Department of Medical Physics, Institut Claudius Regaud-Institut Universitaire du Cancer de Toulouse Oncopole, 1 avenue Irène Joliot Curie, F-31059 Toulouse Cedex 9, France.
Centre de Recherche et de Cancérologie de Toulouse, UMR1037 INSERM-Université Toulouse 3-ERL5294 CNRS, 2 avenue Hubert Curien, F-31037 Toulouse Cedex 1, France.
Phys Med Biol. 2021 Feb 2;66(4):045009. doi: 10.1088/1361-6560/abd22c.
The benefits of using an algorithm that reports absorbed dose-to-medium have been jeopardized by the clinical experience and the experimental protocols that have mainly relied on absorbed dose-to-water. The aim of the present work was to investigate the physical aspects that govern the dosimetry in heterogeneous media using Monte Carlo method and to introduce a formalism for the experimental validation of absorbed dose-to-medium reporting algorithms. Particle fluence spectra computed within the sensitive volume of two simulated detectors (T31016 Pinpoint 3D ionization chamber and EBT3 radiochromic film) placed in different media (water, RW3, lung and bone) were compared to those in the undisturbed media for 6 MV photon beams. A heterogeneity correction factor that takes into account the difference between the detector perturbation in medium and under reference conditions as well as the stopping-power ratios was then derived for all media using cema calculations. Furthermore, the different conversion approaches and Eclipse treatment planning system algorithms were compared against the Monte Carlo absorbed dose reports. The detectors electron fluence perturbation in RW3 and lung media were close to that in water (≤1.5%). However, the perturbation was greater in bone (∼4%) and impacted the spectral shape. It was emphasized that detectors readings should be corrected by the heterogeneity correction factor that ranged from 0.932 in bone to 0.985 in lung. Significant discrepancies were observed between all the absorbed dose reports and conversions, especially in bone (exceeding 10%) and to a lesser extent in RW3. Given the ongoing advances in dose calculation algorithms, it is essential to standardize the absorbed dose report mode with absorbed dose-to-medium as a favoured choice. It was concluded that a retrospective conversion should be avoided and switching from absorbed dose-to-water to absorbed dose-to-medium reporting algorithm should be carried out by a direct comparison of both algorithms.
使用报告介质吸收剂量的算法的益处已经受到临床经验和主要依赖水吸收剂量的实验方案的威胁。本工作的目的是使用蒙特卡罗方法研究不均匀介质中的剂量学的物理方面，并引入一种用于实验验证报告介质吸收剂量算法的形式主义。在两个模拟探测器（T31016 Pinpoint 3D 电离室和 EBT3 光致变色胶片）的敏感体积内计算的粒子注量谱，与放置在不同介质（水、RW3、肺和骨）中的未受扰介质中的谱进行了比较对于 6 MV 光子束。然后，使用 cema 计算为所有介质导出了一个考虑到介质和参考条件下探测器扰动差异以及阻止功率比的不均匀性校正因子。此外，还比较了不同的转换方法和 Eclipse 治疗计划系统算法与蒙特卡罗吸收剂量报告。RW3 和肺介质中探测器的电子注量扰动接近水（≤1.5%）。然而，在骨中更大（约 4%），并影响光谱形状。强调探测器读数应通过不均匀性校正因子进行校正，校正因子范围从骨中的 0.932 到肺中的 0.985。所有吸收剂量报告和转换之间都观察到显著差异，特别是在骨中（超过 10%），在 RW3 中则较小。鉴于剂量计算算法的不断进步，必须将吸收剂量报告模式标准化，以介质吸收剂量作为首选。结论是应该避免回顾性转换，并且应该通过直接比较两种算法来从水吸收剂量报告算法切换到介质吸收剂量报告算法。
