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Basic Science Investigation |
1 INSERM U605, Villejuif, France; 2 Institut Gustave Roussy, Villejuif, France; 3 Université Paris XI, Villejuif, France; 4 University of Pisa, Pisa, Italy; 5 Institut Jean Godinot, Reims, France; and 6 Centre François Baclesse, Caen, France
Correspondence: For correspondence or reprints contact: Florent de Vathaire, INSERM U605 and Institut Gustave Roussy, 39 Rue Camille Desmoulins, 94805 Villejuif Cedex, France. E-mail: fdv{at}igr.fr
| ABSTRACT |
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Key Words: differentiated thyroid carcinoma radioiodine therapy pregnancy outcome radiation dose ovaries
| INTRODUCTION |
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In 1996, we reported the largest study on pregnancies (n = 2,113) occurring after 1970, 258 of which occurred after exposure to radioiodine (6). The only adverse effect observed was an increased incidence of miscarriages in women exposed to therapeutic radioiodine during the year preceding conception. On the basis of that study, we recommended postponing conception for 1 y after the therapeutic administration of radioiodine and until the thyroid hormone status was verified. However, in that same study, only 96 pregnancies occurred after exposure to activities exceeding 3,700 MBq. We present here an update of that study, by doubling the number of pregnancies that occurred after 131I therapy.
| MATERIALS AND METHODS |
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The following features were recorded for each pregnancy: induced abortion, miscarriage, stillbirth, prematurity (defined as a gestational age of <37 wk), birth weight below the 10th percentile for the gestational age, congenital abnormality, and death during the first year of life. Congenital abnormalities were defined according to International Classification of Diseases, 9th Revision (ICD-9), rubrics. Later deaths, thyroid diseases, and tumors at other sites were recorded for live-born children. Only pregnancies occurring after 1970 were considered. A validation study performed on 116 pregnancies confirmed high-quality interviews (6).
Radiation Doses to Ovaries
After radioiodine therapy, the primary sources of radiation doses to the ovaries are the blood, bladder, gut, and 131I uptake in metastases close to the ovaries. Mathematic models of individual patient morphology have estimated doses to the ovaries to be approximately 3-fold higher than the MIRD estimation of 1.4 mGy/37 MBq (1 mCi) (3). Furthermore, hypothyroidism at the time of radioiodine administration can decrease renal iodine clearance and result in more prolonged gonadal exposure.
Women treated with more than 3.7 GBq (100 mCi) of radioiodine generally had lung metastases and had received a mean cumulative activity of 8,800 MBq (237 mCi) before conception. For these women, the dose delivered to the ovaries was estimated to be 4 mGy/37 MBq, assuming 10% lung uptake at 24 h, leading to a mean total dose to the ovaries of approximately 1 Gy.
Statistical Methods
To take into account possible multiple pregnancies and the correlation between the outcomes of pregnancies in the same women, we analyzed pregnancy outcomes by using a generalized estimating equation (7) for repeated measurements. We chose an exchangeable working correlation matrix. This assumption implies that the correlation between distinct pregnancies in the same woman is the same regardless of the rank of the pregnancy. The correlation structure can thus be described with a single correlation parameter, 0 <
< 1. For studying the association between radioiodine activity and the occurrence of adverse events, a binomial distribution was assumed for the observed events. Results were verified by basic logistic regression analysis. Parameters were tested with Wald and likelihood ratio tests. For these calculations in the unexposed women, we used only the pregnancies that occurred before any radioiodine administration.
We investigated the risk factors exclusively for induced abortions and miscarriages and not for thyroid diseases and other cancers in children because satisfying analyses were not possible with the small number of cases.
Because factors other than radiation history may have influenced the outcome of a pregnancy, analyses were adjusted for these variables. Age, smoking, alcohol intake, use of medication (such as antidepressants and β-blockers) during pregnancy, and socioeconomic status were considered. A low socioeconomic status was defined according to International Classification of Professions criteria (8). The following classifications were included: 520–599, 610–640, 710–839, 870–874, 890–910, 930–939, and 950–958. If the definition of a patient's profession was missing, then that of the spouse or partner was considered. All of these analyses were performed with SAS computer software (SAS Institute Inc.).
