Radiation exposure
The absorbed dose is the how much energy that ionizing radiation deposits in a material. The absorbed dose will depend on the type of matter which absorbs the radiation. For an exposure of 1 roentgen by gamma rays with an energy of 1 MeV, the dose in air will be 0.877 rad, the dose in water will be 0.975 rad, the dose in silicon will be 0.877 rad, and the dose in averaged human tissue will be 1 rad. "rad" stands for radiation absorbed dose. This is a special dosimetric quantity used to assess the dose from radiation exposure. Another common measurement for human tissue is Gray (Gy, International or SI unit). The reference for this sentence has a table that gives the exposure to dose conversion for these four materials. The amount of energy deposited in human tissue and organs is the basis for the measurements for humans. These doses are then calculated into radiation risk by accounting for the type of radiation, as well as the different sensitivity of organs and tissues. To measure the biological effects of radiation on human tissues, effective dose or dose equivalent is used. The dose equivalent measures the effective radiation dosage in a specific organ or tissue. The dose equivalent is calculated by the following equation:
Dose equivalent = Absorbed dosage x Tissue weighting factor
Tissue weighting factor reflects the relative sensitivity of each organ to radiation. The effective dose refers to the radiation risk averaged over the entire body. It is the sum of the equivalent dosage of all exposed organs or tissues. Equivalent dose and effective dose are measured in sieverts (Sv). For example, suppose a person's small intestine and stomach are both exposed to radiation separately. The absorbed dose of small intestine is 100 mSv and the absorbed dose of stomach is 70 mSv. The tissue weighting factors of various organs are listed in the following table:
The dose equivalent of small intestine is:
Dose equivalent = 100 mSv x 0.12 = 12 mSv
The dose equivalent of stomach is:
Dose equivalent = 70mSv x 0.04 = 2.8 mSv
The effective dose would then equal dose equivalent (small intestine) + dose equivalent (stomach) = 12mSv + 2.8mSv = 14.8mSv. This risk of harmful effects from this radiation is equal to 14.8mSv received uniformly throughout the whole body. Ionizing radiation is known to cause the development of cancer in humans. Our understanding of this comes from observation of cancer incidence in atomic bomb survivors. The Life-Span Study (LSS) is a long-term study of health effects in Japanese atomic bomb survivors. Also, increased incidence of cancer has been observed in uranium miners. It is also seen in other medical, occupational, and environmental studies. This includes medical patients exposed to diagnostic or therapeutic doses of radiation. It also includes persons exposed to environmental sources of radiation including natural radiation. In the LSS, 105,427 individuals (out of about 325,000 civilian survivors) were followed from 1958 through 1998. During this time, 17,448 cancers were diagnosed. The baseline predicted cancer incidence or number of new cancers is about to 7,000. 850 of these cancers were diagnosed in individuals with estimated doses greater than 0.005 Gy. In other words, they were due to the atomic bomb radiation exposure, which is 11% or 1 in 10 of the cancers diagnosed. The population was defined as those selected to include three major groups of registered Hiroshima and Nagasaki residents:
(1) atomic bomb survivors who were within 2.5 km of the hypocenter at the time of the bombings (ATB),
(2) survivors who were between 2.5 and 10 km of the hypocenter ATB (low- or no-dose group), and
(3) residents who were temporarily not in either Hiroshima or Nagasaki or were more than 10 km from the hypocenter in either city (NIC) at the time of the bombings (no-exposure group). Overall, individuals were exposed to a wide dose range (from less than 0.005 Gy to 4 Gy). There is also a wide range in age. About 45,000 people were exposed to 0.005 Gy or 5mSv. The study shows a linear dose response for all solid tumors. This means the relationship between dose and human body response is a straight line. To see an example, look at the graph titled "Linear graph." Linear dose response also means that the rate of change of human body response is the same at any dose. The International Commission on Radiological Protection (ICRP) describes how deterministic effects, or harmful tissue reactions, occur. There is a threshold dose which causes clinical radiation damage of cells in the body. As the dose increases, the severity of injury increases. This also impairs tissue recovery. The IRCP also describes how cancer develops following radiation exposure. This happens via DNA damage response processes. In recent decades, there have been increased cellular and animal data that supports this view. However, there is uncertainty at doses about 100 mSv or less. It is possible to assume that the incidence of cancer will rise with the equivalent dose in the relevant organs and tissues. Thus, the Commission bases recommendations on this assumption. Doses below this threshold of 100 mSv will produce a direct increase in probability of incurring cancer. This dose-response model is known as ‘linear-non-threshold’ or LNT. To see the model, please see dashed line in the graph "Dose response curve of linear-non-threshold model". Because