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Monte Carlo and Water Calorimetric Determination of Ionization Chamber Correction Factors for Medium Energy X-Ray Beams

J Bancheri1*, S Ketelhut2, L Bueermann2, J Seuntjens1, (1) Medical Physics Unit, McGill University, Montreal, QC, CA, (2) Physikalisch-technische Bundesanstalt (PTB), Braunschweig, Niedersachsen, DE


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: determine chamber correction factors for medium-energy kilovoltage X-ray beams between 50 kV and 300 kV through Monte-Carlo calculations and absorbed dose to water primary standard measurements. This work aims to contribute to the current IAEA TRS-398 update.

Methods: ionization chambers used in this study were the PTW TM30013, NE2571, IBA FC65-G and FC65-P and the Exradin A12. At PTB, the chambers were calibrated against a free-air chamber-based air kerma standard and a water calorimetry-based absorbed dose to water primary standard. The generating potentials of the radiation qualities varied between 50 kV - 300 kV. The in-phantom measurements were done at a depth of 2 cm in a water phantom with a field diameter of 10 cm. The Monte Carlo calculations were carried out with the EGSnrc toolkit. Measured photon spectra of the experimental beams were used in all simulations. The absorbed dose to water and chamber simulations were carried out with the user code ‘egs_chamber’. The water to air mean mass-energy absorption coefficient ratios at a depth of 2 cm and air kerma were calculated with the user code ‘g’. Renormalized photoelectric cross sections were used in all simulations. An in-depth uncertainty analysis was carried out for the calculated chamber correction factors.

Results: calculated chamber correction factors were within 2.2% of unity and assigned a standard uncertainty of 0.3% for all radiation qualities. The measured correction factors were between the calculated values and unity and assigned a standard uncertainty of less than 1%.

Conclusion: calculated and measured chamber correction factors are in agreement, to within standard uncertainties, at 70 kV and 200 kV - 300 kV. Clinical usability of these factors is limited by discrepancies between the data sets at 50 kV and 100 kV - 150 kV.

Funding Support, Disclosures, and Conflict of Interest: Julien Bancheri is a recipient of a Natural Sciences and Engineering Research Council (NSERC) CGSM scholarship. This work was supported financially by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290) and the NSERC RGPIN-2019-06746 operating grant.


Monte Carlo, X Rays, Calibration


TH- External Beam- Photons: Calibration protocol and primary standards

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