使用寬能量中子偵檢器評估標靶真光刀系統之中子周圍等效劑量

Evaluation of Neutron Ambient Dose Equivalent Around the Varian TrueBe am System by Using a WENDI-II Detector

陳欣瑜(Hsin-Yu Chen) ； 陳顯鑫(Hsien-Hsin Chen) ； 廖英蘭(Ying-Lan Liao) ； 金冠瑋(Kuan-Wei Jin) ； 葉健一(Chien-Yi Yeh) ； 蔡惠予(Hui-Yu Tsai)

《放射治療與腫瘤學》 24卷3期 (2017/09) Pp. 239-245

https://doi.org/10.6316/TRO/201724(3)239

摘要

目的：標靶真光刀（Varian TrueBeam）系統是新一代的直線加速器，當高能Ｘ光（＞ 10 MV）與高原子序的組件作用時會產生中子，這些組件可能是整平濾片（flattening filter, FF）或準直儀，標靶真光刀系統設計成可操作在傳統的有整平濾片（flattening filter, FF）模式，或新型的無射束整平濾片（flattening filter free, FFF）模式。本研究的目的是針對 Varian TrueBeam 系統於 FF 與FFF 兩種模式下，使用寬能量中子偵檢器評估中子被誘發後所造成的中子周圍等效劑量，H*(10)。材料與方法：使用 10 MV 的 X 光射線於 FF 與 FFF 兩種模式下進行照射，此時的多葉式準直儀和次要準直儀為完全關閉的狀態。劑量交付的速率在 FF 模式與 FFF 模式下分別為 600 MU/min 與2400 MU/min。H*(10) 的量測是將 WENDI-II 偵檢器置於射束中心點、距射束中心點共平面方向（±X 軸）與正交平面方向（-Y 軸）等三個方向平面的 50 cm 和 100 cm 處，及治療室迷宮走道入口與出口等位置進行評估，所有位置所測得的 H*(10) 是在 200 MU 的照射條件下進行評估。結果：當隨著遠離射束中心，中子周圍等效劑量會隨之降低，且中子的分佈並非對稱的，中子產生量在Ｘ光射束行經方向上是較高的。當由 FF 模式改成 FFF 模式時，射束中心點附近位置的H*(10) 將由 92.5 μSv 降低至 30.8 μSv，而在迷道的 H*(10) 將降低至 6.71 μSv。結論：TrueBeam 系統的 FFF 模式可以降低中子劑量的產生，而研究的數據結果也利於後續評估10 MV 醫用直線加速器的工作人員劑量及輻射防護計畫的參考準則。

關鍵字

中子周圍等效劑量 ； 無射束整平濾片 ； 寬能量中子偵檢器

並列摘要

Purpose : The Varian TrueBeam system is a modern radiotherapy unit. Undesired neutrons are produced by the interaction of high-energy (＞10 MV) photons with high-Z components such as the flattening filter (FF) and jaws. The new design of the Varian TrueBeam system enables the linac to be operated in the flattening-filter-free (FFF) mode, as well as in the FF mode. Therefore, the aim of this study was to evaluate the neutron ambient dose equivalent, H*(10), around the TrueBeam linac in the FF and FFF modes by using a wide energy neutron detection instrument (WENDI-II). Materials and Methods : Ten-megavolt X-ray beams were used in the FF and FFF modes, with the jaws and multileaf collimators fully closed. The dose delivery rates were set to 600 MU/min in the FF mode and 2400 MU/min in the FFF mode. The H*(10) was measured using the WENDI-II located at the isocenter, 50 and 100 cm away from the isocenter, and at the entrance and exit of the maze. The dose delivery was set to 200 MU. Result : The H*(10) was proportional to the monitor unit and declined with increasing distance to the isocenter. The distribution of scattered neutrons did not appear to be isotropic; the H*(10) was somewhat higher in the gun-target direction. The neutron ambient dose equivalent in the FFF mode was 30.8 μSv, lower than the 92.5 μSv detected in the FF mode. In the FFF mode, the neutron ambient dose equivalent in the maze decreased to 6.71 μSv. Conclusion : Operating the TrueBeam system in the FFF mode greatly decreases the level of undesired neutrons. It also corresponds to a reduction in activation within the treatment vault and subsequent exposure to patients and radiotherapists.

並列關鍵字

Neutron ambient dose equivalent ； Flattening filter-free ； Wide energy neutron detection instrument (WENDI-II)