Bolus与皮肤间空腔对浅层组织剂量沉积影响研究

目的 研究Bolus与皮肤间空腔厚度和面积对浅层组织剂量的影响。方法 运用Geant4构建出射6 MV X线的加速器模型,模拟10 cm×10 cm射野下出束情况并记录出射粒子相空间文件。在源轴距水平构建30 cm×30 cm×30 cm水模体,分别在其靠近加速器一侧构建紧贴模体表面及含有不同空腔的30 cm×30 cm×1 cm水膜,以相空间文件作入射粒子源,模拟粒子输运过程,获取水模体中心轴深度剂量分布和射野中心区域不同深度处侧向剂量profile。将含有不同空腔时的模拟数据与水膜和水模体紧密贴合时的数据进行对比。结果 空腔厚度≤5 mm时,影响较小,之后随厚度增加,最大剂量深度(Dmax)增加,对应处百分深度剂量减小,侧向剂量profile受影响深度增加,中心区域剂量减小;随空腔面积增加,Dmax先增大后减小,对应处百分深度剂量先减小后增加,侧向剂量profile受影响深度先增加后减小,中心区域剂量先减小后增加;在远离空腔及深度≥15 mm时侧向剂量profile基本不受影响。结论 使用Bolus时其下空腔厚度应在5 mm以内,面积尽量小。

Research of effect of cavity between bolus and skin on dose deposition in shallow tissues

Objective To evaluate the effect of the thickness and area of the cavity between bolus and skin upon the dose deposition in the shallow tissues. Methods The linear accelerator head model of 6 MV X ray was constructed using Geant4,which simulated the accelerator working condition of 10 cm×10 cm field and recorded the information of emergent particles as phase space file. A water phantom of 30 × 30 × 30 cm³ was designed at the SAD level. A 30 cm × 30 cm × 1 cm water film was constructed adjacent to or with different cavities to the upper surface of the phantom. The phase space file was utilized as particle source to simulate the particle transport process. The axis depth dose distribution and lateral dose profiles in the center area of the field at a depth of 1 mm,5 mm,9 mm,15 mm and 21 mm of the phantom were obtained,respectively. The simulated data obtained from water film with different cavities were compared with those of adjacent to the water phantom. Results When the cavity was ≤ 5 mm,the cavity exerted slight effect. When the thickness was increased,the maximum dose depth (Dmax) was increased,the PDD at Dmax(PDDmax) became less,the depth of influenced lateral dose profiles was increased and the dose in the center area was decreased. Along with the increase of cavity area,the Dmax was initially increased and then decreased,the PDDmax was at first decreased and subsequently increased,the depth of influenced lateral dose profile was initially increased and then decreased,the dose in the center area was first decreased and subsequently increased. The lateral dose profile distant from the cavity or located at a depth ≥15 mm was almost not affected by the cavity. Conclusion The thickness of the cavity between bolus and skin should be less than 5 mm and the cavity area should be small as possible.