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Determination of Chamber Geometry Correction Factors for Phantom-Based Absorbed Dose Determination in External Therapeutic Low Energy KV X-Ray Beams

L Bueermann*, S Ketelhut, Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany


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

Room: AAPM ePoster Library

Purpose: The international IAEA TRS 398 code of practice describes procedures to obtain the absorbed dose to water in clinical therapeutic low energy x-ray beams. Plane-parallel ionization chambers are used for this purpose. They are mounted on the surface of a rectangular PMMA phantom and calibrated in terms of absorbed dose to water for different radiation qualities at fixed reference source to surface distance and field size (SSD,f)_ref. However, different irradiation geometries (SSD,f)_clin prevail in clinical beams, and a chamber geometry correction factor k_g has to be applied. Values of k_g depend on the radiation quality Q, SSD and f. The purpose of this work was to determine and provide values of k_g for the updated TRS 398 code.
Methods: Correction factors k_g(Q,SSD,f) were calculated for the chamber types PTW 23342 and 23344 with the EGSnrc Monte Carlo simulation code for the spectra of weakly-filtered therapeutic x-radiation qualities generated with tube voltages in the range from 10 kV to 100 kV. Calculations were performed for SSD values of 20, 30 and 50 cm and circular field sizes f with diameters 2, 3, 5, 8, 10, 15 and 20 cm. Some calculations were also done for water instead of PMMA phantoms. A subset of the calculated values of k_g was measured in order to validate the calculations.
Results: Calculated and measured values of k_g agreed within the estimated relative standard uncertainties of about 1 %. Comparison of results obtained with PMMA and water confirm that k_g corrects mainly for differences in the backscattering of photons in these two materials.
Conclusion: A comprehensive set of validated geometry correction factors for the chamber types PTW 23342 and 23344 is provided for the updated IAEA TRS 398 code of practice for low energy kilovoltage x-ray beams.


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