Purpose

Isotropy as a quality assurance aspect of intraoperative radiotherapy (IORT) needs to be evaluated accurately. There are some main problems associated with isotropy investigations by two dimensional dosimeters including limited positions and distances in which isotropy can be measured and long irradiation time needed for isotropy evaluation. In this study, isotropic radiation accuracy of spherical applicators of low energy X-ray IORT was estimated by the use of MAGAT gel as a three dimensional gel dosimeter and a specified IORT phantom.

Methods

Designed IORT phantom was manufactured by multi-axis computer numerical control machine (CNC) using Plexiglas. To investigate isotropy, methacrylic acid, gelatin and tetrakis phosphonium chloride (MAGAT) gel dosimeters embedded in the phantom holes were irradiated with a 50 kV X-ray IORT device to 18 Gy at the surface of the vials. Then, relative dose deviations of output doses from their average were extracted from MRI images of the gel vials in different positions and distances from the 50-mm spherical applicator.

Results

The percentages of relative deviations of output doses from the average value were less than 8% in all positions which are within the 15% acceptance limit of isotropy radiation. Moreover, a Pearson-moment correlation analysis showed a Pearson's r equal to 0.99–1 for different polar and azimuth angles representing isotropic radiation emission of X-ray IORT.

Conclusion

Isotropy of IORT can be easily investigated using gel dosimeters by applying the produced phantom which solved the issues facing three-dimensional measurement of isotropic radiation of IORT X-ray source (XRS).

中文翻译：

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MAGAT凝胶剂量计对低能X线术中放疗各向同性辐射的研究

目的

需要准确评估作为术中放疗 (IORT) 质量保证方面的各向同性。使用二维剂量计进行各向同性研究存在一些主要问题，包括可测量各向同性的位置和距离有限，以及各向同性评估所需的辐照时间长。在这项研究中，通过使用 MAGAT 凝胶作为三维凝胶剂量计和指定的 IORT 体模，估计了低能 X 射线 IORT 球形施加器的各向同性辐射精度。

方法

设计的 IORT 体模由使用有机玻璃的多轴计算机数控机床 (CNC) 制造。为了研究各向同性，使用 50 kV X 射线 IORT 设备将嵌入幻影孔中的甲基丙烯酸、明胶和四氯化鏻 (MAGAT) 凝胶剂量计照射到小瓶表面的 18 Gy。然后，从凝胶小瓶的 MRI 图像中提取输出剂量与其平均值的相对剂量偏差，该图像位于距 50 毫米球形施药器的不同位置和距离处。

结果

在各向同性辐射的 15% 可接受限度内的所有位置，输出剂量与平均值的相对偏差百分比均小于 8%。此外，皮尔逊矩相关分析表明，对于代表 X 射线 IORT 各向同性辐射发射的不同极角和方位角，皮尔逊 r 等于 0.99–1。

结论

使用凝胶剂量计可以很容易地研究 IORT 的各向同性，该模型解决了 IORT X 射线源 (XRS) 各向同性辐射三维测量面临的问题。