THE RADAR SITE:
News and Events
RADAR Home Page
RADAR ON-LINE DATA:
On-Line Decay Data
On-Line Kinetic Data
On-Line Model Dose Factors
Occupational Dose Factors
Monte Carlo Applications
External Point Source
Beta Dose to Skin
Immersion in Air
MEDICAL PROCEDURE DOSE CALCULATOR AND RISK LANGUAGE GENERATOR
Internal Dose System
External Dose System
Is it SAFE?????
As the title of this article notes, the issue constantly in the discussion of these regulations is: “Is it safe?” Very soon after the discovery of ionizing radiation, it was learned that radiation can cause harmful effects on the human body. With very high levels of absorbed dose, damage to skin or other organs can be observed within days or weeks, and death can occur as well, if doses are high enough and involve major organ systems like the bone marrow. At lower levels than this, but still at levels that are very much higher than routine exposures that occur in industries like medicine or nuclear power where people may be exposed to significant sources of radiation, no immediate effects will be observed, but decades later, higher rates of cancer may be seen in exposed populations. The most well known case in this category is the populations exposed to the atomic bombings in Japan in 1945. There are other populations included in risk models, but these individuals make up the bulk of the numbers of persons exposed and radiation doses received. The question asked in the 1950s and 1960s, and still being debated today, is “what levels of routine radiation exposure are SAFE” for radiation workers, members of the general public, and others exposed to ionizing radiation due to many beneficial uses of radiation, the most notable being use of radiation in the healing arts. It is important to note that the levels of radiation exposure associated with the practice of nuclear medicine are so small that no observed risk has been documented to these radiation workers or members of the public1. At these levels, the risks we are referring to are stochastic in nature, of which carcinogenesis is known to be the principal radiogenic stochastic risk. Despite any evidence-based radiation associated risk with nuclear medicine procedures, the NRC continues to regulate this medical discipline as if some untoward effects may have gone unnoticed and will soon be evident , despite 70 years of nuclear medicine practice. This is due to the regulatory requirement of ALARA (as low as reasonably achievable). In essence, because of the ALARA policy, NRC regulations continue to impose unnecessary constraints on the practice of nuclear medicine to continuously reduce radiation exposure levels.
At the relatively high dose and dose rate levels experienced by survivors of the Japanese bombings, excess cancers, above those occurring in the population without radiation exposure, were seen above certain dose levels. How to use that information to regulate routine daily exposure of radiation workers and others, who are exposed to dose and dose rate levels that are much lower, is a question that scientists and regulators are still grappling with. The table below shows excess leukemia incidence in the Japanese population as a function of estimated dose to the bone marrow:
Most solid cancers show a dose response function that is termed “linear-quadratic”, as it has a faster growth at higher doses than at lower doses, and a two part curve best fits the data. Leukemia is the cancer thought to be best fit with a linear function through all of the data. In the lowest dose category, including 0.1 Gy (10 rad), there are a total of four estimated excess cancers in about 31,000 persons, in which the normal incidence is nearly 70 cancers. It is important to note that these 4 cases are “estimates” as radiation-induced cancers, should they actually occur, are indistinguishable from those that occur as a natural consequence of life. In the entire population of over 86,000 persons, with some very high exposures (over 1 Gy or 100 rad), a grand total of 94 excess leukemias are observed. This shows that radiation is a weak carcinogen, although it is undoubtedly a carcinogen. The next table, from the recent report of the National Academy of Science’s Biological Effects of Ionizing Radiation (BEIR) committee, shows estimates of lifetime attributable risk, using a linear extrapolation of risks at high doses and dose rates, to an exposure of 0.1 Gy at a low dose rate.
The BEIR Committee’s preferred estimates of lifetime attributable risk (LAR) of incidence and mortality for all solid cancers and leukemia, with 95% subjective confidence intervals are shown in parentheses. These represent the number of cases or deaths per 100,000 exposed persons.
Again, the estimated cases are much smaller than the normal incidence. In fact, the estimated radiation-induced cancer cases are within the statistical uncertainty of the normal incidence cases expected in these populations. Therefore, it is virtually impossible to ascertain if any of these so-called “excess cases” would in reality even be observable. Besides, these estimates are based on the LNT model of radiation risk; such a model has not been applied to other stochastic effects, such as cancer caused by cigarette smoking. We know of no LNT estimates of the likelihood of cancer induction from smoking one cigarette a day or perhaps even one cigarette in a lifetime, yet the LNT model is freely used for regulatory purposes as if its results offer up something meaningful. In fact low doses of radiation, like an occasional glass of wine, may actually be beneficial.
Currently, the NRC is considering revising its regulations based on the latest ICRP recommendations. ICRP Publication 103, recommends that established NRC regulatory exposure limits of 50 mSv (5 rem) per year for radiation workers be lowered to 20 mSv (2 rem) per year. This is based on the observation of more cancers in the Japanese survivors that were reported between the publication of the third report of the Biological Effects of Ionizing Radiation (BEIR) committee of the National Academy of Sciences (NAS) and the fifth BEIR report2,3 . While more cancers were clearly observed, the interpretation of how to apply this observation to persons receiving much lower doses (such as medical patients undergoing diagnostic procedures involving radiation, radiation workers, etc.) is a matter of some discussion. In the BEIR V Executive Summary, it is stated that “The dose dependent excess of mortality from all cancer other than leukemia, shows no departure from linearity in the range below 4 sievert (Sv)…”, the same body of data was reviewed by the French Academy of the Sciences, and their experts expressed “doubts on the validity of using the Linear, No Threshold (LNT) hypothesis for evaluating the carcinogenic risk of low doses (< 100 mSv) and even more for very low doses (< 10 mSv). The LNT concept can be a useful pragmatic tool for assessing rules in radioprotection for doses above 10 mSv; however since it is not based on biological concepts of our current knowledge, it should not be used without precaution for assessing by extrapolation the risks associated with low and even more so, with very low doses (< 10 mSv), especially for benefit-risk assessments imposed on radiologists”4 . In fact, a recent article (Boreham reference, above) stated that increased risk warnings based on LNT extrapolation (and not actual data) need to be viewed with caution, considered hypothetical and even considered detrimental if improperly used for medical advice.
Current radiation regulations for radiation workers permit an annual dose to the whole body (and thus marrow) of 0.05 Sv (5 rem). Doses to members of the general public and others are assigned other numerical values. Take the following quiz, and see how well you know our current NRC regulations in this area.
How did you do? Be honest, we bet you got some of these wrong. And you are not to blame given the current inconsistencies in the various NRC regulations.
It’s VERY hard to say what is “safe”, isn’t it? The ICRP and the BEIR committee continue to take the very conservative approach by assuming that every radiation exposure, no matter how small, causes some small increment of risk, as yet unobserved rather than just “estimated.” This includes exposures received every day by all people on the earth due to background radiation, despite the wide variation throughout the world with no known relationship with cancer induction.
So if you were Dustin Hoffman in the movie, Marathon Man, how would you answer Sir Lawrence Olivier’s question: “Is it safe?”