| |
|||
| Index | Site Map | FAQ | Facility Info | Reading Rm | New | Help | Glossary | Contact Us | |
||
Regulatory Guide 8.36 - Radiation Dose
to the Embryo/Fetus |
(Draft was issued as DG-8011)
July 1992
Section 20.1208 of 10 CFR Part 20, "Standards for Protection Against Radiation," requires that each licensee ensure that the dose to an embryo/fetus during the entire pregnancy, from occupational exposure of a declared pregnant woman, does not exceed 0.5 rem (5 mSv). Paragraph 20.1208(b) requires the licensee to make efforts to avoid substantial variation above a uniform monthly exposure rate to a declared pregnant woman that would satisfy the 0.5 rem (5 mSv) limit. The dose to the embryo/fetus is to be the sum of (1) the deep-dose equivalent to the declared pregnant woman (10 CFR 20.1208(c)(1)) and (2) the dose to the embryo/fetus from radionuclides in the embryo/fetus and radionuclides in the declared pregnant woman (10 CFR 20.1208(c)(2)).
This guide is being developed to provide guidance on calculating the radiation dose to the embryo/fetus. Regulatory Guide 8.13, "Instruction Concerning Prenatal Radiation Exposure," provides instructions concerning the risks associated with prenatal radiation exposure.
Any information collection activities mentioned in this regulatory guide are contained as requirements in 10 CFR Part 20, which provides the regulatory basis for this guide. The information collection requirements in 10 CFR Part 20 have been cleared under OMB Clearance No. 3150-0014.
Calculating the radiation dose to the embryo/fetus from internally deposited radionuclides requires quantitative information about maternal radionuclide intake, placental transfer and kinetics, and resulting embryo/fetus radionuclide concentrations. Intakes of radioactive material occurring prior to the pregnancy may also be important if these materials remain in the pregnant woman during all or part of the gestation period. Transfer kinetics from the mother to the embryo/fetus are modeled as a function of stage of pregnancy, route of intake by the pregnant woman, and time after intake. The stage of gestation (or fetal development) is an important parameter in estimating radionuclide concentrations in the embryo/fetus. The geometry of the embryo/fetus (i.e., size and weight) affects the radionuclide dosimetry.
It is recognized that calculation of prenatal radiation doses from internally deposited radionuclides has many associated difficulties, including a lack of quantitative information about prenatal radionuclide concentrations and transfer across the placenta. The International Commission on Radiological Protection (ICRP) in Publication 56 (Ref. 1) states that, for most radionuclides, preliminary estimates from dosimetric and biokinetic models indicate that the dose to the embryo can be approximated by the dose to the uterus. The dose to the fetus is dependent upon the activity present in both fetal and maternal tissues. ICRP Publication 56 (Ref. 1) also states that, for most radionuclides, the dose to fetal tissue will be similar to or less than the dose to the corresponding maternal tissues.
The current methods available for assessing the radiation dose to the human embryo/fetus from internally deposited radioactive materials in the pregnant woman are subject to a number of uncertainties. Revison 1 to NUREG/CR-5631, "Contribution of Maternal Radionuclide Burdens to Prenatal Radiation Doses--Interim Recommendations" (Ref. 2), provides recommendations and methods for estimating the radiation doses to the embryo/fetus from internal radionuclides. In Revision 1 to NUREG/CR-5631, a number of radionuclides were evaluated. To expedite efforts, the initial evaluation was directed to those radionuclides that were expected to be of greatest significance for prenatal exposure in the work environment. The radionuclides that were identified and included were 3H, 14C, 57Co, 58Co, 60Co, 89Sr, 90Sr, 106Ru, 125I, 131I, 132I, 133I, 134I, 135I, 134Cs, 137Cs, 233U, 234U, 235U, 238U, 238Pu, 239Pu, and 241Am. The methods of Revision 1 to NUREG/CR-5631 are considered interim as efforts continue to further develop the bases and calculational methods for estimating prenatal radiation doses. Revision 1 to NUREG/CR-5631 provides details of the data and bases for the dosimetric features that were used for the radionuclides listed above.
