Radiation Dose and Safety Considerations in Imaging

1

What is the terminology for radiation dose?

Absorbed dose, equivalent dose, and effective dose are all used to describe the effects of radiation on tissue. Absorbed dose specifically refers to the energy transferred to a quantity of tissue by ionizing radiation. Equivalent dose can be derived from absorbed dose by applying a weighting factor that describes the type of radiation, in order to account for the different biological effects of different types of radiation. X-rays, gamma rays, and beta particles, some types of radiation used in medical imaging, are assigned a weighting factor of 1.

Effective dose is the weighted sum of the equivalent doses for all the tissues of the body. The tissue weighting factors are determined by the International Commission on Radiological Protection (ICRP) and periodically updated. ICRP Publication 103 was the most recent update of the tissue weighting factors, which was released in 2007 . Effective dose is meant to confer an estimate of risk of adverse effects due to exposure to ionizing radiation; it does not describe the actual amount of radiation imparted to or absorbed by the body.

2

In what units is radiation dose expressed?

The modern International System of Units (SI) unit for absorbed radiation dose is the gray (Gy), where 1 Gy = 1 Joule (J)/kilogram (kg) = 100 rad. The sievert (Sv) is the SI unit for equivalent dose, which describes the risk of stochastic biological adverse events, such as cancer and genetic damage, when tissue is irradiated. For ionizing radiation used in diagnostic imaging (x-rays, gamma rays), 1 Sv = 100 rem = 1 Gy. Absorbed dose from a medical imaging test is typically on the order of milligrays (mGy), and the associated biological risk is therefore on the order of millisieverts (mSv). The Sv is also the unit for effective dose, which takes into account the different sensitivities of different types of tissue to ionizing radiation.

3

What organizations make recommendations or monitor the use of ionizing radiation in medical imaging?

The International Commission on Radiological Protection (ICRP) is a not-for-profit, volunteer effort based in the United Kingdom. It is responsible for maintaining the International System of Radiological Protection. The ICRP studies and reports on many aspects of radiation protection, including radiation effects and protection of individuals who are exposed to ionizing radiation in the course of their medical care. The ICRP collaborates with the International Commission on Radiation Units and Measurements (ICRU), a standards body that establishes the units of ionizing radiation and the quantities they represent. The National Council on Radiation Protection and Measurements (NCRP) was chartered by the United States Congress. In conjunction with other organizations such as the ICRP and ICRU, it aims to publish information about radiation protection and radiation units. The Nuclear Regulatory Commission (NRC) is a United States government agency responsible for the safe use of nuclear materials, including those used in medical imaging. The International Atomic Energy Agency (IAEA) is an international organization affiliated with the United Nations. Its mission is to promote the safe and responsible use of nuclear energy. The National Electrical Manufacturers Association (NEMA) was involved in the original development of the Digital Imaging and Communications in Medicine (DICOM) standard used for the creation, storage, and transfer of medical images. The Medical Imaging & Technology Alliance (MITA) of NEMA represents manufacturers of medical imaging equipment and recently introduced the computed tomography (CT) Dose Check initiative for alerts and notifications intended to decrease the risk of exposing patients to excess radiation during CT imaging. The Joint Commission (JC) is currently developing standards for diagnostic imaging and educates the public about medical imaging tests. In 2011, they introduced a Sentinel Event Alert regarding the risks associated with radiation exposure from diagnostic imaging, and requiring reporting of adverse events.

4

What are the recommended annual dose limits for radiation exposure?

Annual dose limits for radiation exposure are established by the NRC in the Code of Federal Regulations . Radiation workers may not receive more than 50 mSv per year to the whole body or 500 mSv to an individual organ or to the skin. A lower annual limit of 150 mSv is prescribed for the lens of the eye, because of the increased risk of cataracts. Radiation workers are required to wear thermoluminescent dosimeters (typically embedded within badges or rings) so that their exposure can be monitored. Radiation workers who exceed annual limits will be prevented from continuing in their occupations for the rest of the year if they exceed the annual limit. Members of the general public are limited to 1 mSv per year. In the United States, this does not apply to individuals who are exposed to ionizing radiation from medical imaging, and at present there are no limits on the annual exposure of an individual patient. However, many European countries have established diagnostic reference levels (DRLs), which provide recommended limits on radiation dose indices for diagnostic and interventional radiology examinations as well as nuclear medicine studies.

5

What is the ALARA principle?

