Nedaie H A, Ghahraman A R, Bolouri B, Arbabi A
Radiotherapy Oncology Dept, Cancer Research Center, Tehran University of Medical Sciences, Tehran,Iran.
Medical Physics Dept, Iran University of Medical Sciences, Tehran, Iran.
Med Phys. 2012 Jul;39(7Part4):4638. doi: 10.1118/1.4740178.
Recently, radiation sensitive polymer gels are being used as a reliable dosimetry method for three-dimensional (3D) verification of radiation doses in clinical use. Some properties of gel dosimeters have made them useful in verifying complex situations in electron therapy. The aim of this study was to experimentally evaluate the influence of tissue inhomogeneities on electron beam dose distributions by use of polymer gel dosimetry. Another purpose was to evaluate the appropriateness of polymer gels for electron beam dosimetry applications. A cylindrical phantom filled with MAGIC polymer gel with a polyacrilic wall (ρ = 1.18 g.cm ) was placed in a Perspex water-filled tank exactly underneath the bone inhomogeneity region .Then, the slab phantom was irradiated with a dose of 5Gy of 8MeV electrons to measure the dose distribution beyond the heterogeneity region. Afterwards, another cylindrical gel phantom similar to the above was used and irradiated with the same dose of 15 MeV electrons to measure the dose distribution beyond the same heterogeneity region. The same mentioned setup was repeated for measurement of the dose distribution beneath the air heterogeneity and homogenous phantom. The results of gel dosimetry under bone inhomogeneity have shown a reduction in dose. This is related to the high mass stopping and mass scattering powers of bone tissue. In addition, dose enhancement is seen laterally near the bone-tissue interface, due to increased side scattering of electrons. Hot and cold scatter lobes under heterogeneity regions are other effects that can be seen. The results of gel dosimetry under the air inhomogeneity have shown an increase in dose. This is related to the low mass stopping and mass scattering powers of the air cavity. When a high energy beam passes through a low-density medium or an air cavity, electronic equilibrium is lost along the central axis of the beam .The dose rebuild up is a consequence of this electronic disequilibrium. An overall good agreement was found between measurements with gel and with a diode detector for the single beam experiment. Electron dose distributions are significantly altered in the presence of tissue inhomogeneities such as bone and air cavities which are related to mass stopping and mass scattering powers of heterogeneous materials.
最近，辐射敏感聚合物凝胶正被用作一种可靠的剂量测定方法，用于临床使用中辐射剂量的三维（3D）验证。凝胶剂量计的一些特性使其在验证电子治疗中的复杂情况时很有用。本研究的目的是通过使用聚合物凝胶剂量测定法，实验评估组织不均匀性对电子束剂量分布的影响。另一个目的是评估聚合物凝胶在电子束剂量测定应用中的适用性。将一个填充有MAGIC聚合物凝胶且带有聚丙烯壁（ρ = 1.18 g.cm）的圆柱形体模放置在一个有机玻璃充水箱中，正好位于骨不均匀性区域下方。然后，用8MeV电子的5Gy剂量照射平板体模，以测量不均匀性区域之外的剂量分布。之后，使用另一个与上述类似的圆柱形凝胶体模，并用相同剂量的15MeV电子照射，以测量相同不均匀性区域之外的剂量分布。对上述相同设置进行重复，以测量空气不均匀性和均匀体模下方的剂量分布。骨不均匀性情况下的凝胶剂量测定结果显示剂量降低。这与骨组织的高传能线密度和质量散射能力有关。此外，由于电子侧向散射增加，在骨 - 组织界面附近的横向可见剂量增强。不均匀性区域下方的热散射和冷散射叶是可以看到的其他效应。空气不均匀性情况下的凝胶剂量测定结果显示剂量增加。这与气腔的低传能线密度和质量散射能力有关。当高能束穿过低密度介质或气腔时，沿束的中心轴电子平衡会丧失。剂量重建是这种电子不平衡的结果。在单束实验中，凝胶测量结果与二极管探测器测量结果之间总体上具有良好的一致性。在存在诸如骨和气腔等组织不均匀性的情况下，电子剂量分布会发生显著改变，这与异质材料的传能线密度和质量散射能力有关。
