By Scott Williams, MD

April 3, 2002 --

Radiation Biology

The fundemental physical quantity for relating all biologic effects to radiation exposure is the absorbed dose- the energy imparted per unit mass of tissue (the enegy absorbed per unit of mass over a specified radiated portion of the body) [8,18,20]. Absorbed dose is expressed in units of joules per kilogram (J/kg) and is given the name "gray" (Gy) or milligray (mGy) (or rads) [9]. Effective dose is a parameter that reflects the risk of a nonuniform radiation exposure in terms of of a whole-body exposure [15]. The effective dose is defined as the dose in millisieverts that would have to be delivered uniformly to the whole body to produce the same biologic health consequences as that caused by a dose actually delivered to one or more specific organs [22,24]. The effective dose (measured in millisieverts- mSv or rem- roentgen equivalents human) incorportates the risk of biological effects from ionizing radiation by using a weighting factor specific for the type and energy of the radiation (i.e.: alpha particle, beta particle, or x-ray), and also a tissue-specific weighting factor based upon the radiation sensitivity of each organ or tissue [18]. The use of "effective dose" facilitates making comparisons between different types of radiation exposure [15]. In this manner, CT, radiographic, fluoroscopic, and nuclear medicine studies can all be represented by a "whole body equivalent" dose value (i.e.- the effective dose), eben though the radiation to individual tissues and organs varies greatly among techniques [15]. The effective dose was developed to estimate risk to occupationally exposed individuals and is a useful concept for developing radiation protection standards and setting dose limits for occupationally exposed individuals [22,24]. However, it is not intended for epidemologic studies or predictions of risk to individuals from medical exposures (unfortunately, it is often used in exactly this manner to predict cancer incidence and death in exposed patient populations) [22].

1 Gy = 100 rad (10 mGy= 1rad)
1 Sv = 100 rem (10 mSv= 1rem)

The negative effects of ionizing radiation in humans can be divided into two major categories based on time and exposure- deterministic effects that are seen immediately after large exposures, and stochastic effects seen after a long period of latency 96-25 years) [7]. For radiation effects it is generally assumed that dose and effect are linearly related, even at low doses, and that there is generally not a threshold below which the induction of cancer or genetic damage is not produced (i.e.: even low doses of radiation increase the statistical risk of developing a malignancy) [1]. This is referred to as the linear no-threshold model [22]. BEIR VII recommends the use of a linear no-threshold model for solid tumors and a linear quadratic model for leukemia [17]. This conclusion from BEIR VII replaces the previous linear-quadratic model of radiation effects described in BEIR III. (a linear quadratic model has a slope that increases as radiation exposures accumulate - patients with a large exposure history incur incrementally greater risks with each new radiation exposure [28]). A linear no threshold model is primarily used because of it's simplicity and because it is a conservative approach [22].

However, BEAR VII does note that at doses of 100 mSv or less, statistical limitations make it difficult to evaluate cancer risk in humans [12]. Thus, although we assume a risk at these levels for patient safety, it has not been scientifically demonstrated [12,32]. Additionally, low doses stimulate a repairative protective response by the body [32]. Within minutes, exposure to radiation from CT scans induces an increase in DNA double-stranded breaks, but these breaks are subsequently repaired [35]. In almost all patients these breaks are repaired to less than the initial (pre-CT) background level at some time between 5 and 24 hours [35].

However, others contend that there is strong evidence that a threshold for detrimental effects from radiation does exist and that below this level the effects of radiation are absent or positive [6,10,27]. Biologic data demonstrate that the defense mechanisms against radiation-induced radiation carcinogenesis are powerful and diffuse and their efficacy is much higher for low doses and dose rates [10]. The American Association of Physicists in Medicine and the Health Physics Society concluded that cancer risk estimation should be limited to doses greater than 50 mSv and that the risk sfrom doses below this level are to small to be detectable [27]. At subthreshold doses (up to 50 mSv), the DNA repair responses are error free and can produce an adaptive response that enhances a cells resistance to radiation (elimination of aberrant cells is also expected) [6,19]. Doses that exceed the threshold also elicit cellular defense mechanisms that attempt to minimize DNA damage and ensure cell survival [6]. Unfortunately, with doses above 100mSv these attempts are prone to error and can lead to increased risk for DNA mutation and cancer development [6,19].

