Maternal Medication Use and Neuroblastoma in Offspring

Michael N. Cook1, Andrew F. Olshan1 , Harry A. Guess1, David A. Savitz1, Charles Poole1, Julie Blatt2, Melissa L. Bondy3 and Brad H. Pollock4

1 Department of Epidemiology, School of Public Health, University of North Carolina at Chapel Hill, Chapel Hill, NC.
2 Division of Pediatric Hematology/Oncology, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC.
3 Department of Epidemiology, M. D. Anderson Cancer Center, University of Texas, Houston, TX.
4 Department of Pediatrics, University of Texas Health Science Center at San Antonio, San Antonio, TX.

Received for publication December 30, 2002; accepted for publication November 18, 2003.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The association between a mother’s use of specific medications during pregnancy and lactation and neuroblastoma in her offspring was evaluated in a case-control study. Newly diagnosed cases of neuroblastoma (n = 504) in the United States and Canada were identified between 1992 and 1994 at 139 hospitals affiliated with the Pediatric Oncology Group or the Children’s Cancer Group clinical trial programs. One age-matched control was sampled from the community of each case by means of random digit dialing. Exposure information was ascertained retrospectively from mothers in a structured telephone interview. Odds ratios were estimated using conditional logistic regression, with adjustment for maternal sociodemographic factors. The results did not support an association between neuroblastoma and maternal exposure to diuretic agents, antiinfective agents, estrogens, progestins, sedatives, anticonvulsant drugs, or drugs that may form N-nitroso derivatives. Mothers of cases were more likely to report using medications containing opioid agonists while they were pregnant or nursing than were mothers of controls (odds ratio = 2.4, 95% confidence interval: 1.3, 4.3). Specifically, more mothers of cases reported using medications containing codeine while pregnant or nursing than did mothers of controls (odds ratio = 3.4, 95% confidence interval: 1.4, 8.4). This preliminary finding may be due to bias, confounding, or chance, and additional studies are needed for confirmation.

case-control studies; child; codeine; neuroblastoma; pregnancy; risk factors

Abbreviations: Abbreviations: CI, confidence interval; OR, odds ratio; RDD, random digit dialing.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Neuroblastoma is an embryonal tumor of the sympathetic nervous system arising from the primitive progenitor cells of the sympathetic ganglia and adrenal medulla (1). It is the most common malignancy diagnosed in infants (2) and accounts for approximately 7 percent of all childhood cancers (3). The incidence rate of neuroblastoma is highest during the first year of life (64 cases per million) and declines rapidly thereafter (4). An estimated 650 new cases of neuroblastoma are diagnosed in the United States each year (4). Approximately two thirds of patients with neuroblastoma survive for at least 5 years after diagnosis (4). Factors of prognostic significance in neuroblastoma include the child’s age at diagnosis, the clinical stage of disease, tumor histopathology, and chromosomal abnormalities in tumor cells, such as MYCN proto-oncogene amplification (5). The pathogenesis of human neuroblastoma remains unclear. No single genetic aberration is common to all cases (6). Cytogenetic differences between tumors with different clinical behaviors suggest that neuroblastoma is a heterogeneous disease with at least two distinct biologic subtypes (5, 7).

Environmental factors that influence the incidence of neuroblastoma in human populations have been difficult to identify. The young age of onset for neuroblastoma has led investigators to focus on maternal and paternal exposures that occur before conception or during gestation (8). Epidemiologic studies of neuroblastoma have found inconclusive associations with maternal use of sex hormones (912), vaginal antiinfective agents (11, 13), diuretics (10, 14), analgesics (10, 14), tranquilizers (10, 14), and neurally active drugs (10, 14). The small sample sizes of these studies have prevented full evaluation of the effects of specific drugs and pharmacologic classes of drugs, including drugs that may form potentially carcinogenic N-nitroso derivatives (15).

The purpose of the present study was to screen maternal self-reported medication exposures to evaluate their associations with neuroblastoma. We present the results of a large case-control study that collected detailed information from mothers on their use of specific drugs at specific times during pregnancy. Findings regarding fertility drugs, oral contraceptives, and multivitamin supplements have been reported previously (16, 17).