The observed number of thyroid and other cancers in live-born children was compared with the expected number from incidence data in the French general population. Because no national cancer registry exists in France, we used estimations published by the French Network of Local Cancer Incidence Registries (Francim). These rates were used for all children, including Italian children, because the incidences of cancer below the age of 40, the approximate maximum age of offspring potentially included in the present study, are similar among most countries (9). The standardized incidence ratio was defined as the ratio of the observed number of cancers to the expected number of cancers, and its confidence interval (CI) was estimated by assuming a Poisson distribution for the observed number of cancers and using maximum-likelihood methods (10). We were unable to compare the observed number of deaths during the first year of life to the expected number from external statistics because the definitions of stillbirth differed in France and Italy during the study period and because it was impossible to match information obtained from the interviews with legal definitions. The expected number of deaths among children after the first year of life was computed from French national data and yielded standardized mortality ratios, which express the ratio of observed number of deaths to expected number of deaths. During the study period, these death rates in the general population were similar in France and Italy. These analyses were performed with AMFIT software (Hirosoft Int. Corp.).
| RESULTS |
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The remaining 2,673 pregnancies, registered in 1,126 patients, were included in the present study: 2,078 pregnancies had occurred before thyroid carcinoma treatment, 112 had occurred in patients who had undergone surgery alone for thyroid cancer, and 483 had occurred in patients who had been exposed to radioiodine. Among these last patients, 212 of whom had been treated with 3,700 MBq or more of 131I, 95 (45%) had received 1 treatment, 58 (27%) had received 2 treatments, 33 (16%) had received 3 treatments, and 26 (12%) had received 4–11 treatments. The mean interval between the last 131I treatment and conception was 35 mo (range, 0–243 mo). No progression of thyroid carcinoma, as assessed by follow-up data obtained during pregnancy (clinical examination and thyroglobulin determination on levothroxine treatment), was observed in any of these women during pregnancy.
Induced Abortions (Therapeutic and Elective)
A total of 341 induced abortions (therapeutic and elective) were reported (Table 1), 221 before any treatment, 26 after thyroid surgery alone, and 94 after both surgery and exposure to 131I. Induced abortions were more frequent after surgical treatment both without and with 131I administration than before any treatment (OR = 2.14, 95% CI = 1.67–2.75). After adjustment for age at pregnancy, exposure to higher 131I activity before pregnancy was not associated with a greater probability of an induced abortion (OR [for 3,700 MBq] = 0.83, 95% CI = 0.68–1.00). Nevertheless, both the cumulative radioiodine activity received during the year preceding pregnancy (P < 10–4 for trend) and smoking during pregnancy (OR = 2.15, 95% CI = 1.15–4.0) were found to play significant roles in the probability of an induced abortion. Induced abortions (Table 2) were more frequent in women over 35 y and in those who had not avoided alcohol or tobacco during pregnancies (P < 10–4, OR = 3.3, 95% CI = 2.5–4.3).
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The following parameters were studied after the exclusion of pregnancies that ended in induced abortions.
Miscarriages and Stillbirths
A total of 193 miscarriages (10.4%) were observed among the 1,857 pregnancies that occurred before any treatment. They were more frequent (20.7%) in pregnancies occurring after surgery for differentiated thyroid cancer, but the frequency of miscarriages was 19.0% in the 86 pregnancies that occurred after surgery alone and after no previous exposure to 131I. The only significant factors in the occurrence of miscarriages were the occurrence of a pregnancy after thyroid cancer (with or without radioiodine treatment) (OR = 1.59, 95% CI = 1.19–2.13), the mother's age at pregnancy exceeding 35 y (for both treated and untreated women) (OR = 2.12, 95% CI = 1.54–2.91), and a low birth weight, which was more common in the low-socioeconomic-status group (P < 0.05). After adjustment for age at pregnancy, no correlation was evidenced between cumulative 131I activity and the occurrence of a miscarriage. These results were confirmed when only exposure was considered during the year before conception.
