The Comparison of Absolute Dose due to Differences of Measurement Condition and Calibration Protocols for Photon Beams

The absolute absorbed dose can be determined according to the measurement conditions; measurement material, detector, energy and calibration protocols. The purpose of this study is to compare the absolute absorbed dose due to the differences of measurement condition and calibration protocols for photon beams. Dosimetric measurements were performed with a farmer type PTW and NEL ionization chambers in water, solid water, and polystyrene phantoms using 6 MV photon beams from Siemens linear accelerator. Measurements were made along the central axis of field size for constant target to surface distance of 100 cm for water, solid water and polystyrene phantom. Theoretical absorbed dose intercomparisons between TG21 and IAEA protocol were performed for various measurement combinations on phantom, ion chamber, and electrometer. There were no significant differences of absorbed dose value between TG2l and IAEA protocol. The differences between two protocols are within 1% while the average value of IAEA protocol was 0.5% smaller than TG2l protocol. For the purpose of comparison, all the relative absorbed dose were normalized to NEL ion chamber with Keithley electrometer and water phantom, The average differences are within 1%, but individual discrepancies are in the range of -2.5% to 1.2% depending upon the choice of measurement combination. The largest discrepancy of -2.5% was observed when NEL ion chamber with Keithley electrometer is used in solid water phantom. The main cause for this discrepancy is due to the use of same parameters of stopping power, absorption coefficient, etc. as used in water phantom. It should be mentioned that the solid water phantom is not recommended for absolute dose calibration as the alternative of water, since absorbed dose show some dependency on phantom material other than water. In conclusion, the trend of variation was not much dependent on calibration protocol. However, It shows that absorbed dose could be affected by phantom material other than water.