Lecture 4: Dose Limits for Exposure to Ionizing Radiation

Acronyms/Key Terms:

• Effective Dose Limiting System – EfD

• Effective Dose Limit - EDL

• Cumulative Effective Dose Limit – CumEfD

• Collective Effective Dose – ColEfD

• Quality Factor – Q factor

• Tissue Weighting Factor – WT

• Tissue – T

• Negligible Individual Dose – NID

• Radiation Safety Officer – RSO

• Radiation Safety Committee - RSC

• Equivalent Dose – EqD

• International System of Units: SI – radiation measurement unit that allows the same units to be used among all branches of science throughout the world

Stochastic Effect

• Are mutational, nonthreshold (independent of dose), randomly occurring biological somatic changes; injury to a single cell or small number of cells

  • Chances of occurrence increase with each radiation exposure

    • Cancer

    • Genetic (hereditary) alterations

      • Can occur months or years after high or possibly low exposure

Tissue reactions

• Biological effects of ionizing radiation that can be directly related to the dose received

  • There is a threshold dose below which nothing will occur, and above which the severity of the damage increases as the dose increases

    • i.e., a certain dose of radiation will cause a skin reaction; the higher the dose goes, it will cause a skin reaction to be worse, but a dose below the threshold will not cause a skin reaction

    • Reactions or early or late

      • Tissue reactions typically occur only after large doses of radiation, but can also occur as a result of long-term individual low doses of radiation sustained over several years

      • Can be immediately after radiation or after some time has gone by

• 4 groups that are responsible for evaluating the relationship between equivalent dose (EqD) and induced biological effects: (advisory groups)

• They investigate risk assessments of somatic and genetic effects of radiation

• International Commission on Radiological Protection (ICRP)

• National Council on Radiation Protection and Measurement (NCRP)

• United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)

• National Academy of Sciences/National Research Council Committee on Biological Effects of Ionizing Radiation (NAS/NRC-BEIR)

Dose Limit Agencies cont

• International Commission on Radiological Protection (ICRP)

  • Evaluates information on biologic effects of radiation and provides radiation protection guidance through recommendations on occupational and public dose limits

• National Council on Radiation Protection and Measurement (NCRP)

  • Reviews regulations made by the ICRP and decides how to include them in the U.S. radiation protection criteria, provides general recommendations and publishes the recommendations in the form of NCRP reports

• United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)

  • Evaluates ionizing radiation exposure from a variety of sources and derives radiation risk assessment from data and research and provides information to ICRP for evaluation (radiation risk for radiation-induced cancer and genetic effects)

• National Academy of Sciences/National Research Council Committee on Biological Effects of Ionizing Radiation (NAS/NRC-BEIR)

  • Reviews studies of biologic effects of ionizing radiation and risk assessment and provides information to ICRP for evaluation

Regulatory agencies

• Regulations are mandatory requirements that can apply to individuals, businesses, state or local governments, non-profit institutions, and others

• The regulations provide the cornerstone the radiation oncology departments QI (Quality Improvement) plan

• The radiation department must be familiar with the national, state and professional regulations that affect the departments operations

• Federal and state government, professional, and accreditation agencies mandate standards to ensure all equipment is functional and operates safely and that the operators of the equipment are truly qualified

Regulatory Agencies:

• NRC – Nuclear Regulatory Commission

  • Agreement states

• EPA - Environmental Protection Agency

• FDA - U.S. Food and Drug Administration

• OSHA - Occupational Safety and Health Administration

NRC job:

• oversees nuclear industry

• enforces radiation protection standards

• publishes its rules and regulations in Code of Federal Regulations

• enters into written agreements with state governments allowing them to license and regulate the use of radioisotopes

NRC requirements include:

• Dose limits to radiation workers and members of the public

• Exposure limits for individual radionuclides

• Monitoring and labeling of radioactive materials

• Posting of signs in and around radiation areas

• Reporting of theft or loss of radioactive material

• Penalties for noncompliance with NRC regulations

NRC agreement state

• An NRC Agreement State is a U.S. state that has entered into a formal agreement with the U.S. Nuclear Regulatory Commission (NRC) to assume regulatory authority over most forms of radioactive materials within their state.

• This allows states to regulate radioactive materials that would otherwise fall under the NRC's jurisdiction, creating a single regulatory authority for radiation sources within the state.

• However, the NRC retains oversight of nuclear power plants, high-level waste, and other specific facilities, while Agreement States handle licensing, inspection, and enforcement for other radioactive materials.

NRC agreement state cont:

• State assumes responsibility for enforcing radiation protection regulations through their respective health departments

• Agreement states can enforce their own radiation safety regulations along with the NRC regulations

• Non-agreement states – the state and the NRC enforce radiation protection regulations by sending NRC agents to health care facilities to inspect

• For agreement and non-agreement states, NRC assumes sole responsibility for inspections of nuclear reactors and their adherence to federal regulations

Key characteristics of agreement states

• Assumed Authority:

  • Agreement States regulate byproduct, source, and certain special nuclear materials used by medical, industrial, and academic institutions.

