Radiation Safety and Shielding in Radiotherapy

10.1

Introduction

The National Council on Radiation Protection (NCRP) Report No. 116, Limitation of Exposure to Ionizing Radiation, outlines the goals and philosophies of radiation protection in Chapter 2 of that document. The specific objectives are:

• 1.

  “To prevent the occurrence of clinically significant radiation-induced deterministic effects by adhering to dose limits that are below the apparent threshold levels and

• 2.

  “To limit the risk of stochastic effects, cancer, and genetic effects, to a reasonable level in relation to societal needs, values, benefits gained, and economic factors.”

The goal is not to reduce exposures to zero or near zero levels but rather to reduce levels to ALARA (as low as (is) reasonably achievable) (as defined in Title 10, Section 20.1003, of the U.S. Nuclear Regulatory Commission (NRC) Code of Federal Regulations (10 CFR 20.1003)). The concept of ALARA clearly includes using a risk/benefit approach incorporating societal needs and economic factors to reduce exposures to a reasonable level. The money spent on excessive shielding could be better used to provide direct care to patients with known conditions that can be treated. The risks of radiation exposure largely involve the possibility of an injury that can be immediate or delayed for months or years. It is more beneficial to improve the care for patients with a known condition than to further reduce the possibility of radiation injury to the patient or to others as long as the risk is acceptably low. For example, spending an extra $200,000 on shielding will take away capital that could have been spent on upgrading the treatment planning system (TPS) to a more accurate dose calculation algorithm that would immediately benefit patients.

To develop the exposure limits that are used to keep the risks at a reasonable level, several international agencies regularly review the available data on radiation risk. Among them are the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the International Commission on Radiation Protection (ICRP), and the National Academy of Sciences/National Research Council Committee on the Biological Effects of Ionizing Radiation (BEIR), the most recent report being BEIR VII. UNSCEAR and BEIR develop risk estimates which the ICRP uses to establish recommended dose limits. The NCRP determines how those recommendations will be implemented in the United States.

The dose limits for radiation workers is higher than those for a member of the general public. This is based on the fact that they have received special training in radiation protection, are aware of the associated radiation risks, and have decided to accept them. To determine compliance with the dose limits a facility must either assign a dose monitor to the staff member or perform calculations that show that the worker will not exceed 10% of the annual limit. There are some exceptions to this; staff who work in high risk areas such as interventional radio­logy, nuclear medicine, and Radiation Oncology should be monitored regardless of their expected annual dose level. This is because of the potential for unexpected incidents, particularly with regard to brachytherapy in Radiation Oncology. To ensure the continued safe use of radiation in medicine, staff should receive periodic radiation safety training. This training should include basic radiation safety principles, details specific to their job function, and communication of risk estimates.

To determine compliance with dose limits for the general public, shielding calculations and surveys are done to show that individuals in the areas surrounding the radiation source will not exceed the regulatory limits. The calculations must be confirmed by performing a survey of dose levels surrounding the radiation source after installation using an appropriate instrument. These calculations should be reviewed on a regular basis to confirm that the assumptions made during the design process are still valid.

10.2

Risk Levels and Radiation

The BEIR VII report is a standard reference for understanding the risks of radiation exposure. At very high doses the risks are deterministic in nature with the following endpoints and D50 levels (the dose at which 50% of exposed individuals would be expected to reach a particular endpoint, such as death): central nervous system (CNS) death (>20 Sv), gastrointestinal (GI) death (>10 Sv), and bone-marrow death (>5 Sv). At lower doses stochastic effects occur, the main concern being excess cancer risk with the main types being lung, liver, breast, prostate, stomach, colon, thyroid, and leukemia. BEIR VII quotes the excess number of cases per 100 mSv as being 800 per 100,000 people for men and 1300 per 100,000 people for women. There is a threefold higher risk for small children, with females having a twice higher risk as males. A feature of the BEIR VII report is that it distinguishes between incidence rates (the above numbers) versus mortality, which are, of course, not the same thing. BEIR VII notes that the former is approximately twice the latter. In looking at models of stochastic risk there is active debate as to whether there is a threshold of dose below which there is no risk. The BEIR VII report suggests that there is no evidence for a lower threshold and, as such, used the linear no-threshold (LNT) model for radiation risk.

10.3

Units

The estimation of risk for a given organ is determined by the biological sensitivity to radiation of that organ and the type of radiation. To account for the different density of ionization caused by each type of radiation, a weighting factor is used to modify the physical dose. This equivalent dose (H) is defined as:

where W _R is the radiation weighting factor and D is the physical dose in Gy. The radiation weighting factor, W _R , is defined and used by the ICRP, but an older terminology and calculation method was the quality (or “Q”) factor, a term that is still used by the U.S. NRC. Weighting factors are shown in Table 10.1 .

The SI unit for H is the Sievert (Sv), and an older non-SI unit is the roentgen equivalent man (rem), which is 0.01 Sv. Internationally there is some slight disagreement in the accepted values for the weighting factors as shown in Table 10.1 . The values for neutrons and protons have changed over time to reflect new risk estimates.

Another weighting factor is used to account for the radiation sensitivity of each organ. These factors have also changed over time. Current values are shown in Table 10.2 . Effective dose (E) is defined as:

where W _T is the weighting factor and H _T the equivalent dose for organ T. Units of E are also Sv. Also by definition, Σ _T W _T = 1.

Some authors have suggested that the concept of effective dose is flawed in that it is based on highly subjective judgments, does not reflect age and gender dependencies, and can be confusing and hard to interpret.

10.4

Regulatory Limits

NCRP Report No. 116 from 1993 lists the effective dose limits for radiation workers and the general public. NCRP-116 supersedes the earlier NCRP Report No. 91 from 1987. NCRP-116 dose limits are summarized in Table 10.3 .

The NCRP initially recommended that the annual effective dose limit for the general public should be 0.25 mSv, but a clarification in 2004 stated that the 1.0 mSv per year was justified because of the conservative estimates built into the shielding design methodology (NCRP Statement 10). The NCRP-116 report calls for remedial action for public exposure to natural sources above 5 mSv and states that there is negligible risk below 0.01 mSv.

To put these limits in context, the natural background radiation is approximately 3.1 mSv/year, which is 3 times higher than the public dose limit. The amount of natural background can vary significantly depending on factors such as altitude and radon gas concentrations. In addition, the average person in the United States is exposed to approximately 3 mSv/year in medical uses of radiation. The majority of that exposure comes from computed tomography (CT), although there is also a contribution from procedures performed in nuclear medicine and interventional radiology.

10.5