Tedgren Asa K Carlsson, Ahnesjö Anders
Medical Radiation Physics, Karolinska Institute, Stockholm University, Box 260, SE-171 76 Stockholm, Sweden.
Med Phys. 2003 Aug;30(8):2206-17. doi: 10.1118/1.1587411.
Common clinical brachytherapy treatment planning algorithms perform at best one-dimensional corrections for high Z heterogeneities that will be inaccurate for intermediate energies (60-100 keV). The development of fast methods for a three-dimensional dose calculation to account for high Z materials in this energy range is important, e.g., to fully utilize the potential of patient individualized shields using isotopes such as 241Am and 169Yb. In this work we use the collapsed cone superposition algorithm to calculate the scatter dose contribution around partly lead-shielded point sources at 60, 100, and 350 keV. Methods to scale point kernels for water into kernels for high Z materials are derived. The scaling accounts for scattered photon spectral differences between materials and thus goes beyond the common density scaling approach. Compared to Monte Carlo simulations, the results of our algorithm yield agreements on the unshielded side to within 3% at 350 and 60 keV and to within 7% at 100 keV out to distances of 8 cm from the source. The effect of the shield in the center of the unshielded region is small at 350 keV but significant and occurs at short distances at 100 and 60 keV. At 60 keV, the shield causes a dose reduction of around 10%, 1 cm from the source on the unshielded side. At 100 keV, the reverse effect is seen, the insertion of shields leading to the total dose being increased by about 10% at 1 cm. That one-dimensional algorithms are incapable of predicting these changes shows the importance of accounting for the full three-dimensional geometry in correctly determining the scatter dose contribution.
常见的临床近距离放射治疗治疗计划算法对于高原子序数的不均匀性最多只能进行一维校正，而对于中等能量（60 - 100 keV）来说这种校正会不准确。开发在该能量范围内考虑高原子序数材料的三维剂量快速计算方法很重要，例如，以充分利用使用如²⁴¹Am和¹⁶⁹Yb等同位素的患者个体化屏蔽的潜力。在这项工作中，我们使用坍缩圆锥叠加算法来计算在60 keV、100 keV和350 keV下部分铅屏蔽点源周围的散射剂量贡献。推导了将水的点核缩放为高原子序数材料核的方法。这种缩放考虑了材料之间散射光子光谱的差异，因此超越了常见的密度缩放方法。与蒙特卡罗模拟相比，我们算法的结果在源距离8 cm范围内，在350 keV和60 keV时，未屏蔽一侧的结果一致性在3%以内，在100 keV时在7%以内。在350 keV时，未屏蔽区域中心的屏蔽效果较小，但在100 keV和60 keV时很显著且发生在短距离处。在60 keV时，屏蔽在未屏蔽一侧距离源1 cm处导致剂量降低约10%。在100 keV时，观察到相反的效果，在1 cm处插入屏蔽导致总剂量增加约10%。一维算法无法预测这些变化，这表明在正确确定散射剂量贡献时考虑完整三维几何形状的重要性。
