Number of siblings and survival from childhood leukaemia: a national register-based cohort study from Sweden.

BACKGROUND: Previous studies suggest worse leukaemia survival for children with siblings, but the evidence is sparse, inconsistent and does not consider clinical factors. We explored the associations between number of siblings in the household, birth order and survival from childhood acute lymphoid leukaemia (ALL) and acute myeloid leukaemia (AML). METHODS: In this nationwide register-based study we included all children aged 1-14, diagnosed with ALL and AML between 1991-mid-2015 in Sweden (n = 1692). Using Cox regression models, we estimated hazard ratios (HRs) and 95% confidence intervals (CIs) according to number of siblings and birth order, adjusting for known prognostic and sociodemographic factors. RESULTS: A tendency towards better ALL survival among children with one, or ≥2, siblings was observed, adjHRs (95% CI): 0.73 (0.49-1.10) and 0.63 (0.40-1.00), respectively. However, this was mainly limited to children with low risk profiles. An indication of better AML survival among children with siblings was seen, adjHRs (95% CI) 0.68 (0.36-1.29) and 0.71 (0.34-1.48) but diminished after adjusting for birth order. CONCLUSION: Our results do not support previous findings that a larger number of siblings is associated with poorer survival. Inconsistencies might be explained by underlying mechanisms that differ between settings, but chance cannot be ruled out.