Cancer can be triggered by many different influences, including penetrating radiation such as X rays, neutrons, and alpha-beta-gamma rays emitted by radioactive substances. Biological effects of radiation have been studied for nearly one hundred years. The first such effect, but not cancer, was observed on Pierre Curie, husband of Marie Curie. After carrying a strong radioactive source in his watch pocket he noticed a red spot on his skin, below the source.
The probability of cancer depends on the dose of radiation received. The purpose of this article is to describe effects associated with different doses. The unit of dose is Sievert (Sv). It is defined in terms of energy deposited in a body by penetrating radiation. As a nuclear physicist, I was often exposed to penetrating radiation. Each time I carried a small dosimeter of one kind or another. Smaller doses are expressed in millisieverts (mSv). The old unit of dose, rem, is still widely used by doctors (1Sv=100 rem).
People often ask me "How is harmful nuclear radiation?" My answer is always the same, "It depends on the dose received." Then I elaborate, as summarized below.
A dose of 10 Sv will most likely result in death, within a day or two.
5 Sv would kill about 50% of exposed people.
2 Sv can also be fatal, especially without prompt treatment.
0.25 Sv = 250 mSv is the limit for emergency workers in life-saving operations.
0.10 Sv = 100 mSv dose is clearly linked to later cancer risks.
0.05 Sv = 50 mSv is the yearly limit for radiation workers, like myself.
0.004 Sv= 4 mSv is the typical yearly dose due to natural radiation (cosmic rays, etc).
0.003 Sv= 3 mSV is the typical dose from a mammogram.
Cancer is often treated by X rays. This sounds like a paradox; X rays are causing cancer, how can they be used to cure it? Yes, X rays kill living cells and this can lead to cancer. But cancerous cells are killed more rapidly than healthy cells. With a properly chosen dose, for example 0.5 Sv, a radiologist might kill only about 20 healthy cells for each 100 cancerous cells. Several such treatments, for example, one each month, might give a patient an additional two or three years of life, rather than one or less, with no treatment. A dose of 0.5 Sv would never be prescribed to a healthy person. Doses caused by diagnostic uses of X rays are usually much smaller than those received during therapeutic treatments.
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