Mortality after cancer diagnosis among children with congenital heart disease in Denmark and Sweden.

BACKGROUND Recent decades have witnessed tangible improvements in childhood cancer survival. However, the prognosis for children with congenital heart disease (CHD), the most prevalent birth defect, remains unclear. Due to improved survival of CHD and childhood cancer, evaluating outcomes within this intersection is important for clinical practice. We aimed to assess mortality post-cancer diagnosis among children with CHD. METHODS We conducted a study on the population of Denmark and Sweden, born 1970-2014, with a cancer diagnosis before age 20 in the national cancer registers (end of follow-up 2015; n = 20,665). CHD diagnoses (n = 397) and recorded deaths were retrieved from national health registers. We evaluated the effect of CHD on five-year mortality post-cancer diagnosis fitting Cox proportional hazards regression. RESULTS When excluding children with Down syndrome, children with CHD had a higher five-year mortality post-cancer diagnosis compared to children without (HR 1.48, 95% CI 1.18-1.86). This was particularly notable in children with lymphoma (HR 2.17, 95% CI 1.11-4.25) and neuroblastoma (HR 2.39, 95% CI 1.11-5.15). In more recent decades (post-1990), children with CHD had similar five-year mortality as their counterparts without, except for children diagnosed with lymphoma, where mortality remained elevated (HR 3.37, 95% CI 1.65-6.89). CONCLUSIONS In this large, register-based cohort study, children with CHD fared worse post-cancer diagnosis-particularly lymphoma and neuroblastoma. While a more positive trend emerged in recent years, lymphoma-related mortality remained disproportionately high among children with CHD, underscoring the need for continued research and interventions to improve outcomes for this vulnerable group.