DOE Openness: Human Radiation Experiments: Roadmap to the Project ACHRE Report |
ACHRE Report Part II Chapter 7 The Context for Nontherapeutic Research with Children Risk of Harm and Nontherapeutic Research with Children Beyond Risk: Other Dimensions of the Ethics of Nontherapeutic Research on Children The Studies at the Fernald School Conclusion |
Chapter 7: ConclusionIf an ethical evaluation of human experiments depended solely upon an assessment of the risks to subjects as they could reasonably be anticipated at the time, the radiation experiments conducted on children reviewed in this chapter would be relatively unproblematic.[97]During this time, the association between radiation exposure and the subsequent development of cancer was not well understood, and in particular, little was known about iodine 131 and the risk of thyroid cancer. Both researchers and policymakers appear to have been alert to considerations of harm and concerned about exposing children to an unacceptable level of risk.At the same time, however, the scientific community's experience with radionuclides in humans was limited, and this approach to medical investigation was new. Although the available data about human risk were encouraging and the biological susceptibility of children to the effects of radiation was not appreciated, we are left with the lingering question of whether investigators and agency officials were sufficiently cautious as they began their work with children. This is a difficult judgment to make at any point in the development of a field of human research; it is particularly difficult to make at forty or fifty years' remove. Investigators and officials had to make decisions under conditions of considerable uncertainty; this is commonplace in science and in medicine. Although the biological susceptibility of children was not then known, investigators and officials held the view that children should be accorded extra protection in the conduct of human research, and they made what they thought were appropriate adjustments when using children as subjects. If human research never proceeded in the face of uncertainty, there would be no such experiments. How little uncertainty is acceptable in research involving children is a question that remains unresolved. Today, we continue to debate what constitutes minimal risk to children, in radiation and in other areas of research. The regulations governing research on children offer little in the way of guidance, either with respect to conditions of uncertainty about risk or when risks are known. As best as we can determine, in eleven of the twenty-one experiments we reviewed, the risks were in a range that would today likely be considered as more than minimal, and thus as unacceptable in nontherapeutic research with children according to current federal regulations. It is possible, however, that four of the eleven might be considered acceptable by the "minor increase over minimal risk" standard.[98] In these four experiments, the average risk estimates were between one and two per thousand, the studies were directed at the subjects' medical conditions, and they may well have had the potential to obtain information of "vital importance." Physical risk to subjects is not the only ethically relevant consideration in evaluating human experiments. With the exception of the studies at Fernald, we know almost nothing about whether or how parental authorization for the remaining nineteen experiments we reviewed was obtained. And with the exception of the Fernald studies and the experiment at Wrentham, we know very little about the children who were selected to be the subjects of this research. Therefore, we cannot comment on the general ethics of these other experiments. The experiments at Fernald and at the Wrentham School unfairly burdened children who were already disadvantaged, children whose interests were less well protected than those children living with their parents or children who were socially privileged. At the Fernald School, where more is known, there was some attempt to solicit the permission of parents, but the information provided was incomplete and misleading. The investigators successfully secured the cooperation of the children with offers of extra milk and an occasional outing--incentives that would not likely have induced children who were less starved for attention to willingly submit to repeated blood tests. One researcher speaking almost thirty-five years ago set out the fundamental moral issue with particular frankness and clarity:
. . . we are talking here about first and second class citizens. This is a concept none of our consciences will allow us to live with. . . . The thing we must all avoid is two types of citizenry.[99]
It might have been common for researchers to take advantage of the convenience of experimenting on institutionalized children, but the Committee does not believe that convenience offsets the moral problems associated with employing these vulnerable children as research subjects--now or decades ago.
