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Chapter 15 Radiation Protection

THE ORIGINS OF RADIATION PROTECTION At the Second International Congress of Radiology in 1928 to prepare x-ray protection recommendations, British recommendations were adopted. The 1928 congress set up the International X-Ray and Radium Protection Committee. after World War II was remodeled into two commissions that survive to this day: The International Commission on Radiological Protection (ICRP) The International Commission on Radiation Units and Measurements (ICRU) The U.S. representative Dr. Lauristor Taylor set up a national Advisory Committee on X-Ray and Radium Protection . In 1946, it was renamed the National Council on Radiation Protection and Measurements (NCRP) NCRP reports still form the basis of radiation protection policy in the United States today

ORGANIZATIONS The organization of radiation protection and the interrelation of the various committees : 1: summarize and analyze data and suggest risk estimates for radiation-induced cancer and hereditary effects. 2: there are the committees that formulate the concepts for use in radiation protection and recommend maximum permissible levels.

ORGANIZATIONS Summarize and analyze data: Formulate the concepts : United Nations Scientific Committee on the Effects of Atomic Radiation, usually known as UNSCEAR. United States committee is appointed by the National Academy of Sciences and is now known as the BEIR (Biological Effects of Ionizing Radiations) Committee. These committees are “scholarly” committees in the sense that if information is not available on a particular topic, they do not feel compelled to make a recommendation Example: both committees declined to choose a value for the dose-rate effectiveness factor for carcinogenesis in the human (for which there are no data) and simply described a range of 2 to 10 based on animal studies. Formulate the concepts : At the international level there is the International Commission on Radiological Protection (ICRP), which (together with the International Commission on Radiation Units and Measurements [ICRU]) ICRP often takes the lead in formulating concepts in radiation protection and in recommending dose limits it has no jurisdiction over anyone and can do no more than recommend; it has established considerable credibility

ORGANIZATIONS In the United States, there is the NCRP (also mentioned above), NCRP often, but not always, follows the lead of ICRP In the United States, the Environmental Protection Agency (EPA) has responsibility for providing guidance to federal agencies; it is the EPA that sets, for example, the action level for radon.

QUANTITIES AND UNITS Quantity used to measure the “amount” of ionizing radiation is the absorbed dose Is defined as the energy absorbed per unit mass Unit : Joules per Kilogram, special name : Gray (Gy) Unit used in the past : rad (Radiation Absorbed Dose) Defined as an energy absorption of 100 erg/g. Consequently, 1 Gy equals 100 rad

QUANTITIES AND UNITS Equivalent Dose Effective Dose is the product of the absorbed dose averaged over the tissue or organ and the radiation weighting factor selected for the type and energy of radiation involved Equivalent dose = absorbed dose × radiation weighting factor absorbed dose →gray : equivalent dose→sievert (Sv) absorbed dose→rad : equivalent dose→rem (Rad Equivalent Man) Effective Dose If the body is uniformly irradiated, the probability of the occurrence of stochastic effects is assumed to be proportional to the equivalent dose Sometimes, equivalent doses to various tissues differ substantially, different tissues vary in their sensitivities to radiation-induced stochastic effects ICRP introduced the concept of the tissue weighting factor (WT): relative contribution of each tissue or organ to the total detriment resulting from uniform irradiation of the whole body The sum of all of the weighted equivalent doses in all the tissues or organs irradiated is called the effective dose.

QUANTITIES AND UNITS Committed Equivalent Dose for irradiation from internally deposited radionuclides, the total absorbed dose is distributed over time, as well as to different tissues in the body The dose rate falls off, depending on the physical and biologic half-lives of the radionuclide. To take into account the varying time distributions of dose delivery ICRP defined a C.E.D as the integral over 50 years of the equivalent dose in a given tissue after intake of a radionuclide was chosen to correspond to the working life of a person. Committed Effective Dose If the committed equivalent doses to individual organs or tissues are multiplied by the appropriate tissue weighting factors and then summed → committed effective dose.

