A Brief History of: The killer Therac-25 Radiotherapy machine (Short Documentary)

Summary

The video explores the Therac-25 radiotherapy unit, which due to software flaws, delivered lethal doses of radiation to patients. It discusses the machine's design, its reliance on computer systems over mechanical safety interlocks, and the tragic incidents that occurred between 1985 and 1987. The script also covers the legal actions taken against the manufacturer and the eventual corrective measures implemented to prevent such disasters.

Takeaways

• 🔬 The Therac-25 was a radiotherapy unit that used a medical linear accelerator to treat cancer by accelerating electrons to create a high-energy beam.
• ⚠️ The unit's software was responsible for controlling the machine, including monitoring status, setting up treatment parameters, and activating the beam, but it had critical flaws that led to overexposure incidents.
• 💡 Therac-25's innovation was a double-pass system that allowed a long accelerator to fit into a smaller space, but this came with increased reliance on software for safety.
• 📊 The software was programmed by a single person and was based on the code from previous models, the Therac-6 and Therac-20, with limited testing and no independent review.
• 🚫 Mechanical interlocks that provided a final safety check in previous models were replaced with software controls in Therac-25, which had undiscovered bugs.
• 🤖 Therac-25's computer system did not use a standard operating system and had a proprietary real-time OS, which lacked safeguards against software errors.
• 🆘 Six incidents of incorrect high current electron beams being delivered to patients occurred between 1985 and 1987, leading to severe injuries and deaths.
• 🛠️ After the incidents, the manufacturer, AECL, implemented a corrective action plan including hardware safety interlocks and other hardware and software changes.
• 📝 The incidents highlight the risks of over-reliance on computer systems for safety-critical operations without thorough testing and independent code review.
• 🏥 The consequences of the software flaws in Therac-25 were devastating for the patients involved, resulting in severe disabilities and fatalities.

Q & A

What is the primary function of radiation therapy machines?

Radiation therapy machines are used in the treatment of certain cancers, utilizing ionizing radiation to target and destroy cancer cells while minimizing damage to surrounding healthy tissue.

What safety concerns are associated with radiotherapy units?

Safety concerns include the accurate dosage of radiation, proper storage and disposal of units, and the reliability of the software controlling the machines to prevent overexposure to patients.

How does a medical linear accelerator like the Therac 25 work?

A medical linear accelerator, such as the Therac 25, accelerates electrons to create a high-energy beam used for treating localized areas like tumors. It can switch between electron and X-ray modes for different treatment depths.

What was innovative about the Therac 25's design?

The Therac 25's innovation was the double-pass system, which allowed a long accelerator to be compacted into a smaller space, enabling the delivery of 25 MeV of photons or electrons at various levels.

Why was the software of the Therac 25 considered a critical component?

The software of the Therac 25 was critical because it controlled the entire unit, including machine status monitoring, treatment setup, beam activation, and safety interlocks, replacing the mechanical interlocks of previous models.

What were the consequences of the software glitches in the Therac 25?

The software glitches in the Therac 25 led to several incidents of patients receiving overdoses of radiation, resulting in severe injuries and fatalities.

How did the lack of independent software review contribute to the Therac 25 incidents?

The lack of independent software review meant that issues within the software were not identified and rectified, leading to a false sense of security and complacency among operators.

What actions did AECL take after the incidents with the Therac 25?

AECL eventually implemented a corrective action plan that included a hardware safety interlock and other hardware and software changes to address the identified software and safety issues.

What was the role of the FDA in the Therac 25 case?

The FDA launched a probe into the unit's safety systems, which likely prompted AECL to start looking into the system after multiple incidents of overdose had occurred.

How did the design culture and assumptions about the Therac 25's safety contribute to the incidents?

The design culture assumed that the system would only be linked to hardware failures and that the software was reliable, leading to a lack of consideration for software errors and insufficient testing.