Evaluation of the Characteristics of Ionization Chambers Used for Commissioning in High Dose Rate Linacs

Ahmed B El-Moataz; Abouelenein HS; Ammar H; Khalil MM; El- Nagdy MS

Abstract

Evaluation of the Characteristics of Ionization Chambers Used for Commissioning in High Dose Rate Linacs

Introduction: Recently there has been an increased interest in high dose rate free flattening filter (FFF) linear accelerators which are used clinically in most of the advanced radiotherapy techniques such as VMAT, SRS and SBRT. From previous reports, the calculated ion recombination (Pion) at FFF beams may be inaccurate as calculated from the two-voltage technique. Therefore, Pion was measured for flattened beams of 6 and10 MV and for FFF beams of 6 and 10 MV with the two voltage method and verified by Jaffe Plots.

Materials and Methods: The aim of the work is to investigate the charge collection efficiency of two ionization chambers for photon beams with flatting filter and flattening filter free of Elekta Versa HD Linac with IBA PPC05 and CC13 ionization chambers. Experimental data will be collected from Elekta Versa HD at energies of 6 MV, 10 MV, 6 MV FFF and 10 MV FFF for different field sizes at two different depths applying the two voltage method and verify it using the Jaffe Plots.

Results: After studying the effect of the voltage difference used on the value of electrical charges collected by each of the two chambers at FF and FFF beams, it was found that the effect of the applied voltage is always higher on the cylindrical chamber.

Conclusion: The results showed that the magnitude of that effect increases with both energy and dose rate and it is possible to say that as much as the active volume is small and easy to determine the effective point of measurements as in the parallel plate chamber as much as it showed a significant impact on the accuracy and stability of measurements in most of measurement conditions. Also being suitable for use in absolute measurements makes them better in nonreference or relative measurements at FFF beams.

Author(s):

Ahmed B El-Moataz, Abouelenein HS, Ammar H, Khalil MM and El- Nagdy MS

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