Introduction
For dose measurement in Megavoltage (MV) photon beams with ion chambers, the effect of volume occupied by the air cavity is not negligible. Therefore, the result of measurement should be corrected with a displacement perturbation correction factor (P_dis) or using an effective point of measurement (EPOM). The aim of this study is to calculate the EPOM for cylindrical ion chamber and to evaluate the fixed EPOM that was recommended by standard dosimetry protocols.
Materials and Methods
Percent depth doses (PDDs) for 6 MV and 18 MV were measured with two types of chambers for different depths and field sizes. The EPOM was calculated using results obtained from measurement data for two types of chambers, comparison of the readings, and using dosimetry, mathematical, and statistical consideration. For displacement correction factor
12∆r'> =0,
12∆r'> = 0.6r and different
12∆r'>, the minimum standard deviations ratio (SDRs) were calculated at several depths and field sizes.
Results
Maximum level of SDRs was about 0.38% and 0.49% (when assuming variable
12∆r'>) for 6 MV and 18 MV, respectively (which was less than 0.5% and acceptable). This quantity was greater than one (for assuming 
12∆r'> = 0.6r) and greater than 2 when there was no shift (
12∆r'>  =0)
Conclusion
The results show that the recommended shift for cylindrical ion chamber in dosimetry protocols (upstream of 0.6r) is not correct and using a fixed value for the EPOM at all photon beam energies, depths, and field sizes is not suitable for accurate dosimetry.