Thursday, May 10, 2012

On Why 20 Milliseiverts a Year is Not "Safe," Nor 6 Millisieverts

Schmidt CW 2012. CT Scans: Balancing Health Risks and Medical Benefits. Environ Health Perspect 120:a118-a121.

[excerpts] "CT scanners emit X rays. Different tissue types absorb X rays in varying amounts, and the resulting contrasts provide detailed images of anatomy and disease. Absorbed radiation can break chemical bonds in tissues, liberating charged ions (hence the term “ionizing radiation”) that can damage DNA and produce cancer should cells be unable to repair themselves. Nonionizing radiation—lower-energy radiofrequency waves such as those emitted by microwave ovens and cell phones—doesn’t break chemical bonds....

...The dominant risk assessment model appears in a 2006 report from the National Research Council’s Biological Effects of Ionizing Radiation (BEIR) subcommittee.6 The BEIR VII model postulates there is no safe level of ionizing radiation exposure; carcinogenic effects are assumed to follow a linear dose response, meaning even the smallest exposure carries some level of cancer risk. The BEIR VII model generates so-called lifetime attributable risk (LAR) factors, which estimate the likelihood of cancer in hypothetical individuals as a function of dose. Multiplying the LAR by the number of people exposed to a given dose yields an estimate of expected cancers from that exposure in the population....

The AAPM’s position statement asserts that cancer risks are negligible at effective doses below 50 millisieverts (mSv) for single CT exposures and below 100 mSv for multiple exposures over short durations. But a 2003 paper coauthored by David Brenner, director of the Center for Radiological Research at Columbia University School of Public Health, claims that the atom-bomb survivor data work well for low-dose extrapolation because they’re drawn from large cohorts with well-defined exposures and complete followup.9  And those data, Brenner says, show a statistically significant trend of increasing cancer risk with increasing organ dose between 5 and 100 mSv. Smith-Bindman echoes his conclusions, arguing that cancer risks are established even at effective doses of 10 mSv...

Brenner DJ, et al. Cancer risks attributable to low doses of ionizing radiation: assessing what we really know. Proc Natl Acad Sci USA 100(24):13761–13766. 2003.​0 

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