QUANTITIES AND UNITS Collective Equivalent Dose the collective equivalent dose is the product of the average equivalent dose to a population and the number of persons exposed SI system of units : man-sievert person-sievert, (The old unit was the man-rem.) Collective Effective Dose product of the average effective dose to a population and the number of persons exposed. Unit : person-sievert (formerly man-rem)

QUANTITIES AND UNITS These collective quantities : Collective Committed Effective Dose If a population ingesting or inhaling radionuclides that deposit their dose over a prolonged period of time : the integral of the effective dose over the entire population out to a period of 50 years is called the collective committed effective dose. These collective quantities : can be thought of as representing the total consequences of exposure of a population or group, and they can be thought of as surrogates for “harm.” are much beloved by the bureaucrats because they make it possible to compare different activities or accidents assumes proportionality between dose and biologic effect, which is seldom true. are used widely to give a rough guide to the probability of cancer and hereditary effects in a population exposed to radiation

Summary of Quantities and Units The concept of collective effective dose : allow a rough and quick estimate to be made of the potential health hazards to a population from. Example: an accidental release of radioactivity from a nuclear reactor. these concepts can be used only under conditions in which : reasonable to assume linearity between risk and dose That is risks are directly proportional to the summation of doses from different sources

Summary of Quantities and Units

AIMS AND OBJECTIVES OF RADIATION PROTECTION As stated by the NCRP: 1: to prevent clinically significant radiation-induced deterministic effects by adhering to dose limits that are below the apparent or practical threshold 2: to limit the risk of stochastic effects (cancer and hereditary effects) to a reasonable level in relation to societal needs, values, and benefits gained difference in shape of the dose-response relationships for deterministic and stochastic effects is illustrated

AIMS AND OBJECTIVES OF RADIATION PROTECTION Justification of exposure is one of the basic principles of radiation protection A practice involving exposure to radiation should produce sufficient benefit to the exposed individual or to society to offset the radiation detriment it causes This concept is sometimes difficult to put into practice in the variety of situations in which individuals are exposed: In the case of patients, the diagnostic or therapeutic benefit should outweigh the risk of detriment. In the case of occupational exposure, the radiation risk must be added to and compared with other risks in the workplace. The most difficult situation is exposure for the sake of research, where volunteer subjects may fall into one of three categories: patients who may benefit, patients who may receive no benefit, and healthy volunteers

BASIS FOR EXPOSURE LIMITS the concept of a tolerance dose was used, a dose to which workers could be exposed continuously without any evident deleterious acute effects, such as erythema of the skin. Exposure standards are necessarily based partly on observed effects, but with a great deal of judgment involved LIMITS FOR OCCUPATIONAL EXPOSURE Stochastic Effects No occupational exposure should be permitted until the age of 18 years. The effective dose in any year should not exceed 50 mSv (5 rem). The individual worker's lifetime effective dose should not exceed age in years × 10mSv(1rem). Deterministic Effects 150 mSv (15 rem) per year for the lens of the eye. 500 mSv (50 rem) per year for localized areas of the skin and the hands and feet.

Summery of Recommended Dose Limits

AS LOW AS REASONABLY ACHIEVABLE ALARA AS LOW AS REASONABLY ACHIEVABLE The dose limits referred to previously are all upper limits and subject to the concept of ALARA Radiation is potentially harmful, and exposure to it should be monitored continually and controlled. No unnecessary exposure should be allowed. Equipment and facilities should be designed so that exposure of personnel and the public is kept to a minimum and not up to a standard.

