Consequences of the spectral response of an a‐Si EPID and implications for dosimetric calibration
Bibliographic record
Abstract
One of the attractive features of amorphous silicon electronic portal imaging devices (a-Si EPIDs) as dosimetric tools is that for open fields they are known to exhibit a generally linear relation between pixel value and incident energy fluence as measured by an ion chamber. It has also been established that a-Si EPIDs incorporating high atomic number phosphors such as Gd2O2S:Tb exhibit a disproportionately large response to low-energy (<1 MeV) photons. The present work examines the consequences of this hypersensitivity in a commercially available EPID, the Varian aS500, with respect to energy fluence calibration in a 6 MV radiotherapy beam. EPIDs may be deployed in situations where the spectrum of the incident beam is modified by passing through a compensator or through a patient or phantom. By examining the specific case of a beam hardened by passage through compensator material, we show that the discrepancy between open and attenuated beam calibration curves can be as high as 8%. A Monte Carlo study using a comprehensive model of the aS500 shows that this difference can be explained by spectral changes, and further suggests that it can be reduced by the addition of an external copper plate. We consider configurations with the plate placed directly on top of the EPID cassette and 15 cm above the cassette, supported by Styrofoam. In order to reduce the maximum discrepancy to <4%, it was found that a copper thickness of approximately 0.7 cm was required in the elevated configuration. Improvement was minimal with the copper in the contact configuration. Adding 0.7 cm of copper in the elevated configuration reduced the contrast-to-noise ratio by 19% and the modulation transfer for a given spatial frequency by 30%.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
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 teacher head, 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".