Radiotherapy treatment, like many other fields of medicine, has changed significantly in the last decade with the introduction of more advanced technology and automation. This change has often resulted in aspects of the system which cannot be automated due to technological feasibility and local implementation constraints. This has resulted in a requirement for significant human interaction. This combination of human operations and automation has introduced new error pathways. Traditionally, recommendations to improve the safety of such systems are typically made after the analysis of an adverse event or a significant series of incidents. In contrast, adopting a proactive approach to safety would enable prior identification of potential errors and the specification of appropriate defences against them, thus avoiding costs associated with adverse outcomes. In this paper, a modified version of the proactive Human Reliability Assessment (HRA) method Human Error Assessment and Reduction Technique (HEART) was used to analyse a critical nursing task within a modern radiotherapy system. The modified technique used a participative team approach to complete the assessment in contrast to the normal approach, which uses a single expert assessor. The HEART technique quantifies the likelihood of unreliability of a task and ranks the conditions which most affect the successful completion of that task. HEART has been proposed as a potentially useful HRA tool for applications in healthcare, but such applications have not previously been formally documented. As a result of the modified HEART analysis reported in this paper, remedial measures were identified which were both cost effective and easy to implement. (C) 2011 Elsevier Ltd and The Ergonomics Society. All rights reserved.