Morrison Hali, Menon Geetha, Larocque Matthew P, Jans Hans-Sonke, Weis Ezekiel, Sloboda Ron S
Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta T6G 1Z2, Canada and Department of Oncology, University of Alberta, Edmonton, Alberta T6G 2R3, Canada.
Department of Ophthalmology, University of Alberta, Edmonton, Alberta T6G 2R3, Canada.
Med Phys. 2016 Aug;43(8):4891. doi: 10.1118/1.4959540.
To estimate the total dosimetric uncertainty at the tumor apex for ocular brachytherapy treatments delivered using 16 mm Collaborative Ocular Melanoma Study (COMS) and Super9 plaques loaded with (125)I seeds in order to determine the size of the apex margin that would be required to ensure adequate dosimetric coverage of the tumor.
The total dosimetric uncertainty was assessed for three reference tumor heights: 3, 5, and 10 mm, using the Guide to the expression of Uncertainty in Measurement/National Institute of Standards and Technology approach. Uncertainties pertaining to seed construction, source strength, plaque assembly, treatment planning calculations, tumor height measurement, plaque placement, and plaque tilt for a simple dome-shaped tumor were investigated and quantified to estimate the total dosimetric uncertainty at the tumor apex. Uncertainties in seed construction were determined using EBT3 Gafchromic film measurements around single seeds, plaque assembly uncertainties were determined using high resolution microCT scanning of loaded plaques to measure seed positions in the plaques, and all other uncertainties were determined from the previously published studies and recommended values. All dose calculations were performed using plaque simulator v5.7.6 ophthalmic treatment planning system with the inclusion of plaque heterogeneity corrections.
The total dosimetric uncertainties at 3, 5, and 10 mm tumor heights for the 16 mm COMS plaque were 17.3%, 16.1%, and 14.2%, respectively, and for the Super9 plaque were 18.2%, 14.4%, and 13.1%, respectively (all values with coverage factor k = 2). The apex margins at 3, 5, and 10 mm tumor heights required to adequately account for these uncertainties were 1.3, 1.3, and 1.4 mm, respectively, for the 16 mm COMS plaque, and 1.8, 1.4, and 1.2 mm, respectively, for the Super9 plaque. These uncertainties and associated margins are dependent on the dose gradient at the given prescription depth, thus resulting in the changing uncertainties and margins with depth.
The margins determined in this work can be used as a guide for determining an appropriate apex margin for a given treatment, which can be chosen based on the tumor height. The required margin may need to be increased for more complex scenarios (mushroom shaped tumors, tumors close to the optic nerve, oblique muscle related tilt, etc.) than the simple dome-shaped tumor examined and should be chosen on a case-by-case basis. The sources of uncertainty contributing most significantly to the total dosimetric uncertainty are seed placement within the plaques, treatment planning calculations, tumor height measurement, and plaque tilt. This work presents an uncertainty-based, rational approach to estimating an appropriate apex margin.
使用装有碘-125种子源的16毫米协作性眼部黑色素瘤研究（COMS）模板和Super9模板进行眼部近距离放射治疗，估算肿瘤顶端的总剂量测定不确定度，以确定确保肿瘤获得足够剂量覆盖所需的顶端边缘大小。
采用《测量不确定度表示指南》/美国国家标准与技术研究院的方法，对三个参考肿瘤高度（3、5和10毫米）的总剂量测定不确定度进行评估。研究并量化了与种子结构、源强、模板组装、治疗计划计算、肿瘤高度测量、模板放置以及简单圆顶形肿瘤的模板倾斜相关的不确定度，以估算肿瘤顶端的总剂量测定不确定度。种子结构的不确定度通过围绕单个种子的EBT3变色胶片测量确定，模板组装的不确定度通过对加载模板的高分辨率微型计算机断层扫描来测量模板中种子的位置确定，所有其他不确定度则根据先前发表的研究和推荐值确定。所有剂量计算均使用模板模拟器v5.7.6眼科治疗计划系统，并纳入模板不均匀性校正。
16毫米COMS模板在肿瘤高度为3、5和10毫米时的总剂量测定不确定度分别为17.3%、16.1%和14.2%，Super9模板的相应值分别为18.2%、14.4%和13.1%（所有值的覆盖因子k = 2）。为充分考虑这些不确定度，16毫米COMS模板在肿瘤高度为3、5和10毫米时所需的顶端边缘分别为1.3、1.3和1.4毫米，Super9模板的相应值分别为1.8、1.4和1.2毫米。这些不确定度和相关边缘取决于给定处方深度处的剂量梯度，因此随深度变化而变化。
本研究确定的边缘可作为指导，为特定治疗确定合适的顶端边缘，可根据肿瘤高度进行选择。对于比所研究的简单圆顶形肿瘤更复杂的情况（蘑菇形肿瘤、靠近视神经的肿瘤、与斜肌相关的倾斜等），所需边缘可能需要增加，应逐案选择。对总剂量测定不确定度贡献最大的不确定度来源是模板内种子的放置、治疗计划计算、肿瘤高度测量和模板倾斜。本研究提出了一种基于不确定度的合理方法来估算合适的顶端边缘。
