Dr. Zalis emphasizes that dosage difference for symptomatic patients has not been proven.

Dr. Zalis points out that radiation dosage depends on a patient's weight, and says, "new technology will allow the scanner to adjust the dose per slice based on the patient's cross sectional thickness, permitting even further reduction of dose."

The study will be presented May 7, during the American Roentgen Ray Society Annual Meeting in San Diego.
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Keri Sperry, 703-858-4306
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American College of Radiology

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