Women & Health
ISSN: 0363-0242 (Print) 1541-0331 (Online) Journal homepage: http://www.tandfonline.com/loi/wwah20
Electric Blanket Use and Risk of Thyroid Cancer in the Women’s Health Initiative Observational Cohort Ikuko Kato MD, PhD, Alicia Young MS, Jingmin Liu MS, Judith Abrams PhD, Cathryn Bock PhD & Michael Simon MD, MPH To cite this article: Ikuko Kato MD, PhD, Alicia Young MS, Jingmin Liu MS, Judith Abrams PhD, Cathryn Bock PhD & Michael Simon MD, MPH (2015) Electric Blanket Use and Risk of Thyroid Cancer in the Women’s Health Initiative Observational Cohort, Women & Health, 55:7, 829-841, DOI: 10.1080/03630242.2015.1050545 To link to this article: http://dx.doi.org/10.1080/03630242.2015.1050545
Accepted author version posted online: 21 May 2015. Published online: 21 May 2015. Submit your article to this journal
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Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=wwah20 Download by: [Sichuan Normal University]
Date: 06 November 2015, At: 01:17
Women & Health, 55:829–841, 2015 Copyright © Taylor & Francis Group, LLC ISSN: 0363-0242 print/1541-0331 online DOI: 10.1080/03630242.2015.1050545
Electric Blanket Use and Risk of Thyroid Cancer in the Women’s Health Initiative Observational Cohort
Downloaded by [Sichuan Normal University] at 01:17 06 November 2015
IKUKO KATO, MD, PhD Department of Oncology and Pathology, Wayne State University School of Medicine, Detroit, Michigan, USA
ALICIA YOUNG, MS and JINGMIN LIU, MS Fred Hutchinson Cancer Research Center, Seattle, Washington, USA
JUDITH ABRAMS, PhD, CATHRYN BOCK, PhD, and MICHAEL SIMON, MD, MPH Department of Oncology, Karmanos Cancer Institute at Wayne State University, Detroit, Michigan, USA
Thyroid cancer disproportionally affects more women than men. The aim of this study was to assess whether exposure to extremely low frequency electric magnetic fields from electric blankets (EBs) was associated with the development of thyroid cancer. Data were analyzed from 89,527 women who participated in the Women’s Health Initiative Observational Study and who responded to questions concerning prior use of EBs. During a mean follow-up of 12.2 years, 190 incident cases of thyroid cancer were identified. We estimated the hazard ratio (HR) and 95 percent confidence interval (CI) of incident thyroid cancer associated with EB use by Cox’s proportional hazard model, adjusted for selected covariates. A majority, 57 percent, of the women in the cohort reported the use of EBs while sleeping and/or for warming the bed before sleep. No association was found between use of EBs and subsequent risk of thyroid cancer (HR = 0.98, 95 percent CI 0.72–1.32). Duration of EB use measured in years, months, or hours had no effect on risk. These results did not change when the cases were limited to papillary Received January 23, 2014; revised August 13, 2014; accepted August 31, 2014. Address correspondence to Ikuko Kato, MD, PhD, Department of Oncology and Pathology, Wayne State University School of Medicine, 4100 John R. Street MM04EP, Detroit, MI 48201. E-mail: [email protected] 829
