Lying on the cold metal table, Voyne Ray Cox knew the drill. This was his ninth round of cancer treatment - which is why he was certain that what happened next couldn't be right. He heard a sizzling sound and saw a blue flash. And then - agony. It was like someone had thrust a hot skewer through his shoulder. He cried out in pain, but the operator was down the corridor and she couldn't hear him. She blasted him again and again with the red-hot radiation beam.

Ray wasn't the first patient to be burned by the Therac-25 therapy machine, and he wouldn't be the last. Its dual-purpose design, controlled by a software programme, was supposed to offer hospitals more bang for their buck. But as patient after patient suffered ulcerated skin and yawning lesions, it should have been clear that something was horribly wrong. Why did it take so long for anyone to put this awful puzzle together?

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