|CORAR comments, Part 2 of 2
COUNCIL ON RADIONUCLIDES AND RADIOPHARMACEUTICALS* POSITION PAPER ON SKIN CONTAMINATION DOSE LIMITS
In recent years there has been much interest in the nuclear power industry over discovery of personnel skin exposures from intense external beta sources. The realization that microcurie quantities of beta emitting radionuclides can delivery a very high dose in the order of hundreds or thousands of rads to small areas of the basal layer of the skin without significant effect had prompted the need to revise current dose limitation systems used throughout the world(1) (2). The International Commission on Radiological Protection (ICRP) is, consequently, in the process of reexamining recommendations regarding skin dose limitations. Both the U.S. Nuclear Regulatory Commission (NRC) and the National Council on Radiation Protection and Measurements (NCRP) are reviewing skin response due to high doses delivered to very small areas of the skin to establish appropriate dose limits. All these efforts are expected to result in revised recommendations and regulatory changes.
The primary focus of skin dose limitation is on beta sources commonly encountered in the nuclear power industry(3). However, it is now realized that, a similar situation exists in skin contamination from high specific activity radiochemicals used in other industries, research and medicine. Regulations in the U.S. which control skin dose(4) were originally intended to address large area exposure from external sources such as may be indicated by a dosimeter worn by a radiation worker. Dose limitations were based on the need to prevent deterministic effects and limit stochastic effects. The dose limits were based on the conservative assumption, that large areas of the skin are exposed, and the dose, therefore, could be indicated by the dosimeter located on the part of the body expected to receive the highest dose.
*Note: The Council on Radionuclides and Radiopharmaceuticals, Inc. (CORAR) members include the major manufacturers and distributors of radiopharmaceuticals, radioactive sources and research radionuclides, used in the U.S. for therapeutic and diagnostic medical applications and for industrial, environmental and biomedical research and quality control.
Recognizing that a small beta source can deliver a very high dose to a very small area of skin, the NRC has recommended assessing skin doses averaged over the maximally exposed
l cm2 to test for regulatory compliance(5) (6). In the absence of specific dose-limitations for contaminated skin, the regulatory practice in the U.S. has been to adopt existing regulations for doses to large areas for use in situations where small areas of skin are contaminated. Although there was no specific guidance on how to address skin contamination, some regulatory inspectors have been using the maximally exposed 1 cm2 for assessing compliance in skin contamination cases.
In recently revised NRC regulations(7) and in ICRP recommendations(8), guidance for complying with skin dose limits is clearly defined by recommending that the dose from skin contamination should be averaged over the maximally exposed l cm2. Although this regulation has now changed to an annual limit, the value of the limit is not substantially different from past practice. Applying current applicable limits, intending to limit stochastic effects due to large area exposures, to a 1 cm2 skin area will grossly over estimate the risk due to beta contamination concentrated in tiny areas of skin(1) (2).
In the U.S. the NCRP and NRC are in the process of drafting new recommendations and regulatory guidance, respectively, for skin dose limits. CORAR recommends that new recommendations and regulations be expanded to include not only the power industry but other users of radioactive materials to ensure that the dose due to skin contamination is placed in proper perspective.
Skin Dose Effects
The effects of ionizing radiation on skin are well known and include the induction of skin cancer, erythema, ulceration and late effects. For large-area uniform exposures at low doses, the risk of skin cancer is thought to be proportional to the area irradiated and to the average skin dose. The latent period for skin cancer is greater than 20 years and the mortality rate is very low. Hence, the risk of mortality from skin cancer due to irradiation with ionizing radiation is low compared with other tissues at risk in the body. For this reason, the ICRP assigns a conservative weighting factor, wt, of 0.01 for total body skin irradiation(9). This factor is conservative when mortality from skin cancer is the considered end point. A higher weighting factor my be necessary if the deterministic damage often produced in treating skin tumors is considered unacceptable and is to be avoided(10).
When an area of skin is irradiated non-uniformly or the area is much less than 100 cm2, the risk of stochastic effects is significantly reduced. For very small areas of 1 cm2 or less, stochastic effects are considered relatively unimportant and dose limitations are dominated by deterministic effects(10). Furthermore, repeated high irradiation of small areas due to skin contamination is unlikely in practice and it should normally only be necessary to consider acute effects in single exposure events.
Recent studies on the irradiation of human and pig skin has greatly improved our understanding of acute effects(11) (12). Irradiation of superficial epithelial cells with low energy betas such as from 147Pm (maximum beta energy 225 keV) can result in erythema and transient moist desquamation in a few weeks. Irradiation of epithelial cells and the vascular system at the base of the dermis with more penetrating betas from 90Sr/90Y (maximum beta energy 2.27 MeV) results in erythema and transient moist desquamation initially and dermal necrosis 10-16 weeks after exposure. The thresholds for these acute effects considerably exceed 2,000 rad and depends on the depth of penetration and area irradiated(10). The magnitude of these very high thresholds clearly indicate the need to review current regulatory practice.
Control of radioactive material is a highly developed technology and is generally effective in isolating contamination from personnel. However, significant cases of skin contamination do occasionally occur and should be provided for in the regulations. The nuclear power industry has been exemplary in publicizing incidents involving small beta sources. This has resulted in concerted efforts to assess the significance of these cases and the development of proper radiological perspective appears to be entering a final phase. Other users, handling unsealed radioactivity have not witnessed the difficulties in handling external beta sources that are experienced by the power industry. However, the very complex biochemical applications used in research and medicine, with generally smaller quantities of radioactivity than are present in the fuel cycle, necessarily entails more intimate handling and, consequently, a higher risk of personnel contamination. Of particular interest are high specific activity labeled organic compounds which can penetrate or be carried with solvents through protective clothing and either absorb in skin or attach to the surface of the skin.
