Bueno M, Schuemann J, Duch M, Paganetti H
Universitat Politecnica de Catalunya, Barcelona, SPAIN.
Massachusetts General Hospital, Boston, MA.
Med Phys. 2012 Jun;39(6Part21):3874. doi: 10.1118/1.4735816.
Due to multiple Coulomb scattering in complex geometries, small field dosimetry in proton therapy is challenging. Our goal was to define an indicator for the accuracy of dose delivery based on analytical dose calculations in treatment planning systems for small (e.g. radiosurgery) proton therapy fields.
Seven patients whose treatment involved one or more small fields (below ∼3.6cm in diameter) were selected. We developed a fast methodology to quantify the inhomogeneity of the tissue traversed by a single beam using a heterogeneity index (HI). The implementation was based on the dose calculation approach taken by our pencil beam algorithm. Plans created with the treatment planning system were verified against Monte Carlo dose calculations on a field- by-field basis. DVHs were analyzed and differences in the dose to the GTV were assessed. The correlation between the HI-values and the discrepancies between planning system and Monte Carlo in terms of absolute dose to the target was studied.
Our treatment planning system overestimates the dose within the GTV for very small fields by up to ∼8%, even if proper output factor normalization is done in water. The differences are strongly correlated to HI (Spearman's ρ=0.8, rho<0.0001). More complex heterogeneities within the beam path caused larger errors by the analytical algorithm. With the established correlation a threshold for the HI can be set by choosing a tolerance level.
The HI as defined in this study appears to be a good indicator of the accuracy of proton field delivery in terms of GTV prescription dose when small fields are being delivered. Each HI-value was obtained in less than 2 minutes allowing implementation of the HI algorithm in clinical routine. For HI- values exceeding a certain threshold, either a change in beam incidence or a Monte Carlo dose calculation should be considered.
由于在复杂几何结构中存在多次库仑散射，质子治疗中的小射野剂量学具有挑战性。我们的目标是基于治疗计划系统中针对小（如放射外科）质子治疗射野的解析剂量计算，定义一个剂量传递准确性的指标。
选择了7名治疗涉及一个或多个小射野（直径小于约3.6厘米）的患者。我们开发了一种快速方法，使用不均匀性指数（HI）来量化单束射线穿过组织的不均匀性。该方法基于我们的笔形束算法所采用的剂量计算方法。使用治疗计划系统创建的计划在逐个射野的基础上与蒙特卡罗剂量计算结果进行验证。分析剂量体积直方图（DVH）并评估靶区（GTV）剂量的差异。研究了HI值与计划系统和蒙特卡罗在靶区绝对剂量方面的差异之间的相关性。
即使在水中进行了适当的输出因子归一化，我们的治疗计划系统对于非常小的射野，在GTV内的剂量高估高达约8%。这些差异与HI密切相关（斯皮尔曼相关系数ρ = 0.8，rho < 0.0001）。射线路径内更复杂的不均匀性导致解析算法产生更大的误差。通过建立的相关性，可以通过选择一个容差水平来设置HI的阈值。
本研究中定义的HI似乎是在递送小射野时，就GTV处方剂量而言质子射野传递准确性的一个良好指标。每个HI值的获取时间不到2分钟，这使得HI算法能够在临床常规中得以应用。对于超过某个阈值的HI值，应考虑改变射束入射角度或进行蒙特卡罗剂量计算。
