Errors from radiotherapy machine or software malfunction usually are well documented as they affect hundreds of patients, whereas random errors affecting individual patients are more difficult to be discovered and prevented. Although major clinical radiotherapy incidents have been reported, many more have remained unrecognised or have not been reported. The literature in this field is limited as it is mostly published as a result of investigation of major errors. We present a review of radiotherapy incidents internationally with the aim of identifying the domains where most errors occur through extensive review and synthesis of published reports, unpublished 'Grey literature' and departmental incident data. Our review of radiotherapy-related events in the last three decades (1976-2007) identified more than seven thousand (N=7741) incidents and near misses. Three thousand one hundred and twenty-five incidents reported patient harm of variable intensity ranging from underdose increasing the risk of recurrence, to overdose causing toxicity, and even death for 1% (N=38); 4616 events were near misses with no recognisable patient harm. Based on our review, a radiotherapy risk profile has been published by the WHO World Alliance for Patient Safety that highlights the role of communication, training and strict adherence to guidelines/protocols in improving the safety of radiotherapy process.