830
I. Kato et al.
thyroid cancer, the most frequently occurring histologic type. The results of this study do not support possible health hazards of EBs in regards to thyroid cancer risk. KEYWORDS cohort study, electromagnetic field, postmenopausal women, thyroid cancer
Downloaded by [Sichuan Normal University] at 01:17 06 November 2015
INTRODUCTION Thyroid cancer disproportionally affects women more than men. Particularly for reproductive ages (i.e., 15–44), the female/male ratio of thyroid cancer incidence reaches or exceeds 4.0, and, consequently, thyroid cancer is ranked as the 5th most commonly diagnosed cancer in U.S. women (Howlader et al. 2013). In addition, thyroid cancer incidence in the U.S. has risen more rapidly than other frequently occurring cancers, specifically since 1996 (Howlader et al. 2013) in the absence of any screening efforts suggesting a potential role for new environmental risk factors. Exposure to ionizing radiation is the best established risk factor for thyroid cancer (IARC 2000). On the contrary, to well quantified carcinogenic risk of ionizing radiation to humans (IARC 2000), the effect of nonionizing radiation (except UV) on cancer risk in general has been highly controversial. Nevertheless, two previous expert reviews organized by the International Agency for Research on Cancer classified non-ionizing radiation, namely, extremely low frequency (ELF) and radiofrequency (RF) electric magnetic fields (EMF), as group 2B human carcinogens (possibly carcinogenic). These conclusions were based on the observations from epidemiologic studies on childhood leukemia and adult brain cancer (IARC 2002, 2013), but the data concerning thyroid cancer have been very limited to date. A wide range of household and personal appliances produces non-occupational exposure to EMF, but the exposure rapidly declines with distance from appliances, by the inverse square to inverse cube of distance (IARC 2002). Among various appliances, use of electric blankets (EBs) has raised concern about hazardous health effects because of the combined characteristics of close proximity and prolonged hours of use. Thus far, the primary interest in epidemiologic studies of EB use has been breast cancer risk, owing to postulated effects on reproductive hormones through reduced melatonin secretion by EMF exposure (Cohen, Lippman, and Chabner 1978). These studies have, however, produced inconclusive results (IARC 2002), while a more recent large-scale cross-sectional study in postmenopausal women revealed an association between EB use for 20 years and longer and endometrial cancer (Abel et al. 2007). While reproductive hormones stimulate thyroid growth (Rahbari, Zhang, and Kebebew 2010), investigators have also found that exposure to EMF induces morphological and functional changes in the thyroid glands of rodents
Electric Blanket Use and Thyroid Cancer Risk
831
(Rajkovic et al. 2003; Rajkovic, Matavulj, and Johansson 2005; Wright et al. 1984). Hence, it is plausible that EMF exposure may modulate thyroid cancer risk in humans, although age at exposure may be critical for thyroid carcinogenesis as with ionizing radiation (IARC 2000). This study was the first of which we are aware to examine the association between EB use and thyroid cancer incidence in a prospective cohort study.
MATERIALS AND METHODS Downloaded by [Sichuan Normal University] at 01:17 06 November 2015
Study Participants The women included in this study were a subset of participants in the Women’s Health Initiative (WHI). The WHI was designed to address major causes of morbidity and mortality in postmenopausal women consisting of both multicenter clinical trials and an observational study (OS). This study was based on the OS cohort only. Other publications have described the details of the scientific rationale, eligibility requirements, baseline participant characteristics, and measurement reliability of the WHI OS participants (Hays et al. 2003; Langer et al. 2003). Briefly, participants were women 50–79 years of age, recruited at 40 clinical centers throughout the United States between September 1, 1993 and December 31, 1998. Institutional Review Boards at all 40 clinical centers and at the coordinating center and a study-wide data and safety monitoring board oversaw the study. All participants in the WHI gave informed signed consent. At enrollment, participants completed self-administered questionnaires to provide information concerning demographic, medical, reproductive, and lifestyle factors. In addition, OS participants (not clinical trial participants) answered five questions about EB use, ever use, current use, use only to warm the bed before sleep, number of years used, and average number of months used per year. The definition of EB in this questionnaire was inclusive of electric heating pad, electric mattress pad, and heated water bed in addition to blankets.
Follow-Up and Ascertainment of Cases Incident thyroid cancer cases were identified by self-administered questionnaires annually throughout the study, with all cases confirmed by medical record review (Curb et al. 2003). All primary thyroid cancer cases were coded centrally in accordance with the Surveillance Epidemiology and End Results (National Cancer Institute) coding guidelines. For these analyses, participants were followed up to first thyroid cancer diagnosis, date of death, loss to follow-up, or end of study follow-up (September 30, 2012), whichever
832
I. Kato et al.
occurred first. Only 2.35 percent of the cohort was lost to follow-up at the end of the follow-up.
