A technical write-up - two bugs, one a race condition and one an overflow, both present in previous hardware which had had mechanical interlocks and so not shown the problems.
which links to this 49-page technical treatment:
Medical Devices: The Therac-25, by Nancy Leveson, Uni Washington, an appendix to Safeware: System Safety and Computers
A narrative write-up from 1994:
Radiation Deaths linked to AECL Computer Errors
In 1985 a Canadian-built radiation-treatment device began blasting holes through patients’ bodies.
How a series of simple computer errors sabotaged a state-of-the-art medical wonder.