Chapter 2: Radiation Safety and Protection

The Health Physics Society defines radiation as "energy that comes from a source and travels through space."¹ The source could be atomic particles such as alpha and beta emissions as well as electromagnetic energy associated with AM/FM radio, radar, visible light, ultraviolet light, x-rays, and gamma rays. Radiation with enough energy to remove an electron from an atom is termed ionizing radiation.² A characteristic of x-rays, gamma rays, alpha and beta particles, radiation having this ability can lead to biological damage when absorbed in human tissue.

This chapter will introduce the common units to describe radiation, sources of radiation exposure, radiation dose limits, and an introduction to radiation biology. The chapter will further focus on radiation safety and protection regulations pertinent to the practice of Nuclear Cardiology. These regulations are governed by the Nuclear Regulatory Commission (NRC) and are found in Title 10, Parts 19, 20, and 35 of the Code of Federal Regulations (CFR). NRC NUREG 1556 Volume 9, Revision 2 provides guidance specific to radioactive materials licensing and offers suggested policies and procedures for radiation safety compliance.

There are several conventional terms used when describing radiation. These include exposure, absorbed dose, and dose equivalent. Named after Wilhelm Roentgen, the scientist who discovered x-rays in 1895, the Roentgen (R) is the unit of radiation exposure in air.³ In comparison to the International System of Units (SI), one Roentgen corresponds to the amount of radiation required to liberate 2.58 × 10^-4 Coulombs per kilogram of air.

The Rad, or Radiation Absorbed Dose, is the measure of the amount of energy absorbed by an object as radiation passes through.⁴ The amount of energy absorbed is dependent on the energy of the incident photon and the composition of the material. The f-factor is a tissue weighting factor used to convert exposure in air (R) to absorbed dose (rad) in tissue taking into account the x-ray or gamma ray energy and effective atomic number of the tissue exposed. For example, a 100-keV gamma photon incident on fat will transfer 91% of its energy, whereas the same photon will deliver 96% of its energy to muscle tissue. Therefore, a source of radiation exposing a point in air to 100 R will deliver a dose of 91 rad to fat tissue and 96 rad to muscle tissue at the same reference point.

Dose equivalent is a term used to quantify the amount of energy deposited in tissue along with the associated biological risk from the type of radiation.⁵ The conventional unit for dose equivalent is rem which is calculated by multiplying the radiation absorbed dose (rad) by a radiation quality factor or QF.⁶ Table 2-1 illustrates that the QF for x-rays, gamma rays, and beta particles is equal to 1 whereas the QF for alpha particles is 20.⁷ This means that an absorbed dose of 10 rads from an x-ray, gamma ray, or beta particle equates to 10 rem (10 rad × 1), whereas the dose equivalent would be 200 rem (10 rad × 20) if originating from alpha particulates.

Although the conventional unit to describe absorbed dose and dose equivalent is the rad and rem, the international community now uses the term Gray (Gy) and Sievert (Sv), respectively. 1 Gy = 100 rad and 1 Sv = 100 rem. Table 2-2 provides a summary of the units of radiation exposure, absorbed dose, and dose equivalent.

Ionizing radiation occurs naturally in the earth's soil and rock, in the cosmic rays descending from the sun and stars, as well as in our own body.⁸ This ubiquitous background radiation in the United States totals approximately 3.1 mSv/yr across the population.⁹ Exposure to Radon and its decay products accounts for the majority of the natural background radiation.

In addition to natural background, people are exposed to radiation in manufactured products such as smoke detectors, ceramics, and building materials, as well as from medical sources including diagnostic x-rays and nuclear medicine procedures.¹⁰ The effective radiation dose from these products accounts for an additional 3.1 mSv/yr (Fig. 2-1). It is important to note that these estimates are population based and not all individuals are exposed to radiation from manufactured products and radiological examinations.

The term BERT, or Background Equivalent Radiation Time, has recently been introduced to explain levels of radiation dose to patients relative to natural background radiation exposure. Table 2-3 illustrates radiation doses to patients from Nuclear Cardiology procedures and their associated BERT.

The regulations for dose limits in the United States can be found in Title 10, Part 20 of the Code of Federal Regulations (10 CFR 20). Table 2-4 illustrates these limits, which include limiting radiation to occupational workers, the embryo/fetus of an occupational worker, and the public. Both occupational and public dose limits exclude exposure to natural background radiation.

