Effective Point of Measurement (EPOM) of Some Ionization Chambers for High Energy Photon Beam Dosimetry used in Radiotherapy for the Treatment of Cancer Patient

Abstract

The volumetric effect occupied by the air cavity for the dosimetry of high energy photon beam is impossible to ignore using standard ionization chambers. Hence, the dose measurement should be corrected with a displacement perturbation correction factor (Pdis) or using an Effective Point of Measurement (EPOM). The aim of this study was to calculate the EPOM of some ionization chambers and evaluation of the shift of EPOM that recommended by various international protocols under both reference and non-reference condition. The work was performed with Percentage Depth Dose (PDD) curves by placing chambers (PTW 30013, FC 65G and Semiflex 31010) at the geometrical centers for field size(s) of 5cm × 5cm to 30cm × 30cm at 100cm Source to Surface Distance (SSD) for photon energy 6, 10 and 15MV respectively. The shift of the cylindrical chambers also estimated from PDD values in comparison with reference PDD values by Parallel Plate Chamber (PPC 40 and Murkus 23343) of 100%, 80% and 50% depth in the water. The present study shows that the effective shift is not only varies with chamber materials but also with photon energy. On the other hand the periodical calibration factor of some ionization chambers at standard procedures were compared with manufacturer values also varies with time which is an important issues for the precisional dosimetry in radiotherapy. The details of the EPOM and chamber calibration factor is discussed.