Radiation Safety

The discussion of radiation exposure is complex at best. There has been extensive research and publication on the subject to date. Briefly, the radiation doses delivered to patients for medical imaging are far below levels considered dangerous or proven to cause identifiable adverse effects. A short discussion of the subject has been broken into three sections as below. Please chose the section most applicable for your questions. RIC recommends reading the first section to familiarize yourself with radiation dosing and definitions before moving on.

The following data originates from the collective information available from the American College of Radiology Practice Guidelines, American Nuclear Society, American Cancer Society, Centers For Disease Control, and the Life Span Study regarding observations from the atomic bomb survivors.

  • Environmental and Medical Radiation Exposure and Dosing

      Before a discussion concerning medical radiation exposure can proceed, one must first understand the world in which we live. Aware or not, every substance and living creature on this earth is radiated every second of its existence. The culprit responsible for this fact is the environment we inhabit. Environmental radiation takes three forms: sun, soil and sustenance.

      Cosmic radiation originates from the sun. The absolute amount varies depending upon elevation. Within the United States, the amount is highest in the Rocky mountains and lowest at sea level mainly along the coasts.

      Terrestrial radiation comes from radioactive compounds in the earth mainly uranium and radium. This level varies with thickness of the earth’s surface. Again, lowest near coastal areas and highest in mountainous regions.

      Ingested radiation comes from the substances our bodies take in. Food contains radioactive carbon and potassium. Water contains dissolved radon. The air we breathe contains radon gas.

      The tables below depict the average radiation exposure experienced from the environment as well as average delivered dosage from common medical imaging procedures as a comparison.

      For reference, the unit of radiation exposure is the Rem. 1000 milliRem (mRem) = 1 Rem. The unit of radiation absorption is the Rad. 1 Rad (1000 mRad) = 1 Rem (1000 mRem).

                                                       Environmental Exposure

      Cosmic/yr                    26 mRem (sea level)  -  52 mRem (Denver)

      Terrestrial/yr                16 mRem (coastal)    -   63 mRem (Denver)

      Ingested/yr                                     268 mRem average

       Total/yr                                            310-383 mRem

       Trivia Note                    Airline travel = 0.5 mRem/hr of flight

                                 Smoking 1/2 pack cigarettes/day = 18 mRem/yr

                                                         Imaging Exams

                    X-ray Exams                                    CT Exams

      Chest 2view          10 mRem                     Head             200 mRem

      Mammo                42 mRem                    Chest            700 mRem

      Spine-neck           20 mRem               Abdomen/Pelvis  1000 mRem  

      Spine-lumbar        600 mRem                   Spine           1000 mRem

      Abd 3view            700 mRem              CT Angiogram     2000 mRem

      Abd 1view            250 mRem

      Hip/pelvis             60 mRem

      Extremity             0.5 mRem

      Upper GI              600 mRem

      Lower GI              800 mRem

      As one can see radiation exposure from imaging procedures is comparable to environmental levels in many cases or within a few orders of magnitude. Computed tomography exams deliver the highest doses although these values are ever decreasing with technology advancements. RIC conducts itself by the ALARA axiom which stands for AS LOW AS REASONABLY ACHIEVABLE. Imaging protocols are tailored for each patient and type of exam. You can be sure you or your patients are receiving the lowest possible effective dose needed for diagnosis without compromising accuracy.

  • Adult/Childhood Radiation Effects

      The effects of radiation exposure are classified into deterministic and stochastic categories. Deterministic effects involve injury to tissues or organ structures. The effects do not occur until a threshold dose is reached and severity is determined by how far the threshold is exceeded. The effects can be reversible such as radiation burns, mucus membrane necrosis or bone marrow suppression. Irreversible effects can occur such as organ malformation during development. For the most part deterministic types of injury require very high levels of radiation exposure only seen in relation to nuclear accidents. The levels are orders of magnitude above imaging parameters and will not be discussed further in this section.

      Stochastic effects result from single cell injury involving damage to the cell’s DNA. These effects have no threshold dose and it is believed can occur from any dose of radiation exposure. The results are an all or none phenomena without reversibility or a severity application. These effects mainly deal with cancer induction or reproductive gene mutation.