It is expected that the embryo/fetus dose assessment methods will evolve over the next several years as more research is conducted in this area. As additional research is conducted, better estimates of actual embryo/fetus doses resulting from the exposure of the declared pregnant woman will be possible. For internal doses, research that categorizes the degree of placental transfer, the resulting embryo/fetus/placenta concentrations, and the potential radiation exposures of the embryo/fetus from radionuclides in their more usual chemical forms should simplify assessment of the dose to the embryo/fetus based on the maternal exposure. The ICRP is considering the formulation of dose assessment methods specific for the embryo/fetus.
This regulatory guide provides acceptable methods that may be used in determining the dose to the embryo/fetus. For internal exposure, a simplified approach and a more detailed methodology are presented for conducting dose evaluations. The regulatory position specified in Section 1 provides guidance on the threshold criteria for use in determining when the dose to the embryo/fetus needs to be evaluated. The regulatory position specified in Section 2 presents a simplified approach for estimating the dose to the embryo/fetus from intakes by the declared pregnant woman. The regulatory position specified in Section 3 provides an alternative, more detailed methodology for a limited number of radionuclides, using the gestation-time dependent dosimetric data from Revision 1 to NUREG/CR-5631 (Ref. 2).
A graded approach for determining when to evaluate, with both a simple and more detailed dose assessment methodology, is provided. Both methods are acceptable for evaluating the dose to the embryo/fetus. It is recognized that some licensees will only need to demonstrate that the dose to the embryo/fetus is not likely to exceed the 0.05 rem (0.5 mSv) monitoring threshold of 10 CFR 20.1502, while other licensees may need to determine an embryo/fetus dose for demonstrating compliance with the dose limit of 10 CFR 20.1208 and the recordkeeping requirements of 10 CFR 20.2106(e).
Appendix A provides information on and a table of dose equivalent factors for use in approximating the embryo/fetus dose from radionuclides in maternal blood. Appendix B is a table of blood uptake fractions for ingested activity. Appendix C contains tables of gestation-time dependent doses to the embryo/fetus following introduction of specified radionuclides and chemical forms into maternal blood. Examples of the use of dose assessment methods are provided in Appendix D.
The total radiation dose to the embryo/fetus is the sum of the deep-dose equivalent to the declared pregnant worker and the dose to the embryo/fetus from intakes of the declared pregnant worker. If multiple dosimetric devices are used to measure the deep-dose equivalent to the declared pregnant worker, the results of monitoring that are most representative of the deep dose to the embryo/fetus may be used. The licensee need not use the deep dose to the maximally exposed portion of the whole body of the mother as the deep dose to the embryo/fetus. The licensee may employ temporary or permanent shielding to reduce the deep dose to the embryo/fetus. Alternatively, deep dose to the embryo/fetus may be limited by placing more stringent restrictions on the exposure of the declared pregnant woman than on other members of the occupational work force.
As specified in 10 CFR 20.1208(a), the dose to the embryo/fetus from occupational exposure of the declared pregnant woman during the entire gestation period is not to exceed 0.5 rem (5 mSv). In addition, the licensee is required to make efforts to avoid substantial variation in the monthly exposure throughout the period of gestation. If the dose to the embryo/fetus is found to have exceeded 0.5 rem (5 mSv) or is within 0.05 rem (0.5 mSv) of this dose by the time the woman declares the pregnancy to the licensee, the licensee is required to limit the additional dose to the embryo/fetus to 0.05 rem (0.5 mSv) during the remainder of the pregnancy.
The tables in the appendices to this guide were prepared directly from the computer outputs, which led to the values generally being expressed to three significant figures. This indicates greater accuracy than is warranted by the dosimetry model, but the results are presented in this form to avoid roundoff errors in calculations. In general, final results should be rounded to the nearest thousandth of a rem.
