- 期刊
根據TG-51議定書以不同游離腔量測直線加速器電子輸出劑量之比較
Comparison Different Ionization Chambers by TG-51 Protocol for Clinical Electron Beam Dosimetry
摘要
目的：本文目的主要有二：(1)指出TG-21與TG-51兩種議定書在校正劑量輸出方法上的不同及差異：及(2)依據TG-51進行電子射束輸出劑量之校正，在不同電子能量及游離腔下比較其結果之差異度。 材料與方法：針對Siemens PRIMUS詢問直線加速器的6、9、12、15、18、21MeV等電子能量，分別使用NE2581、IC15圓柱型游離腔及Roos型的PPC35平板式游離腔等，在Wellhofer水假體中進行輸出劑量量測，根據TG-51議定書校正輸出劑量，比較所得之對水吸收劑量值。 結果：根據TG-51議定書使用不同游離腔校正在劑量最大處之電子輸出劑量，變化差異程度隨電子能量大小不同而有不同結果(-1.4%-0.67%)，在電子能量為9MeV時的校正結果差異最大。NE2581與PPC35差 -1.4%，IC15與PPC的差異值為 -1.26。 結論：使用不同游離腔所造成的結果差異所在，除了圓柱型游離腔(NE2581，IC15)和平板式游離腔(PPC35)本身的游離腔構造、材質不同外，對水吸收劑量校正因子(absorbed-dose to water calibration factor，N)的取得亦是一重要因素。其中TG-21議定書是基於暴露(空氣克馬)方法，與許多校正因子的換算去進行劑量校正，而TG-51議定書則是根據對水吸收劑量修正因子N和射束品質轉換因子kQ的觀念，所得之參考輸出劑量校正值直接為水的吸收劑量，其基礎概念較TG-21易瞭解，且不用經過如TG-21的複雜查表計算。
並列摘要
Purpose: The purpose of this paper is two-fold: (1)to indicate specific differences in the techniques between TO-21 and TG-51; and (2) to show the magnitude of the discrepancies in results when following the TG-51 protocol. The discrepancies are compared by different electron energies and chamber types. Materials and Methods: We use three different ionization chambers ( NE 2581, IC 15 and PPC 35) to measure the output of electron beams. To compare the values of absorbed dose to water determined according the newest AAPM TO-51 protocol for electron beams of energies from 6 to 21 MeV. The incident electron beams are from the Siemens PRIMUS3008 linear accelerator. Results: The results are found that differences in determined absorbed dose at Dmax by TO-51 protocol vary with electron beam energies and ion chambers. The discrepancies of the ratios are -1.4 %~0.67 %. The dose determined by the NE 2581 chamber is found to be the largest discrepancies than that of the PPC 35 chamber by -1.4 % at 9 MeV. The corresponding results for the IC 15 chamber are -1.26 % at 9 MeV. Conclusions: We find that at difference s in determined absorbed dose at dmax according to the TG-51 protocol vary with the characteristics of ion chambers. The absorbed-dose to water calibration factor, N, is an important parameter, too. AAPM TO-21 protocol is based on an exposed (air-kerma) standard and multiple energy-dependent correction factors. The TG-51 protocol is based on an absorbed dose to water calibration factor (ND.IV) and an energy-dependent correction factor, kQ. The quantity of interest is absorbed dose to water in this TO-51 protocol. The overall TG-51 protocol is simpler and easier to understand than the TG-21 protocol.