      Since the main concern then is radiation induced cancer let us focus on that discussion. First, we need to understand the origin of the data. Initially, radiation induced cancer rates were tabulated from results generated by the atomic bombing of Japan and Chernobyl accident. The rates were based on the population receiving large doses of radiation (20 Rads and more). In many cases the current statistics represent extrapolations from those doses to imaging levels. This assumes a linear relationship which has not been proven but in the interests of public safety are accepted to even further decrease potential risk to the patient. In reality, most studies have not been able to detect an increased risk of cancer among people exposed to medical doses of radiation.

      The primary radiation induced cancer is leukemia or myeloma followed by solid organ occurrence mainly thyroid, lung, skin and breast/ovary. In children induction is mainly leukemia and thyroid cancer. The calculated lifetime cancer risk from high dose radiation data is 10%/100 Rad which for adults extrapolates to 0.10%/Rad. Other sources estimate a probability of 0.000002/Rad/yr for developing leukemia. The newborn has increased radiation sensitivity and their lifetime risk of developing childhood cancer is reported as 0.4%/Rad. These risk assessments represent only marginal increases to the baseline which are approximately 0.05-0.07% for adults and 0.02-0.03% for the newborn.

      To put these numbers in perspective, the overall cancer risk would be no different between a person from Chicago having an average CT scan and a person who has lived in Colorado for 10 years. In addition, if we are to believe low dose risk data there should be documentable cancer rate differences by geographic region throughout the United States. These differences definitely do not exist, which bears answering whether this is all just a nice mental exercise. Suffice it to say there is a real proven risk to radiation exposure. However, within the realm of medical testing, the parameters set by the radiology community represent extreme precautions to ensure the safety and well-being of its patients.

      While these statements are true of single event exposures, multiple exposures are commonly assumed to be additive over time. The major concern here is a patient who requires multiple recurring CT examinations. Again, no concrete studies exist to guide us on this subject. Considering the background risk is negligible, adding another negligible percentage doesn't seem to be worrisome overall. In addition, the latency period for cancer induction is from 20-40 years. Since the majority of imaging is performed in patients near the last few decades of life, those patients will most likely not survive long enough to experience that risk. For younger patients, we must weigh the necessity to diagnose and treat a potentially life altering disease versus the very small potential of developing a cancer when we are elderly.

  • Radiation Exposure In Pregnancy

      Radiation administration to a pregnant patient is usually associated with extreme anxiety if not frank fear by both the patient and referring physician. To start with, this reaction is unwarranted and unfounded. Firstly, the only imaging exams with more than negligible radiation exposure to the pregnant uterus are radiographs or CT scans of the abdomen and pelvis, lumbar spine and hips. All other exams result in scattered radiation which is effectively blocked from the fetus by appropriate shielding universally utilized by RIC managed departments.

      Referencing the section on imaging dosage, exams directly delivering radiation to the uterus have a maximum of only three to four times yearly background radiation. This level of exposure would not be expected to yield adverse outcome although the potential for radiation effects depends on the stage of pregnancy. The first stage is from conception through week 2 of pregnancy. Research in animals causes miscarriage when radiation exposure occurs during this time period although the doses needed are 100 times those used in radiology exams. Extrapolation suggests a miscarriage rate of 0.5-1.0%/Rad. However, this statistic becomes meaningless when we consider the medically accepted rate of natural miscarriage is 50%.

      The next stage of discussion involves organ system development. Visceral organs develop between weeks 3-8. Radiation injury at this stage leads to growth retardation and mal-development. This is a deterministic type of effect and therefore has a threshold level. Experimentally the level is 15-20 Rad far above imaging doses. Between 9-25 weeks the major concern is the developing central nervous system. High doses of radiation lead to mental retardation and IQ deficits. However, no observed deficiencies have been observed below 10 Rad again far below imaging doses.

      The last stage of development, 26 weeks to term, is primarily concerned with stochastic effects namely cancer induction. In utero exposure may increase the childhood risk of leukemia with doses as low as 1 Rad. Using pelvimetry dose data, the increase is estimated to be from a background rate of 0.2-0.3% up to 0.3-0.7%. However, the data collected regarding children of atomic bomb survivors does not support this statistic. In all we are left with the same conclusion as the prior section. Although a real cancer risk is definable at high doses of radiation exposure, imaging doses are unlikely to pose a reasonable threat to the unborn baby. The risk should be no more than that caused by normal environmental exposure.