Even if one adopts a linear no-threshold dose-risk relationship, it is reasonable to assume that certain factors can modify the ultimate effects of radiation [3]. Factors such as age at time of exposure and the manner in which the radiation was received can affect the risk relationship [3]. In general, low dose radiation induced cancer incidence generally decreases with advancing age [3]. Higher dose rates are generally more damaging than low dose rates- protracted exposures over time are associated with lower cancer risk compared to that of an acute exposure to the same total dose [3]. Finally, the latent period between radiation exposure and cancer death increases with decreasing levels of exposure- and it is possible that for low doses the latent period may exceed the normal life span [3].

Incident radiation can produce damage in one of 2 ways- either through direct cell damage (this effect increases with increasing LET) or via the formation of hydroxyl free radicals (through ionization or excitation reactions) that react with both DNA and RNA to produce molecular alterations (this effect decreases with increasing LET). Radiation damage to DNA is primarily due to indirect action and DNA damage appears to be primarily responsible for the lethal and mutagenic effects of radiation in man. Both moderate hyperthermia and high oxygen tesion increase a cells sensitivity to the effects of radiation. More rapidly dividing cells are also more sensitive (the law of Bergonie and Tribondeau)-  cell cycle sensitivity to radiation effects: M (mitosis) > G2 > S > G1. Certain agents can act as radioprotectants- mostly these are aminothiol compounds which act by inhibiting indirect damage, and facilitating repair.

The present estimate of the doubling dose for genetic effects (the dose needed to double the natural incidence of a genetic or somatic anomaly) is about 100 rads (1 gray).

Background radiation exposure  in the United States has previously been reported to be about 300-360 mrem (3.0-3.6 mSv) per year [1,4]. By far, the greatest contribution to this background exposure is from radon (about 200 mrem/yr). The average dose to the fetus from naturally occurring background radiation over the course of a normal gestation is 0.5-1.0 mSv [20]. Previously, nuclear medicine accounted for about 4% of this exposure, while medical x-rays made up approximately 11%. However, a recent report has suggested that the annual per capita effective radiation dose in the U.S. has increased to between 5.6 and 6.2 mSv [14,22]. About 2.4 mSv of this dose is from natural background, while 3.0 mSv is related to radiologic medical imaging- representing a 6-fold increase from 1980 [14]. Overall, about 50% of the average radiation dose to the population now comes from medical exposures, with about one-fourth due to CT examinations alone [24]. The third major medical source of radiation is nuclear medicine and about 75% of this radiation comes from myocardial perfusion scintigraphy [25].

The estimated dose for a round-trip transatlantic flight between New York and Paris is about 0.12 mSv [4].

Deterministic effects: Deterministic effects involve the loss of tissue function and results in radiation-induced cell death and typically do not occur until the radiation dose exceeds a specific threshold (i.e.- the radiation dose exceeds the capabilities of innnate cellular repair mechanisms) [18,20,26]. Deterministic effects occur soon after exposure and that increase in magnitude with increasing doses above the threshold dose level [9]. Deterministic effects require radiation exposures that are orders of magnitude (>2Gy) above those received from pulmonary CTA (<50 mGy) [26]. Some examples are tumor cell killing, bone marrow suppression, and nephrotoxicity [9]. Deterministic effects are of concern in therapeutic nuclear medicine and radiation oncology where the absorbed doses are high and intended to be cytotoxic [9]. An example would be growth retardation, mental retardation, or death from fetal in utero exposure [21].

Stochastic Effects: Stochastic effects are the result of cellular damage likely at the DNA level causing cancer or other genetic/germ cell mutation [16,31]. These effects have no threshold and the severity of the effect does not vary with the dose [16,20]. Stochastic effects occur randomly, but the probability of occurrence increases as the dose increases and varies depending on the type of ionizing radiation, the tissue receiving the radiation, and the age of the subject [7]. To account for the relative effect per unit absorbed dose that has been observed for different types of radiation, the  International Commission on Radiological Protection (ICRP) has established radiation weighting factors for stochastic effects [9]. The product of the absorbed dose in Gy and the radiation weighting factor is defined as the equivalent dose (expressed in Sieverts - Sv) [9]. For example, alpha particles yield a greater density of ionizing events per unit distance traveled than photons or electrons due to their larger size and higher linear energy transfer [9]. Thus, alpha particles have a higher probability of causing DNA damage and a higher probability of causing stochastic effects per unit of absorbed dose [9]. The weighting factor for alpha particles is 20, while the weighting factor for photons and electrons is 1.

Cancer is a stochastic effect, particularly leukemia which has a latent period of 2 to 10 years post exposure. The risk for leukemia is significantly increased above acute doses of 40 rads (0.4 Gy). There is a longer latent period for solid tumors (10 to 40 years) such as squamous cell carcinoma of the skin and adenocarcinoma of the breast or lung. Decreasing age (younger subjects) clearly increases radiation sensitivity and at all ages women have approximately twice the risk compared to men for the same level of exposure [7]. There may be some associated increased risk for childhood cancer following exposure in utero. Genetic effects are also considered under this category.

