Measurement of ionization chamber absorbed dose factors in megavoltage photon beams
Bibliographic record
Abstract
PURPOSE: Absorbed dose beam quality conversion factors (k(Q) factors) were obtained for 27 different types of ionization chamber. The aim was to obtain objective evidence on the performance of a wide range of chambers currently available, and potentially used for reference dosimetry, and to investigate the accuracy of the k(Q) calculation algorithm used in the TG-51 protocol. METHODS: Measurements were made using the 60Co irradiator and Elekta Precise linac facilities at the National Research Council of Canada. The objective was to characterize the chambers over the range of energies applicable to TG-51 and determine whether each chamber met the requirements of a reference-class instrument. Chamber settling, leakage current, ion recombination and polarity, and waterproofing sleeve effects were investigated, and absorbed dose calibration coefficients were obtained for 60Co and 6, 10, and 25 MV photon beams. Only thimble-type chambers were considered in this investigation and were classified into three groups: (i) Reference chambers ("standard" 0.6 cm3 Farmer-type chambers and their derivatives traditionally used for beam output calibration); (ii) scanning chambers (typically 0.1 cm3 volume chambers used for beam commissioning with 3-D scanning phantoms); and (iii) microchambers (very small volume ion chambers (< or = 0.01 cm3) used for small field dosimetry). RESULTS: As might be expected, 0.6 cm3 thimble chambers showed the most predictable performance and experimental k(Q) factors were obtained with a relative uncertainty of 0.1%. The performance of scanning and microchambers was somewhat variable. Some chambers showed very good behavior but others showed anomalous polarity and recombination corrections that are not fully explained at present. For the well-behaved chambers, agreement between measured and calculated k(Q) factors was within 0.4%; for some chambers, differences of more than 1% were seen that may be related to the recombination/polarity results. Use of such chambers could result in significant errors in the determination of reference dose in the clinic. CONCLUSIONS: Based on the experimental evidence obtained here, specification for a reference-class ionization chamber could be developed that would minimize the error in using a dosimetry protocol with calculated beam quality conversion factors. The experimental k(Q) data obtained here for a wide range of thimble chambers can be used when choosing suitable detectors for reference dosimetry and are intended to be used in the upcoming update/addendum to the AAPM TG-51 dosimetry protocol.
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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 | 0.002 | 0.006 |
| 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 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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 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".