Table 3 shows stillbirths in both treated and untreated women, and Table 4 gives the odds ratios for risk factors regarding abortions and miscarriages for all the women after thyroid surgery (n = 595). During the pregnancies of women who took medications (n = 791), miscarriages were slightly more frequent (9.9% vs. 12.4%, P = 0.06, OR = 1.04, NS) and prematurity was significantly more frequent (10.4% vs. 5.5%, P = 2.10–4, OR = 1.85, 95% CI = 1.34–2.56).
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Seventy-nine live-born children had a malformation. Among them, 9 were born to a previously exposed mother: 3 had a cardiac malformation (222, 8,140, and 740 MBq), 1 had esophageal atresia (222 MBq) and died, 1 had coloboma iridis (11,396 MBq), 1 had Down syndrome (1,702 MBq), 2 had a hip luxation (4,810 and 1,776 MBq), and 1 had a digestive malformation (4,810 MBq) (Table 6). The risk of a malformation was not associated with cumulative 131I activity previously administered for thyroid carcinoma (OR [for 3,700 MBq] = 0.88, 95% CI = 0.55–1.40).
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Thyroid diseases were observed in 42 children (Table 7), 8 of whom were born to exposed mothers. Six children developed autoimmune hypothyroidism, 1 had Graves' disease, 8 had nontoxic goiter, 4 had benign nodules, 14 had a nonoperated nodule, and 3 had thyroid cancer (at the ages of 17, 17, and 21 y). Neither cumulative 131I activity nor the activity administered during the year before conception was associated with an increased risk of thyroid diseases in children, and no thyroid cancer occurred in children born to exposed mothers. Our study was adequately powered (12%) to evidence an increase in risk by a factor of 1.3 (assuming a doubling dose of 1 Gy, applied to an estimated dose of 305 mGy) in the 139 pregnancies that occurred after 131I therapy. The theoretic number of thyroid diseases would be 2.6, a value that can be excluded with a probability of 88%.
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| DISCUSSION |
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We excluded 2,852 pregnancies that occurred before 1970 because the information reported for most of these patients could not be adequately validated. Also, most of these pregnancies occurred before radioiodine administration and would therefore not have been relevant for investigating the risk of 131I exposure in offspring.
In the present study, radioiodine was the only identified source of radiation therapy before pregnancy; all women who reported other types of radiation therapy were excluded.
In contrast, we did not exclude pregnancies in women who had undergone radiographs of the pelvis or abdomen. This decision was made because only information on radiographs during the follow-up of thyroid cancer was available, and this information was probably only part of a lifetime radiologic history. It is highly unlikely that our choice introduced a bias leading to an underestimation of the risk attributable to 131I exposure in the present study. Indeed, in the present study, women who had the most radiographs had also received the largest amounts of 131I: The mean total 131I activity before pregnancy was 518 MBq in women who did not report any radiograph of the pelvis or the abdomen during the follow-up of thyroid cancer, whereas the activities were 4,995 MBq in those who reported 1 radiograph, 5,143 MBq in those who reported 2 radiographs, and 10,175 MBq in those who reported 3–6 radiographs.
This update confirms all of the results of our previous publication (6), except for the notable increased risk of miscarriage with increasing activity administered the year before conception, a finding that was not confirmed in the present study. In fact, since this last publication (6), 6 additional pregnancies have been registered in the highest category of activity (>370 MBq), and none has ended in a miscarriage. In addition, 1 of the 4 miscarriages previously reported in this category occurred after external radiotherapy and was therefore excluded from the present analysis. We use the term "induced abortion" because we were not able to classify these abortions as therapeutic or elective. Indeed, we were able to document the abortion for only 45 pregnancies, all of them for therapeutic purposes (dreaded exposure, ongoing treatment, and malformation suspicion).
In the absence of consensus for another value, we assumed a doubling dose (the dose required to produce genetic damage equal to the spontaneous mutation rate) of 1 Gy for some of our analyses. The 1993 report of UNSCEAR, which mentioned this value, also showed that it is probably conservative and that atomic bomb survivor results led to values 3 times higher and that strong uncertainties remain about this value in humans, in whom it may be much higher (11–14) (UNSCEAR), because humans could be genetically less radiosensitive than mice (15,16).
