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The Software killer Bugs Therac-25 medical device Introduction: The Worst Medical Software Bugs in History in the 1980s Therac-25 manufactured by AECL used a software-based systems rather than hardware controls Software errors result in radiation overdoses As with chemotherapy, the hope is that the effect heals the patient more than it harms them. Background of Therac-25 The Therac-25 is a medical accelerator, that uses radiation to treat cancer patients to remove tumours. Early1970’s, AECL (Canada) and CGR) French to build Medical Linear Accelerators Therac-25 causes radiation overdoses resulting in the death of 6 patients Shocking found that the software appeared to have been written by a programmer with little experience in coding systems. The programmer no longer worked for the company, and could not be found. How Therac-25 works: Generating an Electron Beam The linear accelerator works like the computer monitor The electrons are accelerated by the gun in the back of the monitor and directed at the inside of the screen Medical linear accelerators accelerate electrons to create high-energy beams that can destroy tumours The Therac-25 is designed to fold the beam back and forth to produce long acceleration to fit into smaller space Root Cause Analysis of the Accidents The software code was not independently reviewed. AECL did not consider the design of the software, No proper risk assessment was followed. The user manual did not address error codes, the operator pressed the warning and proceed anyway. When designing the Therac-25, AECL decided to go with computer control they remove many of the manual controls, they also removed the hardware interlocks. Requirements Issues Error messages provided by Therac-25 monitor are not helpful to operators The equipment control task did not properly synchronize with the operator interface task, so race conditions occurred if the operator changed the setup too quickly. Software is required to monitor several activities simultaneously in real-time, such as monitoring input and editing changes from an operator There were no checks that the software was operating correctly The software should be subjected to testing and analysis System testing alone is not adequate; verification would be very valuable. Lessons: Six unfortunate patients of Therac-25 Therac-25 did the unthinkable in 1986 and 1987, overdoses of radiation, killing four and leaving two others with lifelong injuries. The investigation found out the software controlling the machine contained bugs The design of the machine relied on the controlling computer not for safety. There were no hardware interlocks or supervisory circuits to ensure that software bugs couldn’t result in catastrophic failures. Therac-25 has been known as the killer software bug in history. The Aftermath The FDA declared the Therac-25 “defective”. AECL issued software patches and hardware updates which eventually allowed the machine to return to service. The lawsuits were settled out of court. Subject to a class I recall from the FDA. Conclusion: Therac-25 is a software system for cancer treatment Devastating consequences as a result of software bugs Resulting in patients' death and lawsuits Thanks