Wright Tracy, Lye Jessica E, Ramanathan Ganesan, Harty Peter D, Oliver Chris, Webb David V, Butler Duncan J
Australian Radiation Protection and Nuclear Safety Agency, 619 Lower Plenty Rd, Yallambie, Victoria 3085, Australia. School of Chemistry and Physics, University of Adelaide, Adelaide, Australia.
Phys Med Biol. 2015 Jan 21;60(2):883-904. doi: 10.1088/0031-9155/60/2/883. Epub 2015 Jan 7.
The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) has established a method for ionisation chamber calibrations using megavoltage photon reference beams. The new method will reduce the calibration uncertainty compared to a (60)Co calibration combined with the TRS-398 energy correction factor. The calibration method employs a graphite calorimeter and a Monte Carlo (MC) conversion factor to convert the absolute dose to graphite to absorbed dose to water. EGSnrc is used to model the linac head and doses in the calorimeter and water phantom. The linac model is validated by comparing measured and modelled PDDs and profiles. The relative standard uncertainties in the calibration factors at the ARPANSA beam qualities were found to be 0.47% at 6 MV, 0.51% at 10 MV and 0.46% for the 18 MV beam. A comparison with the Bureau International des Poids et Mesures (BIPM) as part of the key comparison BIPM.RI(I)-K6 gave results of 0.9965(55), 0.9924(60) and 0.9932(59) for the 6, 10 and 18 MV beams, respectively, with all beams within 1σ of the participant average. The measured kQ values for an NE2571 Farmer chamber were found to be lower than those in TRS-398 but are consistent with published measured and modelled values. Users can expect a shift in the calibration factor at user energies of an NE2571 chamber between 0.4-1.1% across the range of calibration energies compared to the current calibration method.
澳大利亚辐射防护与核安全局（ARPANSA）已建立了一种使用兆伏级光子参考束进行电离室校准的方法。与采用（60）Co校准并结合TRS - 398能量校正因子相比，新方法将降低校准不确定度。该校准方法采用石墨量热计和蒙特卡罗（MC）转换因子，将石墨中的绝对剂量转换为水中吸收剂量。利用EGSnrc对直线加速器头部以及量热计和水模体中的剂量进行建模。通过比较测量和模拟的百分深度剂量（PDD）及剂量分布来验证直线加速器模型。在ARPANSA束流品质下，校准因子的相对标准不确定度在6兆伏时为0.47%，10兆伏时为0.51%，18兆伏束流时为0.46%。作为关键比对BIPM.RI(I)-K6的一部分，与国际计量局（BIPM）进行比较，6兆伏、10兆伏和18兆伏束流的结果分别为0.9965(55)、0.9924(60)和0.9932(59)，所有束流均在参与者平均值的1σ范围内。发现NE2571型 Farmer电离室的测量kQ值低于TRS - 398中的值，但与已发表的测量和模拟值一致。与当前校准方法相比，用户可以预期在整个校准能量范围内，NE2571型电离室在用户能量下的校准因子会有0.4 - 1.1%的偏移。
