Previous work has shown that PRESAGE® can be used successfully to perform 3D dosimetric measurements of complex radiotherapy treatments. However, measurements near the sample edges are known to be difficult to achieve. This is an issue when the doses at air-material interfaces are of interest, for example when investigating the electron return effect (ERE) present in treatments delivered by magnetic resonance (MR)-linac systems. To study this effect, a set of 3.5 cm-diameter cylindrical PRESAGE® samples was uniformly irradiated with multiple dose fractions, using either a conventional linac or an MR-linac. The samples were imaged between fractions using an optical-CT, to read out the corresponding accumulated doses. A calibration between TPS-predicted dose and optical-CT pixel value was determined for individual dosimeters as a function of radial distance from the axis of rotation. This data was used to develop a correction that was applied to four additional samples of PRESAGE® of the same formulation, irradiated with 3D-CRT and IMRT treatment plans, to recover significantly improved 3D measurements of dose. An alternative strategy was also tested, in which the outer surface of the sample was physically removed prior to irradiation. Results show that for the formulation studied here, PRESAGE® samples have a central region that responds uniformly and an edge region of 6-7 mm where there is gradual increase in dosimeter response, rising to an over-response of 24%-36% at the outer boundary. This non-uniform dose response increases in both extent and magnitude over time. Both mitigation strategies investigated were successful. In our four exemplar studies, we show how discrepancies at edges are reduced from 13%-37% of the maximum dose to between 2 and 8%. Quantitative analysis shows that the 3D gamma passing rates rise from 90.4, 69.3, 63.7 and 43.6% to 97.3, 99.9, 96.7 and 98.9% respectively.

中文翻译：

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3D剂量学中的边缘效应：PRESAGE®不均匀剂量响应的表征和校正。

先前的工作表明，PRESAGE®可成功用于复杂放射治疗的3D剂量测量。然而，已知难以在样品边缘附近进行测量。当关注空气材料界面处的剂量时，例如在调查磁共振（MR）-直线加速器系统提供的治疗中存在的电子返回效应（ERE）时，这就是一个问题。为了研究这种效果，使用常规直线加速器或MR直线加速器对一组3.5厘米直径的圆柱状PRESAGE®样品进行了多次剂量均匀照射。使用光学CT对样品在各部分之间成像，以读出相应的累积剂量。确定了各个剂量计的TPS预测剂量与光学CT像素值之间的校准，该校准是距旋转轴的径向距离的函数。该数据用于制定校正方法，该校正方法适用于用3D-CRT和IMRT治疗计划照射的相同配方的四个其他样品，以恢复显着改善的3D剂量测量。还测试了一种替代策略，其中在辐照之前将样品的外表面物理去除。结果表明，对于此处研究的制剂，PRESAGE®样品具有均匀响应的中心区域和6-7 mm的边缘区域，剂量计响应逐渐增加，在30℃时达到24％-36％的过响应外边界。随着时间的流逝，这种不均匀的剂量反应在程度和大小上都增加。所研究的两种缓解策略均成功。在我们的四个示例性研究中，我们显示了边缘的差异如何从最大剂量的13％-37％减少到2％至8％之间。定量分析显示3D伽玛通过率分别从90.4、69.3、63.7和43.6％上升到97.3、99.9、96.7和98.9％。