The Vanderbilt StudyIn an exceptionally large study[a] at Vanderbilt University in the 1940s, approximately 820 poor, pregnant Caucasian women were administered tracer doses of radioactive iron. Vanderbilt worked with the Tennessee State Department of Health, and the research was partly funded by the Public Health Service.[b] Today, most women take iron supplements during pregnancy. This experiment provided the scientific data needed to determine the nutritional requirements for iron during pregnancy.The radioiron portion of the nutrition study, directed by Dr. Paul Hahn, was designed to study iron absorption during pregnancy.[c] The women, who were anywhere from less than ten weeks to more than thirty-five weeks pregnant, were administered a single oral dose of radioactive iron, Fe-59, during their second prenatal visit, before receiving their routine dose of therapeutic iron.[d] On their third prenatal visit, blood was drawn and tests performed to determine the percentage of iron absorbed by the mother. The infants' blood was then examined at birth to determine the percentage of radioiron absorbed by the fetus. The doses to the women were estimated in the study article, using crude dose-estimation methods available at the time, to be from 200,000 to 1,000,000 countable counts per minute.[e] Although the investigators did not estimate doses to the fetuses in the original study, Dr. Hahn later estimated fetal doses to be between 5 and 15 rad. This estimate, however, has been questioned.[f] There is at least some indication that the women neither gave their consent nor were aware they were participating in an experiment. Vanderbilt study subjects, expressing bitterness at the way they believed they had been treated, testified at an Advisory Committee meeting that the proffered drink, called a "cocktail" by the investigators, was offered with no mention of its contents. "I remember taking a cocktail," one woman said simply. "I don't remember what it was, and I was not told what it was."[g] Although it is not clear what, if anything, the subjects were told, information about the Vanderbilt experiment was available to the general public. In late 1946 news reports appeared in the Nashville press.[h] The actual risk to the fetuses in the Vanderbilt experiment has long been a matter of study. In 1963-1964, a group of researchers at Vanderbilt found no significant differences in malignancy rates between the exposed and nonexposed mothers.[i] However, they did identify a higher number of malignancies among the exposed offspring (four cases in the exposed group: acute lymphatic leukemia, synovial sarcoma, lymphosarcoma, and primary liver carcinoma, which was discounted as a rare, familial form of cancer). No cases were found in a control group of similar size, and approximately 0.65 cases would have been expected on Tennessee state rates, compared to which the three observed cases is a marginally significant excess. This led the researchers to conclude that the data suggested a causal relationship between the prenatal exposure to Fe-59 and the cancer. The investigators also concluded that Dr. Hahn's estimate of fetal exposure was an underestimation of the fetal-absorbed dose. A 1969 study, funded by the AEC and conducted by one of the investigators from the 1963-1964 study, attempted to reconstruct the doses of Fe-59 to the fetuses in the original Vanderbilt study.[j] The investigators observed that the one case of leukemia might have been due to radiation damage, but that the doses in the other two cases were low; therefore, the relationship between the radiation exposure and the cancer in those cases might not have been causal. However, the researchers also noted that due to incomplete data, they could not estimate the dose absorbed by the fetus with confidence and that no definitive conclusions could be drawn from this study as to whether these exposures resulted in damage to the fetus.[k] The Vanderbilt study raises many of the same ethical issues as the experiments reviewed in this chapter. Like these experiments, the Vanderbilt study offered no prospect of medical benefit to the pregnant women or their offspring, raising the question of the conditions under which it is acceptable to put children at risk for the benefit of others, whether before or after birth. What could the investigators reasonably have been expected to know about the risks to which they put their subjects? Did they exercise appropriate caution in exposing fetuses to radiation? What were the pregnant women told, if anything, and was their permission sought? Who were these women, and how were they positioned relative to pregnant women, generally? The Committee did not have the resources to pursue these questions in both research in which children were the subjects and research in which children were exposed as fetuses. We did establish that the Vanderbilt study was not the only experiment during this period to expose fetuses in research that offered no prospect of medical benefit to them or their mothers. While the Committee did not conduct an exhaustive review of the scientific literature, we did find twenty-seven human radiation studies that included pregnant or nursing women as subjects between 1944 and 1974.[l] Of these studies, eight were considered therapeutic, and nineteen offered no prospect of benefit to the subject. Most of the nineteen were tracer experiments. These studies were performed in order to examine human physiology during pregnancy or to study the uptake of radioactive substances by fetuses or nursing infants.[m] They generally addressed valid scientific questions that could not be investigated in other populations. Knowledge of fetal exposure to radioiodine, for example, was relevant to issues such as potential harm to the fetus from maternal uptake of radioiodine in diagnostic tests or to estimate the potential effects of environmental exposure to radioiodine on the human fetus. In other studies, radioactive iron was administered to better understand the physiology of maternal and fetal intake of iron during pregnancy.
