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IV Process of Dose Reconstruction in NTPR Program This chapter discusses methods of dose reconstruction for atomic veterans that have been used in the NTPR program. Consistent with discussions on the principles and process of dose reconstruction in Section I.C, this chapter is organized as follows: Section IV.A discusses development of exposure scenarios; Section IV.B discusses methods used to estimate external dose from exposure to photons, neutrons, and beta particles; Section IV.C discusses methods used to estimate internal dose from intakes of radionuclides; Section IV.D discusses dose reconstructions for occupation forces in Japan; Section IV.E discusses methods used to account for uncertainties in estimates of external and internal dose; Section IV.F discusses the approach to estimating total dose from external and internal exposure, taking into account all radiation types and exposure pathways of concern; and Section IV.G discusses documentation of dose reconstructions and quality assurance. The committee’s evaluations of these aspects of the NTPR dose reconstruction program are presented in Chapters V and VI. As discussed in the standard operating procedures (DTRA, 1997) and in 32 CFR Part 218, the goal of the NTPR program is to obtain upper-bound estimates of dose to atomic veterans, consistent with the policy of giving the veterans the benefit of the doubt in estimating their doses (see Section I.C.3.2). More specifically, the goal is to obtain at least a 95th percentile upper bound of possible doses, taking into account uncertainties in estimating dose. That is, the NTPR program intends that a reported dose should exceed the true dose in at least 95% of all cases and that there should be no more than a 5% chance that the true dose to an individual is higher than the reported value. As discussed in more detail in Sec-
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tions IV.B and IV.C, it is the current policy of the NTPR program to report a central (“best”) estimate of external dose from exposure to photons along with an estimated 95th percentile upper bound, and the same approach has been taken in estimating external dose from exposure to neutrons in dose reconstructions for participant groups. However, only a single estimate of dose is reported when a beta dose to the skin or lens of the eye or an internal dose from intakes of radionuclides is calculated, and this estimate is intended to be at least a 95th percentile upper bound.1 IV.A EXPOSURE SCENARIOS Development of exposure scenarios for participants in the nuclear-weapons testing program involves consideration of assumptions about the locations of the participants of concern, their activities at those locations, and the time spent at each location and assumptions about the radiation environment at the assumed locations of the participants during the time spent at those locations (see Section I.C.2.1). Approaches to development of exposure scenarios used in the NTPR program are described in the standard operating procedures (DTRA, 1997) and in 32 CFR Part 218. The dose reconstruction process requires that the analyst first determine whether a veteran’s records support his qualifying as a “participant” according to the definition in applicable laws and regulations. In this initial stage, military records are used to confirm that the veteran was present at the Nevada Test Site (NTS) or the Pacific test sites during designated intervals before and after tests of nuclear devices, was present in Hiroshima or Nagasaki during the occupation of Japan, or was a prisoner of war near Hiroshima or Nagasaki at the time of the atomic bombings. The burden of proof in establishing a veteran’s status as a participant is stricter if a veteran’s claim is filed for a presumptive disease under 38 CFR 3.309 than for a nonpresumptive disease under 38 CFR 3.311 (see Section I.B.4). Once a veteran’s participation status has been confirmed and a dose reconstruction has been requested (usually by the Department of Veterans Affairs [VA] in response to a claim filed for alleged radiogenic health conditions), a government contractor (currently JAYCOR) undertakes extensive historical research based on archival records to reconstruct the movements and activities of the veteran during the period of participation. 1 An additional concept used in this report to represent the full range of uncertainty in a quantity is the confidence interval. For example, a 90% confidence interval of an uncertain quantity gives the range of values within which the true value should lie in 90% of all cases, and the lower and upper bounds of this confidence interval are the 5th and 95th percentiles, respectively.
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IV.A.1 Unit-Based Dose Reconstructions If nothing in the historical research suggests that the veteran was involved in unusual activities (that is, activities different from those of the other members of his unit), a “unit-based” dose reconstruction may be carried out. An example is participants in units who observed detonations at the NTS from trenches close to ground zero (see, for example, Figure IV.A.1). This one-size-fits-all strategy assigns the same dose to everyone in a given unit, with an upper bound assigned to allow for uncertainty in the estimated dose. FIGURE IV.A.1 Observers in the trench from which they observed a nuclear detonation.