The dose equivalent to the embryo/fetus should be determined based on the monitoring of the declared pregnant woman as required by 10 CFR 20.1502. Specifically, 10 CFR 20.1502(a)(2) requires monitoring the exposure of a declared pregnant woman when the dose to the embryo/fetus is likely to exceed, in 1 year, a dose from external sources in excess of 10% of the limit of 10 CFR 20.1208 (i.e., 0.05 rem). According to 10 CFR 20.1502(b)(2), the licensee must monitor the occupational intakes of radioactive material for the declared pregnant woman if her intake is likely to exceed, in 1 year, a committed effective dose equivalent in excess of 0.05 rem (0.5 mSv). Based on this 0.05 rem (0.5 mSv) threshold, the dose to the embryo/fetus should be determined if the intake is likely to exceed 1% of ALI (stochastic) during the entire period of gestation.
These monitoring thresholds will ensure that any potentially significant exposures to the embryo/fetus are evaluated and, as appropriate, doses are determined. The conditions specified in 10 CFR 20.1502(a) and (b) are based on a 1-year period. Prior to declaration of pregnancy, the woman may not have been subject to monitoring based on conditions specified in 10 CFR 20.1502(a)(1) and 10 CFR 20.1502(b)(1). In this case, the licensee should estimate the exposure during the period monitoring was not provided, using any combination of surveys or other available data (for example, air monitoring, area monitoring, bioassay).
The monitoring criteria contained in 10 CFR 20.1502 do not establish required levels of detection sensitivity. For some radionuclides it may not be feasible to actually confirm by bioassay measurements an intake of 1% of their stochastic ALI. Workplace monitoring, occupancy factors, and access control should be considered as appropriate in evaluating potential exposures and monitoring requirements.
The appropriate dose to be evaluated for the embryo/fetus is the dose equivalent for the duration of the pregnancy. An assessment of the 50-year committed dose is not appropriate. Also, it is not appropriate to use effective dose equivalent or committed effective dose equivalent. (Note: the committed dose equivalent to the uterus may be applied to the embryo/fetus under certain conditions as a simplified approach as described in the regulatory position specified in Section 2.)
According to 10 CFR 20.1208(c)(1), the deep-dose equivalent to the declared pregnant woman will be taken as the external dose component to the embryo/fetus. The determination of external dose should consider all occupational exposures of the declared pregnant woman since the estimated date of conception. The deep-dose equivalent that should be assigned is that dose that would be most representative of the exposure of the embryo/fetus (i.e., in the mother's lower torso region). If multiple measurements have been made, assignment of the highest deep-dose equivalent for the declared pregnant woman to the embryo/fetus is not required unless that dose is also the most representative deep-dose equivalent for the region of the embryo/fetus.
The internal dose to the embryo/fetus should consider the exposure to the embryo/fetus from radionuclides in the declared pregnant woman and in the embryo/fetus. The dose to the embryo/fetus should include the contribution from any radionuclides in the declared pregnant woman (body burden) from occupational intakes occurring prior to conception. The intake for the declared pregnant woman should be determined using air sample data, bioassay data, or a combination of the two. Guidance on bioassay measurements used to quantify intake is being developed and has been issued for public comment as Draft Regulatory Guide DG-8009, "Interpretation of Bioassay Measurements." Specific guidance on workplace air sampling is in Revision 1 to Regulatory Guide 8.25, "Air Sampling in the Workplace."
For continuous or near-continuous exposure to radioactive material that may be inhaled or ingested, the cumulative intake should be quantified and the dose determined at least every 30 days. If significant variation in the exposure levels may have occurred, the time interval for quantifying the intake should be reduced. More frequent evaluations should be considered as the potential dose to the embryo/fetus approaches the limit.
Maternal body burdens resulting from internal occupational exposures prior to conception should be included in determining the embryo/fetus dose. The contribution to the embryo/fetus dose from a maternal burden existing at the time of conception should be evaluated if the maternal burden at the time of pregnancy exceeds 1% of the radionuclide's stochastic ALI value for the appropriate mode of intake and class (for inhalation intakes). For multiple radionuclide burdens, the dose should be evaluated if the sum of the quotients of each burden divided by its stochastic ALI exceeds 0.01. Only body burdens existing at the time of conception need to be considered in evaluating this threshold; radioactive material already eliminated from the body should not be included.
