Physicians at Queen Mary Hospital, the academic hospital of the University of Hong Kong, measured radiation dose for four different heart rates and estimated the lifetime cancer risk for girls and boys imaged at birth, age 5, and age 10. Their findings, published in the August issue of the American Journal of Roentgenology (2009, Vol. 193:2, pp. 539-544), provide guidance for physicians to assess and justify ordering a CTA exam instead of a cardiac MRI exam.
The researchers calculated that the lifetime attributable risk of developing cancer from radiation dose CTA exposure for 5-year-olds living in Hong Kong was 0.22% to -0.33% for boys and 0.61% to 0.85% for girls. For children living in the U.S., the risk was slightly lower at 0.14% to 0.20% for boys and 0.43% to 0.60% for girls.
Lead author Dr. Bingsheng Huang, from the department of diagnostic radiology, and colleagues first measured radiation dose on a standard pediatric phantom of a 5-year-old using 74 thermoluminescent dosimeters distributed in organs, as advised in the 2007 recommendations of the International Commission on Radiological Protection (ICRP).
Exams were performed using four electrocardiogram-gated coronary CTA protocols for heart rates of 40, 60, 70, and 90 beats per minute (bpm) on a 64-detector-row scanner (Discovery VCT, GE Healthcare, Chalfont St. Giles, U.K.). The effective doses with simulated heart rates of 40, 60, 70 and 90 bpm were 16.45, 12.17, 11.97, and 11.81 mSv, respectively.
The heart, lung, breast, thymus, and esophagus had the highest radiation burden because they were directly irradiated. The authors calculated radiation dose for 24 organs, averaging the thermoluminescent dosimetry readings to calculate individual organ dose.
|Organs receiving largest radiation dose in mSv
||Tissue weighting factor
pitch = 0.16
pitch = 0.22
pitch = 0.24
pitch = 0.24
Although the dose results are specific to the protocols and CT scanner used at Queen Mary Hospital, they provide an opportunity for radiologists to make comparisons among different scanners, protocols, and technologies utilized to perform pediatric CTA exams, Huang and colleagues noted.
The researchers used the Biologic Effects of Ionizing Radiation (BEIR) VII report from the National Research Council (NRC) of Washington, DC, to calculate the radiation-induced risk and the lifetime attributable risk of cancer incidence of each organ. These statistics were derived from the U.S. cancer statistics for 2001-2005.
The lifetime attributable risk of cancer incidence calculated for the Hong Kong and U.S. populations were higher than those associated with other CT exams reported in peer-review literature of pediatric patients, according to the authors. The group also found that cancer risk was higher compared with cancer risk reported from adult CTA exams, mainly due to the fact that radiation dose is more detrimental to young children.
|Lifetime attributable risk from CTA for 5-year-olds
||40 bpm heart rate
||60 bpm heart rate
||70 bpm heart rate
||90 bpm heart rate
|Hong Kong boys
|Hong Kong girls
Children living in Hong Kong had a higher lifetime attributable risk due to differences in life table and cancer statistics between Hong Kong and the U.S. Hong Kong residents live longer than U.S. residents, and they have a higher cancer incidence in the lung, colon, liver, and stomach -- key organs exposed to CTA radiation dose.
By Cynthia E. Keen
AuntMinnie.com staff writer
August 21, 2009
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