a . Most of the other tracer studies involving pregnant women and offering no prospect of benefit that were reviewed by the Committee involved fewer than twenty women as subjects. b . William J. Darby, Director of the Tennessee-Vanderbilt Project et al., Summary Report, Section B, Tennessee-Vanderbilt Nutrition Project, July 1, 1946 to December 31, 1946 (ACHRE No. CORP-020395-A), 97-110. This nutrition study summary report notes, "Considerable expansion of the program of study of maternal and infant nutrition has been made possible by a grant of $9,000 per year which was made by the U.S. Public Health Service. These funds were available beginning November 1, 1946." Ibid., 99. The summary observes that the grant was to be used for additional personnel, including the appointment of Dr. Richard Cannon, an obstetrics resident, to the staff of the Division of Nutrition beginning 1 January 1947. Dr. Cannon's name subsequently appears as an investigator in the medical report discussing the radioiron portion of the study, along with Dr. Paul Hahn's and others. c . P. Hahn et al., "Iron Metabolism in Human Pregnancy as Studied with the Radioactive Isotope, Fe-59," American Journal of Obstetrics and Gynecology 61 (March 1951): 477-486. The exact years of the radioiron portion of the nutrition study are uncertain. Minutes from a meeting of the nutrition study investigators indicate the study was to begin in September 1945. Tennessee-Vanderbilt Nutrition Project, Nutrition in Pregnancy Study, "Minutes of Meeting for Discussion of Nutrition in Pregnancy Study, August 17, 1945" (ACHRE No. CORP-020395-A), 17A-C. The radioiron study probably began at approximately that time and appears to have continued until sometime in 1947, based on a review of periodic study summaries. d . The Advisory Committee has not been able to determine whether Dr. Hahn got the radioactive iron used in the study from a private or government source, or both. e . Counts per minute is a measure of the radioactivity detected by a specific counting instrument. The sensitivities of counting instruments vary; a specific instrument may not "see" and count all the radiation coming from a particular substance. Thus, the total amount of radiation emitted by a substance may be calculated by considering the sensitivity of the counter. f . Contemporary estimates of the fetal doses by the Committee and others suggest that the fetal effective dose was a few hundred millirems. g . Wilton McClure, transcript of audio testimony before the Advisory Committee on Human Radiation Experiments, Small Panel Meeting, Knoxville, Tennessee, 2 March 1995, 182. h . "Iron Doses with Radioactive Isotopes Aid to Pregnancy, Experiment Shows," Nashville Banner, 13 December 1946; "VU to Report on Isotopes," The Nashville Tennessean, 14 December 1946 (ACHRE No. CORP-020395-A). i . The investigators identified the hospital records of 751 exposed mothers and 771 unexposed controls, as well as 719 exposed offspring and 734 unexposed offspring, and mailed them questionnaires. Of the exposed mothers, 90.4 percent responded, as did 91.45 percent of the unexposed mothers, 88.2 percent of the exposed offspring, and 89.2 percent of the unexposed. Ruth M. Hagstrom et al.,"Long Term Effects of Radioactive Iron Administered During Human Pregnancy," American Journal of Epidemiology 90 (1969): 1-8. j . Norman C. Dyer and A. Bertrand Brill, "Fetal Radiation Dose from Maternally Administered Fe-59 and I-131," in Radiation Biology of the Fetal and Juvenile Mammal: Proceedings of the Ninth Annual Hanford Biology Symposium at Richland, Washington, May 5-8, 1969, eds. Melvin R. Sikov and D. Dennis Mahlum (Washington, D.C.: GPO, December 1969), 78-88. This study was reviewed in detail by the Committee. The study also investigated fetal absorption of radioiodine because that isotope was and is commonly used in diagnosis and therapy, including in pregnant women. k . Ibid., 85. l . All of the nineteen studies reviewed in detail by the Committee were conducted or at least partially funded by the federal government or were supplied with radioisotopes by the AEC. For the earlier years, the Committee relied on the ACHRE experiments database, AEC isotope distribution lists provided by DOE, and relevant biographies. The Committee also consulted relevant medical indexes and computer databases; the isotope distribution lists provided by DOE did not cover these years. While the computer search would have located nontherapeutic tracer experiments for this period as well, very few were identified. m . Of the nineteen tracer experiments (funded by the government) involving pregnant or nursing women identified by the Committee, only three administered tracer doses to nursing women that offered no prospect of benefit; in at least one of the studies the infants were exposed. In one case, six nursing women were given radioiodine to determine excretion in breast milk, the infants were not given the exposed milk. In another case, two infants were intentionally exposed to the breast milk of their mothers, who were given I-131. An I-131 tracer study on the general population, incidentally included two nursing women. The report indicates that both had been nursing their children, and since there is no indication that the mothers were warned to avoid breastfeeding after the exposure, it is quite probable that the infants were exposed.
