不同材料的定位板对吸收剂量的影响

Influences of positioning-board of different materials on the absorbed dose

目的：探讨有机玻璃与碳纤维定位板对吸收剂量的影响。方法：将固体水模置于治疗床的中心,测量在不同的机架角度（0°-80°）下等中心处加与不加定位板时的吸收剂量,计算出定位板对X射线的衰减。使用治疗计划系统（Treatment Planning System,TPS）软件对不同角度和不同材料定位板进行模拟计算。结果：采用6 MV X射线照射时,碳纤维定位板对吸收剂量的衰减为1.8%-3.1%,而有机玻璃定位板对吸收剂量的衰减为5.1%-8.5%;采用15 MV X射线照射时,碳纤维定位板对吸收剂量的衰减为1.3%-2.2%,有机玻璃定位板对吸收剂量的衰减为3.5%-5.7%。随着射线穿过定位板的深度增加,衰减作用也显著增加,当机架角度为50°-60°时,衰减率达到最大。实测值与TPS模拟的结果一致。结论：有机玻璃定位板对吸收剂量的衰减影响是碳纤维定位板的2倍-3倍。在制定放疗计划时,应针对定位板对吸收剂量的衰减影响进行剂量修正,同时有机玻璃定位板也不宜应用于临床放疗。

Objective To explore the influence of positioning-board of plexiglass and carbon fiber on the absorbed dose. Methods Solid water phantom was fixed at the center of treatment couch. The absorbed doses of the isocenter point with or without positioning-board in different gantry angle changing from 0° to 80° were measured, and the X ray attenuations of different positioning-boards were calculated. Treatment planning system （TPS） software was finally applied to carry out the simulated calculation for different gantry angels and different positioning-boards. Results When 6 MV X ray was applied, the absorbed dose attenuation of carbon fiber positioning-board was 1.8%-3.1%, while that of plexiglass positioning-board was 5.1%-8.5%. When 15 MV X ray was applied, the absorbed dose attenuation of carbon fiber positioning-board was 1.3%-2.2%, while that of plexiglass positioning-board was 3.5%-5.7%. The attenuation was significantly worse with the increasing ray penetration thickness. When the gantry angle was 50°-60°, the attenuation rate reached the highest. Measured results and TPS simulated results were consistent. Conclusion The absorbed dose attenuation of plexiglass positioning-board is 2-3 times than that of carbon fiber positioning-board. Dose correction is necessary for treatment plan design, and plexiglass positioning-board is not appropriate for clinical radiotherapy.