技术说明：带有束流阻挡器的无机架环形伽玛刀的自屏蔽评估和辐射泄漏测量。

Technical Note: Self-shielding evaluation and radiation leakage measurement of a jawless ring gantry linac with a beam stopper.

机构信息

Department of Radiation Oncology, Washington University, St. Louis, MO, 63110, USA.

Varian Medical Systems, 3100 Hansen Way, Palo Alto, CA, 94304, USA.

出版信息

Med Phys. 2021 Jun;48(6):3143-3150. doi: 10.1002/mp.14858. Epub 2021 Apr 9.

DOI:10.1002/mp.14858

PMID:33763897

Abstract

PURPOSE

To characterize the shielding design and leakage radiation from a newly released ring gantry linac (Halcyon, Varian Medical Systems).

METHODS

To assess the radiation leakage surrounding headshield and the radiation level after the beam stopper, measurements were made with GafChromic films. To evaluate the in-room radiation levels, the radiation leakage in the isocenter plane was measured with a large volume spherical ionization chamber (Exradin A6, Standard Imaging). A lead enclosure was constructed to shield the chamber from the low energy scatter radiation from the room. The radiation level at multiple locations was measured with the MLC fully closed and gantry at 0, 45, 90, 135, 180, 225, 270, and 315 degrees. The leakage radiation passing through multiple concrete slabs with various thickness was recorded in a narrow beam geometry to determine the tenth value layer (TVL).

RESULTS

A uniform leakage (<0.05%) at 1 m from electron beam line was measured surrounding the linac head with the maximum leakage measured at the top of the head enclosure. The highest radiation level (<0.08%) was measured near the edge of the beam stopper when projected to the measurement plane. The maximum radiation levels due to the head leakage at 15 locations inside the treatment room were recorded and a radiation map was plotted. The maximum leakage was measured at points that along the electron beam line while the gantry at 90 or 270 degree and at the end of head enclosure (0.314%, 0.4 m from electron beamline). The leakage TVL value is found to be 226 mm in a narrow beam geometry with the concrete density of 2.16 g/cm or 134.6 lb/cu.ft.

CONCLUSION

An overall uniform leakage was measured surrounding linac head. The beam stopper shields the primary radiation with the highest valued measured near the edge of beam stopper. The leakage TVL values are derived and less than the values reported for conventional C-arm linac.

摘要

目的

描述新型环架直线加速器（Halcyon，Varian Medical Systems）的屏蔽设计和漏射线辐射情况。

方法

为评估头架周围的辐射泄漏和束流限束器后的辐射水平，使用 GafChromic 胶片进行了测量。为评估室内辐射水平，使用大体积球形电离室（Exradin A6，Standard Imaging）测量等中心平面的漏射线。构建了一个铅屏蔽罩，以防止来自房间的低能散射辐射进入电离室。在 MLC 完全关闭且机架分别处于 0、45、90、135、180、225、270 和 315 度时，在多个位置测量了辐射水平。在窄束几何形状下记录了穿过具有不同厚度的多个混凝土板的漏射线，以确定十分之一值层（Tenth Value Layer，TVL）。

结果

在距电子束线 1 米处测量到围绕直线加速器头的均匀漏射线（<0.05%），最大漏射线测量值位于头罩顶部。当投影到测量平面时，在束流限束器边缘附近测量到最高的辐射水平（<0.08%）。在治疗室内的 15 个位置记录了由于头漏而导致的最大辐射水平，并绘制了辐射图。在机架处于 90 或 270 度且头罩末端时（距电子束线 0.314%，距离 0.4 米），在电子束线上沿头漏方向测量到最大漏射线（0.314%，距电子束线 0.4 米）。在窄束几何形状下，混凝土密度为 2.16 g/cm³（134.6 lb/cu.ft）时，漏射线 TVL 值为 226 mm。