Purpose

To investigate out-of-field scattered doses from the Integral Quality Monitor (IQM) transmission detector in megavoltage photon beams.

Materials and methods

We measured out-of-field point doses for 20 × 20 cm² 6 ̶ 15 MV photon beams using 10 × 2 cm² Gafchromic EBT3 film strips placed across the surfaces of 5-cm thick water-equivalent RW3 slabs. The films were placed at 10 cm intervals from the central axis (CAX) of each beam, up to 1.0 m away on opposite sides of the CAX. The measurements were conducted at 80 cm and 100 cm source-to-surface distances (SSD) without the IQM, and were repeated with the IQM in the paths of the beams. Measurements were also performed at 90 cm SSD for 20 × 20 and 30 × 30 cm² 15 MV fields. Surface dose profiles were then constructed from the measurements for each beam setup with and without the IQM to examine the differences in scattered dose off-axis. The dose profile for each beam setup was normalised to dose maximum measured on the CAX.

Results

Overall, surface dose profiles acquired with the IQM in the paths of the beams were higher than the corresponding profiles without the IQM. The out-of-field dose increased with increase in photon energy, field size, and shorter SSDs, and decreased with off-axis distance. At 80 cm SSD for the 20 × 20 cm² field, the IQM-induced surface dose ranged from −0.6% ÷ 1.9%, −0.3% ÷ 3.0%, and 0.3% ÷ 6.8% for 6, 10, and 15 MV beams, respectively.

Conclusion

The higher surface dose profiles measured with the IQM attached to the linac compared to the profiles without the IQM indicates that the device is acting as an additional source of scattered radiation.

中文翻译：

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Integral QualityMonitor®在兆伏光子束中的场外散射

目的

要研究从整体质量监控器（IQM）传输检测器在兆伏光子束中的场外散射剂量。

材料和方法

我们使用10×2 cm ² Gafchromic EBT3薄膜条在5厘米厚的等效水RW3平板表面上测量了20×20 cm ² 6 × 15 MV光子束的场外剂量。将胶片与每个光束的中心轴（CAX）间隔10 cm放置，在CAX的相对两侧最远为1.0 m。在没有IQM的情况下以80 cm和100 cm的源到表面距离（SSD）进行测量，并在光束路径中使用IQM重复进行测量。在90 cm SSD上也进行了20×20和30×30 cm ^2的测量15个MV字段。然后，根据使用和不使用IQM的每个光束设置的测量结果构建表面剂量曲线，以检查离轴散射剂量的差异。将每个射束设置的剂量分布标准化为在CAX上测得的最大剂量。

结果

总体而言，用IQM在光束路径中获得的表面剂量分布高于没有IQM的相应分布。场外剂量随着光子能量的增加，场大小和更短的SSD的增加而增加，而随着离轴距离的增加而减小。对于20×20 cm ²场，在80 cm SSD下，对于6、10和15 MV光束，IQM引起的表面剂量范围为-0.6％÷1.9％，-0.3％÷3.0％和0.3％÷6.8％。 ， 分别。

结论

与没有IQM的轮廓相比，将IQM连接到直线加速器所测得的较高的表面剂量轮廓表明该设备正在充当散射辐射的附加来源。