While the dose limit to the public from sources of ionizing radiation is 1 mSv (0.1 rem) per year, this limit may be increased to 5 mSv (0.5 rem) from an infrequent exposure related to another person's medical procedure providing the authorized user determines the exposure is appropriate.

Because it is impractical to set occupational worker dose limits to that of the public, regulators rely on the Linear Non-Threshold risk model to set limits at a point below which the threshold for radiation effects are known and at a point to minimize the theoretical effects from the stochastic risks from ionizing radiation exposure. The dose limits are established at levels of risk already assumed by occupational workers.¹¹ To this point, radiation dose limits are established at a level where the risk of a fatal cancer from occupational exposure to ionizing radiation is similar to the assumed risk of a fatal work accident, which is 1 in 10,000 annually.¹²

In addition to ensuring dose limits are not exceeded, an institution must adopt investigational levels in order to maintain radiation levels consistent with the As Low As Reasonably Achievable (ALARA) philosophy. These quarterly investigational levels may be established by the institution, or the facility may adopt those recommended by the NRC. As illustrated in Table 2-5, there are two investigational levels associated with various monitoring points on the body. An ALARA Level I is a dose equal to 10% of the annual dose limit whereas an ALARA Level II is triggered at 30% of the annual dose limit. To help ensure annual dose limits are not exceeded, these ALARA investigational levels are routinely checked on a quarterly basis.

The NRC recommends the following actions must be taken when an employee exceeds ALARA investigational levels:

ALARA I Exceeded

The RSO or designee should investigate and review actions that might be taken to reduce a recurrence. No further action is necessary unless determined otherwise by the RSO.

ALARA II Exceeded

The RSO should perform a timely investigation into the cause, take action to reduce the recurrence, and submit a report to the institution's Radiation Safety Committee.

Patients administered with radioactive material may be released into the public providing they are not likely to expose any individual to a point where their dose equivalent could exceed 5 mSv (0.5 rem).¹³ Furthermore, the patient or patient's guardian must be provided with a set of instructions to maintain doses to as low as reasonably achievable if the total effective dose equivalent to any other individual is likely to exceed 1 mSv (0.1 rem). Breast-feeding cessation guidelines must also be provided if the dose equivalent to a nursing child could exceed 1 mSv (0.1 rem).

NRC Radiation Guide 8.39, "Release of Patients Administered Radioactive Material"¹⁴ provides guidance for the release of patients, administered activities of radiopharmaceuticals requiring instructions, and breast-feeding cessation recommendations. As illustrated in Table 2-6, patients administered with diagnostic activities of ^99mTc-Sestamibi may be immediately released without radiation safety instructions and do not require stoppage of breast-feeding.

The deleterious risks from ionizing radiation exposure are similar to exposure from other hazards encounter by employees in the workplace, which include both chemical and biological agents.¹⁵ Moreover, the resulting damage to cells from ionizing radiation cannot be distinguished from cellular injury due to these other harmful factors.¹⁶ The killing of cells directly and/or damage to the DNA are dependent on several variables including the caustic agent source and strength, duration of exposure, and stage of cell growth.^17,18 The effects are classified as either stochastic, where the probability of the effect increases with dose, or deterministic, in which the severity increases with dose, after a threshold is exceeded (Fig. 2-2).¹⁹ Stochastic effects include cancer and genetic manifestations whereas deterministic effects include reddening of the skin (erythema), and development of cataracts.

The source of radiation and its strength play a particular role in assessing risks from exposure. While both stochastic and deterministic risk effects are dose dependent, meaning risk increases with radiation energy and duration of exposure, these effects are heightened by the type of radiation. X-rays and gamma rays are high-energy quanta of electromagnetic radiation characterized by short wavelengths and high frequencies that have the ability to ionize an exposed atom. The biological effectiveness, or the risk adjusted for radiation quality, for x-rays, gamma rays, and beta particles are the same (Table 2-1). On the other hand, alpha emissions are high-energy particulate radiation that deposit significantly more energy as it passes through tissue. Because of this increased transfer of energy, alpha particles bear a higher biological risk than x-rays, gamma rays, and beta particles. At equivalent doses of radiation, the associated biological risk from alpha emissions is 20 times that of x-rays, gamma rays, and beta particles.