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Cases
A detailed description of the methods used in this study has been published elsewhere (16). We identified persons younger than age 19 years who received a new diagnosis of pathologically confirmed neuroblastoma between May 1, 1992, and April 30, 1994. Patients were identified from 139 hospitals in the United States and Canada that were members of the Children’s Cancer Group or the Pediatric Oncology Group, two collaborative clinical trials groups sponsored by the National Cancer Institute. The study protocol was approved by the institutional review board regulating human subject research at each participating institution. Permission from the patient’s treating physician was required before a patient’s mother could be invited to participate in this study. Mothers of cases were required to sign and return informed consent forms before they could be interviewed. Case eligibility required that the patient’s home be equipped with a landline telephone and that the patient’s biologic mother be able to conduct an interview in either English or Spanish. A total of 741 eligible cases were identified over the 24-month period of enrollment. The mothers of 538 eligible cases completed interviews (73 percent). Maternal interviews were not obtained for the remaining eligible cases because some physicians refused to permit contact with their patients’ families (n = 90), some mothers refused to be interviewed (n = 57), and some mothers could not be contacted after six attempts (n = 44), or for other reasons (n = 12).

Controls
One age-matched control was sampled from the residential community of each case using a random digit dialing (RDD) procedure that was a modification (18) of the Waksberg-Mitofsky two-stage cluster sampling method (19). We created an ordered list of 99 telephone numbers for each case by replacing the last two digits of the case’s home telephone number with a randomly generated two-digit number. We dialed each number up to six times on different days to enumerate children born within 6 months of the case’s birth date for cases who were aged 3 years or younger or within 1 year of the case’s birth date for cases who were older than age 3 years. Age-matched children enumerated by RDD were eligible to participate as controls if their biologic mothers could be interviewed in either English or Spanish. In the household screening stage of the RDD procedure, 74 percent of operational telephone numbers dialed provided information about the presence or absence of an eligible age-matched child living within the household. Of the 703 eligible age-matched controls identified by RDD, the mothers of 504 children completed interviews (72 percent). Maternal interviews were not completed for the remaining identified eligible controls because some mothers refused to be interviewed (n = 142) and some mothers could not be contacted after six attempts (n = 57).

Exposure assessment
Exposure information was collected retrospectively from mothers in a structured telephone interview conducted by a trained interviewer. Mothers of cases were interviewed between 1992 and 1995. Mothers of controls were interviewed between 1993 and 1996. Among case mothers, 25 percent were interviewed within 1.1 years after the birth of the index child, 50 percent were interviewed within 1.9 years, and 75 percent were interviewed within 3.8 years. Among control mothers, 25 percent were interviewed within 2.1 years after the birth of the index child, 50 percent were interviewed within 3.1 years, and 75 percent were interviewed within 4.7 years. The amount of time elapsed between the index child’s date of birth and the date of the mother’s interview (the recall period) was, on average, 1 year less for cases than for age-matched controls (standard deviation, 0.69 years).

Information on past medication use was ascertained by interviewing mothers using a method similar to that employed in an ongoing case-control study of birth defects (20). In the structured telephone interview, mothers were first asked about use of medications for specific indications. They were then asked to recall the specific brand names or generic names of any medications they had used for 5 days or more for each indication. The 21 indication-specific classes of medications queried about in the interview were: vitamins; iron supplements; antibiotics; cold and cough remedies; antihistamines and allergy remedies; pain relievers and antiinflammatory drugs; morning sickness medicines; laxatives and antidiarrhea medicines; diuretics or water tablets; tranquilizers; antidepressants; sleeping pills; diet pills and appetite suppressants; blood pressure medications; heart disease medications; anticonvulsants; female hormones; other hormones; fertility drugs; corticosteroids; immunosuppressants; oral contraceptives; and acne medications. Mothers were mailed an interview guide in advance of their scheduled telephone interview that listed several brand and generic names of medications as examples for each category. An affirmative response to an indication-specific prompt was followed by a set of detailed questions asking mothers to recall the specific name of the product used, the reason for its use, and the time window(s) in which it was used (2–12 months before pregnancy, the month before pregnancy, the first trimester, the second trimester, the third trimester, or while nursing). Interviewers used the mother’s self-reported date of her last menstrual period to define the starting date of the index pregnancy and the six aforementioned exposure time windows.

