电子束窄条野剂量特性的测定与分析

Dosimetric characteristics of narrow strip electron beams

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 evaluate the dosimetric characteristics of narrow strip electron beams. Methods The percentage depth dose（PDD） curves on the central axis of various energy narrow strip electron beams were measured with radiation scanning system for Varian 2300 C/D linear accelerator. The characteristics of dose parameters of R100, P90, R50 and Rp were analyzed and compared with those of standard square fields. The dose distribution on the central axis plane was measured by film badge. Output factors were measured with NE Farmer 2570 dosimeter and PTW 0.1 cm^3 ion chamber, the observed values were compared with the calculation values according to the square root method. Results The depth of R100 and R90 of narrow electron beam fields shifted to the surface, especially when the short side of narrow beam became smaller and the energy became higher, whereas the R50 and particle range（Rp） changed slightly. The size of the long side showed no significant influence on PDD. The width of 90% isodose curve widened as depth increased, followed by an adduction after reaching the maximum, whereas the width of the lower isodose curve widened with depth only. The deviation of output factor increased while the size of short side decreased according to the square root method. Conclusions The therapeutic depth of narrow electron beams turns superficial and the width of 90% isodose curve widens as depth increases, followed by an adduction after reaching the maximum. The deviation of output factor of narrow strip electron beam between the observed value and calculated value, according to the square root method, is obvious; the output factor should be measured before clinical application.