When exposed to radiation, cells can be affected directly by energy deposited into the atom/molecule or indirectly through ionization of the surrounding medium, which then interacts with the target cells. Whether directly or indirectly via free radical production in cellular water, the toxic effects from radiation exposure include the removal of an electron from DNA molecules causing single- or double-strand breaks. Double DNA strand breaks are overly traumatic and often lead to cellular death. Although single-strand breaks frequently repair themselves correctly, incorrect or mutagenic repairs do occur. A mutagenic repair represents a change in the cellular DNA. This may leave the cell modified yet still viable.¹⁶

Occupational workers exposed to ionizing radiation must practice radiation safety techniques on a daily basis to maintain levels of exposure below federal dose limits. To assess the levels of cumulative exposure, occupational workers are issued personal radiation monitors. When a pregnant worker declares her pregnancy status, an additional monitor is provided to be worn at the waist level for assessing the radiation dose to the fetus.

Over and above the universal precautions common to the healthcare environment, there are three cardinal rules in the protection against ionizing radiation. These are time, distance, and shielding (Fig. 2-3).²⁰

Time

Radiation is emitted as a rate. A radiation source emitting photons or particles at a rate of 80 mGy/hr would expose a particular area to 80 mGy in 1 hour. Understanding this principle, occupational workers limit their time around known radiation sources, which in turn limits their cumulative exposure. In the example above, a worker exposed to an 80 mGy/hr source would only receive 40 mGy of exposure if their time around the source were limited to 30 minutes.

Distance

Like a source of visible light that appears dimmer at increased distances, ionizing radiation decreases in intensity as well. The inverse square law states that the intensity of radiation decreases with the inverse of the square of the distance, or I₁(d₁)² = I₂(d₂)², where I = intensity and d = distance. Radiation is emitted in an isotropic manner. As distance increases, the same fluence of radiation covers a larger surface area, thereby decreasing the exposure to a finite object in the path (Fig. 2-4). Continuing the previous example, a worker exposed to 40 mGy at 1 m from a radiation source will reduce their exposure to 10 mGy at a distance of 2 m.

Example:

Shielding

Shielding is often used to provide a protective barrier between a worker and a radiation source. The shielding material and thickness is dependent upon the radiation source and strength. Inches of concrete or fractions of an inch of lead are commonly used in barriers to provide structural protection to occupational workers and the general public. Partial barriers, lead-lined transport containers, and lead-lined syringe shields are also used to protect radioisotope workers during the preparation and administration of radiopharmaceuticals.

Occupational radiation workers are required to be monitored for radiation exposure if they are likely to enter a high or very high radiation area, are likely to exceed 10% of the annual dose limits, or are a declared pregnant woman likely to receive an annual dose in excess of 1 mSv (0.1 rem).²¹

There are several types of personnel monitors used in Nuclear Cardiology facilities. These include the film badge, a thermo luminescent dosimeter (TLD), and an optically stimulated luminescence (OSL) device. The NRC requires evaluation of these devices by a laboratory that is accredited by the National Voluntary Laboratory Accreditation Program (NVLAP).

Film badges utilize a strip of photographic film contained in a plastic holder. Exposure to radiation will increase the darkening in the developed film. The degree of darkening is proportional to the radiation exposure received. Incorporated into the badge holder are filters such as lead, copper, aluminum, and plastic. The degree of darkening behind each filter helps determine the quality of the radiation received, such as high-energy photons, low-energy photons, and particulate radiation. Film badges are commonly used to measure whole-body irradiation and are comparatively inexpensive. However, they are affected by heat, moisture, and sunlight and must be utilized with care.

TLDs use inorganic crystals such as Lithium Fluoride to measure radiation exposure. When exposed to ionizing radiation, excitation of atoms causes electrons to elevate from their valence band to become trapped in the forbidden band. When heated with high temperatures (annealing), the electrons are released back into the valence band emitting light proportional to the radiation received. By returning to their stable state, the annealing process allows the TLD to be re-used. TLDs do not have the ability to distinguish radiation quality and are commonly used as extremity monitors, such as the ring badge.

OSL combines the benefits of film badge and TLD technology. An aluminum oxide crystalline structure is manufactured into a thin strip and placed between a filter pack. Irradiated atoms in the structure liberate electrons that become trapped and these electrons release light when stimulated by a laser. The release of light is proportional to the radiation received. Similar to that in film badge technology, plastic and metal filters are used to assess the incident radiation quality. OSL devices are commonly used for whole-body radiation monitoring. The devices are sealed in a blister pack and are unaffected by light, heat, and moisture.

