Study characteristics
The systematic search process yielded 117 eligible studies (Figure 1)
that included 509 environmental factor- and childhood cancer risk-
specific meta-analyses. A total of 107 studies were excluded, among
which duplicate studies, namely duplicate meta-analyses on the same
exposure and outcome pair were identified for 10 associations
(Supplementary Table 2). The 117 eligible systematic reviews and
meta-analyses summarized evidence from 3873 individual study estimates
stemming from 1140 primary studies. The vast majority (84.6%) of
original studies were of case-control design (n=964), and the remaining
15.4% were cohort studies (n=176).
The 509 individual meta-analyses of the 117 eligible studies examined
associations between environmental factors and 10 different cancer types
(all cancer sites, hematological malignancies including overall acute
leukemia, acute lymphoblastic [ALL] and acute myeloid [AML]
leukemia, CNS tumors, neural tumors/neuroblastoma, sarcomas, bone
tumors, retinoblastoma, testicular tumors, Wilms tumors and other solid
tumors). The majority of studied outcomes were childhood hematological
malignancies (n=374 associations, 73.5%), followed by CNS tumors (n=91
associations, 17.9%) and neural tumors/neuroblastoma (n=18
associations, 3.5%; Table 1). The potential risk factors were assessed
during all relevant exposure windows, namely during preconception,
pregnancy, delivery and childhood. A median of 11 (range 2-39) study
estimates were combined for each meta-analysis. The average number of
cancer cases in each meta-analysis was 3688 (range 19-31,610; Table 1).
Around two-thirds of the 509 meta-analytical associations (62.9%) were
assessed through random effects models, whereas 23.8% of these
associations were evaluated only through fixed effects models.