fascinatinghorror

The Therac-25 Accidents

Added 2023-05-09 10:15:01 +0000 UTC

Comparable doses

I did a little research into how the doses of radiation accidentally delivered by the Therac-25 compared to the doses that were supposed to be delivered, as well as to some other incidents of radiation exposure.

During a typical chest x-ray, for example, you might receive less than one rad (or Radiation Absorbed Dose) of radiation. In most cases the intended dose for a patient receiving radiotherapy with the Therac-25 was in the range of 80 – 200 rads. A dose of 500 rads is considered lethal in many circumstances.

But what about the dose actually given when the machine malfunctioned? In the six accidents described in the video the dose patients received ranged from around 15,000 to 20,000 rads – easily a hundred times of more the dose they were supposed to get.

The quick fix

At one point in time, AECL were aware that there was an issue caused by operators going back and changing the operating mode of the machine. While they worked on a permanent fix for this, they sent out a memo advising of a temporary solution to be applied in the meantime. Here’s an extract from that memo:

“Effective immediately, and until further notice, the key used for moving the cursor back through the prescription sequence must not be used for editing or any other purpose. To avoid accidental use of this key, the key cap must be removed and the switch contacts fixed in the open position with electrical tape or other insulating material.”

The temporary solution was quite a simple one – remove a keycap so that operators could not, even accidentally, backtrack through the setup sequence. It’s an astonishingly low-tech quick fix for an extremely serious problem with an extremely advanced piece of technology.