Hong Kong children more susceptible to CT dose

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Children in Hong Kong, compared with U.S. pediatric patients, have a greater risk of cancer from the radiation dose delivered during coronary CT angiography (CCTA), according to a study in the August issue of the American Journal of Roentgenology

Bingsheng Huang, MD, from the department of diagnostic radiology at Queen Mary Hospital, the University of Hong Kong in Hong Kong, and colleagues sought to measure the radiation dose from ECG-gated CCTA and to estimate the cancer risk associated with the radiation dose for children.

The researchers measured organ doses with a pediatric phantom (representing a five-year-old) and thermoluminescent dosimeters on a 64-slice CT scanner. They studied four retrospectively ECG-gated CCTA protocols with four simulated heart rates and the corresponding pitches.

The investigators estimated the lifetime attributable risk of cancer incidence for children in the U.S. and Hong Kong, based on the National Academies Biologic Effects of Ionizing Radiation VII report.

According to the researchers, the effective doses were 16.45, 12.17, 11.97, and 11.81 mSv for the four protocols, respectively.

Huang and colleagues estimated that the corresponding lifetime attributable risks for five-year-old U.S. boys and girls were approximately 0.17 percent and 0.52 percent, respectively, and were about 0.28 percent and 0.73 percent, respectively, for five-year-old Hong Kong boys and girls.

In relation to the total cancer incidence (baseline cancer incidence plus lifetime attributable risk), the lifetime attributable risk from CCTA radiation exposure contributed up to 0.99 percent and 3.51 percent for Hong Kong boys and girls, respecitvely, and up to 0.46 percent and 1.57 percent for U.S. boys and girls, respectively.

Based on their findings, the authors concluded that the percentage contribution of lifetime attributable risk in total lifetime cancer incidence for Hong Kong children is higher than for U.S. children because Hong Kong children have a higher lifetime attributable risk of cancer incidence and a lower baseline cancer incidence.

Also, Huang and colleagues noted that the doses and cancer risks for the four heart rates were different, mainly because of the difference in pitch.

"In most scanners and across various vendors, pitch is set to increase when heart rate increases (except when the increase is small; e.g., on our scanner the pitch remains at 0.22 for heart rates between 53 and 60 bpm). This is to ensure that all phases of the heart are scanned in order to avoid discontinuity in the anatomic coverage of the heart between images from consecutive cardiac cycles," they wrote.