Radiation Dose Limits and Safety

Dose Limits

• Exposure to radiation must be limited for the general public, patients, and radiation workers to minimize the risk of harmful biologic effects.
• Occupational and nonoccupational Effective Dose (EfD) and Equivalent Dose (EqD) limits have been developed for radiation safety for tissues and organs like the eye lens, skin, hands, and feet.
• An EfD limiting system is incorporated into Title 10 of the Code of Federal Regulations, Part 20, a document from the U.S. Office of the Federal Register.

Basis of EfD Limiting System

• The concept of radiation exposure and the associated risk of radiation-induced malignancy serve as the basis for the effective dose limiting system.
• The National Council on Radiation Protection and Measurements (NCRP) Report No. 116 and International Commission on Radiological Protection (ICRP) Report No. 60 are resources for revised recommendations.
• Medical imaging professionals should be familiar with radiation safety guidelines because they share the responsibility for patient safety from radiation exposure and are subject to radiation exposure while performing their duties.
• The maximum limit for occupational dose is 50 mSv

Radiation Protection Standards Organizations

• 4 Advisory organizations are responsible for evaluating the relationship between radiation EqD and induced biologic effects, and also provide recommendations for radiation protection.
• The 4 Advisory organizations are:
  • International Commission on Radiological Protection (ICRP)
  • National Council on Radiation Protection and Measurements (NCRP)
  • United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)
  • National Academy of Sciences/National Research Council Committee on the Biological Effects of Ionizing Radiation (NAS/NRC-BEIR)

Advisory Organizations Functions

• National Academy of Sciences/National Research Council Committee on the Biological Effects of Ionizing Radiation (NAS/NRC-BEIR):
  • Studies biologic effects of ionizing radiation and risk assessment and provides information to organizations such as the ICRP for evaluation.
• United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR):
  • Evaluates human and environmental ionizing radiation exposure and derives radiation risk assessments from data and research, providing information to organizations such as the ICRP for evaluation.
• International Commission on Radiological Protection (ICRP):
  • Evaluates information on biologic effects of radiation and provides radiation protection guidance through general recommendations on occupational and public dose limits.
• National Council on Radiation Protection and Measurements (NCRP):
  • Reviews regulations formulated by the ICRP and decides ways to include those recommendations in U.S. radiation protection criteria.

U.S. Regulatory Agencies

• Responsible agencies enforce radiation protection standards for the general public, patients, and occupationally exposed personnel.
• 5 Regulatory agencies:
  • Nuclear Regulatory Commission (NRC)
  • Agreement states
  • Environmental Protection Agency (EPA)
  • U.S. Food and Drug Administration (FDA)
  • Occupational Safety and Health Administration (OSHA)
• Nuclear Regulatory Commission (NRC):
  • Oversees the nuclear energy industry, enforces standards, publishes rules in Title 10 of the US Code of Federal Regulations, and has agreements with state governments.
• Agreement States:
  • Enforce radiation protection regulations through their respective health departments.
• Environmental Protection Agency (EPA):
  • Facilitates development and enforcement of regulations in the environment to control radiation
• US Food and Drug Administration (FDA):
  • Conducts an ongoing product radiation control program
  • Regulates the design and manufacture of electronic products, including x-ray equipment
• Occupational Safety and Health Administration (OSHA):
  • Functions as a monitoring agency in places of employment, predominantly in industry.
  • Ensures the safety of employees

Radiation Safety Program

• Facilities must have an effective and detailed radiation safety program to ensure adequate safety.
• The NRC mandates that facilities establish a radiation safety committee (RSC).
• Functions of the radiation safety committee (RSC):
  • Provides guidance
  • Facilitates ongoing operation of the program, and selects a qualified person to serve as a radiation safety officer (RSO)

Radiation Safety Officer (RSO)

• Responsibilities of an RSO:
  • Overseeing the program's daily operation
  • Providing for formal review of the program
  • Maintaining radiation-monitoring records for all personnel
  • Being available to provide counseling for individuals
• Typical RSO's include:
  • Medical physicist
  • Health physicist
  • Radiologist
  • Other individuals qualified through adequate training and experience
• RSO must be approved by the NRC and the state.

