Determination of the effective point of measurement of cylindrical ionization chambers for electron beams with different energies

Document Type : Original Article


1 Department of Radiology, Medical Physics, Ahvaz Jondishapur University of Medical Sciences, Ahvaz, Iran.

2 Department of Radiotherapy & Oncology, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran.


Background and Objective: In order to measure the absorbed dose due to electron beam irradiation at the reference point of a medium, part of this medium is replaced by a cylindrical ionization chambers. Thus changes will appear in attenuation and scattering of the radiation. In order to assess the displacement effect, an effective point of measurement uses as the reference point. This effective point is taken upstream of the geometrical center of the chamber. This effective point of measurement depends on every detail of the chamber design, energy and field size. So, in the different researches on this point, various values has been reported for cylindrical ionization chamber. The purpose of this study was to analyse the dosimetric information of electron beams and determine the analytical effective point of measurement in CC13 ionization chamber.
Subjects and Methods:  In this study a CC13 ionization chamber was used for dose determination of 9,12 and 15MeV electron beams of Varian accelerator of Ahwaz  Golestan hospital  in different field sizes of 6×6cm2up to 25×25cmapplicators. Measurement was made in blue phantom, up to 30mm depth. The PDD curve was plotted separately for all treatment electron fields. The table curve 2D software was used for analyzing experimental data. Then critical point for each curve was calculated.
Results: The torsion area is caused by changing environment from air to phantom (water) during measuring of ionization.Also, with increased energy and field size, mean effective point of measurement is altered.
Conclusion: For CC13 ionization chamber, the mean value is obtained as 0.9r.


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