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目的探讨电子束窄条野的剂量特性。方法用辐射野扫描系统对瓦里安2300C/D直线加速器多种能量电子束窄条野进行中心轴深度剂量扫描,分析R100、R90、R50及RP等参数的变化规律,并与标准方野进行比较。用胶片法测不同窄条野不同能量下射野中心轴平面等剂量分布,分析等剂量曲线特点。用NE Farmer 2570剂量仪和PTW 0.1 cm3电离室测量窄条野输出因子,并与方根式法计算值比较。结果窄条野使高值剂量深度(R100、R90)移向表面,且当窄条野短边变小和在高能时尤为明显;而对低值剂量深度(R50)和电子射程(RP)影响不大,窄条野长边对百分深度剂量影响很小。90%等剂量曲线宽度随深度的增加由窄变宽,至最大后向内收缩,底部为弧形;低值等剂量曲线随着深度的增加向两侧扩展。方根式法计算窄条野输出因子的偏差随窄条野短边减小而增大。结论窄条野与方野相比治疗深度变浅,并在一定深度处90%等剂量曲线变宽;方根式法计算电子束窄条野输出因子有偏差,临床应用建议实际测量。
Objective To investigate the dose characteristics of electron beam narrow strip field. Methods Radiation field scanning system was used to scan the narrow beam field of Varian 2300C / D linear accelerator for the axial depth dose scanning. The variation regularities of R100, R90, R50 and RP were analyzed and compared with the standard square field Compare Using the film method to measure the dose distribution of the center axis of the radiating field with different energy of different narrow strip fields, the characteristics of isodose curve were analyzed. The narrow field output factors were measured with a NE Farmer 2570 dosimeter and a PTW 0.1 cm3 ionization chamber and compared with the calculated root-mean-square method. Results The narrow field shifted the high dose depth (R100, R90) to the surface, especially when the short side of the narrow field became smaller and at high energy. However, the effects of low dose depth (R50) and electron range (RP) Small, narrow field of the long side of the percentage of the depth of dose has little effect. The width of 90% isodose curves broadened from narrow to narrow with the increase of depth, then contracted inwardly to the maximum and curved at the bottom. The low-dose isodose curve spreads to both sides as the depth increases. The variance of the output factor of the narrow field calculated by the square root method increases as the short edge of the narrow field decreases. CONCLUSIONS: The depth of treatment is narrower than that of lateral field, and the 90% isodose curve broadens at a certain depth. The deviation of the output factor of electron beam narrow field is calculated by the square root method, and practical measurement is recommended in clinical practice.