This threshold of 1% ALI provides a simplified approach for determining when pre-existing body burdens should be evaluated. At this threshold, it is unlikely that any resultant dose to the embryo/fetus would be significant (i.e., greater than 10% of the 0.5 rem (5 mSv) limit). As an alternative, the dose assessment methods presented in the regulatory position specified in Section 3 of this guide may be used for determining whether a pre-existing body burden represents a potentially significant dose (i.e., greater than 0.05 rem (0.5 mSv)).
The determination of the dose to the embryo/fetus from the intake of radioactive material by the pregnant woman should be based on the best available scientific data. At present, the NRC staff considers Revision 1 to NUREG/CR-5631 (Ref. 2) to provide such data. For most radionuclides, the dose to the embryo/fetus will be similar to or less than the dose to the maternal uterus (Ref. 1). However, the data in Revision 1 to NUREG/CR-5631 indicate that for some radionuclides the embryo/fetus dose may be significantly different, either greater than or less than the dose to the uterus.
Based on these premises (uterus dose similar to fetal dose and the data in Revision 1 to NUREG/CR-5631 (Ref. 2)), a set of dose factors has been developed for use in calculating an embryo/fetus dose. Except for those radionuclides addressed in Revision 1 to NUREG/CR-5631 (Ref. 2), the dose factors presented in Appendix A to this guide represent the committed dose equivalent to the uterus per introduction of unit activity into the first transfer compartment (i.e., blood) of the woman.1 For the radionuclides in Revision 1 to NUREG/CR-5631, the dose factors in Appendix A represent the maximum dose equivalent to the embryo/fetus for the gestation period from the introduction of unit activity into the first transfer compartment of the woman at any time during the gestation period.
The dose limit for the embryo/fetus is expressed as a 9-month gestation dose equivalent. Particularly for certain radionuclides with both long radiological half-lives and long-term biological retention, the committed dose equivalent to the uterus may be significantly different from a 9-month gestation dose equivalent to the embryo/fetus. Several radionuclides of this type have been evaluated in Revision 1 to NUREG/CR-5631 (Ref. 2), and data have been developed for calculating an embryo/fetus gestation dose instead of using the committed dose equivalent to the uterus.
For demonstrating compliance with the dose limits of 10 CFR 20.1208, the dose factors in Appendix A may be used for approximating the embryo/fetus dose equivalent for the entire gestation period.
The steps for determining the embryo/fetus dose, using the simplified method, are as follows:
2.1 Include all the intakes by the declared pregnant woman at any time during the gestation period in the calculation of the embryo/fetus dose.
2.2 For ingested radionuclides, determine the activity uptake by the first transfer compartment (blood) by multiplying the intake (I) by the appropriate uptake factor (f1) from Appendix B (adapted from Federal Guidance Report No. 11, Table 3 (Ref. 4)). The uptake factor, f1, is the fraction of an ingested compound of a radionuclide that is transferred into the first transfer compartment (i.e., blood uptake fraction).
2.3 For inhaled radionuclides, determining the fraction of initial intake that is transferred to the blood involves an evaluation of the deposition in the three compartments of the lung and the subsequent time-dependent transfer to the body fluids and to the GI tract. Unless it is known otherwise, it should be assumed that the transfer from the lung to body fluids and from lung to GI tract to body fluids follows the ICRP 30 (Ref. 3) modeling (which is the basis for this guide).
