| Summary: | 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.
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