Example of Therac-25

The Therac-25 was a medical device used to destroy remaining tumour growth, after patients had had the majority removed through manual surgery, by firing electrons or x-rays at a targeted location. This machine was computer controlled and remotely operated. For shallow growths a low rad mode ‘e’ was used; for deep growths a high rad mode ‘x’ was used in conjunction with a metal plate to transform the electrons into x-rays. The Therac-25 also automated more of the safety features, which in previous models had been manually operated. In 1986, Ray Cox went in for one of his follow-up treatments, of which he had already had some. The operator accidently set the machine to ‘x’, but immediately realised his error and changed the setting from ‘x’ to ‘e’. Due to this quick change in settings, the metal plate used to change the electrons to x-rays retracted; however, the machine was still set to high rad mode. The operator, in another room, delivered a dose to the patient, but due to the setup the computer responded with an error. The operator, going off prior knowledge, believed this to mean that the machine did not deliver the dose, so they delivered it again. A second time the error message came up, and so the operator delivered another dose. At this point, Ray Cox removed himself from the machine after receiving three painful blasts. Due to untested software, no human-based safety checks, and no hardware interlocks Ray Cox died 4 months later due to major radiation burns. This was just one of many cases in which fatal levels of radiation were delivered to patients over the lifespan of the machine. Had a human factors study been carried out the risk involved with an operator quickly changing from one mode to another and putting the machine into an unknown state should have been discovered, which would have prevented the accidental loss of lives.