热塑膜对X射线治疗剂量影响的研究

目的 研究热塑膜对χ射线治疗剂量的影响.方法 运用为模拟放射源而专门开发的BEAMnrc大型蒙特卡罗程序,研究热塑膜在χ射线治疗肿瘤时对皮肤表面剂量的影响.选用本科使用的国产热塑膜(密度为1.12 g/cm3)和模拟密度为1.38 g/cm3的膜,比较不同密度膜对χ射线表面剂量的影响.膜的形态结构分两种,其中一种膜无网孔,另一种膜有网孔.膜的网孔大小有两种,网孔面积分别为0.1 cm×0.1 cm和0.1 cm×0.2 cm,其中网孔与网孔之间的膜材料宽度均为0.1cm.结果 热塑膜的使用主要影响χ射线治疗剂量的建成区,其中0.24 cm厚度下1.38、1.12g/cm3密度无孔膜的表面剂量分别为74.9%、57.0%;1.12 g/cm3密度下0.24、0.12 cm厚度无网孔膜的表面剂量分别为57.0%、41.2%;1.12 g/cm3密度、0.24 cm厚度下无网孔和有网孔膜的表面剂量分别为57.0%、44.5%;1.12 g/cm3密度、0.24 cm厚度、0.1 cm ×0.1 cm、0.1 cm×0.2 cm网孔面积的表面剂量分别为54.1%、44.5%.结论 放疗中热塑膜的使用对患者身体表画吸收的剂量存在较大影响,这种影响的大小和热塑膜材料、孔径和厚度等有关.医生和物理师在做放疗计划设计时,应考虑到热塑膜对患者皮肤剂量影响及其生物效应,必要时采取措施修正,否则可能导致较严重皮肤反应.

Impact of thermoplastic mask on X-ray surface dose calculated with Monte Carlo code

Objective To calculate the effects of thermoplastic mask on X-ray surface dose.Methods The BEAMnrc Monte Carlo Code system, designed especially for computer simulation of radioactive sources, was performed to evaluate the effects of thermoplastic mask on X-ray surface dose.Thermoplastic mask came from our center with a material density of 1.12 g/cm2. The masks without holes,with holes size of 0. 1 cm× 0. 1 cm, and with holes size of 0. 1 cm × 0. 2 cm, and masks with different depth (0.12 cm and 0.24 cm) were evaluated separately. For those with holes, the material width between adjacent holes was 0. 1 cm. Virtual masks with a material density of 1.38 g/cm3 without holes with two different depths were also evaluated. Results Thermoplastic mask affected X-rays surface dose. When using a thermoplastic mask with the depth of 0. 24 cm without holes, the surface dose was 74. 9% and 57.0% for those with the density of 1.38 g/cm3 and 1.12 g/cm3 respectively. When focusing on the masks with the density of 1.12 g/cm3, the surface dose was 41.2% for those with 0.12 cm depth without holes;57.0% for those with 0. 24 cm depth without holes;44. 5% for those with 0. 24 cm depth with holes size of 0.1 cm ×0.2 cm;and 54.1% for those with 0.24 cm depths with holes size of 0.1 cm ×0.1 cm.Conclusions Using thermoplastic mask during the radiation increases patient surface dose. The severity is relative to the hole size and the depth of thermoplastic mask. The surface dose change should be considered in radiation planning to avoid severe skin reaction.