Comment on “QA procedures in radiation therapy are outdated and negatively impact the reduction of errors” [Med. Phys. 38, 5835 (2011)]
Bibliographic record
Abstract
To the Editor, In the November 2011 Point/Counter Point debate, both authors referred to the incident in Tyler TX as events where “QA procedures were not followed” or where “horrific medical physics errors occurred.”1 As the physicist of record in Tyler at the time of the Therac 25 Malfunction 54, I would like to clarify the record regarding this event. In contrast to the statements made by the authors, Malfunction 54 was not the result of a medical physicist error or failure to perform QA. The incident was the result of a well documented software error that permitted theTherac 25 linac to operate in photon mode without a target or flattening filter in the beam, resulting in a near-instantaneous dose of ∼16 000 cGy. There was no warning for the therapist and very little information from the Therac 25 that it had delivered such a high dose; the Malfunction 54 code was a general indicator that either too high or too low a dose had been delivered. As one of the first digitally operated linacs, spurious malfunction codes occurred periodically; the manufacturer [Atomic Energy of Canada Ltd (AECL)] indicated these were due to machine noise and that there were no actual problems. Following the first event, three AECL engineers examined the Therac 25 and could find no problems. At the time of the second incident, the therapist was teaching a student and recalled changing the mode on the Therac 25 from photons to electrons. With her recall of the keystrokes used for the patient, we were able to reproduce the Malfunction 54. These instructions were subsequently used to reproduce the Malfunction 54 type event at all Therac 25 locations. There was a similar Therac 25 error in Yakima, Washington resulting from a different series of events—toggling from light field to treatment mode caused the target to be left out—that also resulted in a patient death. The Therac 25 errors have been described in detail,2 were the result of manufacturer software deficiencies, and had nothing to do with the lack of a qualified medical physicist or attention to QA. Additional details on the Therac 25 and other linac accidents will be described in an upcoming review article for Medical Physics.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.010 | 0.038 |
| Meta-epidemiology (narrow) | 0.002 | 0.002 |
| Meta-epidemiology (broad) | 0.003 | 0.002 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.004 | 0.004 |
| Scholarly communication | 0.004 | 0.005 |
| Open science | 0.006 | 0.002 |
| Research integrity | 0.046 | 0.050 |
| Insufficient payload (model declined to judge) | 0.009 | 0.011 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".