醫用直線加速器以多葉式準直儀執行強度調控放射治療所造成中子污染的劑量之研究

Study of the Dose of Neutron Contamination to Patients Treated with Intensity Modulated Radiation Therapy

葉世安(Shyh-An Yeh) ； 蕭光吟(Kuan-Yin Hsiao) ； 吳嘉明(Jia-Ming Wu) ； 郭忠民(Chung-Ming Kuo) ； 陳清江(Ching-Jiang Chen) ； 趙敏(Max-Min Chao)

《台灣應用輻射與同位素雜誌》 6卷3期 (2010/09) Pp. 907-915

https://doi.org/10.29832/TJARI.201009.0002

摘要

醫學物理研究的領域中，包含放射治療、放射診斷、與核子醫學，過去十年來有諸多重要的發展，並且廣泛的應用於臨床診療中。以放射治療爲例，由早期的鈷六十治療機，到現在的高能電子直線加速器(linear accelerator, LINAC)，配合治療計畫以三度空間順形治療(3-D conformal radiation therapy)，與強度調控放射治療(intensity modulated radiation therapy, IMRT)等技術，可使輻射劑量準確的施於腫瘤位置，並大幅減低正常組織的副作用。此外，先進的醫學影像攫取系統，如電腦斷層掃描、核磁共振造影、單光子斷層掃描與正子斷層掃描等整合發展，裨同時於影像解析，影像融合，與功能性標定上，提供病患最大的利益。除資訊科技的發展帶來如上的利基外，加速器設計與製造技術的進步亦不遑多讓。高能電子直線加速器的最大能量可達20~35MeV，提供高通率的電子或光子進行治療，裨於短時間內完成複雜的治療計畫。不同於一般核子設施，醫用直線加速器設立於人員稠密的醫療院所中，並且每天治療眾多的病人。因此，醫用加速器設施的輻射防護作業，更應審慎評估，不只是要保障醫事人員的安全更需要知道病患除了接受電腦治療計劃所規劃的輻射劑量外，同時是否也接受中子污染的劑量造成額外輻射的傷害。醫用加速器中，對高能電子或光子引發的中子而言，足以阻擋光子的主屏蔽亦能有效阻擋中子，惟仍需考量中子在迷道中的散射與川流(streaming)情形。醫用電子直線加速器，對於中子污染評估的數據與方法，多年來未曾更新與改良。有鑑於此，本研究將針對科內18MV的高能光子加速器在執行強度調控放射治療時，以不同數量的射束、不同的入射角度及不同的監控單位(monitor unit)，進行詳盡的中子污染特性研究，再參考蒙地卡羅計算機程式的試算結果，與最近的中子產生截面器所產生的各種元素對中子的反應截面，用模擬試算光子因爲活化金屬物質及高能光子在機頭處所造成的中子污染對病患所造成的劑量加成和實測的結果做驗證，以便日後對活化產物及中子的污染劑量做一個全面的瞭解。

關鍵字

放射治療 ； 強度調控 ； 中子通量及劑量分佈圖

並列摘要

This study will investigate the neutron contaminations from medical accelerators, The evaluation methods and data for neutron dose to patients will be presented and compared with the Monte Carlo simulation results in order to set up a model using for evaluating patients unwanted neutron doses. For medical electron linear accelerators, photoneutrons produced by photonuclear interaction between X-ray＞8 MeV and gantry materials. In this study, photo-neutron production from a 18 MV LINAC gantry head and transport in accelerator facility will be simulated using the FLUKA Monte Carlo code, and then compared with the measurement by using He-3 proportional counter, TLDs and Cd-Au and neutron bubble detectors. The preliminary measurement study indicate that the neutron yields increase with the decreasing filed size collimated by the secondary collimators. Neutrons produced from upper gantry components will be attenuated to some extent by the lower collimators, the leakage neutron fluence under beam outlet decrease with the field size reducing from 10×10 cm^2. Neutron fluence per unit monitor unit does not increase significantly due to the using of multi-leaf collimator in 3-D conformal therapy and intensity modulated radiation therapy. The measured variation up to 20% of the neutron leakage as a function of gantry position at the maze exit was reproduced by the calculation. The neutron dose and/or counting rates for detectors along the maze path were compared between measurements and Monte Carlo calculations as well as Kersey empirical method. In this study, the characteristics of neutron sources and their attenuation in patients will be investigated in 18 MV photon striking on target materials of C, N, Al, Fe, Cu and W. The results of source term and the corresponding attenuation lengths provide for simply estimation of effective dose in patients at the direction of neutron emission. For photon energies below ~30 MeV the variation of the attenuation length is very small and could be treated as a constant value (29.57 g•cm^(-2)). This study also measured in several sets of treatment portals different gantry incident angle and different monitor units to see the influence of neutron contamination dose in patients. This study adopt Pinnacle treatment planning system and Elekta linear accelerator. PTW 300013 Farmer type chamber and Unidose electrometer was used as dose measurement devices. Solid water phantom was adopted to perform variable different thickness to simulate the treatment portable irradiate to the patient. When the treatment planning has been done in Plato planning system, the beams parameter will have to transfer to linear accelerator for dose deliver. In treatment console, beams were made copy in clinical mode then paste to service mode in order to do dose verification check. At this moment, all the patient irradiation beams were shift to irradiate from vertical direction, so, all the mechanical tolerance has to be set non-resistance, which meant the irradiation can be deliver without any angle resistance. When all the beams irradiated with variable depth according to the treatment planning, the reading was converted to actual absolutely dose and accumulate every beam's readings to total dose and then compare to the planning desired dose for analysis.

並列關鍵字

radiotherapy ； IMRT ； neutron contamination