Study of the Effect of Ionization Chamber Effective Point on Percentage Depth dose Measurements
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- Category: Vol. 77, June 2009
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Study of the Effect of Ionization Chamber Effective Point on Percentage Depth dose Measurements, ABO SALEH ABO SALEH M. EL-AWADY, YASSER BAYOUMI and AIDA R. TOLBA
Abstract
Introduction: The use of cylindrical ionization chamber for relative dosimetry is common in the radiotherapy centers. IAEA TRS-398 recommended that when measuring the per-centage depth dose (PDD) using this type of chambers, the chamber effective point of measurement (EPOM) must be taken into account. This requires the complete depth–ionization distribution be shifted towards the surface a distance equal to 0.6r for photon beam, where r is the internal radius of the chamber cavity.
Purpose: The aim of this work is to study experimentally the effect of using EPOM shift and without shift on the PDD measurements beyond the build up region for clyndrical chambers of small volumes.
Materials and Methods: The PDD measurements are done using Varian linear accelerators series 2100C/D at energies 6 MV and 18 MV for field sizes 5x5 cm2, 10x10 cm2, 20x20 cm2 and 30x30 cm2, PTW dosimetry system, MEPHYSTO software, and PTW semiflex and PinPoint ionization chambers of volumes 0.125 cc and 0.015 cc respectively are used.
Results: For 6MV, the maximum absolute percentage difference (MAPD) between the measurements using PinPoint chamber with (at EPOM) and without shift (at the center of the chamber) decreases with increasing field size. The maxi-mum (Max) and minimum (Min) values of MAPD are 0.94% and 0.54% at field sizes 5x5 cm2 and 30x30 cm2 respectively. For 18 MV, the MAPD decreases with increasing the field size with Max and Min values 1.07% and 0.37% at field sizes 5x5 cm2 and 30x30 cm2 respectively. For semiflex chamber, MAPD decreases with increasing the field size for both 6 MV and 18 MV. The Max and Min of MAPD are 1.65% and 0.63% at field sizes 5x5 cm2 and 30x30 cm2 respectively for 6 MV; and 1.02% and 0.25 at field sizes 5x5 cm2 and 30x30 cm2 respectively for 18 MV. The correlation factor (R) for all fields and energies is 0.9999 for both chambers.
However, when the PTW MEPHESTO software is used for shifting PDD data by 0.6r from original data without shift, the MAPD for all fields and energies between the MEPHESTO shifted data and actual measuring data with shift are inagree-ment where r=0.9999 and r=1 for PinPoint and semiflex chambers respectively.
Conclusion: This study shows that, in the case of PDD measurements beyond the build-up region, the shift effect of the chamber effective point of measurement for cylindrical ionization chambers of small volumes (0.015 cc and 0.125 cc) is not significant either using the shift experimentally or the dosimetery software process.