Formally known as misadministrations, medical events are defined by the NRC. The NRC must be notified of a reportable medical event no later than the next calendar day after occurrence. The telephone notification must be followed by a written report within 15 days. Pertinent to Nuclear Cardiology, the following events must be reported to the NRC if they result in a dose equivalent to the patient that exceeds 0.05 Sv (5 rem) or a dose to the skin or an organ that exceeds 0.5 Sv (50 rem). Events meeting these definitions but not exceeding the reportable dose limits need only be reported to the facility's administrators.

1. An administration of a dose that is greater than 20% difference from the prescribed dose

2. An administration of the wrong radiopharmaceutical

3. An administration of a radiopharmaceutical to the wrong person

4. An administration of a radiopharmaceutical by the wrong route (i.e., oral vs. intravenous)

There are several postings required in a Nuclear Cardiology laboratory. These include the "Notice To Employees" (NRC Form 3) and appropriate radiation caution signage.^22–24 NRC Form 3 (Fig. 2-5) is required to be posted in a conspicuous manner and replaced if defaced or altered. This form should be posted in an area frequented by workers either to or from the area where radiation is used or stored. This form is commonly found in employee lounges and entrance doors to radiation areas.

A "Caution Radioactive Materials" sign (Fig. 2-6) is required to be posted in any area where radioactive materials are used or stored that exceed 10 times the quantity listed in Appendix C of Title 10, Part 20 of the Code of Federal Regulation. This equates to 1 mCi of ⁵⁷Co, 0.1 mCi of ¹³⁷Cs, and 10 mCi of ^99mTc and ²⁰¹Tl. A "Caution Radiation Area" sign is required to be posted in each area where radiation levels could result in an individual receiving 0.05 mSv (5 mrem) in 1 hour at 30 cm from a radiation source. Caution signs are commonly located on the entrance doors to the hot lab, imaging room, and injections areas such as the treadmill room. If a radioactive material is used in a room for less than 8 hours per day and is under constant supervision, the area is not required to be posted with the caution sign. Furthermore, a radiation area sign is not needed if the source of the exposure is from a patient meeting the criteria to be released into the public.

Ambient radiation level surveys must be performed at the end of each working day in areas where radioactive materials are used and stored. The most common instrument for performing ambient radiation surveys in the Nuclear Cardiology department is the Geiger–Müller (GM) meter, a gas-filled detector that produces and collects ion pairs when ionizing radiation passes through the detector and deposits energy. The NRC recommends an ambient exposure trigger limit of 5 mR/hr in restricted areas and 0.1 mR/hr in unrestricted areas.²⁵

Removable contamination surveys (wipe tests) should be performed weekly using an appropriate gamma-counting device such as sodium iodide (NaI) well counter. While Appendix R of NRC NUREG 1556 Volume 9,²⁵ suggests an action level of 20,000 dpm/100 cm² for ^99mTc and ²⁰¹Tl, many Nuclear Cardiology facilities use the 2000 dpm/100 cm² action level required for other isotopes common in general nuclear medicine departments. It is also important to verify ambient radiation and contamination limits in your individual area of practice as many states have more restrictive requirements.

When ambient radiation levels or contamination surveys exceed limits, a radiation spill has occurred requiring action. The NRC recommends establishing an activity threshold for a major and minor spill and provides guidance in Appendix N of NRC NUREG 1556 Volume 9.²⁵ The suggested procedures are as follows:

Minor Spill (less than 100 mCi of ^99mTc/²⁰¹Tl)

1. Notify persons in the area that a spill has occurred.

2. Prevent the spread of contamination by covering the spill with absorbent paper.

3. Wear gloves and protective clothing and clean up the spill using absorbent paper.

4. Survey the area with an appropriate radiation detection instruments.

5. Report the incident to the RSO.

Major Spill (greater than 100 mCi of ^99mTc/²⁰¹Tl)

1. Clear the area. Notify all persons not involved in the spill to vacate the room.

2. Prevent the spread of contamination by covering the spill with absorbent paper labeled "caution radioactive material," but do not attempt to clean it up.

3. Shield the source if possible. Do this only if it can be done without further contamination or a significant increase in radiation exposure.

4. Close the room and lock or otherwise secure the area to prevent entry.

5. Notify the RSO immediately.

6. Decontaminate personnel by removing contaminated clothing and flushing contaminated skin with lukewarm water, then washing with mild soap. If contamination remains, the RSO may consider inducing perspiration. Then wash the affected area again to remove any contamination that was released by the perspiration.