The International ICRP has suggested that there is a stochastic risk of 5% per sievert for inducing malignancy from radiation exposure [4]. This means that 5 of 100 individuals exposed to a dose of 1 Sv (100 rem) would develop cancer (or a 1:2000 risk at an exposure of 10 mSv [18]) [4]. If this concept is applied to a lower-level radiation exposure, the one would have to assume an additional 5 radiation induced malignancies among 100,000 individuals exposed to an effective dose equivalent of 1 mSv/yr (0.1 rem/yr) [4,26]. To put this into perspective, the the cancer risk associated with radiation exposure should be compared to the natural risk of cancer [17]. In the US, approximately 22% of the population will die from cancer, thus the relative increased risk for cancer from medical radiation exposure is very small (for instance, a child who underwent TC-MDP bone scan would have only a 0.04% lifetime increased cancer risk) [17]. According to the BEIR VII lifetime risk model, one person would be expected to develop cancer out of 100 people after a single exposure to 100 mSv (or 1 person per 1000 for a dose of 10 mSv [27,30]), whereas 42 of the 100 people would be expected to develop cancer from non-radiation causes [20]. According to BEIR VII, the lifetime attributable risk for the exposed population would be 510 excess cancers per 100,000 people exposed to 100 mGy (approximately 5 in 1000 LAR for fatal cancer) [27]. According to the NCRP report 115 the excess relative risk is 5 x 10-5 per person per mSv [33]. Thus, if each person in a population of one million received an effective dose of 10 mSv (1 rem), the expected number of fatal excess-cancer cases in the population over their remaining life-span would be 500, or an increase in the overall incidence of only 0.17% [33]. See other causes and their lifetime risk of death.

Much of the data regarding radiation risks stems from atomic bomb survivors (the Life Span Study [13]). The Radiation Effects Research Foundation (RERF) has studied the effects of radiation exposure to the atomic bomb survivors from Hiroshima abd Nagasaki [22]. That data demonstrated a significant increase in the incidence of various types of cancer in Japanese survivors that received whole body doses of 100 mSv or more [22]. However, there is disagreement regarding extrapolation of nuclear explosion data to low-level exposure [7]. The actual amount of radiation received by bomb survivors cannot be confirmed as there was no on-site radiation dose measurements [7]. Another key point to remember is that the Japanese survivors were exposed to whole-body radiation, a mixture of radiations more complex than the relative low energy x-rayt beams used in CT and medical imaging, and to radioactive fallout [22,24]. Therefore, the type and flux of radiation from atomic bombs is of a different quality compared to that from x-ray based medical imaging [7,19]. Atomic bomb survivors were also exposed to neutrons that are more carcinogenic [10].

There has been no evidence to support the theory that low level radiation produces life shortening effects in man. This is because cancer is common - for a standardized U.S population, BEIR VII predicted a baseline cancer incidence of 42% and a cancer mortality of 20% (1 in 5 risk of a fatal malignancy) [1,11]. The lifetime attributable risk (LAR) is used to estimate the likelihood for of radiaiton induced cancer over the lifetime of individuals exposed to ionizing radiation [22]. A dose of 0.01 Sv (10 mSv or 1 rem) would cause only 1 in 1000 lifetime cancers for all age groups according to the BEIR VII report [1,9]. The population weighted average lifetime excess risk of death from cancer following an acute whole body dose of 10 rems (0.1 Sv) increases by 0.8% (i.e.: There are 800 excess cancers per 1 million person-rems). However, to put this in perspective- the risk of dying from a pedestrian accident is 1.6 per 1000, and the risk of drowning is 0.9 per 1000 [19]. The French Academy of Science concluded there was not sufficient evidence to support an increased cancer risk with exposures less than 20 mSv [7]. A 15 country study of radiation workers found an excess relative risk for all-cause mortality of 0.42/Sv (0.00042/mSv) that was mainly due to an increase in cancer mortality [7].

