SU‐E‐T‐110: Quantitative Image Based Measurement of Electron Beam Spot Position
Bibliographic record
Abstract
Purpose: Develop an image based method to measure electron beam spot position w.r.t the Collimator Axis Of Rotation (CAOR). Methods: Two stainless steel rods, 3 mm in diameter, are mounted in a jig attached to the linac collimator. The rods lay in different planes normal to the CAOR, one at 159 cm SSD and the other at 70 cm SSD, and are offset 10 and 6 cm from the CAOR respectively. Images of both rods are acquired at multiple coll rotations. At each angle the rods project a tangent to an inscribed circle, the centre of which is calculated. The lower rod is very close the plane of the EPID and the centre of its inscribed circle defines the mechanical CAOR. The centre of the inscribed circle calculated from multiple images of the upper rod will be displaced from the CAOR by some distance proportional to the distance the beam source is from the CAOR. Test measurements were performed on an Elekta 6 MV where the In Plane beam spot position was moved a known distance using steering fine and beam spot position was measured.Results: Independent beam spot measurement (Flexmap Image Registration ball bearing) reported the initial beam spot at 0.29 mm from CAOR, moving to 0.91 mm (delta =0.62 mm) with steering. The rod tests showed the initial beam spot to be .31 mm from the CAOR and 1.17 mm from CAOR after adjustment. Note that in our companion paper we report similar values (initial 0.13mm going to 0.92 mm and initial value of 0.09mm going to 0.79) with our two co‐rotational dosimetric beam spot tests Conclusions: A simple method for measuring beam spot position with respect to CAOR is described. It provides its own internal reference and could be incorporated into routine QA using the EPID. Jason Smale is an employee of Elekta Canada
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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 | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.003 | 0.001 |
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".