Radioactive waste generated in the Nuclear Cardiology department may be disposed by various methods. The two most common are decay-in-storage and return to an authorized recipient. Decay-in-storage is allowed for isotopes with a half-life of 120 days or less.²⁶ Storage of waste for decay requires that the waste to be held for a minimum of 10 half-lives in a shielded container and prior to final disposal the waste must be surveyed to ensure it is consistent with background radiation. All radioactive labels must be removed or defaced, and a record of the waste disposal must be maintained for 3 years. A summary of the minimum length of time radioactive waste must be held in storage for isotopes common to Nuclear Cardiology is summarized in Table 2-7.

Radioactive waste, including used and unused doses, may also be transferred to an authorized recipient. Transfer of the waste requires verification the recipient is licensed to receive the specified radioactive material. Furthermore, when transferring this radioactive material the facility must follow Department of Transportation (DOT) shipping guidelines. A summary of these requirements is outlined in the "Shipping of Radioactive Packages" section.

Receipt of radioactive packages is outlined in Title 10, Part 20.1906 of the Code of Federal Regulations. Radioactive packages must be monitored within 3 hours of delivery if received during normal working hours and no later than 3 hours from the beginning of the next working day if received after normal hours. Proper monitoring requires surveying the package for external radiation exposure and wipe testing for removable contamination. Radiation surveys must be less than 10 mR/hr at 1 m and less than 200 mR/hr at the package surface. Contamination wipe testing must be performed if there is suspicion the package is damaged or the integrity is degraded in any manner. The wipe test results must be less than 6600 dpm/300 cm². Should the wipe test or survey results exceed limits, the delivery carrier and the NRC must be notified immediately.²⁷

In addition to receiving radioactive materials, the Nuclear Cardiology department must routinely ship radioactive materials. These shipments include the return of spent and unused patient radiopharmaceutical doses as well as the return of sealed calibration sources. Shipping of radioactive materials is regulated by the DOT under Title 49 of the Code of Federal Regulations (49CFR).

There are four DOT labels used for shipping radioactive materials (Fig. 2-7). The use of each label is determined by the quantity of material in the package and the radiation exposure at the surface and 1 m from the package.

The most common radioactive package shipped from the Nuclear Cardiology department is the "UN2910—Excepted Package, Limited Quantity." For a package to ship under the specifications of UN2910, the maximum quantity in the package cannot exceed the limits derived from the table in 49CFR173.425. Furthermore, the following criteria must be met:

1. The surface of the package must not exceed 0.5 mR/hr

2. A wipe test of the package must not exceed 6600 dpm/300 cm²

The limited quantity package limits for common isotopes derived from 49CFR173.425 are illustrated in Table 2-8. The limits apply to radioactive material in both solid and liquid forms. For packages containing more than one isotope, the lower limit applies to the entire contents of the package.

A radioactive package that exceeds the excepted package quantities, has a surface exposure of less than 0.5 mR/hr, an exposure at 1 m that is indistinguishable from background, and a wipe test of less than 6600 dpm/300 cm² receives a Radioactive I "White" label.

A Radioactive II "Yellow" label is used for packages with a surface exposure of 0.5 to 50 mR/hr, a 1 m exposure of less than 1 mR/hr and a wipe test of less than 6600 dpm/300 cm². The Radioactive II labeled is commonly used when shipping calibration sources back to the vendor for disposal.

Packages with a surface exposure of 50 to 200 mR/hr, a 1 m exposure of less than 10 mR/hr and a wipe test of less than 6600 dpm/300 cm² must be labeled with a Radioactive III "Yellow" label.

Radioactive labels must be placed onto two sides of the package and packages requiring the Radioactive I, II, or III label must be a classified as a "Type A" shipping container. A "Type A" shipping container is a strong package that has met certain criteria including a water spray test, drop test, compression test, and penetration test. A summary of the radiation exposure limits for each DOT label is illustrated in Table 2-9.

The NRC requires licensees to develop a policy for the safe use of radioactive materials. Appendix T of NRC NUREG 1556 Volume 9 Revision 2 provides a sample policy that facilities may choose to adopt in lieu of developing their own. The NRC policy for the safe use of radioactive materials illustrated in Figure 2-8 provides a basis for establishing a safe working environment.