Dosimetry
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Dosimetry is the measurement of absorbed dose in matter and tissue resulting from the exposure to ionizing radiations.
More specifically, it is a scientific field, closely related to medical physics, that is involved in the calculation of doses delivered in radiation therapy.
Dose is measured in grays (Gy) for matter or sieverts (Sv) for biological tissue, where 1 Gy or Sv = 1 joule per kilogram. Unfortunately, non-SI units are still somewhat prevalent in this field; dose is often measured in rads and dose equivalent in rems, where 100 rad = 1 Gy and 100 rem = 1 Sv.
The distinction between absorbed dose (Gy) and dose equivalent (Sv) is based upon the biological effects of the radiation in question. For different types of radiation, the same absorbed dose (measured in Gy) may have very different biological consequences. Therefore, a radiation weighting factor (denoted wr) has been established, which compares the relative biological effects of various types of radiation. By definition, x-rays and gamma rays are unity, such that 1 Gy = 1 Sv. Values of wr are as high as 20 for alpha particles. For the same absorbed dose, alpha particles are 20 times as biologically potent as x or gamma rays! Dose equivalent is further explored with tissue weighting factors (w t), which account for sensitivities of various organs exposed to a given dose of radiation; after all, not all organs are equally radiosensitive.
Radiation dose refers to the amount of energy deposited in matter and/or biological effects of radiation, and should not be confused with the unit of radioactive activity (becquerel, Bq). Doses can be measured in places other than where the radioactive decay occurred.
The worldwide average background dose for a human being is about 3.5 mSv per year [1], mostly from cosmic radiation and natural isotopes in the earth. The largest single source of radiation exposure to the general public is naturally-occurring radon gas, which comprises approximately 55% of the annual background dose. It is estimated that radon is responsible for 10% of lung cancers in the United States.
There are several ways of measuring doses from ionizing radiation. Workers that come in contact with radioactive substances or use radioactivity routinely carry personal dosimeters. These dosimeters contain materials that can be used in thermoluminescent dosimetry (TLD) or optically stimulated luminescence (OSL). The equipment used in radiotherapy (linear particle accelerator in external beam therapy) is routinely calibrated using ionization chambers.
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