PROTECTION OF THE EMBRYO/FETUS NCRP : a monthly limit of 0.5 mSv (50 mrem) to the embryo or fetus once a woman declares her pregnancy. ICRP : a limit of 2 mSv (200 mrem) to the surface of the woman's abdomen (lower trunk) for the remainder of the pregnancy EMERGENCY OCCUPATIONAL EXPOSURE Under normal conditions, only actions involving the saving of life justify acute exposures in excess of the annual effective dose limit. NCRP and ICRP recommendation of 0.5 Sv (50 rem) should be applied EXPOSURE OF PERSONS YOUNGER THAN 18 YEARS OF AGE an annual effective dose limit of 1 mSv (100 mrem) should be maintained (NCRP) EXPOSURE OF MEMBERS OF THE PUBLIC (NONOCCUPATIONAL LIMITS) everyone is exposed to natural background radiation of about 1 mSv (100 mrem) annually, excluding radon, which may result in a mortality risk of 10-4 to 10-5 annually. NCRP : limits for human-made sources other than medical : frequent exposure : the annual effective dose should not exceed 1 mSv (100 mrem). infrequent exposure :a maximum annual effective dose equivalent of 5 mSv (500 mrem) for calculation of effective dose, the hands and feet, localized areas of the skin, and the lens of the eye are also subject to an annual dose limit of 50 mSv (5 rem).

EXPOSURE TO INDOOR RADON Radon levels vary enormously with different localities, depending on the composition of the soil and the presence of cracks or fissures in the ground, which allow radon to escape to the surface. Many homes in the United States and Europe consequently contain an appreciable quantity of radon gas. enters the living quarters through the basement. Insulating and sealing houses increased greatly as a result of the escalating cost of heating oil in the 1970s In a confined space such as a basement, the decay of radon leads to the accumulation of progeny that are solids, which stick to particles of dust or moisture and tend to be deposited on the bronchial epithelium. These progeny emit α-particles and cause intense local irradiation. The EPA has set the “action level” at about 148 Bq/m3 (4 pCi/L) Of the deaths that can be attributed to radon, perhaps one third could be avoided by reducing radon in homes in which it is above the “action level” of 148 Bq/m3 (4 pCi/L) recommended by the EPA

DE MINIMIS DOSE AND NEGLIGIBLE INDIVIDUAL DOSE Collective dose to a population has little meaning without the concept of de minimis dose. The idea is to define some very low threshold below which it would make no sense to make any additional effort to reduce exposure levels further The term de minimis comes from the legal saying De minimis non curat lex, which roughly translates to “The law does not concern itself with trifles.” Concept (by NCRP) : negligible individual dose. defined here : The dose below which further efforts to reduce radiation exposure to the person are unwarranted. The NCRP considers an annual effective dose of 0.01 mSv (1 mrem) to be a negligible individual dose RISKS ASSOCIATED WITH CURRENT RECOMMENDED LIMITS The possible deleterious effects of long-term occupational exposure to radiation include: reduction of life expectancy (a combination of the probability of developing a fatal cancer and the number of years lost if it occurs) morbidity, that is, decreased quality of life, associated with nonfatal cancers and hereditary effects. The ICRP has coined the term detriment to cover all of these effects. The total detriment (life lost and quality of life impaired) for adult radiation workers amounts to about 4.7 × 10-2 per sievert. the average annual equivalent dose to monitored radiation workers with measurable exposures is about 2 mSv (200 mrem), which results in a total detriment per year of less than 2 × 10-4.

NCRP AND ICRP COMPARED Both bodies recommend a maximum of 50 mSv (5 rem) in any 1 year but : NCRP adds a lifetime cumulative limit of the person's age × 10 mSv (1 rem) ICRP adds a limit of 20 mSv (2 rem) per year averaged over defined periods of 5 years. Under NCRP recommendations, a new radiation worker could receive 50 mSv (5 rem) in each of several consecutive years until the limit of age × 10 mSv (1 rem) kicks in. Under ICRP rules, the average cannot exceed 20 mSv (2 rem) per year over a 5-year period, so one or two 50-mSv (5-rem) years would have to be followed by several years at very low exposure levels. Under the NCRP, a person occupationally exposed from 18 to 65 years of age could receive a total dose of 650 mSv (65 rem). Under the ICRP, the same person could receive 940 mSv (94 rem), but less would be received in the early years and more at later ages