Duty of candour and clinically significant accidental or unintended exposures: revisiting the definition of moderate harm for patient safety incidents involving ionizing radiation.

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To explore a quantitative interpretation of the term 'moderate harm' as applied to the triggering of the duty of candour associated with clinically significant accidental and unintended exposures to ionizing radiation. Current definitions of 'moderate harm' were matched to the lay descriptions of disease and injury states used in the calculation of detriment as disability-adjusted life years (DALY) by the World Health Organization, to obtain a value of detriment associated with 'moderate harm'. Published conversion factors between effective dose and DALY were used to calculate the effective dose associated with the same detriment. The DALY loss associated with a moderate harm incident is estimated as 0.0216 years. This corresponds to the detriment resulting from exposure to ionizing radiation of 21 mSv. An effective dose of 21 mSv relates to a probability of induced cancer of 0.0012. The results obtained closely match existing guidance although the method used is completely different. It is concluded that there is no evidence to change the existing guidance on the triggering of DoC in radiation incidents. An alternative approach to linking 'moderate harm' and radiation detriment has reinforced existing guidance.