Evaluating heterogeneity in the impact of the reformulated gasoline program on childhood and young adult lymphoma incidence: A Bayesian causal inference analysis.

Observational studies of the relationship between traffic-related air pollution (TRAP) and childhood hematologic cancer have been suggestive but inconclusive. Yet recent quasi-experimental studies have found that US environmental policies targeting reductions in TRAP, including the reformulated gasoline program of the 1990s, have reduced incidence of childhood and young adult (CYA) lymphoma in affected areas. However, no studies to date have assessed potential heterogeneity in these effects. This study investigates the heterogeneous impacts of the reformulated gasoline program on the incidence of CYA lymphoma across disease type and demographic strata. We employ recently-proposed Bayesian causal Gaussian process models, applied to population cancer registry data, to estimate effects of the program on CYA lymphoma incidence across strata defined by cancer type, sex, race, Hispanic ethnicity, and age group. Our analytic framework allows for stable estimation of stratum-specific effects via data-driven information sharing across space, time, and strata. Effects are reported on both the absolute and relative scales. We find evidence that the largest program-attributable reductions in lymphoma incidence rates occurred for Hodgkin lymphoma, and among individuals who are male, white, and/or aged 20-29. The finding of larger reductions in Hodgkin lymphoma is notable since prior TRAP studies have primarily focused non-Hodgkin lymphoma.