Depending on the machine settings, the organ being studied typically receives a radiation dose in the range of 15 millisieverts (mSv) (in an adult) to 30 mSv (in a neonate) for a single CT scan, with an average of two to three CT scans per study. At these doses, as reviewed elsewhere, the most likely (though small) risk is for radiation-induced carcinogenesis.
Most of the quantitative information that we have regarding the risks of radiation-induced cancer comes from studies of survivors of the atomic bombs dropped on Japan in 1945. Data from cohorts of these survivors are generally used as the basis for predicting radiation-related risks in a population because the cohorts are large and have been intensively studied over a period of many decades, they were not selected for disease, all age groups are covered, and a substantial subcohort of about 25,000 survivors received radiation doses similar to those of concern here — that is, less than 50 mSv. Of course, the survivors of the atomic bombs were exposed to a fairly uniform dose of radiation throughout the body, whereas CT involves highly nonuniform exposure, but there is little evidence that the risks for a specific organ are substantially influenced by exposure of other organs to radiation.
There was a significant increase in the overall risk of cancer in the subgroup of atomic-bomb survivors who received low doses of radiation, ranging from 5 to 150 mSv; the mean dose in this subgroup was about 40 mSv, which approximates the relevant organ dose from a typical CT study involving two or three scans in an adult.
Although most of the quantitative estimates of the radiation-induced cancer risk are derived from analyses of atomic-bomb survivors, there are other supporting studies, including a recent large-scale study of 400,000 radiation workers in the nuclear industry who were exposed to an average dose of approximately 20 mSv (a typical organ dose from a single CT scan for an adult). A significant association was reported between the radiation dose and mortality from cancer in this cohort (with a significant increase in the risk of cancer among workers who received doses between 5 and 150 mSv); the risks were quantitatively consistent with those reported for atomic-bomb survivors.
The situation is even clearer for children, who are at greater risk than adults from a given dose of radiation , both because they are inherently more radiosensitive and because they have more remaining years of life during which a radiation-induced cancer could develop.
In summary, there is direct evidence from epidemiologic studies that the organ doses corresponding to a common CT study (two or three scans, resulting in a dose in the range of 30 to 90 mSv) result in an increased risk of cancer. The evidence is reasonably convincing for adults and very convincing for children.
No comments:
Post a Comment