2.4 For simplicity and conservatism in the modeling, the total uptake into the blood from the maternal intake is assumed to be instantaneous. However, for radionuclides with lung clearance class of W (10- to 100-day half-life clearance) or Y (greater than 100-day half-life clearance), the actual translocation from the lung and uptake in the blood may occur over a time period that exceeds the gestation period. Clearance from the lung may take up to several years. All the initially deposited material is not immediately available for uptake by the first transfer compartment (blood). However, an incremental transfer from the lung to the blood may be assessed based on the lung model as described in ICRP Publications 30 and 19 (Refs. 3 and 5).2
Table 1, adapted from the data in Figure 5.2 of ICRP 30 (Ref. 3), may be used for determining the total transfer from the lung to the first transfer compartment (i.e., blood), where f1 is the blood uptake fraction from Appendix B.3 The lung clearance class (D, W, or Y) for a particular chemical form of a particular radionuclide may be obtained from Appendix B to 10 CFR 20.1001-20.2401.
| Table 1 | |
|---|---|
| Transfer Fraction of Inhaled Activity to First Transfer Compartment | |
| Class | Transfer Fraction (TF) |
| D | 0.46 + 0.15 f1 |
| X | 0.12 + 0.51 f1 |
| Y | 0.05 + 0.58 f1 |
2.5 Based on the determination of the maternal intake, the dose to the embryo/fetus for the entire gestation period should be calculated using the following equations:
| DE = Σ Ii x f 1,i x DF1 | (Equation 1) |
| DE = Σ Ii x TFi x DF1 | (Equation 2) |
where:
DE = dose equivalent to the embryo/fetus for the entire gestation period from the acute intakes of all radionuclides during the gestation period (rem)
Ii = intake of radionuclide i by the declared pregnant woman at any time during the gestation period (µCi)
DFi = dose factor for use in approximating the dose equivalent to the embryo/fetus for the entire gestation period from the introduction of unit activity (1 µCi) into the maternal blood at any time during the gestation period, from tabular data presented in Appendix A to this guide (rem/µCi in maternal blood)
f1,i = the fraction of radionuclide i reaching the body fluids following ingestion (i.e., the fraction of ingested activity of radionuclide i that enters the blood), from data presented in Appendix B to this guide
TFi = transfer fraction of inhaled activity to the first transfer compartment (i.e., the fraction of inhaled activity of radionuclide i that enters the blood, see Table 1 of this guide)
2.6 For pre-existing body burdens, the total burden determined to exist at time of pregnancy should be assumed to be available for uptake in the blood of the woman. The dose should be assigned to the embryo/fetus as if the maternal blood uptake occurs within the first month of pregnancy. The embryo/fetus dose is calculated by multiplying the maternal burden of the radionuclide by its dose factor from Appendix A using the equation:
| DE = Σ Ai x DF1 | (Equation 3) |
where:
DE = dose equivalent to the embryo/fetus
Ai = maternal burden existing at time of pregnancy (µCi)
DFi = dose conversion factor (Appendix A)
This method provides a simplified and conservative approach for evaluating the significance of pre-existing conditions. If the embryo/fetus is likely to receive a dose in excess of 25% of the limit from pre-existing burdens (i.e., greater than 0.125 rem (1.25 mSv)), more detailed modeling should be considered.4
2.7 Doses from multiple nuclides or multiple intakes should be evaluated on a frequency corresponding to the determination of the intake. Multiple dose determinations should be added to determine the total dose. Doses may need to be reevaluated if better estimates of intakes are provided by followup bioassay measurements.
As an alternative to the simplified methods presented above, a gestation-time dependent dose to the embryo/fetus may be calculated for the radionuclides addressed in Revision 1 to NUREG/CR-5631 (Ref. 2). Revision 1 to NUREG/CR-5631 presents dosimetric methods for calculating the dose to the embryo/fetus following the instantaneous introduction of unit activity into the first transfer compartment (blood) of the pregnant woman at successive stages of gestation. These methods include the contribution to the embryo/fetus dose from the resultant body burdens of the declared pregnant woman and from activity in the embryo/fetus resulting from transfer across the placenta. Refer to Revision 1 to NUREG/CR-5631 (Ref. 2) for a detailed description of the modeling.
The methods and data of Revision 1 to NUREG/CR-5631 (Ref. 2) may be used for determining the dose to the embryo/fetus from maternal intakes at successive stages of gestation for the radionuclides 3H, 14C, 57Co, 58Co, 60Co, 89Sr, 90Sr, 106Ru, 125I, 131I, 132I, 133I, 134I, 135I, 134Cs, 137Cs, 233U, 234U, 235U, 238U, 238Pu, 239Pu, and 241Am.
