Use-Error Focused Risk Analysis for Medical Devices: A Case Study of the Therac-25 Radiation Therapy System

Risk analysis or hazard analysis has been used as an engineering tool for many years to identify system risks and control system modes of failure. In alignment with the recent emphasis on patient safety, the tools of risk analysis have seen increased attention. These tools and related methods have been applied to understanding “use-errors” made with medical devices. Use-errors are defined as a pattern of predictable human errors that can be attributable to inadequate or improper design. Use-errors can be predicted through analytical task walkthrough techniques and via empirically based usability testing. This paper explores the methodology of use-error focused risk analysis and some of its history. An example is offered on how it can apply to a well-known but no longer marketed medical device, the Therac-25 computer controlled radiation therapy system, which was the inspiration for Steve Casey's highly regarded book Set Phasers on Stun.