In general, few women had received a dose to the ovaries approximating 1 Gy in the present study: Only 139 women had received more than 140 mGy (mean dose: 305 mGy). Whatever the particular event studied, the present study was not sufficiently powered to detect an increase in risk.
We did not find any increase in untoward pregnancy outcomes, such as stillbirths or malformations. Our results concerning stillbirths are consistent with the last publication on stillbirths in the offspring of female Japanese atomic bomb survivors, which included 55,303 births after a mean dose approximating 140 mGy; 894 of those pregnancies ended in a stillbirth (17). Our results are also consistent with those of the Childhood Cancer Survivor Study, which included 4,029 pregnancies; 37 of those pregnancies ended in a stillbirth (18). Up to now, studies regarding the consequences of the Chernobyl accident have been less informative because they are only temporal trend studies without individual dose estimations: An increased risk of stillbirths was found in some Eastern European countries during a period starting 1 mo after the Chernobyl accident and ending 2 y thereafter, but the results were considered by the authors to be inconsistent with the low level of effective radiation doses received by these populations (0.1–0.2 mSv/y) (19). Similar results for similar dose levels were found in Bavaria and throughout Germany during the year after the accident, but total infant mortality gradually decreased during the same period (19–21). In contrast to but consistent with the Japanese atomic bomb survivor results, the stillbirth rate appeared to be unaffected by the Chernobyl accident in Finland, despite a well-documented monthly dose estimation (22), in Sweden (23), and in Norway (24).
We failed to find increases in all-cause mortality and cancer mortality among live-born children. These results are consistent with those observed for the offspring of female Japanese atomic bomb survivors: 1,281 deaths, 278 of which were from cancer (25). Similar results were observed for cancer incidence in the offspring of female Japanese atomic bomb survivors in a study including 709 cancers (26).
In the present study, only 9 malformations were observed in 309 live births to exposed women (2.9%), compared with the 12.9 malformations expected from the rate observed in offspring conceived before thyroid cancer or iodine therapy, in the absence of the effect of exposure, and compared with the 16.9 malformations expected with the assumption of a doubling dose of 1 Gy. Despite these low values, we can exclude the value of 16.9 with a probability of 88%. Our findings are consistent with the fact that preconception irradiation is not related to malformations in live births, as highlighted in a previous, larger study (17). Similarly, our cohort included only 8 thyroid diseases in 309 live births to exposed women (2.5%). Among the 2009 live births in our cohort, only 3 developed differentiated thyroid carcinoma, of whom none were born to the 309 previously exposed mothers. Therefore, we were not able to demonstrate that radiation exposure of the mother detectably increased the thyroid cancer incidence.
Differentiated thyroid cancer is one of the nonfamilial cancers that generates the highest heritability: The proportion of such cancer attributable to genetic susceptibility was estimated to be 53% in an analysis based on the national Swedish Family Cancer Database, a value higher than that for any other cancer (27). Because our study included only 2009 live births, we were not able to join the debate about differentiated thyroid cancer heritability, by comparing the incidence of thyroid cancer among the offspring in our cohort with that expected in the general population.
We did not analyze the neuropsychological development of the offspring, which cannot be investigated by a face-to-face interview with the mother. Nevertheless, published data consistently document a relationship between perturbation of maternal thyroid hormone status and developmental outcomes in offspring (28). These results could have major implications in thyroid cancer offspring, and therefore further studies should investigate this issue.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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| References |
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F. de Vathaire, J.-P. Garsi, and M. Schlumberger Reply: Pregnancy Outcome After 131I Therapy J. Nucl. Med., February 1, 2009; 50(2): 327 - 328. [Full Text] [PDF] |
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D. R. Boreham and J.-A. Dolling Risks Associated with Therapeutic 131I Radiation Exposure J. Nucl. Med., May 1, 2008; 49(5): 691 - 693. [Full Text] [PDF] |
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