Ferrer Carlos, Huertas Concepción, Plaza Rodrigo, Aza Zulima, Corredoira Eva
Department of Medical Physics and Radiation Protection, H.U. La Paz, Madrid, Spain.
J Appl Clin Med Phys. 2018 Nov;19(6):124-132. doi: 10.1002/acm2.12456. Epub 2018 Sep 25.
This work aims to study the effect on surface dose and dose distribution caused by the Elekta Fraxion cranial immobilization system. The effect of Fraxion inclusion in Elekta Monaco treatment planning system and its calculation accuracy is also checked. To study the dose attenuation, a cylindrical phantom was located over the Elekta Fraxion with an IBA CC13 ionization chamber placed in the central insert at the linac isocenter. Dose measurements at multiple gantry angles were performed for three open fields, 10 × 10 cm, 5 × 5 cm and other smaller 2 × 2 cm. Measured doses were compared with the ones calculated by Monaco. Surface dose and dose distribution in the buildup region were measured placing several Gafchromic Films EBT3 at linac CAX between the slabs of a RW3 phantom located over Fraxion and read using FilmQA Pro software. Measures were performed for two open field sizes and results were compared with Monaco calculations. Measurements show a 1% attenuation for 180° gantry angle but it can be as high as 3.4% (5 × 5 open field) for 150°/210° gantry angle, as with these angles the beam goes through the Fraxion's headrest twice. If Fraxion is not included in the calculation Monaco calculation can result in a 3% difference between measured and calculated doses, while with Fraxion in the calculation, the maximum difference is 0.9%. Fraxion increases 3.7 times the surface dose, which can be calculated by Monaco with a difference lower than 2%. Monaco also calculated correctly the PDD for both open fields (2%) when Fraxion is included in the calculation. This work shows that the attenuation varies with gantry angle. The inclusion of Fraxion in Monaco improves the calculation from 3% difference to 1% in the worst case. Furthermore, the surface dose increment and the dose in the buildup region are correctly calculated.
本研究旨在探讨医科达Fraxion头部固定系统对表面剂量和剂量分布的影响。同时也检测了在医科达Monaco治疗计划系统中纳入Fraxion后的效果及其计算准确性。为研究剂量衰减情况，将一个圆柱形模体置于医科达Fraxion上方，并在直线加速器等中心的中央插入件中放置一个IBA CC13电离室。针对三个开放野（10×10 cm、5×5 cm和其他较小的2×2 cm）在多个机架角度进行了剂量测量。将测量得到的剂量与Monaco计算的剂量进行比较。通过在位于Fraxion上方的RW3模体的平板之间的直线加速器中心轴处放置几片Gafchromic Films EBT3来测量建成区的表面剂量和剂量分布，并使用FilmQA Pro软件进行读取。针对两种开放野尺寸进行了测量，并将结果与Monaco的计算结果进行比较。测量结果显示，对于180°机架角度，剂量衰减为1%，但对于150°/210°机架角度，剂量衰减可能高达3.4%（5×5开放野），因为在这些角度下射线会两次穿过Fraxion的头枕。如果在计算中不纳入Fraxion，Monaco计算的测量剂量与计算剂量之间可能会有3%的差异，而在计算中纳入Fraxion时，最大差异为0.9%。Fraxion使表面剂量增加了3.7倍，Monaco计算的差异低于2%。当在计算中纳入Fraxion时，Monaco还正确计算了两种开放野的百分深度剂量（相差2%）。这项研究表明，衰减随机架角度而变化。在Monaco中纳入Fraxion可将计算差异在最坏情况下从3%改善到1%。此外，表面剂量增量和建成区的剂量也得到了正确计算。