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If no badge records are available, a unit-based dose reconstruction can be based on radiation-monitoring data that were obtained at the time of an operation as part of the test itself. With computer models, the measurements are interpolated and smoothed across space and time (allowing for the physics of radioactive decay) and then combined with historical summaries of the activities of the unit, including the likely path of the unit through the radiation environment, to reconstruct the dose for the unit. If the exact times spent in various locations are not known, assumptions are sometimes applied on the basis of the presumption that radiation-safety policies in force at the time of the test were followed. IV.A.2 Individualized Dose Reconstructions If a participant was involved in unusual activities, an individualized dose reconstruction is required. In such instances, there may have been complete or nearly complete badging during the entire time of participation.2 If so, and if the issue and turn-in dates for the badges of record are complete and cover the veteran’s entire time at the site, the badge readings are simply summed, and their variances are combined with a method called quadrature, in which the variance (error) of the summed dose is taken to be the sum of the variances of the individual readings, assuming independence of errors. The per-badge biases and variances are based on modifications of methods proposed in a previous National Research Council report on film badge dosimetry in atmospheric nuclear tests (NRC, 1989). One issue that often arises in the dose reconstruction process is related to the fact that participants often had a “permanent” badge, which was supposed to be worn throughout their entire time in an operation, plus occasional “mission” badges, which were issued on particular occasions when radiation safety personnel determined that a participant was likely to encounter an unusual potential for exposure. If the “permanent” badge was not worn on such occasions, the proper way to combine the two types of badge readings would be to sum them. If, instead, the two badges were worn contemporaneously, the mission badges can be ignored because any additional dose experienced on a particular mission presumably was already captured by the permanent badge. A dose reconstruction policy requiring the benefit of the doubt to be given to the veteran would require summing the two, and this was sometimes done. Because badging often was not complete or uncertainties remained (for example, because the issue or turn-in dates were missing—a common problem), an individualized dose reconstruction is required for some intervals of the veteran’s time of participation. The analyst must reconstruct the particular activities and locations of activities that the veteran would have undertaken in the 2 Complete badging of participants happened infrequently, most often for short-term participants during and after 1956 Operation REDWING in the Pacific.
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assumed radiation environment and apply modeled radiation levels to those activities and locations. Difficulty often arises in the reconstruction of the veteran’s experiences. In some cases, “cohort” film badging is used to assign a unit-based dose. When only a few members of the unit wore a badge during an operation, the mean of those few badge readings can be assigned to all members of the same cohort. The uncertainty in such dose assignments is computed from the variability in the badge measurements. Dose assignments based on film-badge data are discussed in more detail in Section IV.B.1 and IV.E.2. IV.A.3 Individualized Reconstruction of Scenarios The sources of historical information that can be used to reconstruct exposure scenarios include, in addition to such official documents as morning reports and ship logs, narratives written at the time, such as reports of unexpected changes in wind or fallout that complicated the management of radiation exposure for participants at specific tests. Individual information about a veteran’s job type or specific mission responsibilities is sometimes available. Other documents can contribute information on a person’s exposure scenario, such as questionnaires filled out by the veteran (especially early in the NTPR Program) or statements the veteran provided in support of his claim. Occasionally, other people are consulted to clarify uncertainties in what was experienced by a particular veteran, such as his commanding officer or others in the same unit. Surviving widows or children are sometimes contacted when the veteran is deceased, although they usually do not provide much detail. Some of the veterans had been sworn to secrecy for national-security reasons and never described their experiences even to their spouses. IV.B ESTIMATION OF EXTERNAL DOSE All estimates of external dose to participants are based on film-badge readings or surveys with field instruments. If the participant wore a film badge and the data could be located, the external gamma dose of record is generally based on those data. If no acceptable film-badge data are available or if the film-badge data do not cover all potential exposures, the external dose for these exposures is based on a “scientific” dose reconstruction that relies on survey data. For most participants, the reconstructed gamma and neutron dose from external exposure is based on a generic dose reconstruction performed for their particular units’ activities during a given test series, modified as appropriate to conform to a participant’s duties and period of exposure. It is important to note that the method used in the NTPR program to estimate external doses changed over time as shown below (Schaeffer, 2001a).
