Radiation Dose Limits and Guidelines

Dose Reference

• Average radiation dose per examination is used as a guide for general practice.
• This aims to protect patients from unnecessarily high radiation doses and prevent negligence from radiology technologists.

Dose Limits

• The effective dose limit for radiation exposure is 1 mSv per year.
• In special circumstances, the effective dose can be increased to 5 mSv in a single year, provided that the average over five consecutive years is less than 1 mSv per year.
• The equivalent dose limit to the lens of the eye is 15 mSv per year.
• The equivalent dose limit to the skin is 50 mSv per year.

Dose Limits & Linear, Non-Threshold Response

• The basis for dose limits assumes a linear, non-threshold response to radiation, meaning that any exposure, regardless of how small, poses a risk.
• All unnecessary radiation exposure must be avoided.

Dose Limit Sources

• The National Council on Radiation Protection and Measurements (NCRP) assesses risks based on data from the BEIR Committee and the National Safety Council.
• State and federal government agencies typically adopt these recommended dose limits as legal requirements.

Dose Limits & Deterministic/Stochastic Effects

• Dose limits are established to prevent deterministic effects, which have a threshold dose below which they don't occur, and limit the risk of stochastic effects, which have no threshold.
• Dose limits are specified for occupational exposures only.

Dose Limits & Patient Exposure

• Dose limits are not applicable to patients undergoing medical procedures.
• A direct risk versus benefit assessment is made for patients.
• Patient exposure should be kept as low as reasonably achievable (ALARA).

Radiation Dose Limits 1987 & 1991

• The NCRP limits established in 1987 were 50 mSv per year (5000 mrem).
• The International Commission on Radiation Protection (ICRP) limits established in 1991 were 20 mSv per year (2000 mrem).

Dose Limits - Skin & Eyes

• Certain organs have higher dose limits than the whole body dose limit.
• The dose limit for skin is 500 mSv per year.
• The dose limit for eyes is 15 mSv per year.

Dose Limits - General Population

• The dose limit for individuals in the general population is 5 mSv per year (500 mrem per year) if the exposure is infrequent.
• For frequent exposure, such as for hospital employees working near radiology, the limit is 1 mSv per year (100 mrem per year).
• This 1 mSv per year dose limit is used by physicists to calculate the thickness of protective barriers.

Dose Limits & Students under 18

• Students under the age of 18 may not receive more than 1 mSv per year during their educational activities.
• It is a general practice not to accept underage students for radiation technology programs.

Radiation Dose Limits & Pregnancy

• Two situations in diagnostic radiology require special care and action due to the potential risks associated with pregnancy:
  • The severity of the potential response to radiation in utero is both time and dose dependent.

Responsibilities to the Patient (Pregnancy)

• Estimate the fetal dose.
• Determine the stage of gestation at the time of exposure.
• Determine which alternative to recommend to the patient (continue to term or terminate the pregnancy).

Fetal Dose & Pregnancy

• A series of x-ray examinations would rarely exceed a fetal dose of 5 rad.
• Below 10 rad, a therapeutic abortion is not indicated unless other risk factors exist.
• Above 25 rad, the risks of latent injury may justify termination of the pregnancy.
• Between 10 and 25 rad, the precise time of irradiation and the emotional state of the patient must be considered.

Pregnancy & X-ray Examinations

• Never knowingly examine a pregnant patient with x-rays without a documented decision to do so.
• Discuss the need for the examination with a radiologist before performing x-ray examinations on a potentially pregnant patient.
• Determine the patient’s menstrual cycle and withhold the exam until any questions about pregnancy are answered.

10-Day Rule

• The safest time to x-ray a female of childbearing age is within ten to fourteen days of the onset of menses.
• This is recommended for high-dose examinations where the dose to the uterus is likely to exceed 10 mGy.
• High-dose examinations are performed within 10 days of the start of the period to ensure the patient is not pregnant.

28-Day Rule

• Any request for radiological examinations of female patients of reproductive capacity that places the uterus in or near the primary x-ray beam (irradiation between the diaphragm and pubis) will be referred back to the doctor for discussion if the patient has missed a period.

Objectives of Radiation Protection (ALARA)

• Prevent clinically important radiation-induced deterministic effects by adhering to dose limits below the threshold level.
• Limit the risk of stochastic effects to a conservative level as weighted against societal needs, values, benefits acquired, and economic considerations.
• This is known as the ALARA concept (As Low As Reasonably Achievable).

ALARA Principle

• The National Council on Radiation Protection and Measurements (NCRP) established this principle to minimize radiation exposure.
• The continuation of good radiation protection programs and practices has been effective in keeping average and individual exposures for monitored workers well below the limits.

ALARA & Radiologic Practice

• The ALARA concept should guide the selection of technical radiographic and fluoroscopic exposure factors for all patient imaging procedures.
• The goal is to keep radiation exposure and consequent dose to the lowest possible level.

Responsibility for Maintaining ALARA

• It is the responsibility of the employer to provide the necessary resources and appropriate environment to execute an ALARA program.
• Management should perform periodic exposure audits to ensure proper reduced radiation exposure practices are being applied.
• Radiation workers must be knowledgeable of the applicable rules and regulations governing their work situation.

Optimisation of Practice (ALARA)

• The main goal of the ALARA program is to reduce both external and internal radiation doses to a level that is ALARA – as low as reasonably achievable.
• The three basic protective measures used to reduce external exposure (Cardinal Principles of Radiation Protection):
  • Minimizing time in a field of radiation
  • Maximizing the distance from a source of radiation
  • Using shielding

ALARA & Risk

• Assume that any radiation exposure carries with it some risk.
• ALARA balances the assumed risks of radiation exposure against the benefits of performing the work.
• The risk from receiving the exposure should be "worth" the net benefit of the activity.

Preventing Unnecessary Exposure (ALARA)

• ALARA means preventing unnecessary exposure as well as overexposure.
• The implementation of the ALARA concept is the responsibility of all employees.
• The success of the ALARA program depends on each radiological worker’s attitude and actions.

Minimizing Time & Dose

• Exposure = Exposure rate x exposure time
• During radiography, exposure time can be reduced to minimise motion blur.
• During fluoroscopy, exposure time can be reduced to minimise patient and personnel exposure.

This quiz covers essential concepts related to average radiation doses, effective dose limits, and regulations aimed at protecting patients and technologists in radiology. Test your understanding of the linear, non-threshold response principle and the sources of dose limit guidelines.