Pappas Eleftherios P, Zoros Emmanouil, Moutsatsos Argyris, Peppa Vasiliki, Zourari Kyveli, Karaiskos Pantelis, Papagiannis Panagiotis
Medical Physics Laboratory, Medical School, National and Kapodistrian University of Athens, Athens 115 27, Greece.
Phys Med Biol. 2017 May 21;62(10):4160-4182. doi: 10.1088/1361-6560/aa6a01. Epub 2017 Mar 29.
There is an acknowledged need for the design and implementation of physical phantoms appropriate for the experimental validation of model-based dose calculation algorithms (MBDCA) introduced recently in Ir brachytherapy treatment planning systems (TPS), and this work investigates whether it can be met. A PMMA phantom was prepared to accommodate material inhomogeneities (air and Teflon), four plastic brachytherapy catheters, as well as 84 LiF TLD dosimeters (MTS-100M 1 × 1 × 1 mm microcubes), two radiochromic films (Gafchromic EBT3) and a plastic 3D dosimeter (PRESAGE). An irradiation plan consisting of 53 source dwell positions was prepared on phantom CT images using a commercially available TPS and taking into account the calibration dose range of each detector. Irradiation was performed using an Ir high dose rate (HDR) source. Dose to medium in medium, [Formula: see text], was calculated using the MBDCA option of the same TPS as well as Monte Carlo (MC) simulation with the MCNP code and a benchmarked methodology. Measured and calculated dose distributions were spatially registered and compared. The total standard (k = 1) spatial uncertainties for TLD, film and PRESAGE were: 0.71, 1.58 and 2.55 mm. Corresponding percentage total dosimetric uncertainties were: 5.4-6.4, 2.5-6.4 and 4.85, owing mainly to the absorbed dose sensitivity correction and the relative energy dependence correction (position dependent) for TLD, the film sensitivity calibration (dose dependent) and the dependencies of PRESAGE sensitivity. Results imply a LiF over-response due to a relative intrinsic energy dependence between Ir and megavoltage calibration energies, and a dose rate dependence of PRESAGE sensitivity at low dose rates (<1 Gy min). Calculations were experimentally validated within uncertainties except for MBDCA results for points in the phantom periphery and dose levels <20%. Experimental MBDCA validation is laborious, yet feasible. Further work is required for the full characterization of dosimeter response for Ir and the reduction of experimental uncertainties.
对于设计和制作适用于近期在铱近距离放射治疗计划系统（TPS）中引入的基于模型的剂量计算算法（MBDCA）实验验证的物理体模，人们已经认识到有这样的需求，而本研究探讨了这一需求是否能够得到满足。制备了一个聚甲基丙烯酸甲酯（PMMA）体模，以容纳材料不均匀性（空气和聚四氟乙烯）、四个塑料近距离放射治疗导管，以及84个氟化锂热释光剂量计（MTS - 100M 1×1×1 mm微型立方体）、两张放射变色胶片（Gafchromic EBT3）和一个塑料三维剂量计（PRESAGE）。使用市售TPS并考虑每个探测器的校准剂量范围，在体模CT图像上制定了一个由53个源驻留位置组成的照射计划。使用铱高剂量率（HDR）源进行照射。使用同一TPS的MBDCA选项以及使用MCNP代码和基准方法的蒙特卡罗（MC）模拟来计算介质中的介质剂量，[公式：见原文]。对测量和计算得到的剂量分布进行空间配准并比较。热释光剂量计、胶片和PRESAGE的总标准（k = 1）空间不确定度分别为：0.71、1.58和2.55 mm。相应的总剂量学不确定度百分比分别为：5.4 - 6.4、2.5 - 6.4和4.85，这主要归因于热释光剂量计的吸收剂量灵敏度校正和相对能量依赖性校正（位置相关）、胶片灵敏度校准（剂量相关）以及PRESAGE灵敏度的依赖性。结果表明，由于铱和兆伏级校准能量之间相对固有的能量依赖性，氟化锂存在过度响应，并且在低剂量率（<1 Gy/min）下PRESAGE灵敏度存在剂量率依赖性。除了体模周边点和剂量水平<20%的MBDCA结果外，计算结果在不确定度范围内得到了实验验证。实验性MBDCA验证虽然费力，但却是可行的。需要进一步开展工作以全面表征铱的剂量计响应并降低实验不确定度。