of this uncertainty at low doses, the Commission does not calculate the hypothetical number of cancer cases. The risk of low dose radiation in medical imaging is unproven. It is difficult to establish risks associated with low dose radiation. One reason why is that a long period of time occurs from exposure to radiation and the appearance of cancer. Also, there is a natural incidence of cancer. It is difficult to determine whether increases in cancer in a population are caused by low dose radiation. Lastly, we live in environment where other powerful carcinogens may affect the results of these studies. This includes chemicals, pollutants, cigarette smoke, and more. See table for effective doses from common medical diagnostic imaging exams. This compares to background levels of radiation. Background radiation is from naturally radioactive materials and cosmic radiation from space. People are exposed to this radiation from the environment continuously, with an annual dose of about 3 mSv. Radon gas is a radioactive chemical element that is the largest source of background radiation, about 2mSv per year. This is similar to a head CT (see table). Other sources include cosmic radiation, dissolved uranium and thorium in water, and internal radiation (humans have radioactive potassium-40 and carbon-14 inside their bodies from birth). Aside from medical imaging, other man-made sources of radiation include building and road construction materials, combustible fuels, including gas and coal, televisions, smoke detectors, luminous watches, tobacco, some ceramics, and more in the reference. Below is an example from the US Nuclear Regulatory Commission on how different types of food contain small amounts of radiation. The sources of radiation are radioactive potassium-40 (40K), radium-226 (226Ra), and other atoms: The embryo and fetus are considered highly sensitive to radiation exposure. The highest risk of lethality is during the preimplantation period. This is up to day 10 postconception. Malformations generally occur after organogenesis. This is the phase of development where the three germ layers (the ectoderm, endoderm, and mesoderm) form the internal organs of the fetus. The estimated dose threshold is 0.1 Gylow-linear-energy-transfer (LET) radiation, and this period generally occurs from day 14-50. Animal data supports the idea that malformations are induced at a dose of around 100 mGy. Another risk is reduction of intelligence quotient (IQ). The most sensitive period is weeks 8–15 postconception. IQ reduces by 30 IQ points/Sv, which can lead to severe mental retardation (SMR). Malformations begin to occur at a dose threshold of at least 300 mGy. Cancer can also be induced by irradiation, which generally occurs from day 51-280 of pregnancy. Most X-rays occur during the third trimester of pregnancy. There is sparse information on radiation exposure from the first trimester of pregnancy. However, data suggests that the relative risk is 2.7. Relative risk is a measure of probability of an outcome in one group versus the other. In this case, the risk of cancer formation in the first trimester is 2.7 times higher than the risk of cancer formation in the third trimester. In addition, the United Nations Scientific Committee on the Effects of Atomic Radiation calculated excess relative risk in the first trimester. It is 0.28 per mGy. Excess relative risk is the rate of disease in an exposed population divided by the rate of disease in an unexposed population, minus 1.0. This means that the risk of cancer from irradiation in the first trimester is 28% higher than in the third trimester. There are multiple benefits from using radiation from medical imaging. Screening imaging exams are used to catch cancer early, reducing the risk of death. It also reduces the risk of having serious life-limiting medical conditions, and avoiding surgery. These tests include lung cancer screening, breast cancer screening, and more. Radiation is also used as therapy for many different types of cancer. About 50% of all cancer patients receive radiation therapy. Radiation therapy destroys cancer cells, stopping them from growing. Aside from cancer, many types of medical imaging are used to diagnose life-threatening diseases, such as heart attacks, pulmonary embolism, and pneumonia. The gamma ray field can be characterized by the exposure rate (in units of, for instance, roentgen per hour). For a point source, the exposure rate will be linearly proportional to the source's radioactivity and inversely proportional to the square of the distance,
F = Γ×α / r2
where "F" is the exposure rate, "r" is the distance, "α" is the source activity, and "Γ" is the exposure rate constant, which is dependent on the particular radionuclide used as the gamma ray source. Below is a table of exposure rate constants for various radionuclides. They give the exposure rate in roentgens per hour for a given activity in millicuries at a distance in centimeters. The following table shows radiation quantities in SI and non-SI units:
Although the United States Nuclear Regulatory Commission permits the use of the units curie, rad, and rem alongside SI units, the European Union European units of measurement directives required that their use for "public health ... purposes" be phased out by 31 December 1985.
Risk to embryo and fetus
The embryo and fetus are considered highly sensitive to radiation exposure.