As Low as Reasonably Achievable (ALARA) Concept

• The ALARA, put forth in 1954 by the NCRP, refers to the "continuation of good radiation protection programs and practices which traditionally have been effective in keeping the average and individual exposures for monitored workers well below the limit".
• Medical imaging personnel and radiologists share the responsibility to keep occupational and nonoccupational dose limits ALARA.
• EfDs and EqDs should be well below maximal allowable levels.

Model for the ALARA Concept

• The ALARA concept represents a conservative model regarding the relationship between ionizing radiation and potential risk.
• The relationship is assumed to be completely linear nonthreshold and it is safer to overestimate rather than underestimate the risk of injury.

Categories of Radiation-Induced Responses

• Somatic effects are biologic effects of ionizing radiation.
• Severity of effect is directly proportional to the dose and have a threshold.
• Effects are only after large doses, or from long-term individual low doses.
• Effects can be early or late tissue reactions.

Early and Late Tissue Reactions

• Nonlinear threshold
• Early Tissue Reactions:
  • Erythema
  • Blood changes like decrease in white blood cell count (leukopenia).
  • Decrease in lymphocytes or platelets.
  • Epilation, or loss of hair.
• Severe Early Tissue reactions include ARS subsyndromes:
  • Hematopoietic
  • Gastrointestinal
  • Cerebrovascular
• Late Tissue Reactions:
  • Cataract formation
  • Fibrosis
  • Organ atrophy
  • Loss of parenchymal cells
  • Reduced fertility
  • Sterility caused by a decrease in reproductive cells

Stochastic Effects

• Can be somatic or genetic, linear nonthreshold, and randomly occurring biologic changes.
• Probability of effect is directly proportional to the dose.
• Examples include:
  • Cancer
  • Genetic alterations
  • Mutagenesis (irradiation of DNA of somatic cells leading to abnormalities in new cells as they divide in that individual)

Current Radiation Protection Philosophy

• Both genetic and somatic responses to ionizing radiation are considered in developing EfD limiting recommendations.
• The current philosophy assumes a linear nonthreshold relationship between radiation dose and biologic response (ALARA).
• The EfD limiting system controls the risk of biologic damage to radiation workers and the general public.
• The effective dose limit (EDL) is the upper-boundary dose with negligible risk of bodily injury or hereditary damage.

EfD Limiting System

• Limits may be expressed for whole-body exposure, partial-body exposure, and exposure of individual organs.
• Upper limits minimize the risk to humans in terms of early and late tissue reactions and stochastic effects; limits do not include natural background and medical radiation.
• Upper boundary radiation exposure limits for occupationally exposed persons are associated with risks similar to those in other industries such as manufacturing, trade, or government, which are safe.

Vulnerability of the Embryo-Fetus to Radiation Exposure

• Embryo-fetus in utero is particularly sensitive to radiation exposure.
• Epidemiologic studies of atomic bomb survivors showed a dose-dependent increase in severe intellectual disability for fetal doses greater than approximately 0.4 Sv.
• The greatest risk for radiation-induced intellectual disability occurred when the embryo-fetus was exposed 8 to 15 weeks after conception.

Current NCRP Recommendations

• A. Occupational Exposures:
  • EfD Limits:
    • Annual: 50 mSv
    • Cumulative: Age in years x 10 mSv
    • Lens of eye: 150 mSv
    • Localized areas of the skin, hands, and feet: 500 mSv
• B. Embryo and Fetus Exposures: - Monthly: 0.5 mSv - Entire gestation: 5 mSv
• C. Negligible individual dose (annual): 0.01 mSv
• D. Public Exposures (annual): - EfD limit, continuous or frequent exposure: 1 mSv - EfD limit, infrequent exposure: 5 mSv - Lens of eye: 15 mSv - Localized areas of the skin, hands, and feet: 50 mSv
• E. Education and training exposures (annual): - EfD limit: 1 mSv - Lens of eye: 15 mSv - Localized areas of the skin, hands, and feet: 50 mSv