Nasopharyngeal IrradiationNasopharyngeal irradiation,[a] introduced by S. J. Crowe and J. W. Baylor of the Otological Research Laboratory at the Johns Hopkins University, was employed from 1924 on as a means of shrinking lymphoid tissue at the entrance to the eustachian tubes to treat middle ear obstructions, infections, and deafness. For this treatment, intranasal radium applicators (sealed ampules containing radium salt) were inserted (at least three insertions per treatment cycle) into the nasopharyngeal area for twelve-minute periods.[b] The therapeutic effect of the treatments resulted from the penetrating radiation emitted from the radium source (gamma and beta rays), not from the internal deposition of radium itself. Crowe and his colleagues reported that "under this treatment, the lymphoid tissue around the tubal orifices gradually disappeared, marked improvement or complete return of the hearing followed, and in many the bluish discoloration of the tympanic membrane also disappeared."[c] This method was used for more than a quarter century as a prophylaxis against deafness, for relieving children with recurrent adenoid tissue following tonsillectomy and adenoidectomy, and for children with chronic ear infections. Asthmatic children with frequent upper respiratory infections were also often considered for this type of irradiation.An average of 150 patients a month, mostly children, were given the treatment at the Johns Hopkins clinic over a period of several years.[d] Many children received the treatment more than once as recurrent lymphoid tissue was considered an indication for treatment. Crowe and his colleagues reported that the results following irradiation of the nasopharynx alone were not only as good as, but often better than, those following removal of tonsils and adenoids.[e] In review articles, they noted that approximately 85 percent of treated patients responded with decreased numbers of infections and/or improved hearing when treated at young ages. They also concluded that "it is effective, safe, painless, inexpensive and has proved particularly valuable for prevention of certain ear, sinus and bronchial condition in children."[f] Although early articles by Crowe and colleagues indicate that nasopharyngeal radium treatments were accepted as standard procedure for the prevention of childhood deafness, these treatments, like most standard interventions in medicine, had not been subjected to formal scientific evaluation. A controlled study was conducted from 1948 to 1953 by Crowe and his colleagues to determine "the feasibility of irradiation of the nasopharynx as a method for controlling hearing impairment in large groups of children associated with lymphoid hyperplasia in the nasopharynx; to draw conclusions concerning the per capita cost of such an undertaking as a public health measure."[g] Crowe et al. wrote in an NIH "Notice of Research" that "the procedure of treatment is not new, as an individual measure; this is the first adequately controlled experiment of sufficient size for accurate statistical analysis."[h] This work was funded by NIH for the entire period of study. As recorded in an NIH grant application, the study involved approximately 7,000 children screened for hearing impairment.[i] Of those screened, approximately 50 percent were selected for further study based on the chosen criteria for hearing loss. Half of this study group was irradiated with radium, while the other half served as a control group. Crowe and colleagues reportedly concluded from this study (published in 1955) that the radium treatments did shrink swelling of lymphoid tissue and improve hearing.[j] This type of therapy was ultimately discontinued because of newly available antibiotics and the use of transtympanic drainage tubes, as well as awareness of the potential risks of radiation treatment. In addition to the targeted lymphoid tissue, the brain and other tissues in the head and neck region, including the paranasal sinuses, salivary glands, thyroid, and parathyroid glands are also exposed to significant doses of radiation during the radium treatments, prompting concern that these treated individuals might have been placed at increased risk for radiation-induced cancers at these sites. Dale P. Sandler et al., in their 1982 study of the effects of nasopharyngeal irradiation on excess cancer risk for children treated at the Johns Hopkins clinic, found "a statistically significant overall excess of malignant neoplasms of the head and neck among exposed subjects," based however on only four cases in comparison with 0.57 expected.[k] This excess was accounted for mainly by three brain tumors that occurred in the irradiation subjects. One other malignant tumor, a cancer of the soft palate, was also reported. The Department of Epidemiology at the Johns Hopkins University has undertaken a further follow-up study of the Crowe et al. cohort of children irradiated there, previously studied by Sandler et al.[l] Verduijn et al., in their 1989 study of cancer mortality risk for those individuals (mostly children) treated by nasopharyngeal irradiation with radium 226 in the Netherlands, reported that "the present study has found no excess of cancer mortality at any site associated with radium exposure by the Crowe and Baylor therapy. Specifically, the finding of Sandler et al. of an excess of head and neck cancer was not found in this study group."[m]
