The Therac-25: The Software Bug That Killed Patients

The Therac-25 Disasters: When Software Failure Turned Deadly

In the mid-1980s, at the dawn of the computer age in medicine, a machine called the Therac-25 was meant to be a marvel of modern technology. It was a radiation therapy linear accelerator, designed to destroy cancerous tumors with high-energy beams. It promised precision and efficiency. Instead, it became infamous for causing one of the most horrific and instructive tragedies in the history of software engineering.

Between 1985 and 1987, the Therac-25 massively overdosed at least six patients. Four of them died from their injuries. The victims weren’t just statistics; they were people like Ray Cox, a 58-year-old man undergoing treatment for a back lesion, who was left paralyzed, in agony, and eventually killed by the very machine meant to heal him.

The investigations that followed uncovered not a single “glitch,” but a perfect storm of software errors, flawed system design, and devastating human complacency. This is the story of the Therac-25, a catastrophe that forever…