Personnel may occasionally contaminate large areas of skin, but more typically fixed skin contamination is confined to the hands to very small areas from 0.01 to 5 cm2 in extent. In addition unprotected forearms and facial surfaces can be contaminated by small spattered droplets. Immediate attempts to decontaminate may remove contamination from the general contaminated surface area but leave small spots elsewhere that are more difficult to remove. Contaminated skin areas less than 1 cm2 are often due to migration through pin holes in apparently intact and appropriate gloves. Discussions with a wide range of radioactivity users have implied that small area skin contamination can be easily missed during routine surveillance, although this is less likely since large-area "pancake" - GM tubes and NaI (Tl) detectors are becoming more common in conjunction with more careful self monitoring.
Most skin contamination can be easily removed simply by washing with soap and water. In most cases, routine washing at the end of an operation will suffice to remove the contamination in time to prevent a significant dose. However, there is an obvious benefit for proper and frequent monitoring to ensure that any skin contamination is quickly detected and removed. In many facilities the workers may have to carry out this monitoring in the presence of a radiation field or may have to leave and subsequently re-enter a radiation field to monitor possible contamination during the course of an operation. Hence extra vigilance in contamination monitoring can result in extra external exposure due to extra time in entering and leaving radiation areas in addition to the time needed to complete productive work. It is clear that proper radiation protection must be based on a correct balance between the risk associated with small area skin contamination and the risk due to external dose to larger areas of the body. For this reason it is important that an appropriate dose-limitation system is applied to contaminated skin.
Although most significant cases of skin contamination involve the irradiation of very small areas of skin, it is recognized that there are other less frequent situations that should be treated differently. Uniform wide area contamination should involve a more restrictive dose limit than non-uniform localized contamination. This also applies to cases of contaminated clothing that may have moved during activities and consequently caused a wide area of skin to be irradiated. Certain labelled compounds, especially halogenated acids and other compounds that readily complex with proteins, will penetrate protective clothing and skin and then prove difficult to remove unless harsh decontamination methods are used. Normally, emissions from low energy beta emitters, such as 14C, 35S and 3H, deposited on the surface of thick skin cannot penetrate the skin to reach live tissue. If, however, the radionuclide is absorbed into the skin, the close proximity of these beta emitters to live tissue can result in prolonged significant irradiation. These, less common, skin contamination situations can result in a wide range of doses according to the specific circumstances. They might require careful study to derive a correct dose assessment. Generally, it is not difficult to identify these special situations. Although dose assessment may be complex in certain rare situations, the dose-effect relationships are similar.
CORAR believes that it is now practicable to establish a dose limit for skin contamination that properly reflects the risk and that is compatible with dose limits established for other tissues and organs in the body. The dose limit for skin contamination is particularly needed for beta emitting radionuclides and should be considered for those radiochemicals that can contaminate the surface of the skin and for those that can be partially absorbed into the skin.
CORAR urges that skin contamination dose limit recommendations should be established to enable the development of appropriate regulations applicable to all radioactive material users in both industry and institutions.
1. Moeller, D.W. "The Hot particle" Problem -- A Continuing Challenge", Radiation Protection Management, Vol. 6, No. 5 (Sept./Oct. 1989) pp. 57-61.
2. Chabot, G.E., and Skrable, K.W. "Beta-gamma point source on the skin problem - activity estimation and dose analysis", Health Physics, Vol. 55, No. 5 (November), 1988, pp. 729-739.
3. NCRP, "Biological Effects and Exposure Limits for "Hot Particles"", NCRP Report No. 130, 1999.
4. U.S. Nuclear Regulatory Commission, "10 CFR 20.101(a)".
5. U.S. Nuclear Regulatory Commission, Information Notice No. 86-23, "Excessive Skin Exposures Due to Contamination with Hot Particles", 1986.
6. U.S. Nuclear Regulatory Commission, Information Notice No. 90-48, "Enforcement Policy for Hot Particle Exposures", 1990.
7. U.S. Nuclear Regulatory Commission, "10 CFR 20.1003, 20.1201 (a)(2)(ii)".
8. ICRP "1990 Recommendations of the International Commission on Radiological Protection", ICRP Publication 60, Annals of the ICRP, Vol. 21, No. 1-3, 1990.
9. ICRP "Statement from the 1978 Stockholm Meeting of the ICRP", ICRP Publication 28, Annuals of the ICRP, Vol. 2, No. 1, 1978.
10. Charles, M.W. "General considerations of the choice of dose limits, averaging areas and weighting factors for the skin in the light of revised skin cancer risk figures and experimental data on non-stochastic effects", INT. J. RADIAT. BIOL., 1990, Vol. 57, No. 4, pp. 47-51.
11. Hopewell, J.W., Coggle, J.E., Wells, J., and Hamlet, R., Williams, J.P., and Charles, M.W., "The acute effects of different energy beta emitters on pig and mouse skin", British Journal of Radiology, Suppl. 19, 1986, pp. 47-51.
12. Charles, M.W., Hopewell, J.W., and Coggle, J.E., "Recent trends in radiobiology of skin and repercussions for dose limitation and personal dosimetry", Radiation Theory and Practice, Proceedings of the 4th International Symposium of the Society for Radiological Protection, Malvern 1989 (IOP, Bristol) pp. 419-424.