热塑料面膜对皮肤剂量的影响研究

Study on influence of thermoplastic mask on skin dose

目的定量研究鼻咽癌放射治疗时,热塑料面膜在与皮肤各种距离的情况下对皮肤剂量及靶区剂量的影响,对使用面膜后皮肤反应加重的临床现象进行探讨.方法使用VARiAN2100C/D加速器产生的X6与E9射线;比利时ORFIT工业公司生产的U-PLAST材料的固定热塑料面膜;半导体探头;DPD-510半导体现场测量仪及Farmer 2570/1剂量仪和258l电离室(0.6cm3)采用源轴距(SAD)和源皮距(SSD)的方法,测量面膜与皮肤不同距离时对皮肤剂量和最大剂量点剂量的影响.结果 (1)X线入射线对皮下2 mm处剂量有一定影响,可提高11%的剂量,电子线(E9)则在面膜与皮肤有一定距离(2～2.5 cm)时产生一个高剂量峰,在皮下2 mm处最大可产生35%左右的剂量增幅.(2)使用面膜后对最大剂量点的剂量影响在X6和E9分别为0.5%和3%.(3)射出线束对皮肤剂量最大增加量为1.3%.结论 X6和E9的入射线会对皮肤剂量产生较大影响.而X6在使用面膜后无论出射线还是入射线对最大剂量点的剂量影响均不大,可以认为使用面膜不必对开机量进行校正;而E9则有3%的剂量增加,考虑为面膜提升PDD值所致,剂量计算时要将此因素考虑进去.

Objective To study how much the relative skin dose is changed with the application of thermoplastic mask. Methods 6 MV X-ray and 9Mev electron,thermoplastic mask, semiconductor detector （ scanditronix DPD 510） and Farmer 2570/1 dosimeter were used. Skin dose was measured by SAD and SSD method. Results There was obvious influence of the mask on dose. A 11% dose increase was found 2mm beneath the surface in 6MV X-ray while a 35% dose increase in 9MeV electron. The dose increase at maximum dose point was 0.5% and 3% respectively in 6MV X-ray and E9 electron. Conclusions The thermoplastic mask has no influence on the point of maximum dose when 6 MV X-ray was used both in entry and exit beam,but its influence on the build-up range beneath the entry beam is large. Especially in 9MeV with an air gap between mask and skin.