Study of the Effect of Ionization Chamber Effective Point on Percentage Depth dose Measurements

Introduction: The use of cylindrical ionization chamber for relative dosimetry is common in the radiotherapy centers. IAEA TRS-398 recommended that when measuring the percentage depth dose (POD) 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 POD measurements beyond the build up region for clyndrical chambers of small volumes. Materials and Methods: The POD measurements are done using Varian linear accelerators series 2100C/D at energies 6 MV and 18 MV for field sizes 5x5 cm^2, 10x10 cm^2, 20x20 cm^2 and 30x30 cm^2, 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 maximum (Max) and minimum (Min) values of MAPD are 0.94% and 0.54% at field sizes 5x5 cm^2 and 30x30 cm^2 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 cm^2 and 30x30 cm^2 respectively. For semiflex chamber, MAPD decreases with increasing the field size for both 6 MY and 18 MY. The Max and Min of MAPD arc 1.65% and 0.63% at field sizes 5x5 cm^2 and 30x30 cm^2 respectively for 6 MV; and 1.02% and 0.25 at field sizes 5x5 cm^2 and 30x30 cm^2 respectively for 18 MY. 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 POD 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 inagreement where r=0.9999 and r=1 for PinPoint and semiflex chambers respectively. Conclusion: This study shows that, in the case of POD 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 dosimetcry software process.