Study of the influence of phantom material and size on the calibration of ionization chambers in terms of absorbed dose to water

Mehenna Arib,Youcef Boudouma,Toufik Medjadj

doi:10.1120/jacmp.v7i3.2264

Abstract

In the International Atomic Energy Agency's (IAEA) code of practice (TRS 398) and the American Association of Physicists in Medicine's dosimetry protocol (TG‐51), full‐scatter water phantoms are recommended for the determination of the absorbed dose for both photon and electron beams and, consequently, for the calibration of the user's ionization chambers. This procedure is applied in the Secondary Standard Dosimetry Laboratory, where the calibration is performed on a C60o gamma beam, in comparison with reference chambers whose absorbed dose‐to‐water calibration coefficients, ND,w, are known. In this work, we present the results of the calibration of 10 Farmer‐like ionization chambers calibrated in three water phantoms (sizes 20×20×15 cm3, 30×30×30 cm3, and 35×35×37 cm3) and two plastic phantoms (size 20×20×20 cm3) polymethyl methacrlyate (PMMA) and polystyrene). Calibrations are performed by the substitution method using an ionization chamber whose ND,w has been supplied by the IAEA's reference laboratory. It is shown that the results, expressed as the percentage ratio of the calibration coefficient in a given phantom to that of the standard IAEA phantom, is less than 0.35% for all investigated chambers, and that the standard deviation of the mean of the ND,w calibration coefficients determined in all five phantoms is less than 0.06%, except for one nylon‐walled ionization chamber, where the observed 0.34% value could be explained by the hygroscopic properties of nylon. Furthermore, a chamber‐to‐chamber dependence of the calibration coefficient has been shown to vary by up to 2.8%. These results emphasize that the phantom dimensions and its material are not sensitive criteria for the calibration of cylindrical ionization chambers in terms of absorbed dose to water. The results also show that generic calibration coefficients could not be considered for a given type of chamber.PACS number: 87.53.Dq

Highlights

It has been established that the required accuracy of the absorbed dose delivered to the target volume in radiotherapy is considerably improved if the calibration of a clinical beam is performed with ionization chambers calibrated in terms of absorbed dose to water.(1) In this context, the recent dosimetry protocols published by the International Atomic Energy Agency(2) (IAEA) and the American Association of Physicists in Medicine(3) (AAPM) are based on the use of an ionization chamber calibrated in terms of absorbed dose to water in a standard laboratory’s reference quality beam, generally taken as a 60Co gamma ray beam
Compared to the overall uncertainty over the absorbed dose-to-water calibration coefficient, which is around 1.1%, the difference between the calibration coefficients obtained in this study for the different phantoms and those for all the investigated ionization chambers is not significant
The absorbed dose-to-water calibration coefficients can be determined in any water or solid phantom whose dimensions are at least 20 × 20 × 20 cm[3]

Summary

Introduction

It has been established that the required accuracy of the absorbed dose delivered to the target volume in radiotherapy is considerably improved if the calibration of a clinical beam is performed with ionization chambers calibrated in terms of absorbed dose to water.(1) In this context, the recent dosimetry protocols published by the International Atomic Energy Agency(2) (IAEA) and the American Association of Physicists in Medicine(3) (AAPM) are based on the use of an ionization chamber calibrated in terms of absorbed dose to water in a standard laboratory’s reference quality beam, generally taken as a 60Co gamma ray beam.56 Arib et al.: Influence of phantom on the calibration of ionization chambers...Both cylindrical and plane-parallel ionization chambers are recommended as reference instruments for the calibration of the user’s 60Co gamma ray beam, and water is recommended as the reference medium for absorbed dose measurements.The accuracy and traceability of the calibration of radiotherapy dosimeters are of great concern, since absorbed dose cannot be obtained with non-calibrated instruments. 56 Arib et al.: Influence of phantom on the calibration of ionization chambers Both cylindrical and plane-parallel ionization chambers are recommended as reference instruments for the calibration of the user’s 60Co gamma ray beam, and water is recommended as the reference medium for absorbed dose measurements. The IAEA’s code of practice recommends that the calibration coefficient used to determine the absorbed dose be obtained in a water phantom. We investigate the influence of the phantom size and its material over the calibration coefficient. For this purpose, 10 Farmer-like ionization chambers were calibrated in five different phantoms using the substitution method, and the calibration coefficients were compared to those obtained in the IAEA standard water phantom

Methods

Results

Conclusion