Names of medications reported by the mothers were coded for analysis using a drug dictionary developed and maintained by the Slone Epidemiology Center at Boston University (20). The cross-mapping tables included in this electronic drug dictionary were used to identify individual drugs in multicomponent products and to categorize drugs into pharmacologic classes according to the American Hospital Formulary Service Pharmacologic-Therapeutic Classification (American Society of Health-System Pharmacists, Inc., Bethesda, Maryland). Drugs with the potential to form stable N-nitroso derivatives were identified by Dr. William Lijinsky (National Cancer Institute-Frederick Cancer Center, Frederick, Maryland), who used published and unpublished laboratory data and his knowledge of chemistry to determine the nitrosatability of each drug (list of nitrosatable drugs provided by Dr. Roberta McKean-Cowdin, University of Southern California-Norris Comprehensive Cancer Center, University of Southern California; personal communication, 2000).

Data analysis
We screened generic drugs to identify exposures for which there were at least five exposed cases. Because the relevant induction period for neuroblastoma is not known, the screening procedure included all person-time in which children could have been exposed to a medication via their mothers’ circulation or breast milk. For each generic drug, exposure was defined as any reported use of a product containing the drug while the mother was pregnant or nursing. For each drug class (e.g., all opioid agonists combined), exposure was defined as any reported use of a product containing a drug in that class while the mother was pregnant or nursing. The conditional maximum likelihood estimator of the common odds ratio and its 95 percent confidence interval were calculated using conditional logistic regression analysis for matched-pairs data. Adjustments were made for the effects of mother’s age at the child’s birth (<20, 20–24, 25–30 (referent), 31–39, or >=40 years), mother’s ethnicity (White (referent), Black, Hispanic, or other), and mother’s educational level (less than high school, high school only, or any college (referent)). To minimize the potential for statistical bias (21), we did not estimate the adjusted odds ratio using maximum likelihood methods for exposures with fewer than five case-exposed/control-unexposed discordant pairs and five case-unexposed/control-exposed discordant pairs.

In the hypothesis screening procedure, an exposure was considered to be associated with neuroblastoma if the odds ratio was above 2.0 and the estimated standard error (sampling error) of the natural logarithm of the odds ratio estimate was less than 0.5. For drug exposures selected in the screen, the exposure prevalence among cases and controls was tabulated for different exposure time windows. In addition, we conducted separate analyses in subsets of matched pairs defined by the case’s age at diagnosis (<=1 year, >1 year) and tumor MYCN amplification status (amplified, normal) to investigate potential effect measure heterogeneity across different biologic subtypes of neuroblastoma. MYCN copy number was determined by Southern blot analysis (22) or fluorescence in situ hybridization (23) at the central laboratories of the Children’s Cancer Group or the Pediatric Oncology Group. Pathologic data on MYCN copy number were available for 381 cases, of which 67 had MYCN amplification.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The analyses presented in this report were based on 504 matched case-control pairs for which we had complete maternal interview data. The distribution of selected demographic characteristics of the cases and controls and their biologic mothers is presented in table 1. A higher proportion of neuroblastoma cases were boys in comparison with controls (55.4 percent vs. 49.8 percent). Cases were less likely to have a mother with college education than controls (21.6 percent vs. 26.8 percent).


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TABLE 1. Demographic characteristics of children and their mothers, case-control study of neuroblastoma, United States and Canada, 1992–1996
 
Table 2 lists the individual generic drugs reported by at least five case mothers at any time during pregnancy or lactation. Adjusted odds ratios are presented in table 2 for the 24 drugs that met the minimum sample size requirements for maximum likelihood estimation. Codeine was the only drug screened that showed a positive association with neuroblastoma. Twenty-two (4.4 percent) case mothers as compared with six (1.2 percent) control mothers reported using a product containing codeine while they were pregnant or nursing (odds ratio (OR) = 3.4, 95 percent confidence interval (CI): 1.4, 8.4). Almost all exposure to codeine originated from combination analgesics that contained both codeine phosphate and acetaminophen. However, no association was found with maternal acetaminophen use (OR = 1.1, 95 percent CI: 0.8, 1.5). Twenty-six (93 percent) of the 28 case and control mothers who reported using a medication containing codeine stated that they were prescribed the medication by a physician.