The risk of cancer from ionizing radiaiton is thought to decrease with advancing age and the potential lifetime hazard of a given dose of radiation is higher in children and young adults (perhaps by a factor of 2-3, depending on age) [1,13,19,22]. A retrospective study demonstrated an increase in leukemia and brain cancer in children who underwent multiple CT scans at ages younger than 15 years, with an excess absolute risk of 0.83 excess cases of leukemia and 0.32 excess cases of brain cancer in 10,000 children recieving 10 mGy from a CT scan [22]. According to BEIR VII, if 1,000,000 10-year old children reveive a dose of 10 mSv, twenty-five will die as a result of this exposure at age 60 years [13]. This risk is 2.1 times higher than that for an individual receiving the same radiation dose at an age over 30 years [13]. The risk curve is felt to become relatively flat around the age of 40 years [19]. Certain patients with genetic mutations may also be at increased risk from radiation exposure [24]. Certain patients with ataxia-telegiectasia (a neurodegenerative disease) have an increased sensitivity to ionizing radiation that causes irreparable cell mutations that may place that at increased risk for carcinogenesis [24].

Mutagenesis, or radiation effects in the off-spring of radiated individuals, has not been demonstrated in humans (BEER 5), although it can be seen in plants and animals at high doses.

Non-stochastic (Deterministic) Effects: The severity of the effect varies with the dose (i.e.- increased risk with increasing dose) and the effects are not seen below a certain threshold level of radiation [16]:

1- Cataracts: A minimum dose of 200 rem (2 Sv) is required for the development of cataracts. The threshold for developing a vision impairing cataract under conditions of prolonged, fractionated exposure is felt to be 800 rem (8 Sv). However, an article that studied atomic bomb survivors, suggests that a radiation effect for the development of vision imairing cataracts can be seen at doses below 1 Gy (100 rads) and that standards for protection of the eye from brief radiation exposures should be 0.5 Gy (50 rads) or less [23].

2- Sterility: Temporary sterility in males can be seen following acute doses of 15 rem (0.15 Sv), while permanent sterility can be seen after doses of 350 rem (3.5 Sv). In females, permanent sterility can be seen following doses of 250 to 600 rem (2.5-6.0 Sv).

3- In utero exposure: Fetal doses (age 0-3 months) from phantom studies using a 16 slice CT are: renal stone 4-12 mGy, appendix 15-40 mGy, CT PE 0.24-0.66 mGy) [16- Invited commentary)]. It is not until a dose of 100-150 mGy (10-15 rad) that it is recommended the pregnancy be assessed for the need for intervention (termination) [16,20]. Theoretical risks at this level include a less than 3% chance of cancer development, a 6% chance of mental retardation, loss of IQ points by 30 points per 100 mGy, and a 15% chance of microcephaly, however, the risk s depend on the fetal age at the time of exposure [16]. See Table for Gestational Radiation Effects

Bystander Effects: Radiation induced bystander effects are biological responses in cells that were not traversed by an ionizing radiation track- that is- these responses occur in non-irradiated cells [5]. These effects may be the result of secreted signals from the irradiated cells [5]. These effects can be potentially beneficial (such as radioprotection) or detrimental (such as cytotoxicity) [5].

Potential Effects of In Utero Exposure: See also discussion in V/Q imaging in pregnancy

Long term effects of in utero radiation exposure include sterility, genetic effects, the induction of malignancies, and neurologic impairment. The teratogenicity of radiation is dose dependent, with the risk of fetal malformation increasing significantly at fetal doses above 150-200 mGy (15-20 rads) and fetal damage occurring at exposures greater than 500 mGy (50 rads) [20]. Prenatal death occurs if the radiation is received during perimplantation. During the first or second postconceptus week, in the preimplantation and preorganogenesis stages, a fetal dose of 50-100 mGy (5-10 rads) may cause the failure of blastocyst implantation and result in spontaneous abortion [20]. However,if the embryo were to survive, the radiation dose would not likely result in deterministic or stochastic effects in the liveborn child because the cells of the blastocyst are omnipotent and can replace damaged cells [20]. The most sensitive period for neonatal death to occur following radiation exposure is between 3 to 5 weeks gestation. The most sensitive or vulnerable period for congenital abnormalities is during the period of organogenesis (8-15 weeks gestation) [20]. During this period, exposures above 100-200 mGy (10-20 rads) can be associated with intrauterine growth retardation and CNS defects such as microcephaly and. mental retardation [20]. After the 15th gestational week, the fetus is less sensitive to radiation effects on the CNS [20].

The risk of malignancy, mis-carriage, or major malformations is negligible in fetuses exposured to 50 mGy or less (5 rads) [20]. Exposures below 50 mGy (5 rads) have not been shown to be associated with different pregnancy outcomes compared to fetuses exposed to background radiation alone [20]. However, other authors suggest a safe dose being 1 mGy or less (1 rad) as this dose is associated with an incremental risk of carcinogenesis of less than 1 in 10,000 [21]. Most radiologic procedures increase the risk for childhood cancer by less than one in 1000 [21]. Doses of 20-50 mGy (2-5 rads) can theorectically increase the overall risk of carcinogenesis to less than one in 250 [21].