ALARA stands for “As Low as Reasonably Achievable” . It is the principle that is intended to guide the use of ionizing radiation in medical imaging. According to ICRP Publication 105, there are two principles of radiation safety: justification and dose optimization. Justification indicates that the benefit of the imaging procedure outweighs the risk associated with exposing the patient to a small amount of ionizing radiation. Dose optimization is often summarized as ALARA, but it dictates that the exam should be tailored to the clinical question, patient size, and the anatomy of interest. Furthermore, it also describes the need for proper equipment maintenance and testing, to assure that the amount of radiation to which a patient is exposed is appropriate for the study type.

6

What is the inverse square law?

The inverse square law is a geometric relationship ( Figure 7-1 ) that expresses the intensity of energy emitted by a point source as a function of the distance from that source. It assumes that energy dissipates equally in all directions and that its intensity is inversely proportional to the square of the distance r from the source (i.e., 1/ r ² ). The inverse square law is applicable to different types of electromagnetic radiation, including radio waves, light, x-rays, and gamma rays. For this reason, it is a tenet of radiation safety for radiation workers who use fluoroscopy: by doubling one's distance from the x-ray tube of the fluoroscope, one's radiation exposure is decreased by 75%.

7

What are stochastic and nonstochastic effects of radiation exposure?

There are two types of adverse effects from radiation exposure: nonstochastic (also known as deterministic) and stochastic (also known as probabilistic). Nonstochastic effects are nonprobabilistic. They have a known minimum threshold of radiation exposure. If this threshold is not exceeded, it is extremely rare for deterministic effects to occur. However, if the threshold is exceeded, the severity of the deterministic effect will depend on the dose of radiation to which the individual has been exposed. This is commonly described as a dose-related response. Examples of deterministic effects include erythema, epilation (hair loss), cataracts, and, at sufficiently high doses, death. By comparison, stochastic effects are probabilistic. The probability of the occurrence of a stochastic effect is greater at higher doses of radiation exposure, but the severity of the effect is similar whether it occurs from exposure to more or less radiation. The two categories of stochastic effects include cancer induction and genetic mutation. Stochastic effects are not presently believed to have a specific exposure threshold, although this is a subject of debate.

8

What is the BEIR VII report?

The Biological Effects of Ionizing Radiation (BEIR) VII report was issued by the National Research Council in 2006. It describes the relationship between low doses of radiation exposure and the associated risk of solid cancers . Data from Japanese atomic bomb survivors, Chernobyl survivors, patients who underwent radiation therapy for lung and breast cancers, radiation workers (including pilots, medical professionals, and nuclear workers), and individuals living at high altitudes were reviewed in developing the model. The data from the radiation workers showed a protective effect, while the individuals living at high altitudes did not show an increased cancer incidence; data from the former were not included in the model. The report states that “At doses less than . . . 100 mSv, statistical limitations make it difficult to evaluate cancer risk in humans,” and the committee felt that the safest conclusion was that the risk would continue linearly toward zero. This resulted in the so-called linear no-threshold model for cancer risk estimation.

9

What is the linear no-threshold model?

The linear no-threshold (LNT) model ( Figure 7-2 ) effectively states that there is no safe level of exposure to ionizing radiation, and that cancer risk can be increased at any level of exposure. It was the primary conclusion of the BEIR VII report. Alternative models include a linear model with threshold, a super-linear model, and radiation hormesis. These models were not felt to be supported by the data analyzed for the BEIR VII report. There has been much confusion and controversy about deriving cancer risk based on radiation exposure since the introduction of the LNT model . In addition, the appropriateness of the LNT model continues to be questioned, while the lifetime attributable cancer risks based on the BEIR VII report are often incorrectly quoted as scientific fact rather than as “subjective confidence intervals” that incorporate opinion in addition to numerical analysis.

10

What is radiation hormesis?

Radiation hormesis is a hypothesis that suggests that exposure to low levels of radiation expxosure is actually beneficial to biological tissues by stimulating their innate protective mechanisms . In this context, low levels of exposure are defined as those similar to background radiation exposure. The protective mechanisms include: detoxification of reactive oxygen species, repair of DNA damage, induction of apoptosis, increased immune-mediated cell death, induction of terminal differentiation, and increased expression of “stress response” genes. The hypothesis is that exposure to low levels of ionizing radiation stimulates the housekeeping functions of the immune system and persists for hours to days after the exposure. There continues to be a great deal of controversy surrounding this hypothesis, as it is contrary to the LNT hypothesis that was previously introduced. At present, there is no conclusive evidence for either hypothesis.