Downloaded by [Sichuan Normal University] at 01:17 06 November 2015
Statistical Analysis We computed hazard ratios (HRs) and 95 percent confidence intervals (CIs) using the Cox proportional hazard model. We tested potential covariates one at a time in a model that also included an indicator variable for participation in the WHI extension study period. We selected these explanatory variables because they were basic demographic variables or a known strong risk factor (benign thyroid disease) or were reported as potential risk factors for thyroid cancer or as causal confounders with electric blanket use in earlier studies (Abel et al. 2007; Dal Maso et al. 2009; Kabat et al. 2012; Kitahara et al. 2012; Rahbari, Zhang, and Kebebew 2010). The “final” multivariable model included potential risk factors that altered the regression coefficient for the main exposure variable, electric blanket use, by at least 10 percent in addition to a known strong risk factor, a history of benign thyroid disease. We computed summary exposure indices (cumulative months/hours of use) using median values of each category of the number of years of use (
ISSN: 0363-0242 (Print) 1541-0331 (Online) Journal homepage: http://www.tandfonline.com/loi/wwah20
Electric Blanket Use and Risk of Thyroid Cancer in the Women’s Health Initiative Observational Cohort Ikuko Kato MD, PhD, Alicia Young MS, Jingmin Liu MS, Judith Abrams PhD, Cathryn Bock PhD & Michael Simon MD, MPH To cite this article: Ikuko Kato MD, PhD, Alicia Young MS, Jingmin Liu MS, Judith Abrams PhD, Cathryn Bock PhD & Michael Simon MD, MPH (2015) Electric Blanket Use and Risk of Thyroid Cancer in the Women’s Health Initiative Observational Cohort, Women & Health, 55:7, 829-841, DOI: 10.1080/03630242.2015.1050545 To link to this article: http://dx.doi.org/10.1080/03630242.2015.1050545
Accepted author version posted online: 21 May 2015. Published online: 21 May 2015. Submit your article to this journal
Article views: 35
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=wwah20 Download by: [Sichuan Normal University]
Date: 06 November 2015, At: 01:17
Women & Health, 55:829–841, 2015 Copyright © Taylor & Francis Group, LLC ISSN: 0363-0242 print/1541-0331 online DOI: 10.1080/03630242.2015.1050545
Electric Blanket Use and Risk of Thyroid Cancer in the Women’s Health Initiative Observational Cohort
Downloaded by [Sichuan Normal University] at 01:17 06 November 2015
IKUKO KATO, MD, PhD Department of Oncology and Pathology, Wayne State University School of Medicine, Detroit, Michigan, USA
ALICIA YOUNG, MS and JINGMIN LIU, MS Fred Hutchinson Cancer Research Center, Seattle, Washington, USA
JUDITH ABRAMS, PhD, CATHRYN BOCK, PhD, and MICHAEL SIMON, MD, MPH Department of Oncology, Karmanos Cancer Institute at Wayne State University, Detroit, Michigan, USA
Thyroid cancer disproportionally affects more women than men. The aim of this study was to assess whether exposure to extremely low frequency electric magnetic fields from electric blankets (EBs) was associated with the development of thyroid cancer. Data were analyzed from 89,527 women who participated in the Women’s Health Initiative Observational Study and who responded to questions concerning prior use of EBs. During a mean follow-up of 12.2 years, 190 incident cases of thyroid cancer were identified. We estimated the hazard ratio (HR) and 95 percent confidence interval (CI) of incident thyroid cancer associated with EB use by Cox’s proportional hazard model, adjusted for selected covariates. A majority, 57 percent, of the women in the cohort reported the use of EBs while sleeping and/or for warming the bed before sleep. No association was found between use of EBs and subsequent risk of thyroid cancer (HR = 0.98, 95 percent CI 0.72–1.32). Duration of EB use measured in years, months, or hours had no effect on risk. These results did not change when the cases were limited to papillary Received January 23, 2014; revised August 13, 2014; accepted August 31, 2014. Address correspondence to Ikuko Kato, MD, PhD, Department of Oncology and Pathology, Wayne State University School of Medicine, 4100 John R. Street MM04EP, Detroit, MI 48201. E-mail: [email protected] 829