• Integrated Oversight:

  • States can consolidate oversight of radioactive materials with their existing jurisdiction over other radiation sources, such as X-ray machines.

• State-Run Programs:

  • States are responsible for issuing licenses, setting fees, and enforcing regulations, though the NRC provides technical training and periodic reviews.

• Benefits:

  • States gain control over licensing fees and bring regulation closer to their constituents. The NRC benefits by leveraging state expertise and resources, allowing the agency to focus on areas of higher national concern.

EPA (Environmental Protection Agency)

• facilitates the development and enforcement of regulations pertaining to the control of radiation in the environment

  • Protecting the health of humans and the natural environment from industrial practices and harmful waste disposal

  • Directs federal agencies

  • Oversees the general area of environmental monitoring

  • Determines the level of action for radon

FDA (U.S. Food and Drug Administration)

• Conducts an ongoing radiation control program, regulating the design of electronic products like X-ray machines, and protects us against faulty manufacturing

  • Conducts on-site inspections of X-ray equipment

OSHA (Occupational Safety and Health Administration)

• functions as a monitoring agency in places of employment in industry through Part 1910 of Title 29 of the US Code of Federal Regulations (29 CFR 1910)

  • Regulates occupational exposure

  • Regulates training programs in the work place

  • Ensures “employees right to know” with regards to hazards in the work place and covers:

    • Hazardous substances

    • Infectious agents

    • Ionizing radiation

    • Nonionizing radiation

• OSHA requires employers to evaluate their work places for harmful agents and to provide training and written information to their employees

U.S. Department of Transportation (DOT) (Also works with the NRC)

Regulate and ensure that hazardous materials are moved safely by all forms of transportation

Radiation safety program

• All radiation therapy facilities must have a radiation safety program to ensure the safety of patients and radiation workers

  • Administration will:

    • Make sure there is a budget for radiation safety

    • Oversee the development of the policies and procedures

    • Provide equipment necessary to begin and maintain the program

Radiation Safety Committee (RSC)

• The NRC mandates that a RSC be established for a facility

  • RSC provides guidance for the program and facilitates its ongoing operations.

  • Serves as the governing body, establishing those policies and providing oversight to ensure compliance with regulations.

    
    

Radiation Safety Officer (RSO)

• Will be appointed to oversee the program’s daily operations and provide a formal annual review of the program

  • RSO is usually a Medical or health physicist, radiologist or an individual with adequate training

  • Is appointed by the facility and approved by the state and NRC

Responsibilities of the RSO:

• Developing a radiation safety program that is in compliance with the internationally accepted guidelines for radiation safety

• Makes sure that all are protected from unnecessary exposure, especially pregnant individuals

• The RSO maintains and monitors the radiation exposure records for all personnel and provides counseling when needed (declared pregnant workers and if individuals exceed allowable limits)

What does the NRC require for the RSO?

• The RSO should be given the authority to stop operations that are considered unsafe

• The NRC requires the name of the RSO be put on the radioactive material license to ensure a qualified individual can directly interact with the NRC during inspections and if there are any questions regarding the radiation safety program of a facility

• Usually the RSO is a full-time employee of the licensed facility.

Consumer –Patient Radiation Health and Safety Act of 1981 (title IX of Public Law 97-35)

• The primary goal was to ensure the competence of those administering radiologic procedures through education and training, thereby safeguarding the public from excessive and unnecessary radiation exposure from diagnostic and therapeutic procedures.

1. Requires the establishment of minimal standards for the accreditation of education programs for persons who perform radiologic procedures and certification of such persons

2. insures that medical and dental radiologic procedures are consistent with rigorous safety precautions and standards.

Not all states have adhered to this standard since there are no penalties for noncompliance (state of MO being one of those)

What is EfD?

• a dose quantity in the International Commission on Radiological Protection (ICRP) system of radiological protection

• EfD is an attempt to provide a quantity that is a measure of general harm in humans.

• It takes into account the amount of absorbed dose (The amount of energy that is deposited in a material per unit mass of the material), that is received by a human, the exact type of radiation (alpha, beta, protons, neutrons, are all somewhat different at the same absorbed dose levels), and the specific organs or organ systems irradiated.

EfD units

• EfD is the most effective way of measuring the biological effects of radiation. The unit of EfD is mSv, a subunit of the sievert.

• Irradiation of internal organs has more consequences than irradiation of extremities. So in deriving the EfD, the contribution of absorbed dose that affects different organs and systems, as well as the type of radiation is considered in determining the EfD.

• Unit of mSv is used for radiation protection purposes

EfD formula

• Effective dose, tolerance dose, conversion of units (rad to Gy in SI units     

• EfD=D x W_r x W_t

Effective dose limit (EDL)

• Is the upper boundary dose of ionizing radiation that results in a negligible risk of:

  • Bodily injury

  • Hereditary damage

• Upper boundary safe radiation exposure limits for occupationally exposed persons are associated with risks that other employees in other industries would encounter

• Industries included:

  • Manufacturing

  • Trade

  • Civil Service

How can EDL be expressed?