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TABLE 2. Adjusted odds ratio for neuroblastoma in relation to maternal use of selected medications, United States and Canada, 1992–1996
 
Adjusted odds ratios are presented in table 3 for the 11 drug classes that met the minimum sample size requirements for maximum likelihood estimation. Among the 11 drug classes screened, opioid agonists were the only class of drugs found to have an odds ratio above 2.0. The opioid agonist drug class included codeine, dextromethorphan, diphenoxylate, hydrocodone, loperamide, meperidine, morphine, oxycodone, and propoxyphene. Forty-one case mothers (8.1 percent) and 17 control mothers (3.4 percent) reported using an opioid agonist at least once while they were pregnant or nursing (OR = 2.4, 95 percent CI: 1.3, 4.3).


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TABLE 3. Adjusted odds ratio for neuroblastoma in relation to maternal use of medications, by pharmacologic action, United States and Canada, 1992–1996
 
The distribution of case and control mothers who were exposed to codeine and other opioid agonists in different exposure periods is presented in table 4. Exposure to opioid agonists was numerically higher among case mothers than among control mothers in the first trimester, in the third trimester, and during the postnatal period of breastfeeding. The reported prevalence of codeine exposure also was numerically higher among case mothers during these same three exposure periods. The low number of exposed controls in each time window prevented the estimation of period-specific odds ratios with adjustment for maternal sociodemographic variables and inter-time-window confounding.


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TABLE 4. Maternal use of codeine and other opioid agonists around the time of and during pregnancy, by exposure period, United States and Canada, 1992–1996
 
The analysis of effect measure heterogeneity across different case subgroups stratified by age at diagnosis and MYCN oncogene amplification status was limited by the small number of exposed cases and controls in each subgroup analysis. The odds ratio estimates for maternal exposure to codeine and to all opioids combined were both reasonably homogeneous across neuroblastoma case subgroups, although the subgroup-specific estimates were highly imprecise (data not shown).


    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
To our knowledge, the present study is the largest case-control study of neuroblastoma to date that has evaluated the potential etiologic role of maternal medication use. Our screen of self-reported medication exposures among case and control mothers did not reveal any noteworthy associations, except for a 2.4-fold association with the use of opioid agonists during pregnancy and lactation. This finding was attributable, in large part, to a 3.4-fold association between maternal codeine use and neuroblastoma.

A case-control study of 104 neuroblastoma cases identified between 1970 and 1979 from the Greater Delaware Valley Tumor Registry found a higher prevalence of maternal use of prescription pain medication during pregnancy among neuroblastoma cases than among their siblings (12 exposed cases; OR = 6.0, 90 percent CI: 2.0, 18.1) (10). This study did not provide the names of the drugs that mothers reported as "prescription analgesics." A second case-control study of 101 neuroblastoma cases identified between 1979 and 1986 at St. Jude Children’s Research Hospital in Memphis, Tennessee, found higher use of tranquilizers prescribed for pain among mothers of neuroblastoma patients than among mothers of children with other forms of childhood cancer (10 exposed cases; OR = 3.2, 95 percent CI: 1.4, 7.5) (14). This study also did not identify the specific analgesics that mothers reported as "tranquilizers prescribed for pain." No findings regarding prescription analgesics were reported in two recent case-control studies of neuroblastoma conducted in Germany (8) and New York State (11). The observation of imprecise associations with mothers’ self-reported use of prescription analgesics in two other case-control studies of neuroblastoma (10, 14) is interesting in light of the observed association with codeine in our study. Codeine is one of the most widely used prescription drugs among pregnant women in the United States (24, 25).

Our study did not confirm many associations found in previous case-control studies of neuroblastoma. The strong association with diuretics reported in two case-control studies of neuroblastoma (10, 14) did not materialize in our study or in the German case-control study of neuroblastoma (26). The number of mothers who reported using benzodiazepines and barbiturate tranquilizers in our study was too small to effectively address the association between tranquilizers and neuroblastoma suggested by the study conducted at St. Jude Children’s Research Hospital (14). The case-control study of 183 neuroblastoma patients conducted in New York State reported an association with drugs used to treat vaginal infection (11); however, no association with azole antifungal agents was present in our data. Our study did not confirm an association between neuroblastoma and metronidazole use during pregnancy, which was suggested by an analysis of Tennessee Medicaid claims data (13). The results of our study also did not indicate that neuroblastoma cases were more likely to have been exposed in utero to phenytoin than controls, as was suggested by several case reports in the literature (27).