Whenever discussing the potential risk s of inutero radiation exposure, one must also keep in mind that the risk of spontaneous abortion in normal pregnancy is about 15%, major malformation 3%, prematurity and growth retardation 4%, and mental retardation is 1% [20].

Gallium is the diagnostic nuclear medicine agent associated with the highest fetal radiation exposure, but the amount is still below 5 rads.

Mental Retardation:

Mental retardation can be seen following in utero radiation exposure and is the most commonly documented abnormality in humans who are prenatally exposed. The fetus is most susceptible to radiation delivered during the 8 to 15 week period, and somewhat less susceptible between 16 and 25 weeks. The risk of mental retardation is dose related and is only about 4% for a 10 rem (0.1 Sv) dose, while it may be as high as 43% for a 100 rem (1 Sv) dose. There may be a threshold for retardation between 20 and 40 rem (0.2-0.4 Sv), but this has not yet been proven.

Malignancy:

The carcinogenic effect of in utero exposure to ionizing radiation is less well established [20]. The increased risk of cancer in the fetus is estimated to be approximately 500 deaths before age 10 years among 1 million children exposed shortly before birth to 1 rad (2 x 10-4 per rad). The Oxford survey of childhood cancers demonstrated a 39% increase in malignancy from in utero exposure to 10 mSv [18]. The leukemia risk for the child is greatest if radiation was received during the first trimester.

 According to theICRP, the best quantitative estimate of risk is about one cancer per 500 fetuses exposed to 30 mGy of radiation, or 0.2% (this exposure would be at the high end ot the dose form a CT scan of the abdomen and pelvis) [20]. The ACR states that a dose of 20 mGy represents an additional projected life-time risk of about 40 additional cancers per 5000 babies, or 0.8% [20]. Other authors suggest that an exposure of 50 mGy doubles the risk of childhood carcinogenesis from 0.1% to 0.2% [31]. The relative risk of developing a fatal childhood cancer is higher following first trimester exposure, compared to exposure in the second or third trimester [20].

Low Level Radioactive Waste:

The major source of low level radioactive waste is the nuclear power industry- both in volume and activity. Medical sources produce the second largest volume of waste, but the activity is very low.

The maximum exposure to any member of the population near a LLNW facility must be less than 25 mrem/yr to the whole body, and less than 75 mrem/yr to the thyroid.

Radiation Workers:

BEIR V has replaced BEIR III.
 

The yearly permissible radiation dose:

The NRC defines the annual dose limits for radiation workers [29]. Persons that are likely to receive more than 10% of the occupational dose limit need to be monitored- most commonly with dosimeter badges [29].

    Radiation Workers:
     

      Whole Body (United States): 5 rem (50 mSv/0.05 Sv)/year [2,29] or 1 rem multiplied by age in years (cumulative dose). The International Commission on Radiation Protection recommends an occupational effective dose of 2 rem (20mSv)/year

      Lens: Originally 15 rem (150 mSv or 0.15 Sv), but ICRP has suggested a lens dose threshold for cataractogenesis of 0.5 Gy and recommended an equivalent dose limit for the lens of the eye of 20 mSv/yr averaged over defined 5 year periods, with no single year to exceed 50 mSv [34]

      Hands/Forearms: 50 rem (500 mSv/0.5 Sv)

      Skin: 50 rem (500 mSv/0.5 Sv)

      Other organs: 50 rem (500 mSv/0.5 Sv)

      Pregnancy and Lactation:
       

        A pregnant radiation worker cannot receive more than 0.5 rems (5 mSv [20,29]) during their entire pregnancy and the dose should be uniform monthly- 0.05 rems/month once the pregnancy is discovered.

        The National Council on Radiation Protection and Measurements (NRCP) in report No. 54 states that at doses less than 10 rem (0.1 Sv) any increased risk from radiation would be very small in comparison to the normal risks of pregnancy.

        Many radiopharmaceuticals are secreted in breast milk including pertechnetate, iodine, and gallium. As a general rule, lactating women should be instructed not to nurse until there is no detectable activity in the milk. For most technetium containing preparations the combined biologic clearance and physical decay make it safe to resume breast feeding after 24 hours. Gallium, however, requires cessation for weeks, and most authorities recommend that breast feeding be terminated after I-131 administration.


The occupational dose limits for minors are 10% of the dose limit for adults [29].