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Year Change in Methodology 1978 Individual services (Army, Navy, Air Force, and Marines Corps) report external doses based on film-badge dosimetry 1980 Neutron dose reconstruction added for unit dose reconstructions 1984 Statistical application of military-unit film-badge readings used when a veteran’s film-badge readings are missing 1988 Dose reconstruction applied to periods of incomplete film-badge coverage 1989 Upper-bound doses for individual film-badge data applied 1990 Doses from damaged film badges superseded by reconstructed doses 1992 Total upper-bound doses coupling the estimated uncertainty from film-badge data and reconstructed doses are calculated 1996 Upper-bound doses included in all reports in response to VA claims 1998 Skin-dose calculations added for all skin-cancer claims in response to a VA request When changes in policy or method are adopted, there is no systematic way to review earlier dose reconstructions or to apply the changes retroactively; this is discussed in more detail in Section VI.E. The method currently used in the NTPR program to estimate the most likely external gamma dose based on film-badge dosimetry, most likely gamma dose based on a scientific dose reconstruction, most likely neutron dose, and upper-bound beta dose to skin or lens of the eye are discussed in the following paragraphs. The method used to estimate upper bounds of gamma and neutron doses is discussed in Section IV.E.2. IV.B.1 External Dose Estimation from Film-Badge Data It is the policy of the NTPR program that if a film badge was issued and worn and valid film-badge data can be located, the film-badge reading is to be considered the dose of record for the period when the badge was worn (see Brady and Nelson, 1985). Thus, the policy for reconstructing a dose to a test participant is first to search for film-badge data. However, during the earlier test series, only a small fraction of test participants were badged. Attempts were made beginning in the 1956 Operation REDWING in the Pacific and the 1957 Operation PLUMBBOB in Nevada to badge all participants. During the earlier test series, mission badges were issued to participants to be worn when some radiation exposure was expected to occur because of the particular duties to be performed, such as maintenance on contaminated aircraft or recovery of contaminated equipment from displays. Civilian and military participants at Operation CROSSROADS numbered about 43,000, but only about 7,000 film badges were issued to the persons thought most likely to receive radiation exposure. Because multiple badges were issued to many of those personnel, only a small percentage of CROSSROADS participants had film-badge records. Often, one or more members of a unit per-
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forming similar duties would be issued a “cohort” film badge. For example, only one or two members of each platoon participating in maneuvers during tests at the NTS were badged (for example, see Frank et al., 1982). The data from this cohort badge would provide an estimate of the external dose to the entire group. Even if permanent or mission badges were issued (see Figure IV.B.1), often the badges or the data from them can no longer be located. For example, although many film badges were issued during Operation UPSHOT-KNOTHOLE in 1953, most of the data from them were lost, and only summary data can be located (for example, see Edwards et al., 1985). When film-badge data are reported but the data are considered suspect, the NTPR program requests a re-examination of the film by the Department of Energy. If the film is still on file, it is re-examined by a health physicist, and a determination is made as to whether the reading is questionable or highly suspect. Often, film was damaged by heat, water, or humidity, particularly in the Pacific during the REDWING and DOMINIC test series. That was particularly the case if a badge was worn for more than a few weeks (NRC, 1989). Problems with calibration errors also caused film data to be suspect (NRC, 1989). According to current policy of the NTPR program (Schaeffer, 1995; 2002b; 2002e), suspect film-badge data are discarded in favor of a scientific dose reconstruction. In a 1989 report, a committee of the National Research Council reviewed the method used in the NTPR program to analyze film-badge data (NRC, 1989). The FIGURE IV.B.1 Example of film badges worn by participants at atomic tests.
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review examined possible calibration errors and reported heat and water damage, and it recommended that bias and uncertainty factors be applied for each test series. It also recommended how the film-badge data should be reported and how estimates of uncertainty in individual readings and sums of multiple readings should be treated. The NTPR program claims in its letters to VA and the veterans that its reported film-badge data conform to the recommendations of the National Research Council report. However, the NTPR program does not follow the recommendations exactly but has modified them somewhat. In particular, the report recommended that a reported film-badge reading be divided by a bias factor to convert to a whole-body equivalent dose in rem. Most of the bias factor is intended to convert a film-badge measurement of exposure in air in roentgens (R) to a whole-body equivalent dose in rem. However, the NTPR program assumes that the film-badge exposure in R is a direct estimate of the shielded whole-body dose in rem (Klemm, 1989; Flor, 1992).3 Furthermore, all badge readings, rather than the bias-corrected doses inferred from them, are summed to get the total dose. The NTPR program asserts that that is done to preserve a one-to-one correlation with the film-badge record so that a veteran can see evidence that original records are being used in the dose reconstruction and to avoid the perception that the program is lowering recorded doses (Flor, 1992; Schaeffer, 2002b). Estimates of film-badge doses used in dose reconstructions thus are higher, by a factor of about 1.3 or more, than if recommendations by the National Research Council committee had been followed precisely. In assigning doses based on film-badge data, the NTPR program usually assumes that if a participant was issued both a mission badge and a permanent badge encompassing the same interval, the badges were worn concurrently, as required. Thus, doses recorded by mission badges were generally assumed to be included in the permanent-badge reading. However, if the sum of all mission-badge doses is greater than the permanent-badge reading, the higher value is to be used (Flor, 1992). Similarly, when mission badges were issued but no permanent badge was issued, the mission-badge data generally were adjusted by subtracting the reconstructed dose from routine exposure to fallout during the period that the mission badge was supposed to have been worn. This procedure is used because that dose presumably would have been included in the mission-badge reading. IV.B.2 External Gamma-Dose Estimation Based on Dose Reconstruction Because only a fraction of participants were issued film badges during the earlier test series and the time when badges were worn often covered only a portion of the time of potential exposure, methods have been developed to recon 3 The bias correction from exposure in air in roentgens to a whole-body equivalent dose in rem, which is approximately a factor of 0.7, was applied to film-badge data in 1990 as recommended by the 1989 National Research Council report, but was rescinded by the NTPR program in 1992.