Among the Japanese atomic bomb survivors, no excess of brain
tumors was found. However, several studies have noted an increased risk of
both benign and malignant brain tumors following therapeutic doses of radiation
to the head and neck region during childhood.[n] From the Committee's own
limited risk analysis of these experiments, we concluded that the brain and
surrounding head and neck tissues would be put at highest risk and estimated
the lifetime risk at approximately 4.35 per 1,000 and an increased relative
risk of 62 percent.[o]
The Hopkins nasopharyngeal study raises different ethical
issues
than those posed by the other experiments reviewed in this chapter, all of
which offered no prospect of medical benefit to the children who served as
subjects. By contrast, the nasopharyngeal irradiation experiment was designed
to determine whether children at risk for hearing loss would be better off
receiving radiation treatments or not receiving such treatments. A central
issue here was whether it was permissible to withhold this intervention from
"at risk" children. The application of radium was at this point a common, but
scientifically unproven, treatment for children at risk of hearing loss; the
risks of the treatment were not well characterized. If it was really unknown
which was better for children--receiving radium or no intervention--then the
medical interests of the children were best served by being subjects in the
research because, as a consequence, they would have a 50 percent chance of
receiving the better approach. The nasopharyngeal experiment thus belongs to a
class of research the Committee did not investigate--therapeutic research with
children.
Relying on the risk estimate developed in the
Sandler study, Stewart Farber, a radiation-monitoring specialist with a
background in public health, has projected 51.4 excess brain cancers over a
fifty-year period in the 7,613 servicemen irradiated in the Navy and Army Air
Forces studies noted above. Stewart Farber, Consulting Scientist of the Public
Health Sciences, to Stephen Klaidman, ACHRE Staff, 8 March 1995
("Nasopharyngeal Radium Irradiation-Initial Radiation Experiments Performed by
DOD on 7,613 Navy and Army Air Force Military Personnel during 1944-45"). Alan
Ducatman, M.D., of the University of West Virginia School of Medicine, who
coauthored a letter with Farber to the New England Journal of Medicine
regarding the radium exposure of military personnel, wrote that he found "no
convincing evidence of excess cancer in the exposed population." He added,
however, "there is also no good evidence for the null hypothesis." Alan
Ducatman, West Virginia University School of Medicine, to Duncan Thomas, Member
of the Advisory Committee on Human Radiation Experiments, 22 February 1995
("I'm sorry I could not respond . . .") (ACHRE No. WVU-021795-A).
Han K. Kang, with the Environmental Epidemiology Service of the Veterans
Health Administration, is currently conducting a study to assess the
feasibility of an epidemiologic study of Navy veterans who received radium
treatments. Han K. Kang, Environmental Epidemiology Service, Veterans Health
Administration, "Feasibility of an Epidemiologic Study of a Cohort of
Submariners Who Received Radium Irradiation Treatment," 23 August 1994. It is
not clear, however, that sufficient numbers of treatment-documented personnel
can be identified, as a group representing submariners has apparently been able
to identify only six former Navy personnel from of a pool of twenty-seven whose
records indicate they received radium treatment. (It is not clear whether the
data being collected by the VFW with the support of Senator Joesph Lieberman of
Connecticut will be from a representative sample of respondents. If, in fact,
these data are from a highly nonrepresentative sample, the study may not be
considered scientifically valid.) However, the Veterans of Foreign Wars
organization apparently is now processing hundreds of surveys filled out by
veterans who say they underwent nasopharyngeal radium treatment. Once this
task is completed, Senator Lieberman plans to present the data to the
Department of Veterans Affairs with a recommendation that an epidemiologic
study be conducted.
b . Samuel J. Crowe, "Irradiation of the Nasopharynx," Annals of
Otology, Rhinology and Laryngology 55 (1946): 31.
c . Ibid., 30.
d . Ibid., 33.; Dale P. Sandler et al., "Neoplasms Following Childhood
Radium Irradiation of the Nasopharynx," Journal of the National Cancer
Institute 68 (1982): 3-8.
e . Ibid., 33.
f . Ibid.
