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Background: Non-Hodgkin lymphoma (NHL), the most common hematologic malignancy, consists of numerous subtypes. The etiology of NHL is incompletely understood, and increasing evidence suggests that risk factors may vary by NHL subtype. However, small numbers of cases have made investigation of subtype-specific risks challenging. The International Lymphoma Epidemiology Consortium therefore undertook the NHL Subtypes Project, an international collaborative effort to investigate the etiologies of NHL subtypes. This article describes in detail the project rationale and design. Methods: We pooled individual-level data from 20 case-control studies (17 471 NHL cases, 23 096 controls) from North America, Europe, and Australia. Centralized data harmonization and analysis ensured standardized definitions and approaches, with rigorous quality control. Results: The pooled study population included 11 specified NHL subtypes with more than 100 cases: diffuse large B-cell lymphoma (N = 4667), follicular lymphoma (N = 3530), chronic lymphocytic leukemia/small lymphocytic lymphoma (N = 2440), marginal zone lymphoma (N = 1052), peripheral T-cell lymphoma (N = 584), mantle cell lymphoma (N = 557), lymphoplasmacytic lymphoma/Waldenström macroglobulinemia (N = 374), mycosis fungoides/Sézary syndrome (N = 324), Burkitt/Burkitt-like lymphoma/leukemia (N = 295), hairy cell leukemia (N = 154), and acute lymphoblastic leukemia/lymphoma (N = 152). Associations with medical history, family history, lifestyle factors, and occupation for each of these 11 subtypes are presented in separate articles in this issue, with a final article quantitatively comparing risk factor patterns among subtypes. Conclusions: The International Lymphoma Epidemiology Consortium NHL Subtypes Project provides the largest and most comprehensive investigation of potential risk factors for a broad range of common and rare NHL subtypes to date. The analyses contribute to our understanding of the multifactorial nature of NHL subtype etiologies, motivate hypothesis-driven prospective investigations, provide clues for prevention, and exemplify the benefits of international consortial collaboration in cancer epidemiology.

Non-Hodgkin lymphoma (NHL) is the most common hematologic malignancy and the fifth most common type of cancer in more developed regions of the world (1). Numerous NHL subtypes with distinct combinations of morphologic, immunophenotypic, genetic, and clinical features are currently recognized (2,3). The incidence of NHL subtypes varies substantially by age, sex, and race/ethnicity (4–7). However, the etiological implications of this biological, clinical, and epidemiological diversity are incompletely understood. The importance of investigating etiology by NHL subtype is clearly supported by research on immunosuppression, infections, and autoimmune diseases, which are the strongest and most established risk factors for NHL. Studies of solid organ transplant recipients and individuals infected with HIV demonstrate that risks are markedly increased for several—but not all—NHL subtypes (8–13). Some infections and autoimmune diseases are associated with a single specific subtype [eg, human T-cell lymphotropic virus, type I (HTLV-I) with adult T-cell leukemia/lymphoma (14), celiac disease with enteropathy-type peripheral T-cell lymphoma (PTCL) (15–17)], whereas others [eg, Epstein–Barr virus, hepatitis C virus (HCV), Sjogren’s syndrome (18–21)] have been associated with multiple subtypes. In the last two decades, reports from individual epidemiological studies of NHL have suggested differences in risks among NHL subtypes for a wide range of risk factors, but most studies have lacked the statistical power to assess any differences quantitatively and have not systematically evaluated combinations of subtypes. One study assessed multiple risk factors and found support for both etiologic commonality and heterogeneity for NHL subtypes, with risk factor patterns suggesting that immune dysfunction is of greater etiologic importance for diffuse large B-cell lymphoma (DLBCL) and marginal zone lymphoma than for chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) and follicular lymphoma (22). However, that analysis was limited to approximately 1300 NHL cases and considered only the four most common NHL subtypes. Pooling data from multiple studies through the International Lymphoma Epidemiology Consortium (InterLymph) have provided substantial insight into associations between specific risk factors and NHL subtypes, with evidence that family history of hematologic malignancy, autoimmune diseases, atopic conditions, lifestyle factors (smoking, alcohol, anthropometric measures, and hair dye use), and sun exposure are associated with NHL risk (19,21,23–32). However, no previous study has compared patterns of risk for a range of exposures for both common and rarer NHL subtypes. We undertook the InterLymph NHL Subtypes Project, a pooled analysis of 20 case–control studies including 17 471 NHL cases and 23 096 controls, to advance understanding of NHL etiology by investigating NHL subtype-specific risks associated with medical history, family history of hematologic malignancy, lifestyle factors, and occupation. The detailed risk factor profiles for each of 11 NHL subtypes appear in this issue (15–17,33–40). In this report, we assess risk factor heterogeneity among the NHL subtypes and identify subtypes that have similar risk factor profiles.