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struct the external gamma dose to unbadged participants on the basis of available monitoring data and physical models. Generic average external doses have been estimated for all major units participating in each test series. The results of the generic assessments and the methods used to estimate external dose have been published in a series of reports issued mainly in the 1980s. Examples of such unit dose reconstructions include calculations for observers at NTS tests (Barrett et al., 1987), maneuver troops at NTS tests (Edwards et al., 1983; 1985), garrisons on the headquarters islands at the Pacific test sites on Enewetak and Bikini atolls (Thomas et al., 1982; 1983a; 1983b; 1984), sailors on support ships (Weitz et al., 1982; Thomas, 1983a; 1983b), boarding parties on target vessels (Weitz et al., 1982), and occupation troops at Hiroshima and Nagasaki (McRaney and McGahan, 1980). The published unit-dose reports referred to above are supplemented by internal memoranda in which daily doses are estimated for specific ships and islands and for smaller units at NTS tests (Frank, 1982; Ortlieb, 1991; 1995; Phillips, 1983; Thomas, 1985; Weitz, 1995a; 1995b; 1997). For example, the Ortlieb (1995) memorandum gives daily dose tables for seven support units at Operation UPSHOT-KNOTHOLE: the 505th Signal Services, the 412th Engineer Construction Battalion, the 3623rd Ordinance Company, the 77th Army Band, the 93rd Army Band, the 371st Evacuation Hospital, and the 163rd Quartermaster Laundry Company. On the basis of those unit dose reconstructions, the NTPR program assigned a generic dose to all participants in the units. Unless a formal dose reconstruction is requested as a consequence of a VA claim or a specific participant request to the Defense Threat Reduction Agency, the participant’s dose of record is generally based on either a film-badge measurement or the assigned average dose for the participant’s unit. The applicable unit dose reconstructions are usually the starting point for a scientific dose reconstruction for a specific person. IV.B.2.1 Unit Dose Reconstructions at the NTS External doses to military units participating as observers or in maneuvers at NTS tests were based on estimates of the location of troops versus time, as obtained from documented unit activity histories, mission plans, and rehearsals (Goetz et al., 1979; 1980; 1981). Shielding provided by vehicles, trenches, and so on, was estimated from radiation transport calculations (Edwards et al., 1983). Computerized interpolation schemes were used to estimate dose rates at various grid locations and times from the available exposure-rate data (Edwards et al., 1985). Separate dose estimates have been made for direct exposure to prompt gamma and neutron radiation in trenches, from exposure to fallout and from overhead debris clouds during observation of a shot from trenches or during post-shot maneuvers, from exposure to fallout-contaminated fields from previous shots during pre-shot rehearsals, and from observation of contaminated displays set up
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to study blast and radiation effects. For example, Table IV.B.1 summarizes calculated unit doses and upper and lower confidence limits of a 90% confidence interval for maneuver troops participating in tests during the UPSHOT-KNOTHOLE series at the NTS (Edwards et al., 1985). As indicated in the table, external doses from prompt radiation during observation of the tests from trenches were generally smaller than doses received from residual gamma radiation during post-shot maneuvers and touring of contaminated display areas (see Figures IV.B.2 and IV.B.3). The estimated confidence intervals are discussed further in Section IV.E.2. TABLE IV.B.1 Summary of unit dose reconstruction for maneuver troops at Operation UPSHOT-KNOTHOLE (Edwards et al., 1985); BCT = Battalion Combat Team. Estimated lower and upper confidence limits shown are doses to add or subtract from tabulated dose to obtain 5th and 95th percentiles of distribution, respectively.
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FIGURE IV.B.2 Troops leaving a trench shortly after a detonation. FIGURE IV.B.3 Army personnel examining equipment damaged during a nuclear detonation.
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Most of the estimated uncertainty is generally due to uncertainty in the assumed average exposure rate on a ship or an island or to uncertainty in field measurements of exposure rates for maneuver troops or observers. However, those averages are often based on sparse data. IV.E.2.2.1 Upper bounds in unit dose reconstructions at the NTS The generic unit dose reconstructions discussed in Section IV.B.2 generally also included estimates of uncertainty. As stated in Barrett et al. (1987), “where errors are determined, they are estimates of uncertainty in mean dose associated with the group activities. No attempt is made to predict the distribution of dose within a unit [and] departures by individuals from the average activity scenario are not considered.” Table IV.B.1 in Section IV.B.2.1 lists the total uncertainty (actually, the doses to be added or subtracted from the indicated dose to obtain the 5th and 95th percentile doses) estimated in a unit dose reconstruction for maneuver troops at Operation UPSHOT-KNOTHOLE at the NTS (Edwards et al., 1985). Maneuver troops observed the tests from trenches several thousand meters from ground zero. Shortly after the blast, they left the trenches and marched toward ground zero to predetermined objectives. After reaching their objectives, the troops proceeded to display areas to observe the effects of the test on various types of equipment. The calculated uncertainty range arises from two basic sources: the uncertainty in the gamma-radiation field and the uncertainty in the space-time scenario of troop movements. Errors in position, time, and gamma intensity are not independent, owing to the radiation-safety (rad-safe) constraint that limited troops to areas with exposure rates less than 2.5 R h−1 (except at Shot GRABLE). It was assumed that there was no violation of rad-safe limits, except for Battalion Combat Team (BCT)-B at Shot NANCY, so the assumption that troops kept close to the limits when detouring is expected to result in “high-sided” estimates of dose. Thus, the exposure rate of 2.5 R h−1 is assumed to apply without error under these conditions. The uncertainty is all assumed to be in the duration and path length of any detour. In the display areas, limits of advance were not always reported. An assumption was then made that all the displays within rad-safe limits were inspected; this assumption should tend to overestimate exposures. The timing of the troops’ march was based on the reported time of attack, time of arrival at the objective, and arrival at the pickup point. Reasonable march speeds (usually about 70 ± 20 m min−1) and display-area stay times (usually 5 min with a range of 2.5−10 min) were assumed to construct a scenario consistent with the known times. The most important influence of timing on the uncertainty in dose was assumed to be the time spent at the positions of highest exposure. The various sources of uncertainty were combined approximately because it was asserted that they could not be combined rigorously, owing to the disparity of
