Unintended doses in radiotherapy-over, under and outside?

Radiotherapy is a safe treatment; nevertheless, national reporting of serious incidents allows investigation of potential harm to individuals and failing safety culture. UK guidance has previously been limited to overexposures, but underexposures will be included in the new legislation, and positioning errors have also been explicitly included in recent guidance. This commentary reviews current guidance and suggests practical approaches to the additional categories, including the definition of a local error margin.