830
I. Kato et al.
thyroid cancer, the most frequently occurring histologic type. The results of this study do not support possible health hazards of EBs in regards to thyroid cancer risk. KEYWORDS cohort study, electromagnetic field, postmenopausal women, thyroid cancer
Downloaded by [Sichuan Normal University] at 01:17 06 November 2015
INTRODUCTION Thyroid cancer disproportionally affects women more than men. Particularly for reproductive ages (i.e., 15–44), the female/male ratio of thyroid cancer incidence reaches or exceeds 4.0, and, consequently, thyroid cancer is ranked as the 5th most commonly diagnosed cancer in U.S. women (Howlader et al. 2013). In addition, thyroid cancer incidence in the U.S. has risen more rapidly than other frequently occurring cancers, specifically since 1996 (Howlader et al. 2013) in the absence of any screening efforts suggesting a potential role for new environmental risk factors. Exposure to ionizing radiation is the best established risk factor for thyroid cancer (IARC 2000). On the contrary, to well quantified carcinogenic risk of ionizing radiation to humans (IARC 2000), the effect of nonionizing radiation (except UV) on cancer risk in general has been highly controversial. Nevertheless, two previous expert reviews organized by the International Agency for Research on Cancer classified non-ionizing radiation, namely, extremely low frequency (ELF) and radiofrequency (RF) electric magnetic fields (EMF), as group 2B human carcinogens (possibly carcinogenic). These conclusions were based on the observations from epidemiologic studies on childhood leukemia and adult brain cancer (IARC 2002, 2013), but the data concerning thyroid cancer have been very limited to date. A wide range of household and personal appliances produces non-occupational exposure to EMF, but the exposure rapidly declines with distance from appliances, by the inverse square to inverse cube of distance (IARC 2002). Among various appliances, use of electric blankets (EBs) has raised concern about hazardous health effects because of the combined characteristics of close proximity and prolonged hours of use. Thus far, the primary interest in epidemiologic studies of EB use has been breast cancer risk, owing to postulated effects on reproductive hormones through reduced melatonin secretion by EMF exposure (Cohen, Lippman, and Chabner 1978). These studies have, however, produced inconclusive results (IARC 2002), while a more recent large-scale cross-sectional study in postmenopausal women revealed an association between EB use for 20 years and longer and endometrial cancer (Abel et al. 2007). While reproductive hormones stimulate thyroid growth (Rahbari, Zhang, and Kebebew 2010), investigators have also found that exposure to EMF induces morphological and functional changes in the thyroid glands of rodents
Electric Blanket Use and Thyroid Cancer Risk
831
(Rajkovic et al. 2003; Rajkovic, Matavulj, and Johansson 2005; Wright et al. 1984). Hence, it is plausible that EMF exposure may modulate thyroid cancer risk in humans, although age at exposure may be critical for thyroid carcinogenesis as with ionizing radiation (IARC 2000). This study was the first of which we are aware to examine the association between EB use and thyroid cancer incidence in a prospective cohort study.
MATERIALS AND METHODS Downloaded by [Sichuan Normal University] at 01:17 06 November 2015
Study Participants The women included in this study were a subset of participants in the Women’s Health Initiative (WHI). The WHI was designed to address major causes of morbidity and mortality in postmenopausal women consisting of both multicenter clinical trials and an observational study (OS). This study was based on the OS cohort only. Other publications have described the details of the scientific rationale, eligibility requirements, baseline participant characteristics, and measurement reliability of the WHI OS participants (Hays et al. 2003; Langer et al. 2003). Briefly, participants were women 50–79 years of age, recruited at 40 clinical centers throughout the United States between September 1, 1993 and December 31, 1998. Institutional Review Boards at all 40 clinical centers and at the coordinating center and a study-wide data and safety monitoring board oversaw the study. All participants in the WHI gave informed signed consent. At enrollment, participants completed self-administered questionnaires to provide information concerning demographic, medical, reproductive, and lifestyle factors. In addition, OS participants (not clinical trial participants) answered five questions about EB use, ever use, current use, use only to warm the bed before sleep, number of years used, and average number of months used per year. The definition of EB in this questionnaire was inclusive of electric heating pad, electric mattress pad, and heated water bed in addition to blankets.
Follow-Up and Ascertainment of Cases Incident thyroid cancer cases were identified by self-administered questionnaires annually throughout the study, with all cases confirmed by medical record review (Curb et al. 2003). All primary thyroid cancer cases were coded centrally in accordance with the Surveillance Epidemiology and End Results (National Cancer Institute) coding guidelines. For these analyses, participants were followed up to first thyroid cancer diagnosis, date of death, loss to follow-up, or end of study follow-up (September 30, 2012), whichever
832
I. Kato et al.
occurred first. Only 2.35 percent of the cohort was lost to follow-up at the end of the follow-up.
Downloaded by [Sichuan Normal University] at 01:17 06 November 2015
Statistical Analysis We computed hazard ratios (HRs) and 95 percent confidence intervals (CIs) using the Cox proportional hazard model. We tested potential covariates one at a time in a model that also included an indicator variable for participation in the WHI extension study period. We selected these explanatory variables because they were basic demographic variables or a known strong risk factor (benign thyroid disease) or were reported as potential risk factors for thyroid cancer or as causal confounders with electric blanket use in earlier studies (Abel et al. 2007; Dal Maso et al. 2009; Kabat et al. 2012; Kitahara et al. 2012; Rahbari, Zhang, and Kebebew 2010). The “final” multivariable model included potential risk factors that altered the regression coefficient for the main exposure variable, electric blanket use, by at least 10 percent in addition to a known strong risk factor, a history of benign thyroid disease. We computed summary exposure indices (cumulative months/hours of use) using median values of each category of the number of years of use (
