平衡帽材料对准直器散射因子测量值的影响探讨

The study of effects of the material of buildup cap to the measured value of output factor

目的:探讨平衡帽材料对准直器散射因子（Sc）测量值的影响。材料和方法:采用由A-150塑料和铜制做成的两种平衡帽。用Farmer2570剂量仪分别测量6MV X线不同射野条件下电离室的电离值,并归一到参考射野,得到两组Sc值进行比较。结果:使用铜材平衡帽时,电离室的电离值明显高于使用A-150塑料平衡帽时电离室的电离值,最大高出14％,但两组Sc值的差异并不显著,均在1％以内。结论:虽然高密度材料和空气严重不等效,但对Sc测量值的影响较小,因此选用高密材料的平衡帽测量Sc（尤其在小野条件下）也不失为一种较好的方法。

Objective:To study the effects of the material of a buildup cap to the determined value of Output Factor(Sc).Material and Methods: Using two buildup caps made of plastic A-150 and copper respectively .The ionizing values of ionization chamber were measured with Farmer 2570 electrometer in the fields of all kinds of sizes respectively and calibrated to reference field . Two groups of values of Sc were obtained and compared. Results: The ionizing values measured with the buildup cap of copper is evidendy larger than with the buildup cap of plastic A-150 and the maximum is 14%, but the difference between two groups of the values of Sc is not much evident and all is in the range of 1%. Conclusions: Though the material of high density is not equivalent with air , the effects of that to the measured value of Sc is less .So using the buildup cap made of the material of high density for the measurement of Sc is also a quite good method, specially in the fields of the small sizes.