• These exposure limits can be expressed in whole body, partial-body or individual organs

• There are separate limits set for occupationally exposed workers and the general public

  • Do not include natural background and medical exposure

Effective dose limiting system (EfD)

system is the current method for assessing radiation exposure and associated risk of biologic damage to radiation workers and the general public, both external and internal

Radiation Risk

Determined from the complete injury caused by radiation exposure

• Taken into account in formulating occupational risk standards are:

  • Potential for terminal cancer

  • Hereditary imperfections triggered by reproductive cell mutations

  • Shortening of life span due to induction of cancer or other abnormalities

  • Overall poorer quality of life

Occupational Risk

• A radiation protection program is designed to ensure the hazards to radiation workers is no greater than the hazard to the general public

  • The program will prevent individual workers from having a total external plus internal cumulative EfD (CumEfD) in excess of their age in years times 10 mSv

    • 10 mSv x Age

  • *CumEfD pertains to the whole body

    • CumEfd = cumulative effective dose limit

• Is the lifetime EfD

• EfD’s do not include: natural background radiation or exposure from an individual receiving a medical procedure

• Does include external and internal exposures

Equivalent dose (EqD)

• Radiation quantity used for radiation protection when a person receives radiation exposure from multiple sources of ionizing radiation, unit is mSv

• EqD formula: EqD=D x Wr

  • Wr is radiation weighting factor, defined by regulation

  • D is absorbed dose

Cumulative effective dose limit (CumEfD)

• Determine the CumEfD limit to the whole body (in SI units) of an occupationally exposed person who is 37 years old

  • EqD = 10 mSv x age (in years)

  • EqD = 10 mSv x 37

  • EqD = 370 mSv

• This represents the lifetime limit for a 37-year-old radiation worker

• The recommended annual EfD for occupationally exposed individuals is 50mSv

• A typical radiation worker will not even approach 50 mSv in any given year

Determine the CumEfD limit to the whole body (in SI units) of an occupationally exposed person who is 40 years old

• EqD = 10 mSv x age (in years)

• EqD = 10 mSv x 40

• EqD = 400 mSv

  • This represents the lifetime limit for a 40 year old radiation worker

• The radiation worker has been employed at a nuclear power plant for 10 years. He had previously been employed as a radiation worker in another industry, in which he received a cumulative EfD of 100 mSv, therefore the radiation protection program for his current positon should have ensured that he has not accumulated a total EfD greater than 300 mSv during his 10 years of employment

Basis for EfD limiting system:

• Any organ (not just those considered critical structures) in the human body is vulnerable to damage from exposure to ionizing radiation

  • Sensitivity of organs varies (TD 5/5 table)

Tissue Weighting Factor (Wt)

• Represents the relative contribution of a specific organ or tissue to the total health detriment from ionizing radiation, accounting for its sensitivity and size, as determined by the International Commission on Radiological Protection (ICRP).

  • Whereas, the EfD limiting system is an attempt to compare the various risks of cancer and genetic effects on the tissues or organs that are exposed to radiation

  • The EfD takes into account that differences in tissue and organ sensitivities; thus the tissue weighting factor is used

  • The Wt factor “indicates the ratio of the risk of stochastic effects attributable to irradiation of a given organ or tissue (T) to the total risk when the whole body is uniformly irradiated”

    • (More about tissue weighting in an upcoming lecture)

There are also EDL’s (effective dose limits) to non-occupationally exposed individuals not undergoing medical procedures:

• Spouse, parent or guardian accompanying a patient

• Annual EDL is 1 mSv for continuous or frequent exposures from artificial sources other than medical irradiation and natural background

• The limit is 5 mSv annually for infrequent exposure to non-occupational individuals

Recommendation for pregnant radiation workers:

• Monthly EqD not to exceed .5 mSv to the embryo and fetus

• Limit during the entire pregnancy not to exceed 5.0 mSv after declaration of a pregnancy

  • Excludes both medical and background radiation

  • Exceeding the recommended limits could result in the fetus having a small head or mental retardation

  • Restricts the total lifetime risk of leukemia and other malignancies

Annual occupational dose limits for tissue reactions, for tissues and organs exposed selectively or together have been set to prevent excessive doses to these organs or tissues:

• 150 mSv to lens of eyes

• 500 mSv for localized areas of the skin, hands and feet

Negligible Individual Dose (NID)

• There is a 0.01 mSv/year per source or practice that has been set

  • An annual EfD that provides a low exposure cut-off level so agencies can dismiss a level of effective dose as being negligible risk; below this level, a reduction of individual exposure is unnecessary

  • The concept of NID is applied in order to deliberately and specifically curtail the radiation exposure risk level of an individual. It provides a lower limit for application of the ALARA process.

Action Limits for Exposure

• Healthcare facilities set limits to ensure the personnel dose limits do not approach EfD limits

  • Limits are set well below the actual limits, usually 1/10 of the limit

  • Healthcare facilities set their own action limits that are well below actual allowable limits

  • The limits are meant to trigger an investigation that should uncover any high exposures

    • Usual reasons for high exposure readings:

      • Film badge left in treatment, sim or brachy room

        • Fell off lab coat or uniform, left lab coat in room