Specific N-nitroso compounds (e.g., ethylnitrosourea) induce nervous system tumors in rodents following transplacental exposure (2830). We did not find an overall association with mothers’ use of drugs classified as having the potential to form stable N-nitroso derivatives. Acknowledging that most potentially nitrosatable drugs would have produced a nitrosamine derivative, our findings are consistent with animal experiments that have not shown nitrosamines to be transplacental carcinogens (31). Sulfamethoxazole, the only drug that could have produced a nitrosamide derivative, was not associated with neuroblastoma in our data.

Our study should be evaluated in light of its potential limitations. Selection bias was possible because only 68 percent of eligible cases were included in the analysis (504 of 741 eligible cases), and the response proportion among eligible controls was estimated to be 53 percent (74 percent RDD screening fraction multiplied by 72 percent participation among the identified eligible controls). A case-control study of childhood leukemia noted that parents of enumerated RDD controls who agreed to be interviewed were more often White and better educated than those who declined to be interviewed (32). Although we did not collect demographic information on nonresponders, the control mothers who did participate were more likely to be college-educated than the case mothers in our study. To minimize the potential impact of differential selection of controls based on socioeconomic status, we used multivariable modeling techniques to adjust odds ratio estimates for correlates of maternal socioeconomic status. Nonetheless, we cannot exclude selection bias as a possible noncausal explanation for the observed association with codeine.

Drug use during pregnancy is known to be underreported when women are asked to recall this information after delivery (3338). The limitations of autobiographic memory have been reviewed extensively in recent texts (39, 40). We did not validate mothers’ self-reports against their medical records, because most drug exposures reported were over-the-counter medications (41). We did try to improve the completeness of mothers’ drug recall by including prompts in the interview that were designed to elicit memories of past illnesses that might have been treated with medications (42) and by providing lists of medication names in the interview guide as memory aids (43, 44). Nonetheless, the mothers who participated in our study could not have been expected to systematically remember the exact names of all medications they had used during pregnancy.

The extent of underreporting of past use of medications containing codeine among the mothers in our study is not known. Researchers in the Third National Health and Nutrition Examination Survey (1988–1994) estimated that 0.95 percent of the US adult population used a combination product containing codeine phosphate and acetaminophen in a given month (45). A survey of 2,752 mothers in the Baltimore-Washington, DC, area (1980–1987) in which 75 percent of the women were interviewed within 7 months after delivery found that 1.6 percent, 1.2 percent, and 1.1 percent of mothers reported using an analgesic or cold remedy containing a narcotic in the first, second, and third trimesters of pregnancy, respectively (46). The proportion of control mothers who reported using medications containing codeine in our study was somewhat lower than these survey-based estimates (i.e., 0.4 percent, 0.6 percent, and 0.4 percent for exposure in the first, second, and third trimesters, respectively). In the Tennessee Medicaid program (1983–1988), codeine was dispensed to 4.3 percent, 3.7 percent, and 4.0 percent of pregnant women during the first, second, and third trimesters of pregnancy, respectively (24). In the Michigan Medicaid program (1981–1983), codeine was dispensed to 6.8 percent, 5.0 percent, and 4.5 percent of pregnant women during the first, second, and third trimesters, respectively (25). Incomplete recall may explain why the estimated utilization of codeine in the source populations of our study and of another study that conducted postnatal interviews (46) was so much lower than in studies of Medicaid pharmacy claims data (24, 25).

Recall bias (47, 48) could fully explain the observed association with codeine in our data if case mothers recalled the names of medications containing codeine 3.4 times more completely than control mothers. The recall period for case mothers was, on average, 1 year shorter than that for matched control mothers because of practical considerations in the sampling of controls using RDD. Case mothers also may have had greater motivation to remember past medication use than control mothers. Validation studies have not found substantial differences in the underreporting of past medication use between case and control mothers in retrospective studies of pregnancy outcomes (36, 4954). When case-control differences in recall accuracy have been detected for measurable drug exposures (i.e., exposures reported by at least five control mothers), they have not inflated the odds ratio estimate by more than a factor of 1.9 (54). We attempted to avoid spurious associations that could be artifacts of recall bias by setting the threshold of our hypothesis screening procedure to a criterion that was above this expected level of error (i.e., OR > 2.0).

