full free access: Proportion of second cancers attributable to radiotherapy treatment in adults: a cohort study in the US SEER cancer registries : The Lancet Oncology

Note: there is also chemotherapy-induced secondary cancers (not part of this study) but putting all treatment-related therapies (adverse events) in perspective
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Interpretation
A relatively small proportion of second cancers are related to radiotherapy in adults, suggesting that most are due to other factors, such as lifestyle or genetics.