Therac-25 Software STEM Case Study

Investigate the Therac-25 medical crisis with this 6-page STEM packet covering software bugs, race conditions, radiation dose math, and safety ethics.

Engage your middle and high school students with a comprehensive, hands-on deep dive into the most infamous software bug in medical history. This complete printable unit bridges computer science, medical physics, and biomedical ethics to teach Therac-25 radiation overdose causes, software race conditions, the removal of mechanical hardware interlocks, arithmetic variable overflow errors, human-computer interface (HCI) flaw patterns, and medical device safety standards.

What’s Included in this 6-Page Unit Packet:

Page 1: Historical Case Study & Technical Breakdown – Full narrative of the 6 fatal radiation overdose accidents across clinics in the US and Canada, computer-controlled medical linear accelerator mechanics, removal of backup mechanical relays, PDP-11 assembly code race conditions, “Malfunction 54” screen alerts, and comprehension check questions.

Page 2: Day in the Life Scenario Challenge – Roleplay assignment where students step into the shoes of a Hospital Medical Physicist at Tyler, Texas in March 1986, analyzing operator fast-keying logs, turntable positioning timing windows, uncalibrated dose spikes, and device shutdown protocols.

Page 3: Hands-On Unplugged Simulation & Applied Math – Step-by-step math calculations comparing high-current raw electron beam power versus low-current X-ray target beam power, radiation dose multiplication factors in Rads, and 1-byte integer counter overflow timing windows with 0-255 rollover.

Page 4: Engineering Ethics & Workplace Dilemma – Critical thinking exercise exploring software code reuse versus medical device safety testing, analyzing AECL’s initial denial of software bugs, blaming human operators for “Malfunction 54” errors, re-using Therac-6 assembly code without verification, and FDA recall authority reform.

Page 5: Redesign Brief & Innovation Lab – Creative blueprinting assignment where students design “Therac-25 Medical Linac V-2,” incorporating independent hardware-limit interlocks, redundant optical turntable position switches, secondary watchdog microcontroller circuits, and fail-safe UI keypress delay buffers.

Page 6: Teacher Answer Key & Scoring Rubric – Complete answer key for all analytical questions and math calculations, along with a 10-point grading rubric for evaluating student blueprints on Page 5.

Teacher Benefits:

100% Print-Friendly Format: Clean text layout with zero dark background boxes or awkward formatting for easy editing and printing.

Multi-Day Classroom Value: Designed to cover 3 to 5 full class periods. Perfect for Computer Science, Physical Science, Biomedical Engineering, Health Science, CTE electives, or emergency sub plans.

High-Interest Real-World Content: Combines radiation dosage math, assembly code logic simulation, hospital diagnostic roleplay, software safety ethics, and medical device redesign!