The steps for determining the embryo/fetus dose using the Revision 1 to NUREG/CR-5631 (Ref. 2) methods are as follows:
3.1 The methods presented in the regulatory position in Sections 2.1 through 2.4 should be used for determining the uptake in the first transfer compartment (blood) of the declared pregnant woman.
3.2 Equations 1 and 2 of the regulatory position specified in Section 2.5 may be used for determining the embryo/fetus dose with the following clarifications:
3.2.1 For Equations 1 and 2, in place of the dose factor parameter, DF, the dose values should be taken from Appendix C to this guide for the time period representing the time of intake relative to stage of gestation. The data in Appendix C to this guide are for an absorbed dose (in rads) from the introduction of 1 µCi of the radionuclide into the first transfer compartment (blood) of the woman at the beginning of the specified month of gestation. To convert from an absorbed dose (rad) to a dose equivalent (rem), the data in Appendix C should be multiplied by the appropriate quality factor from Table 1004(b).1 of 10 CFR Part 20. For 3H, 14C, 57Co, 58Co, 60Co, 89Sr, 90Sr, 106Ru, 125I, 131I, 132I, 133I, 134I, 135I, 134Cs, a quality factor of 1 should be applied. For 233U, 234U, 235U, 238U, 238Pu, 239Pu, and 241Am, a quality factor of 20 should be applied, recognizing that most of the embryo/fetus dose results from alpha decay.
For some radionuclides (e.g., 235U), a blood uptake at the beginning of the gestation period results in a negligible dose contribution to the embryo/fetus. These radionuclides are identified in the tables in Appendix C to this guide by an "N" entry in the row for the 0-day of gestation at radionuclide introduction (i.e., the first row of dose factor data). For an intake of these radionuclides within the first month of gestation, a time-weighted dose factor using the second month data (31-day row) should be used. The 31-day dose factor should be multiplied by the quotient of the days-to-date in the first gestation month at time of intake divided by 30 days. For example, assuming a maternal intake of 14C resulting in a 1-µCi blood uptake on the 20th day of the pregnancy, the embryo/fetus dose should be determined by multiplying the cumulated dose from an intake at day 31 (i.e., Table C3, Cumulated Dose column, 1.89E-04 rads) by the ratio of 20 days to 30 days (i.e., 20 divided by 30).
3.2.2 For using the tabular dose data in calculating the embryo/fetus dose, it may be assumed that all intakes occurring within any of the 30-day periods of gestation occur at the beginning of that period.5 The cumulated dose column should be used in order to determine the total dose for the remainder of the gestation period.
3.2.3 For pre-existing body burdens from occupational exposure, the total burden determined to exist at time of pregnancy should be assumed to be available for uptake in the blood of the woman. The dose should be assigned to the embryo/fetus as if the maternal blood uptake occurs within the first month of pregnancy. The embryo/fetus dose is calculated by multiplying the maternal burden of the radionuclide by its dose factor (Equation 3). The dose factor to be used from the Appendix C tables is that factor corresponding to the cumulated dose for a 0-day of gestation at radionuclide introduction (i.e., right-most column, first data entry). However, for those radionuclides with an "N" for this 0-day entry, the entry for the second gestation month should be used (i.e., the right-most column, second data entry). Alternatively, time-dependent release kinetics may be used for calculating that fraction of the body burden that is translocated to the blood through the duration of the pregnancy. The time-dependent release is described in ICRP Publications 30 and 54 (Refs. 3 and 6). This approach is complex, involving interlinking differential equations, and is considered outside the scope of a routine health physics program.
3.3 Doses from multiple nuclides and multiple intakes should be evaluated with a frequency corresponding to the intake (i.e., at least once every 30 days). Multiple dose determinations should be added to determine the total dose. Doses may need to be reevaluated if better estimates of intakes are provided by followup bioassay measurements.