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their associated distributions. For each source of uncertainty, the limits on dose are interpreted in terms of error factors on the best-estimate doses given in Table IV.B.1. For example, for BCT-B (forward unit) at Shot NANCY, the dose to add to or subtract from the total dose of 2.4 rem (−0.7 rem and +1.5 rem) was determined by combining the error factors for the components described below:17 The contribution due to uncertainty in stay times at halt points and displays: −0.5 to +1.0 rem. The contribution due to uncertainty in march speed: −0.3 to +0.5 rem. The contribution due to uncertainty in gamma exposure rate: −0.0 to +0.4 rem. For both BCTs at Shot SIMON, the total uncertainty was assumed to be dominated by the time spent at the 2.5-R h−1 rad-safe limit. The total uncertainty in this case thus is assumed to be due entirely to the uncertainty in march speed and arrival and stay times. The error factors due to march-time uncertainty were estimated to be −0.06 and +0.12 rem, and the estimated error factors due to stay time were −0.13 and +0.27 rem, with a combined uncertainty range in total dose of 3.1 − 0.2 (= 2.9) to 3.1 + 0.3 (= 3.4) rem. The uncertainty analyses for other NTS unit dose reconstructions are similar to that described above, and the central (“best”) estimates generally are asserted to be “high-sided.” However, the estimated uncertainty ranges for various parameters vary from shot to shot and among units, depending on the available information and specific exposure scenario. IV.E.2.2.2 Upper bounds in unit dose reconstructions at Pacific test sites The Pacific-test-site unit dose uncertainties are usually based on the estimated coefficient of variation (CV) in the measured post-shot exposure rate at various locations. In some of the dose reconstructions for the later test series, where most participants were badged, the upper bound for reconstructed doses is based on a comparison with the standard deviation of the available film-badge measurements (for example, see Weitz, 1995b; 1997). The upper bounds based on the measured post-shot monitoring data generally assume that multiple exposures to fallout from the same shot are independent, that is, that a participant is exposed at random locations each time he is outdoors or topside on a ship. The upper bound is calculated by assuming that the uncertainty in the dose incurred during each interval of exposure on deck or outdoors to the same fallout (assuming three intervals per day totaling 40% of the day topside for ships and 60% outdoors for islands) is completely independent of the previous exposure, that is, that the participant’s location with respect to the distribution of fallout exposure-rate 17 The details of how the error factors were combined and any correlations assumed are not given by Edwards et al. (1985).
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measurements is completely random. The uncertainty in the sum of all doses outdoors or on deck is calculated by summing the variances in each individual dose (for example, see Thomas et al., 1982; 1984). The dose incurred below decks is assumed to be very low because of an assumed 90% shielding factor. The consequence of those assumptions is that the more time the participant is assumed to be exposed to the same fallout, the smaller the estimated fractional uncertainty in the total dose received. A default CV of 50% is usually applied to measured average exposure rates on the basis of survey data from ships on which about 30 or more measurements were made (for example, see Thomas et al., 1984). However, the CV based on the distribution of measurements on some ships or islands for some fallout events often was much greater. For many ships, only an average exposure rate was reported. If data are not available for a particular ship, data from a nearby ship or island are used because it is assumed that the amount of fallout would have been similar. Although an additional systematic uncertainty of a factor of 1.2 is asserted for the effective shielding factor18 (Thomas et al., 1984), it appears that this was not usually applied in practice. No uncertainty is apparently assumed in the decay rate or in the default CV in calculating these upper bounds. Upper-bound factors (ratios of upper bounds to central estimates) for participants exposed for an entire test series on a particular ship or island based on the above assumptions are tabulated in the relevant unit dose-reconstruction reports. The values are usually applied and referenced in the individual dose-reconstruction reports. Although the upper-bound factor should be greater than the tabulated values for participants exposed for only a part of the test series, it appears that the same upper-bound factor was used regardless of the time exposed. If it is assumed that all exposures outdoors (and indoors on islands) are random, an estimated average upper bound for participants exposed over an entire test series will be only about 10–20% greater than the central estimate (Thomas et al., 1982; 1984). IV.E.2.3 Upper-Bound Estimates of Neutron Dose An upper bound of the dose from external exposure to neutrons is also estimated in the relevant unit dose reports and is based on the estimated uncertainty in the transport-calculated exposure and the uncertainty in the shielding correction. Uncertainty in the quality factor for neutrons (an uncertainty in the relative biological effectiveness of fission neutrons versus gamma rays) has not been taken into account in estimating the upper-bound neutron dose. 18 The effective shielding factor is the product of the shielding factor weighted by the fraction of time spent indoors or below decks. For the default shielding factor of 0.5 used for troops billeted on residence islands and assumed to be indoors 40% of the time, the effective shielding factor is (0.5 × 0.4) + 0.6 = 0.8; for sailors on ships and assumed to be below deck 60% of the time, it is (0.1 × 0.6) + 0.4 = 0.46.
