Comparing Method Of Calibration Of Absorbed Dose In Water Between Difference Dosimetry Protocols In Radiotherapy

Objective To study the difference in the calibration of the absorbed dose in water between the dissymmetry protocols of IAEA TRS-398and TRS-277for high-energy photon and electron beams using several cylindrical chambers and plane-parallel chamber. This study provided guidance for clinical accelerator calibration from TRS-277to TRS-398transition. Methods1.Experimental measurements were used to compare the differences in absorbed dose from3photon beams and6electron beams according to IAEA TRS-398and IAEA TRS-277.2.The calibration factors ND,W,Q0in terms of absorbed dose to water were calculated from the air exposure calibration factors AX, and were compared with the measured in European standard laboratory.3. IAEA TRS-398recommended that to calibrate of a plane-parallel chamber in user electron beams against a reference cylindrical chamber which was directly calibrated under a60CO gamma radiation and derive the calibration factor in terms of absorbed dose to water under cross-calibration. Comparing the differences in absorbed dose according to cross-calibration procedure and TRS-277. Results1.For photon and electron beams, the distinction of water absorbed doses obtained according to the two different protocols was0.5～1.2%and0.2%～2.4%respectively, and was primarily due to their different calibration factors.2.The difference of cylindrical chamber between calibration factors ND,W,Q0calculated from the air exposure calibration factors NX theoretically and ND,W,Q0measured in European standard laboratory was1.1%and0.65%. The deviation between calculated and measured calibration factor of Parallel plate chamber was larger, up more than2%.3.After cross-calibrated with a parallel plate chamber with the measured dose recommendation method based TRS-277to compare the results of measurements using in the range0.5%difference2.2%. Conclusions The discrepancy in measured absorbed doses between IAEA TRS-398and TRS-277dissymmetry protocols has some differences, which was clinically acceptable. TRS-398allows a more convenient localization than TRS-277for photon beams. For electron beams, the measurement position of ionizing chamber is relatively complex. TRS-398provides a more simple formalism and a more accurate measurement. The deviation between measured and calculated values of calibration factor of cylindrical ionization chamber in terms of absorbed dose to water is insignificant. In the case the experimental conditions which can not directly measured chamber calibration factors in terms of absorbed dose to water, can use the calculated value instead of the theoretical value.The calculated and measured values of calibration factor of parallel plate chamber haslarge deviation, it is not recommended to use their calculated values. It is better to follow the IAEA TRS-398protocol in china and consistent with international dissymmetry, which has large important significance for carrying out precise radiotherapy.