目的:本文以PTW N23333游離腔在空氣中測得Ir-192射源的曝露率為基準，探討Gammamed後荷近接治療機組提供的有限體積圓柱型plexiglass假體用於校正Ir-192射源活度時應修正的參數及此假體在臨床應用上的優點。材料與方法:先以PTW N23333游離腔，根據AAPM Report 41的報告所敘述的參數修正後,測得距射源8公分處之空氣中曝露率，加入研究中額外參數的修正後，求得介質內之劑量率;再測量位於Plexiglass假體內,有無附加回散射水假體之曝露率，求得此假體之回散射因子,以探討在plexiglass假體內測量應做的修正。結果:使用Plexiglass假體時，其回散射修正因子(BSF)經測量應為1.081。劑量梯度校正因子(Cgrad)在射源至游離腔的距離(SCD)等於8公分處之修正值為1.158。結論:使用Plexiglass假體校正Ir-192射源活度，在臨床應用上比在空氣中測量有較高的穩定性,重現性及方便性,然而在使用前需先與其他測量方法相互印證，求得其回散射等因子才能準確用於定期射源活度之檢測。

　　　　　Purpose: In this paper, the PTW N2333 ionization chamber was used to calibrate the inair exposure rate of Ir-192 HDR brachytherapy source in a Gammamed 12I remote afterloadingmachine. The exposure rate of the source in finite cylinder plexiglass phantom that was equippedwith the system was measured. The calibration factor of the plexiglass phantom for Ir-192 sourceactivity calibration is discussed and the clinical advantages of the phantom is summarized.Materials and Methods: The in air exposure rate of the source was measured with the PTW N2333ionization chamber connected to the Capintec electrometer at SCD equal to 8 cm. Source activitywas calculated from the measured exposure according to the suggested parameters of AAPMReport 41. The in phantom exposure rate were measured with and without water phantom tocalculated the back-scatter-factor of the plexiglass phantom. Results; The back-scatter-factor isequal to 1.081 and the factor Cgrad =1.158 should be considered to calibrate the scatter from theupper and button side of the phantom which was less due to the lack of scatter and dose gradientcorrection.Conclusion: Calibrations in plexiglass phantom of Ir-192 HDR brachy-therapy source allow morestability and reproducibility. The calibration factors of the plexiglass phantom needs to beconsidered in routine source calibration.