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IV.E.3 Uncertainty in Estimates of External Beta Dose The NTPR program does not perform uncertainty analyses for beta-particle dosimetry, relying instead on an assumption that estimates of dose are upper bounds (“high-sided”). Beta doses from contaminated ground or other surfaces, for example, are calculated by multiplying a presumably overestimated beta-to-gamma dose ratio by an upper-bound gamma dose. As noted in Section IV.B.4, the current methodology for assessment of beta-particle dose from sources external to the body is described in Barss (2000). The method has remained substantially the same since routine assessment of skin dose began in 1998, although numerical values of the beta-to-gamma dose ratios have evolved. IV.E.3.1 Exposure to Contaminated Ground As noted in Section IV.B.4.1, beta dose to the skin or lens of the eye from external sources is accrued simultaneously with gamma dose from radioactive fallout, contamination, or neutron-induced radionuclides. As a result, the beta dose is proportional to the gamma dose, and its magnitude can be mathematically expressed by a beta-to-gamma dose ratio. Uncertainty in the beta-to-gamma dose ratios is discussed by Barss (2000), and sources of uncertainty are identified, with emphasis on how simplification of the assessment process leads to estimates that are higher than the likely actual doses. It is stated that the uncertainty most difficult to quantify is that in the reduction in beta-particle fluence between the source and receptor locations, which is much more dependent than gamma fluence on shielding material, chemical and physical properties of the radionuclide and the surface, and distance from the source deposited on a surface. An example offered by Barss (2000) concerns deposition locally on the ground, for which large particles associated with tower shots provided substantial self-shielding. As a result of weathering, environmental transport, and dispersion, fallout particles may also penetrate to such a depth in soil as to substantially reduce beta doses compared with such material being on the surface. The inability to model the magnitude of each of these reduction effects adequately, and their degree of interdependence, are said to limit the usefulness of any attempt to quantitatively model their associated uncertainties. Another example offered by Barss (2000) concerns initial decontamination techniques used on ships and aircraft surfaces (washdown systems and fire hoses), which tended to remove loosely bound or attached particles but would “fix” residual material to the surface to an extent proportional to the surface porosity or accessible surface area. The implication is that after washdown the remaining contamination would be largely fixed in surficial pores, causing more attenuation of beta particles than of gamma rays and resulting in a lower beta-to-gamma dose ratio than if the material were truly on top of a surface.
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The Barss (2000) report indicates that calculated beta-to-gamma dose ratios 1 and 2 years after detonation are substantial overestimates because they ignore the effects mentioned in the two foregoing examples. The report further indicates that an enormous expenditure of resources would be needed to adequately describe and quantify the uncertainties in model parameters, given the high degree of variability in the environmental interaction with residual radionuclides. Additional resources would be required to further propagate the uncertainty associated with each model parameter to obtain an estimate of the overall uncertainty in a calculated beta dose. The report notes that although there are environmental models that attempt to achieve the objectives (quantify parameters and propagate uncertainty), their usefulness remains inversely proportional to their degree of scientific debate and interpretation. The discussion concludes that the best resolution of the dilemma (quantification of uncertainty), in the absence of a rigorous and scientifically appropriate approach to quantify and apply modification factors for environmental and particle effects, is consideration of field measurements. It is stated that, in some comparisons, the current beta-to-gamma dose ratios are in reasonably good agreement with previous calculations and available measurements and, at worst, overestimate the measurements by a factor of 2-3. There is no discussion of factors that might cause underestimation of beta doses, such as errors in estimating time since detonation and underestimates of distances from contaminated surfaces or exposure times. It is clear from Barss (2000) that application of the method is expected to result in a “high-sided” dose. IV.E.3.2 Immersion in Contaminated Air or Water As noted in Section IV.B.4.2, beta doses from immersion in contaminated air or water are calculated by using dose coefficients, durations of exposure, and composite beta-spectrum radiation energies associated with a reconstructed gamma exposure or film-badge reading. The calculated beta dose is added to the upper-bound gamma dose for the corresponding period. There is no discussion of uncertainty by Barss (2000) related to beta-particle doses from immersion, although it seems clear from examination of Figure IV.B.8 (see Section IV.B.4.2) that a small uncertainty in the time of onset of exposure could lead to a large uncertainty in a composite dose coefficient, particularly during the period shortly after detonation. IV.E.3.3 Skin Contamination As noted in Section IV.B.4.3, dose coefficients from Kocher and Eckerman (1987) can be used to calculate beta dose from skin contamination, with adjustments for backscatter and for the case in which a glove is contaminated. The
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VARSKIN code (Durham, 1992) can be used to calculate skin dose for specific source geometries. Skin-contamination measurements are recommended as the best source of contamination data from which to calculate dose, but methods are also suggested for using dose or exposure-rate measurements to estimate contamination. Barss (2000) does not discuss uncertainty related to beta-particle doses from skin contamination. IV.E.4 Uncertainty in Estimates of Internal Dose Estimates of uncertainty in calculated internal doses are not presented in dose reconstructions for individual atomic veterans or in unit dose reconstructions for participant groups. In all dose reconstructions that include an estimate of internal dose, the calculated dose is presented as a single value without uncertainty. Uncertainties in internal doses are also not evaluated or discussed in any detail in reports documenting the calculation methods (Egbert et al., 1985; Barrett et al., 1986). Thus, the treatment of uncertainty in estimated internal doses differs from the approach to addressing uncertainty in estimated doses from external exposure to photons. As discussed in Section IV.E.2, dose reconstructions