Non-radiation worker: 0.1 rem (100 mrem or 1mSv)/year (not including patients who received diagnostic or therapeutic radiopharmaceuticals). The dose exposure in any unrestricted area should not exceed 2 mrem/hr.


Monitoring devices are recommended for adults that are likely to receive a dose in excess of 10% pf the annual limit, declared pregnant patients that are likely to receive more than 0.1 rem during their entire pregnancy, and in individuals entering high or very high radiation areas.

A optically stimulated luminescence body badge can detect as little as 1 mrem for gamma/x-rays and 10 mrem for beta particles and are reusable. A thermoluminescent display is often used in a ring badge, can only be read once, and has a minimum detection of 30 mrem for gamma or x-rays and 40 mrem for beta particles. Other monitoring devices include pocket ionization chambers and electronic dosimeters.

Acute Radiation Sickness:

The onset of symptoms, severity of the illness, and survival are all dose dependent. A higher dose produces more severe symptoms, more rapidly. Acute radiation sickness is not seen until doses of about 100 rem at which time patients may experience anorexia and nausea. ARS begins with a prodromal phase of nausea and vomitting which can begin within minutes to hours of exposure and subsides over a few hours. A latent phase, or asymptomatic period follows and can last hours to weeks depending on the dose of radiation received. Subsequently, patients will develop overt ARS syndromes and either eventually recover or die.

ARS syndromes include:

1- Bone marrow suppression: Occurs with doses of about 100 rem

2- GI syndrome: Associated with doses of about 800 rem

3- CNS/Cardiovascular syndrome: Doses oof 2000-3000 rem. Characterized by deteriorating consciousness, respiratory depression, and increased vascular permeability.

The minimum lethal dose in man is about 200 rem. The median lethal dose or LD50 (ie: the dose required to kill 50% of the population within 60 days) is between 300 to 500 rem and is mostly related to the loss of bone marrow function. With antibiotic therapy the dose is about 450 rem. With very aggressive treatment, the LD50 can be as high as 1100 rem. Doses in excess of 1500 rem are always fatal, regardless of the amount oof intervention.

Radiation Posting:

1- Caution Radioactive Materials: Denotes an area or room in which radioactive material is used or stored.

2- Radiation Area: Denotes an area in which an individual may receive in excess of 5 mrem/hr (0.05 mSv/hr) or 100 mrem (1 mSv) in 5 consecutive days at 30 cm from the radiation source [29].

3- High Radiation Area: Denotes an area in which an individual may receive in excess of 100 mrem/hr (1 mSv/hr) at 30 cm from the radiation source [29].

4- Very high radiation area: Denotes an area in which an individual could receive an absorbed dose in excess of 500 rad (5 Gy) in one hour at 1 m from the radiation source [29].

Written Directive:

Certain radiopharmaceuticals require a written directive- an authorized users written order or administration of byproduct materials [29]. A written directive must be dated and signed by an authorized user before that administration of more than 1.11 mBq (30 uCi) of I131, any therapeutic dose of unsealed byproduct material, or any therapeutic dose of radiation from byproduct material other than I131 (including alpha emitters) [29]. The directive must include the radioactive drug, dose, and route of administration [29]. Written revision to an existing written directive may be made if the revision is dated and signed by an authorized user before administration of the dose of radiopharmaceutical [29]. Copies of written directives should be kept for at least 3 years [29]. If because of the emergency nature of a patients condition, a delay in providing a written directive would jeopardize the patient's health, an oral directive is acceptable [29]. A written directive must be prepared within 48 hours of the oral directive [29].

Release Criteria:

If the total effective dose equivalent to any other individual from exposure to the released individual is not likely to exceed 5 mSv, the individual can be released [29]. If the total effective dose equivalent to any other individual is likely to exceed 1 mSv, the released individual, or their parent or guardian, should be provided with instructions, including written instructions, on how to maintain ALARA to other individuals [29]. If the total effective dose equivalent to a nursing infant or child could exceed 1 mSv, written and oral instructions should also include guidance on interruption or discontinuation of v=breast feeding and information on the consequences of failure to follow the guidance [29].

Medical Event (Misadministration):

A licensee is required to determine the activity of a dose before administering it to a patient and is responsible for ensuring that the dose falls within a prescribed range or dose not differ from the prescribed dose by more than 20%, but this can vary by state and in some states the dose has to be within 10% [29]. A facility that obtains unit doses from a central nuclear pharmacy does not have to assay those doses in a dose calibrator (although most commonly do) [29]. If the dose is not directly measured in a dose calibrator, a calculation is required that is based on the dose determined by the licensed pharmacy or manufacturer and takes decay into account [29].

