Backgrounds: Multidisciplinary therapy has increased the risk of subsequent late effects, but detailed analyses on secondary cancers in childhood cancer survivors (CCSs) are limited in Asian countries. Methods: This was a retrospective cohort study comprising 10,069 CCSs who were diagnosed between 1980 and 2009 across 15 Japanese hospitals. We conducted secondary analyses to estimate the incidence of secondary cancer according to each primary malignancy and to elucidate the association between primary and secondary cancers. We also explored the risk factors for the development of secondary cancer in each independent primary malignancy. Results: The cumulative incidence of secondary cancer at 20 years varied among primary cancers: hematological malignancy, 3.1% (95% CI 2.2–4.3); retinoblastoma, 6.6% (95% CI 1.5–16.8); pediatric solid tumor, 2.5% (95% CI 1.3–4.2); brain tumors, 5.2% (95% CI 1.7–11.8) bone/soft tissue sarcoma, 5.2% (95% CI 2.3–10.1); and others, 3.3% (95% CI 1.6–6.0) (p = 0.015). The cumulative incidence of secondary cancers is highest in those with osteosarcoma (13.1%) followed by those with hepatoblastoma (8.4%) and retinoblastoma (6.6%). Close association between the primary and secondary cancer diagnoses was found. The risk factors for secondary cancer development depended on the primary cancer, but autologous/allogeneic stem cell transplantation was a relatively common risk factor. Conclusion: The cumulative incidence of secondary cancer varied among primary cancers. The primary cancer was closely associated with the secondary cancer but stem cell transplantation was a common risk factor for secondary cancers among CCSs.
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