for individual veterans often provide an estimated upper bound of the external photon dose, especially if the veteran filed a claim for compensation. Many generic dose reconstructions for participant groups also include a quantitative analysis of uncertainty in external photon dose. An upper-bound estimate of external photon dose is intended to represent a 95% confidence limit, and the difference between the upper bound and the central estimate indicates the magnitude of uncertainty. Upper-bound estimates of dose are important because, in accordance with the policy that the veteran will be given the benefit of the doubt (see Section I.C.3.2), the NTPR program intends that upper bounds will be used in evaluating claims for compensation. In the absence of a quantitative analysis of uncertainty in estimated internal doses, this uncertainty is addressed in the NTPR program by using an alternative approach mentioned in Section I.C.2.4. An argument is made that methods used to estimate internal doses incorporate assumptions that should result in overestimates of internal doses to participants. For example, the method of estimating dose from inhalation of resuspended fallout that was previously deposited on the ground or other surfaces (see Section IV.C.2.1) relies mainly on an assumption that resuspension factors that are applied in various exposure scenarios greatly overestimate the actual extent of resuspension of deposited fallout. On the basis of that type of argument, estimates of internal dose obtained in dose reconstructions are assumed to represent suitable upper bounds for use in evaluating claims for compensation; that is, the estimated doses are assumed to be “high-sided.” As discussed in Section IV.E.3, essentially the
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same approach to accounting for uncertainty is used in estimating external beta dose to the skin or lens of the eye. The committee reiterates that an approach of relying on “high-sided” assumptions to estimate credible upper bounds of possible doses, rather than an approach involving a quantitative analysis of uncertainty in a central estimate, is a reasonable way to address uncertainty. Furthermore, an upper bound so obtained is appropriate for use in evaluating claims for compensation. However, it is a valid approach to addressing uncertainty only if estimated doses are indeed “high-sided.” That is, on the basis of available information and scientific judgment, there must be a high degree of confidence that calculated internal doses do not underestimate actual doses to participants. Thus, an evaluation of methods used in the NTPR program to estimate internal doses to atomic veterans essentially involves an assessment of the extent to which the methods are likely to overestimate doses. The committee’s evaluation of the methods of estimating internal dose is presented in Section V.C. IV.F ESTIMATES OF TOTAL DOSE AND UNCERTAINTY FOR INDIVIDUAL PARTICIPANTS Although many participants have received a dose assessment from the NTPR program based on film-badge data in their medical records or their unit’s generic dose reconstruction, VA may request a formal dose reconstruction from DTRA to evaluate a claim for compensation (see Section III.B). A veteran may also request a detailed dose reconstruction by directly contacting DTRA. An individual dose reconstruction attempts to determine all possible pathways and sources of exposure for the participant on the basis of his military records and personal statement. The analyst reviews the assumed exposure scenario for the participant and modifies or recalculates the unit dose reconstruction according to the time exposed (which may have differed from the time assumed in the generic reconstruction), special duties or missions, available film-badge data, and so on. The analysis and dose estimate are reported in a detailed memorandum that specifies the assumed exposure scenario, exposure rates and decay rates, references to the applicable unit dose reports, and any other analysis methods applied (see Section IV.G.1). If the participant was exposed at various times and places, the memorandum reports the estimated dose from each exposure and sums the individual doses to determine the total for all exposures from all test series that the veteran participated in.19 The neutron dose in rem is added to the estimated whole-body gamma dose, and the total is reported to VA or the veteran when an external dose is reported. 19 For example, some veterans participated in multiple test series, both in the Pacific and at the NTS; some were exposed at various locations during the same test series, such as on different ships or islands; and some were on leave during parts of the time during a test series.
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Reconstructions of external dose that were done in recent years (1998 and later) also often include an assessment of dose to the skin or lens of the eye, particularly for participants claiming compensation for skin cancer or cataract, and these assessments include the contributions from beta exposure. Appendix A contains two examples of dose-reconstruction memoranda from sample cases reviewed by the committee. Some individual dose reconstructions are unique—that is, not covered by a generic unit dose-reconstruction method—and require a fairly complex dose assessment. For these cases, the dose-reconstruction memorandum details the assumptions made in estimating the dose. The total dose reported to VA or the veteran consists of the best estimate of the gamma-plus-neutron equivalent dose from all sources of external exposure and an upper-bound (95th percentile) estimate that combines the estimated upper bounds from each source of exposure and from estimates based on film-badge data and reconstructions. The neutron and gamma upper-bound estimates also are combined to estimate an upper bound for the sum. As discussed in Section IV.B, total upper bounds for external gamma-plus-neutron dose have been consistently reported in a formal dose reconstruction since 1996. The upper-bound calculations typically assume that exposures to different shots, or at different locations, are not correlated and can be combined in quadrature by summing the variances (Flor, 1992). In addition, DTRA is often asked by VA to provide upper-bound estimates for generic or film-badge doses that were previously provided to the veteran or VA for which upper bounds had not been estimated. Those upper-bound requests often result in the reporting of a revised central estimate based on a new method or new exposure scenario information. If a skin or eye dose from beta exposure is calculated, it is reported separately. Estimates of beta dose are already considered to be “high-sided,” so no additional upper bound is reported. If a claim involves a disease of a specific organ, and an internal (inhalation) dose has been calculated for that organ, the calculated organ dose is also reported separately. As discussed in Sections IV.C.2.1.7 and IV.E.4, this estimate is also intended to be “high-sided,” so no additional upper bound is reported. (Often, even though an actual “high-sided” inhalation dose estimate is provided, the inhalation dose is also reported as less than the screening criterion of 0.15 rem; see discussion of the low-level internal dose screen in Section VI.C). When no specific organ dose is calculated, a committed effective dose equivalent from inhalation is sometimes calculated (see Section IV.C.2). Again, it considered to be a “high-sided” estimate, and no additional upper bound is reported. Although the NTPR program does not combine external and inhalation dose estimates to estimate the total and upper-bound doses to a specific organ, the VA practice is to sum the reported external-dose upper bound (if an upper bound is provided) with the reported “high-sided” inhalation (internal) dose estimate to