Uncontrolled confounding also must be acknowledged as a potential limitation. Medical conditions that caused mothers to use analgesic agents could confound the estimation of the effect of codeine if these conditions, in and of themselves, increase the risk of neuroblastoma. The association with these conditions would need to be equal to or greater than the odds ratio estimated for codeine in magnitude in order to fully explain this observed association. None of the pregnancy- or birth-related characteristics measured in our case-control study that might have prompted analgesic use, including cesarean delivery (OR = 1.4, 95 percent CI: 1.0, 1.9), had effect estimates of a magnitude comparable to that found with codeine (55). However, mothers were not asked in the interview to provide a complete history of all medical conditions causing pain during pregnancy and lactation that might have led to analgesic use. The fact that no associations were observed with nonopioid analgesics (aspirin, acetaminophen, and ibuprofen) downplays the argument that the indications for using pain medication were independently associated with neuroblastoma in our study population.

It is conceivable that the association with codeine was observed purely by chance. We made no attempt to employ traditional (e.g., Bonferroni) or empirical Bayes, semi-Bayes, or Bayesian adjustments (56, 57) for multiple comparisons to reduce the overall false-positive error rate. The fact that our study did not observe many of the associations that have been found in previous case-control studies of neuroblastoma emphasizes the potential of observing a spurious association by chance in any single study.

Interpretation of whether the observed association with maternal codeine use in our data is causal depends in part on biologic mechanisms for such a relation (20). In the National Toxicology Program, codeine did not induce chromosomal aberrations in Chinese hamster ovary cells, it was not mutagenic in any of the four strains of Salmonella typhimurium, and it had no measurable carcinogenic activity in 2-year feeding studies conducted with rats and mice (58).

Although codeine is unlikely to be a carcinogen, codeine does have neuroendocrine effects that could disrupt adrenal gland development in the fetus and neonate. Codeine is known to cross the placenta and enter the fetal compartment at concentrations that can influence neurologic development (59). Codeine also is excreted in breast milk at concentrations 1.5–2.4 times higher than in the maternal circulation (60, 61). Phenanthrene opioids (morphine and codeine) are synthesized endogenously at picomolar concentrations in mammalian tissues (62), and the adrenal gland has the highest tissue concentration of endogenous codeine production in the rat (63). Studies conducted with the human neuroblastoma cell line SK-N-SH showed that opioid peptides can inhibit cell proliferation by binding to a specific opioid receptor (64), and blockade with naltrexone can increase tumor growth in mice injected with these cultured neuroblastoma cells (65). Codeine significantly increased adrenal gland weights in male and female rats after 13 weeks of chronic exposure and in male rats after 15 months of chronic exposure (58). In addition, the incidence of benign adrenal gland pheochromocytoma was significantly reduced in male rats following 2 years of chronic codeine exposure (58). In light of these experimental findings, the observed association between neuroblastoma and maternal codeine use in our study cannot be dismissed on the grounds of a lack of biologic plausibility.

On the basis of this study and two other case-control studies of neuroblastoma that have found imprecise associations with mothers’ self-reported use of prescription pain medications (10, 14), the hypothesis of a potential cause-effect relation between opioid analgesics and neuroblastoma deserves additional research. Analyses of state Medicaid claims data or other large health-care databases with mother-child linkages would be the preferred method of evaluating this hypothesis, because such studies would avoid the potential for both selection bias and recall bias.


    ACKNOWLEDGMENTS
 
This study was supported in part by grants CA57004 and T32-CA-09330 from the National Cancer Institute. Additional support was provided by a grant (P30ES10126) from the National Institute of Environmental Health Sciences.

The authors thank the staff of Battelle’s Centers for Public Health Research and Evaluation (Durham, North Carolina) for study coordination and interviewing. They thank Joanna Smith for programming support. The authors gratefully acknowledge the assistance of the principal investigators, oncologists, and coordinating staff at the participating institutions affiliated with the Children’s Cancer Group and the Pediatric Oncology Group. A list of principal investigators, institutions, and grant numbers is provided for each collaborative clinical trials program in Appendix tables 1 and 2.

A1


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APPENDIX TABLE 1. Participating principal investigators—Pediatric Oncology Group
 
A2


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APPENDIX TABLE 2. Participating principal investigators—Children’s Cancer Group
 


    NOTES
 
Correspondence to Dr. Andrew F. Olshan, Children’s Oncology Group, P.O. Box 60012, Arcadia, CA 91066-6012 (e-mail: andy_olshan{at}unc.edu). Back


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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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