The purpose of this investigation was to analyse incidents discovered in our radiation therapy department by means of human factor analysis and classification system (HFAC S). We adapted the original framework of the HFAC S and apply it to the analysis of incidents discovered in our radiotherapy department during a five-year period. Results showed that recurrent causal factors of incidents were attention failures and distracted/overconfidence behaviour as well as loss of situational awareness and mental fatigue. In our radiation therapy department the HFAC S allowed to highlight recurrent errors causal factors. Consequently we defined corrections factors for operators behaviour and implemented an operational protocol which improve operators attitude.