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obtain an estimate of the upper bound of the organ dose. As discussed in Section III.E, the sum is used in the process of evaluating whether it was at least as likely as not that a veteran’s disease was caused by the radiation exposure. IV.G DOCUMENTATION AND QUALITY ASSURANCE IV.G.1 Documentation of Dose Reconstructions The documentation of dose reconstructions for individuals required by the NTPR program is specified in the standard operating procedures (SOPs) (DTRA, 1997). However, the discussion in the SOPs appears to be limited to the documentation that is to be sent to the veteran or his representative, rather than a complete documentation requirement. The SOPs state that: “In order to consistently serve the veteran, the veteran (or his representative) needs disclosure of the information that leads to his dose.” The documentation requirements are summarized as follows: Documentation pertaining to relevant generic (unit) dose reconstructions. All scenario and radiological information pertinent to the dose determination (explicitly or by reference). Detailed information or analysis not fully covered in previous documents, which is to be communicated in an individual dose memorandum attached to the case correspondence or in the body of the correspondence. Information that is too complex or generic or that otherwise detracts from the presentation of the individual dose memorandum or correspondence, which is to be covered by fact sheets (or other written material) distributed to the correspondent. Information on availability of cited formal reports and unpublished documents (subject to Privacy Act-related redactions), which is to be included in the case correspondence. Appropriate disclosure of other information, including representations from the time of the operation, such as operational summary data or data entered into individual records, even if such information is not corroborated; explanation of when this information, if it is not the most credible, is not retained in the final analysis; and other types of information that do not necessarily furnish the dose of record, such as dose entries in medical records and information contradicted elsewhere in records. Beyond the referenced information, an individual dose reconstruction or synopsis should include an explanation of what is specific to the veteran’s case, for example: The adaptation from a published report of the dose for the veteran’s period of participation.
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The principal source of uncertainty that affects the upper-bound dose. That the potential for an internal dose has been considered, even if the finding is of no internal dose. That a finding obtained from internal dose screening (see Section VI.C) applies to the veteran’s target organ. That internal dose assessments do not apply to assessments of dose to skin or lens of the eye. The reason for a change in estimated dose from previous correspondence if the change is based on new data (a change that results from a procedural change is addressed in the correspondence but not in a dose-reconstruction memorandum). Reporting of total doses in a dose summary, which is in tabular form if there are multiple contributions to external gamma or neutron dose. The specifications for documentation discussed above are related to what should be provided to the veteran. Also of concern to the committee is the detailed internal documentation of the dose reconstructions themselves, which is necessary to make detailed, independent reviews possible. This is discussed in Section VI.A. IV.G.2 Quality Assurance The committee did not see details of a formal quality assurance (QA) program in the SOPs (DTRA, 1997), and the files of individual dose assessments reviewed by the committee did not contain the expected indications of a systematic QA process. The committee was informed (Schaeffer, 2001a) that: “There are no additional quality assurance written procedures other than those provided to the committee. The SOP indicates what constitutes a quality dose reconstruction and directs review for conformity with the SOP’s procedures, and appropriateness and responsiveness to the correspondence or request received by the NTPR program. The DTRA Program Manager conducts the final review/approval.” The committee notes that the SOPs (DTRA, 1997) constitute more of a program overview than a detailed document that could guide specific day-to-day work, and they have little to say about QA. They do, however, specify the documentation discussed in the previous section that should accompany a dose assessment, to serve the veteran consistently, and that the assessment is to be reviewed (but not by whom). On further inquiry by the committee, additional information on QA for the dose reconstruction program was given in a letter from DTRA (Schaeffer, 2002e), which is provided in Appendix D. The letter indicates that QA had always been a key element in management and direction of the NTPR program and that the DTRA solicitation for NTPR program support contained a program-management
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requirement for QA monitoring, which was one of the contract-evaluation factors for award. In response to the solicitation, the contractor submitted a technical proposal that specified QA measures. The statement of work included in the DTRA solicitation for NTPR program support indeed contains the following requirement for quality assurance: “The contractor shall provide quality assurance monitoring for the NTPR Program in the areas of database management, dose assessment, and veteran assistance.” As stated by DTRA (Schaeffer, 2002e): “In response to the solicitation, JAYCOR/SAIC submitted a technical proposal that specified quality assurance measures in the program task areas of database management, radiation exposure assessment, and veteran assistance.” The committee did not have the opportunity to review the technical proposal submitted by JAYCOR and SAIC and consequently did not see the specified QA measures.
Representative terms from entire chapter: