[en] The use of ionising radiation (IR) for medical diagnosis and treatment procedures has had a major impact on the survival of paediatric patients. Although the benefits of these techniques largely outweigh the risks, the evidence to date suggests that children are more sensitive than adults to the carcinogenic effects of IR. Therefore, there is a need to better understand the long-term health effects of such exposures in order to optimise treatment in these young patients and reduce the risk of late toxicities. HARMONIC aims to improve our understanding of the health effects of exposure to medical IR in children, specifically cancer patients treated with modern radiotherapy techniques, and cardiac patients treated with cardiac fluoroscopy procedures (CFP). HARMONIC also develops dosimetric data collection software tools to allow dose reconstruction in both CFP and radiotherapy. The project builds on a multi-disciplinary collaboration to investigate long-term outcomes (endocrine dysfunction, cardiovascular and neurovascular damage, quality of life (QoL) and social impacts, and secondary cancers) of paediatric cancer patients after the application of modern radiotherapy modalities. Instruments for harmonised demographic, clinical and dosimetric data collection were defined serving as a pilot phase for a future pan-European registry. The cardiac component of HARMONIC builds a pooled cohort of approximately 100,000 patients who underwent CFP in 7 countries, while aged under 22 years. The cohort, based on data collection from hospital records and/or insurance claims data, will be followed-up using national registries and insurance records to determine vital status and cancer incidence. Where available, information on organ transplantation (a major risk factor for cancer development in this patient group) and/or other conditions predisposing to cancer will be obtained from national or local registries and health insurance data. The relationship between estimated radiation dose and cancer risk will be investigated using regression modelling. With its prospective design and the creation of a biobank for the collection of biological samples, HARMONIC also aims at providing a mechanistic understanding of radiation-induced adverse health effects and identify potential biomarkers indicative of vascular adverse effects and secondary cancer. These biomarkers could ultimately contribute to early diagnosis, treatment and prevention of adverse effects. (authors)