When a cancer patient is treated with radiation in a radiotherapy clinic, the tumour dies after absorbing a dose of more than 40 sieverts. During the treatment, healthy tissue and organs near the tumour get an incidental dose of some 20 sieverts, which is 20,000 times the recommended annual limit and at least five times the dose that proved fatal at Chernobyl.
How can tissue survive this friendly fire? A radiation dose is the same in principle, whether received in a hospital or elsewhere. But the critical point is that the therapeutic dose is spread over four to six weeks, giving cells time to repair the damage. Each day the healthy cells receive about 1 sievert, and just manage to repair themselves. The tumour cells receive a higher dose, and just fail to do so.
Allison goes on to advocate a change to radiation safety recommendations. Current international recommendations are "1 millisievert per year above the natural background level of about 2.5 millisieverts per year." The author suggests more granularity in the timespan portion of the limit, and a much higher overall limit: "I suggest the upper limit might be reset at a lifetime total of 5 sieverts, at no more than 0.1 sievert per month. That would be a fraction of a radiotherapy dose, spread over a lifetime."
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