A medical event (formally a misadministration) is said to have occurred if one or all of the first three conditions occur in association with one or both of the 4th and 5th conditions [29]:

(1) the administration of a radiopharmaceutical other than the intended one; or (2) administration of a radiopharmaceutical to the wrong patient; or (3) administration of a radiopharmaceutical by a route other than the intended one;

AND (4) the dose administered differs from the prescribed dose by 20% or more (too high or too low) or falls outside the prescribed dose range; and/or (5) the effective dose equivalent is greater than 5 rem (50 mSv) or an organ or tissue dose is greater than 50 rem (500 mSv) or the shallow dose equivalent to the skin is greater than 50 rem (500 mSv).

For all medical uses of NRC-licensed radioactive materials, a "medical event" occurs if BOTH: 

  1. The dose administered differs from what was prescribed, and the difference meets the NRC's reporting requirements, AND

  2. One or more of the following occur:

  • the dose given is off by at least 20 percent from the prescribed dose, either too high or too low
  • dose exceeds 5 rem, 50 rem to an organ/tissue, or 50 rem shallow dose equivalent; exposure >5 rem to an embryo; exposure >5 rem to a nursing child
  • the wrong radioactive drug is used
  • the radioactive drug is administered by the wrong route
  • the wrong individual receives the dose
  • a dose is administered to the wrong part of the body and exceeds by 50 percent or more the dose that area should have received
  • a sealed source used in the treatment leaks.

For therapeutic medical events (misadministrations), the licensee must notify NRC, the patient's referring physician, and the patient or guardian (unless the referring physician feels that this would be inappropriate) within 24 hours or as soon as possible. A written report must then be sent to the NRC.

Diagnostic medical events (misadministrations must be promptly investigated by the radiation safety officer and the report kept for 10 years. The licensee must also notify the NRC and the referring physician within 15 days if the misadministration involves (1) the use of a by-product material not intended for medical uuse, (2) the administration of a dose over 5 times that prescribed, or (3) the patient received an organ dose of over 2 rem, or a whole body dose over 500 mrem.

Receipt of Radioactive Materials:

Licensees must ensure that packages containing radioactive materials are opened safely, and must keep records of package surveys [29]. Survey package at surface and at 1 meter using a survey meter and tested for removable contamination via a wipe test [29]. Taking possession and monitoring should occur within 3 hours of package receipt (or within 3 hours of the start of business if the package is delivered overnight) [29]. If activity in the package exceeds accepted levels (more than 2000 disintegrations per minute on the wipe test or an exposure rate above the allowed limits for the package (other also state that if contamination is suspected, a wipe test should be performed and no more than 0.01 uCi/100 cm2 should be removed) the shipper/package carrier and the NRC or state must be notified [29].

Shipping labels: Radioactive I - ≤ 0.005 mSv/hr or 0.5 mrem/hr; Radioactive II - > 0.005 mSv/hr, but ≤ 0.5 mSv/hr or 50 mrem/hr; Radioactive III - > 0.5 mSv/hr, but ≤ 2 mSv/hr or 200 mrem/hr [29].

Major versus minor spills:

The spill can be minor or major depending on the volume of the spill, amount of spilled radioactivity, size of the spill, and internal radiation hazard to personnel [29]. The definition depends on the radiopharmaceutical


Tc-99m: Minor < 100mCi; Major > 100 mCi
Tl-201:  Minor < 100 mCi; Major > 100 mCi
Ga-67:   Minor < 10 mCi; Major > 10 mCi
In-111:   Minor < 10 mCi; Major > 10 mCi
Sm-153: Minor < 10 mCI; Major > 10 mCi
I-131 :    Minor < 1 mCi; Major >1 mCi
P-32:      Minor < 1 mCi; Major > 1 mCi

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(2) J Nucl Cardiol 2006; Schleipman AR, et al. Occupational radiation dose associated with Rb-82 myocardial perfusion positron emission tomography imaging. 13: 378-384

(3) AJR 2006; Mascalchi M, et al. Risk-benefit analysis of x-ray exposure associated with lung cancer screening in the Itallung-CT trial. 187: 421-429

(4) J Nucl Med 2007; Schoder H, Gonen M. Screening for cancer with PET and PET/CT: potential limitations. 48: 4S-18S

(5) J Nucl Med 2007; Sgouros G, et al. MIRD continuing education: bystander and low-dose-rate effects: are these relavant to radionuclide therapy? 48: 1683-1691