g . S. J. Crowe et al., The Johns Hopkins University School of Medicine
and School of Hygiene and Public Health, to Federal Security Agency, Public
Health Service, National Institutes of Health, July 1948 ("The Efficiency of
Nasopharyngeal Irradiation In the Prevention Of Deafness in Children, Notice of
Research Project, Grant No. B-19") (ACHRE No. HHS No. 092694-A).
h . Ibid.
i . Ibid.
j . Ibid.
k . For the combination of benign and malignant neoplasms, there were 23
cases, for a relative risk of 2.08 with a 95 percent confidence interval of
1.12 to 3.91. Sandler, "Neoplasms Following Childhood Radium Irradiation,"
5.
l . Jessica Yeh and Genevieve Matanowski, fax to Anna Mastroianni (ACHRE),
7 July 1995 ("Nasopharyngeal Power Analysis"), 1-3.
m . Verduijn et al., "Mortality after Nasopharyngeal Irradiation,"
Annals of Otology, Rhinology, and Laryngology 98 (1989): 843.
n . S. Jablon and H. Kato, "Childhood Cancer in Relation to Prenatal
Exposure to Atomic-Bomb Radiation," The Lancet, ii (1970): 1000-1003.;
M. Colman, M. Kirsch, and M. Creditor, "Radiation Induced Tumors," in Late
Biological Effects of Ionizing Radiation, Vol. 1 (Vienna: International
Atomic Energy Agency, 1978), 167-180; R. E. Shore, R. E. Albert, and B. S.
Pasternak, "Follow-up Study of Patients Treated by X ray Epilation for Tinea
Capitis: Resurvey of Post-Treatment Illness and Mortality Experience,"
Archives of Environmental Health 31 (1976): 17-24; and C. E.
Land,"Carcinogenic Effects of Radiation on the Human Digestive Tract and Other
Organs," in Radiation Carcinogenesis, eds. A. C. Upton et al. (New York:
Elsevier, 1986), 347-378.
o . The radiation dose estimate to the head and neck region was
calculated according to the following assumptions: (1) Source description: 50
mg of radium, active length 1.5 cm, filtered by 0.3 mm of Monel metal. (2)
Average treatment: 60mg/hrs; based on three 12-minute treatments (radium
applicators inserted through
both nostrils)= (12x3x50x2)/60 mins per hour= 60 mg-hrs. (3) Dose rate at
points in a central orthogonal plane surrounding the source: for distances up
to 5 centimeters dose estimated using published data (Quimby Tables, Otto
Glasser et al., Physical Foundations of Radiology, 3d ed. [New York:
Paul Hoeber, Inc., 1961]) for linear radium sources with dose increased by 50%
to allow for the reduced filtration provided by the applicator wall and
converting roentgen to rad by a multiplication factor of 0.93. For distances
greater than 5 centimeters, the dose rate is reduced in accordance with the
inverse square law, with a proportionality constant of 690 rad-cm2. There was
no dose correction for attenuation of the gamma rays by tissue absorbtion,
which has been calculated to be about 2%/cm (yielding a dose reduction of about
20% at 10 cm).
The local gamma dose to the head and neck region was assumed to be
distributed
according to an inverse square law d(r) = 690/r2 rad. The Committee
approximated the exposed region of the body by a sphere with radius 10
centimeters. This was felt to be a conservative assumption, because although
the dose does not go to zero at the base of the neck, a 10-centimeter sphere
would also extend outside the skull. Averaging this dose distribution over the
exposed sphere, the average dose to the head was found to be 20.7 rad. The
exposed volume is about 4189 cm3, or 29 percent of the total body, so the
average whole body dose is about 6.0 rad. Multiplying this by the BEIR V risk
coefficient for children exposed at age five, 1.4/1,000 person-rad, produces a
lifetime risk of about 8.4/1,000. This calculation assumes that the brain and
other head tissues have average radiosensitivity. BEIR V also gives
absolute-risk coefficients for brain cancer ranging from 1 to 9 per million
person-year-rad, with 3 being a reasonable average. Applying this figure to an
average head dose of 20.7 rad, the Committee estimates a lifetime risk of about
4.35/1,000. The corresponding relative risk coefficients average about 3
percent per rad, so this dose would correspond to an excess relative risk of 62
percent.
Table 2. Summary and Risk Analysis for Studies Examined by the Advisory Committee
* Risk estimates are reported as average values for each experiment; maximum values ( ) are reported when available. |