The purpose of this section is to provide information to applicants and licensees regarding the NRC staff's plans for using this regulatory guide.
Except in those cases in which an applicant proposes an acceptable alternative method of complying with specified portions of the Commission's regulations, the methods described in this guide will be used
in the evaluation of applications for new licenses, license renewals, and license amendments and for evaluating compliance with 10 CFR 20.1001- 20.2401.
Appendix B -- Blood Uptake Fractions for Ingested Activity
Appendix D -- Examples of Embryo/Fetus Dose Calculations
1. International Commission on Radiological Protection, "Age-Dependent Doses to Members of the Public from Intake of Radionuclides: Part 1," ICRP No. 56, Pergamon Press Inc., 1989.
2. M. R. Sikov et al., "Contribution of Maternal Radionuclide Burdens to Prenatal Radiation Doses--Interim Recommendations," NUREG/CR-5631, Revision 1 (PNL-7445), U.S. Nuclear Regulatory Commission, March 1992.
3. International Commission on Radiological Protection, "Limits for Intakes of Radionuclides by Workers," ICRP No. 30, Parts 1 through 4, including supplements, Annals of the ICRP, Volume 2, No. 3/4, Pergamon Press Inc., 1979.
4. K. F. Eckerman, A. B. Wolbarst, and A. C. B. Richardson, "Limiting Values of Radionuclide Intake and Air Concentration and Dose Conversion Factors for Inhalation, Submersion, and Ingestion," Environmental Protection Agency, Federal Guidance Report No. 11 (EPA- 520/1- 88-020), September 1988.
5. International Commission on Radiological Protection, "The Metabolism of Compounds of Plutonium and Other Actinides," ICRP No. 19, Pergamon Press Inc., May 1972.
6. International Commission on Radiological Protection, "Individual Monitoring for Intake of Radionuclides by Workers: Design and Interpretation," ICRP No. 54, Annals of the ICRP, Volume 19, No. 1-3, Pergamon Press Inc., 1988.
Footnotes
1 The committed dose equivalent factors for the uterus presented in Appendix A were calculated based on the modeling employed during the development of the ICRP 30 (Ref. 3) data. It is recognized that the metabolism of the pregnant woman may not be adequately represented by the standard metabolic model. However, partly because of the lack of more definitive data, this modeling has been used for determining the dose commitment factors for the uterus that may be used for evaluating compliance with the embryo/fetus dose limit.
2 As modeled in ICRP Publications 19 and 30, the clearance from the different lung compartments is assumed to follow first-order kinetics. This approach is complex, involving interlinking differential equations, and is considered outside the scope of a routine operational health physics program.
3 The coefficients for the transfer fraction equations in Table 1 are applicable to particles with a 1-micrometer activity median aerodynamic diameter (AMAD). As a default, these equations may be used for all particle sizes. However, if the actual particle size distribution is known, transfer fractions for other AMAD particle sizes may be derived from data in Figure 5.2 of ICRP 30 (Ref. 3).
4 This approach for evaluating pre-existing body burdens does not specifically address time-dependent releases as could occur for certain radionuclides with both a long biological retention and radiological half-life. However, the assumption of blood uptake of the total burden in the first month of the gestation period provides a simple method with reasonable assurance that any actual dose to the embryo/fetus will not be significantly underestimated. More detailed evaluations may be needed for unusual circumstances in which a pre-existing body burden could present a significant source of exposure to the embryo/fetus. An evaluation of this nature should be conducted by individuals knowledgeable in the area of internal dosimetry. Such a detailed evaluation could consider the element retention functions as presented in ICRP Publications 30 and 54 (Refs. 3 and 6). Also, the modeling presented in Revision 1 to NUREG/CR-5631 (Ref. 2) could be applied. The details of this type of an evaluation are beyond the types of analyses that are considered routinely required and, as such, are outside the scope of this guide.
5 The correlation of intake to actual stage of gestation can only be roughly estimated. For this reason, it is believed that the correlation should be limited to the best estimate of the month of gestation.
|
Privacy Policy |
Site Disclaimer |