(6) J Nucl Med 2008; Boreham DR, Dolling JA. Risks associated with therapeutic 131I radiation exposure. 49: 691-693

(7) AJR 2009; Mayo JR, Leipsic JA. Radiation dose in cardiac CT. 192: 646-653

(8) J Nucl Med 2009; Sgouros G, et al. MIRD commentary: proposed name for a dosimetry unit applicable to deterministic biological effects- the Barendsen (Bd). 50: 485-487

(9) AJR 2009; Pflederer T, et al. Image quality in a low radiation exposure protocol for retrospectively ECG-gated coronary CT angiography. 192: 1045-1050

(10) Radiology 2009; Tubiana M, et al. The linear no-threshold relationship is inconsistent with radiation biologic and experimental data. 251: 13-22

(11) Radiology 2009; Sodickson A , et al. Recurrent CT, cumulative radiation exposure, and associated radiation-induced cancer risks from CT of adults. 251: 175-184

(12) AJR 2009; McCollough CH, et al. In defense of body CT. 193: 28-39

(13) J Nucl Med 2009; Fahey FH. Dosimetry of pediatric PET/CT. 50: 1483-1491

(14) Radiology 2009; Mettler FA, et al. Radiologic and nuclear medicine studies in the United States and worldwide: frequency, radiation dose, and comparison with other radiation sources- 1950-2007

(15) AJR 2010; McCollough CH, et al. How effective is effective dose as a predictor of radiation risk? 194: 890-896

(16) Radiographics 2010; Wieseler KM, et al. Imaging in pregnant patients: examination appropriateness. 30: 1215-1233

(17) J Nucl Med 2011; Fahey FH, et al. Minimizing and communicating radiation risk in pediatric nuclear medicine. 52: 1240-1251

(18) J Nucl Cardiol 2011; Williams KA, Ballapuram K. Radiation exposure in diagnostic imaging- use, misuse, or abuse? Part I: the background and science of medical radiation. 18: 566-569

(19) Radiology 2012; Brink JA, et al. Informed decision making trumps informed consent for medical imaging with ionizing radiation. 262: 11-14

(20) AJR 2012; Wang PI, et al. Imaging of pregnant and lactating patients: part I, evidence-based review and recommendations. 198: 778-784

(21) Radiographics 2012; Tremblay E, et al. Quality initiatives. GUidelines for use of medical imaging during pregnancy and lactation. 32: 897-911

(22) Radiology 2012; Hendee WR, O'Connor MK. Radiation risks in medical imaging: separating fact from fantasy. 264: 312-321

(23) Radiology 2012; Neriishi K, et al. Radiation dose and cataract surgery incidence in atomic bomb survivors, 1986-2005. 265: 167-174

(24) Radiology 2012; Boone JM, et al. Radiation Exposure from CT Scans: How to Close Our Knowledge Gaps, Monitor and Safeguard Exposure--Proceedings and Recommendations of the Radiation Dose Summit, Sponsored by NIBIB, February 24-25, 2011. 265: 544-55

(25) J Nucl Cardiol 2012; Mercuri M, et al. Tracking patient radiation exposure: challenges to integrating nuclear medicine with other modalities. 19: 895-900

(26) AJR 2013; Mayo J, Thakur Y. Pulmonary CT angiography as first-line imaging for PE: image quality and radiation considerations. 522-528

(27) AJR 2013; Costello JE, et al. CT radiation dose: current controversies and dose reduction strategies. 201: 1283-1290

(28) AJR 2014; Eisenberg JD, et al. The fisherman's cards: how to address past and future radiation exposures in clinical decision making. 202: 362-367

(29) AJR 2015; Baldwin JA, et al. All you need to know as an authorized user. 205: 251-258

(30) AJR 2015; Lam Dl, et al. Communicating potential radiation-induced cancer risks from medical imaging directly to patients. 205: 962-970

(31) Radiology 2017; Tailor TD, et al. Imaging heart disease in women. 282: 34-53

(32) J Nucl Med 2017; Siegel JA, et al. Subjecting radiologic imaging to the linear no-threshold hypothesis: a non sequitur of non-trivial proportion. 58: 1-6

(33) J Nucl Med 2017; Weber W, Zanzonico P. The controversial linear no-threshold model. 58: 7-8

(34) AJR 2017; Parikh JR, et al. Potential radiation-related effects on radiologists. 208: 595-602

(35) J Nucl Med 2017; Siegel JA, et al. Dose optimization to minimize radiation risk for children undergoing CT and nuclear imaging is misguided and detrimental. 58: 856-868


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