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Childhood Social Environment and Risk of Non–Hodgkin Lymphoma in Adults

¹ Department of Medical Epidemiology and Biostatistics, Karolinska Institutet; ² Department of Pathology and Oncology, Karolinska Institutet and University Hospital, Stockholm, Sweden; ³ Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark; ⁴ Northern California Cancer Center, Fremont, California; ⁵ Division of Epidemiology, Department of Health Research and Policy, Stanford University School of Medicine, Stanford, California; ⁶ Department of Oncology, Radiology, and Clinical Immunology, University of Uppsala, Uppsala, Sweden; and ⁷ Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts

Requests for reprints: Karin Ekström Smedby, Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, SE-171 77 Stockholm, Sweden. Phone: 468-517-707-06; Fax: 468-517-793-04; E-mail: karin.ekstrom.smedby{at}ki.se.

Better hygiene and sanitation and decreasing family size parallel the increasing incidence of non–Hodgkin lymphoma (NHL) in many populations around the world. However, whether sibship size, birth order, and crowding are related to adult NHL risk is not clear. We investigated how family structure and childhood social environment were related to the risk of NHL and NHL subtypes in a large Scandinavian population–based case control study with 6,242 participants aged 18 to 74 years. Detailed exposure information was obtained through telephone interviews. Odds ratios (OR) and 95% confidence intervals (95% CI) were calculated using logistic regression, and all statistical tests were two-sided. Having four or more siblings was associated with a moderately increased risk of NHL, compared with having no siblings (OR 1.34, 95% CI 1.11-1.62, P_trend < 0.001). Having four or more older siblings was associated with a similar risk increase (OR 1.33, 95% CI 1.12-1.59, P_trend = 0.003) compared with being the oldest, whereas number of younger siblings was unrelated overall. The associations were independent of other environmental exposures and did not vary by country, age, or sex. High household crowding was also positively associated with risk of NHL. Results were slightly stronger for diffuse large B-cell and T-cell lymphomas than for other major NHL subtypes. Our findings add to the evidence that large sibship size, late birth order, and childhood crowding are associated with an elevated risk of NHL. Effect mechanisms may be related to early age at onset and high frequency of specific infections or total microbial exposure in childhood. [Cancer Res 2007;67(22):11074–82]

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