應用高解析測量式蒙地卡羅評估強度調控放射治療於 鼻咽癌之耳朵劑量

鼻咽癌(Nasopharyngeal carcinoma, NPC)的強度調控放射治療(Intensity modulated radiation therapy, IMRT)因為是涉及在鼻咽部和顱底部位給予高輻射劑量。而且頭頸部區域的特徵在於包括組織，骨骼和空氣界面的複雜解剖結構，因此本研究的第一個目的是研究組織異質性對與IMRT治療NPC病人劑量分佈的影響。此外，耳朵是位於高劑量區附近的一個非常重要的危急器官並容易因輻射誘導產生副作用。所以本研究的第二個目的是開發一種高解析度(High resolution, HR)的假體以評估IMRT在耳朵的劑量分佈。本研究改良測量式蒙特卡羅(Measurement-based Monte Carlo, MBMC)方法。MBMC研究方法的主要內容包括：(1) 利用BEAMnrc模擬Varian 21EX直線加速器治療機頭的X光束傳輸，(2)以DOSXYZnrc模擬患者劑量和(3)透過電子成像裝置(electronic portal imaging device, EPID)，用於測量IMRT治療時輻射經過不同權重加權後的不均勻通量分佈圖。本研究所使用之設備為童綜合醫院放射腫瘤科所有的Varian 21EX直線加速器，EPID型號為 aS1000。研究期間共收集十位執行IMRT治療治畫的NPC病患進行評估，通過Eclipse 計劃系統中的非均向解析演算法(Anisotropic Analytical Algorithm, AAA)和 MBMC方法計算得到的劑量分佈比較，評估因組織不均勻性對劑量分布的影響。為了進一步評估高劑量區周圍較小的危急器官對於劑量的反應，在十位NPC患者中挑選三位以及一位額外的顱底腫瘤病患，以MATLAB(R2010a版本)設計HR假體進行MBMC模擬。以電腦斷層(computed tomography, CT)在病人頭頸部和顱底區域分別取得3毫米和1毫米的影像。再採用MATLAB中的imresize功能將影像進行雙三次插值演算法縮放，將假體的像素大小在顱底區修正為0.05×0.05×0.1立方厘米，頭部其他部分則為0.05×0.05×0.3立方厘米。以 MBMC 模擬十位鼻咽癌患者IMRT 治療計畫的比較結果顯示MBMC 都略低於AAA，在PTV1 接受95％以上處方劑量的體積(VPTV95)，MBMC 和AAA計算結果分別為98.7％以及99.0％，在PTV1(D95％) MBMC 和AAA 計算結果分別為6832cGy 以及6895cGy。另一方面，在頭頸部治療應注意的危急器官，包括：視神經、水晶體、眼球、脊髓、顳下頜關節、腮腺和中耳等，MBMC 模擬結果和AAA 比較，上述的器官會得到更高的劑量分佈。MBMC 和 AAA 平均劑量之間的差異指出，治療時應再確認危急器官的劑量，以避免嚴重的併發症。對小體積危急器官(體積<1立方厘米)以HR-MBMC模擬三位鼻咽癌患者的結果顯示，在PTV1平均D95％的HR-MBMC和AAA計算結果分別為6763cGy以及6847.1cGy。第八對顱神經，半規管，耳蝸的平均劑量，與AAA比較 HRMBMC 增加了287.5 cGy。模擬顱底腫瘤病患的結果也顯示耳朵的劑量HRMBMC比AAA高。HR-MBMC預測平均劑量對右側第8對腦神經，右側耳蝸以及右半規管與AAA相比分別為751.5 cGy、732.2 cGy，532.5 cGy、468.2 cGy以及870.7 cGy、 817.0 cGy。這說明HR-MBMC在顱底高劑量梯度區域中，作為分析小體積危急器官劑量分布工具的潛力。 MBMC 劑量模擬方法可以作為用於IMRT 治療範圍內具有複雜的組織成分和小體積的危急器官劑量評估參考，因為它利用EPID 可以測量非常精細的通量分佈圖。而HR-MBMC 方法則非常適合於小體積的器官，如耳朵劑量分佈的詳細評估。

關鍵字

蒙地卡羅模擬；鼻咽癌；強度調控放射治療；測量式蒙地卡羅；耳朵；非均向解析演算法；電子成像裝置

並列摘要

Intensity modulated radiation therapy (IMRT) of nasopharyngeal cancer (NPC) involves delivering a high radiation dose to the nasopharynx, and parts of the skull base. As the head and neck region is characterized by complex anatomical structures that involve tissue, bone, and air interfaces, the first objective of this study was to study the effects of tissue heterogeneity on dose distribution in NPC patients treated with IMRT. Moreover, the ears are a very small critical organ located near the high dose area and are prone to radiation-induced toxicity. The second objective of this study, therefore, was to develop a high-resolution (HR) voxel phantom to evaluate the dose distribution of the auditory apparatus during IMRT. The measurement-based Monte Carlo (MBMC) method was adopted for the study. The major components of the MBMC technique involves (1) the BEAMnrc code for beam transport through the treatment head of a Varian 21 EX linear accelerator, (2) the DOSXYZnrc code for patient dose simulation and (3) an electronic portal imaging device (EPID) for measuring the efficiency map which describes non-uniform fluence distribution of the IMRT treatment beam. Beam parameters and the aS1000 EPID at Tungs’ MetroHarbor hospital were commissioned for this study. Ten NPC IMRT plans were evaluated for dose effect due to tissue heterogeneity by comparing dose distributions calculated with Eclipse plans and those obtained from the Measurement-based Monte Carlo (MBMC) method. The Eclipse plans were based on the anisotropic analytical algorithm (AAA). For further evaluation of dose effect to small critical structures, MBMC simulations using HR phantoms (HR-MBMC) were applied to the IMRT treatment plans of three of the ten NPC patients and an additional skull base tumor. In-house MATLAB (R2010a) program was developed to create the HR voxel phantoms. CT slices 3mm in thickness in the patient’s head and neck region and 1 mm in the skull base area were obtained. The CT images were rescaled using bicubic interpolation of the 'imresize' function in MATLAB. The voxel size of the HR phantoms was 0.05 x 0.05 x 0.1 cm3 in the skull base area and 0.05 x 0.05 x 0.3 cm3 for other parts of the phantoms. MBMC simulation on 10 NPC patients revealed that in PTV1 the mean value of the volume receiving at least 95% of the prescribed dose (VPTV95), was slightly lower for MBMC (98.7%) than that for AAA (99.0%). The dose to 95% of PTV1 (D95%) also showed lower mean dose for MBMC (6832 cGy) when compared with AAA (6895 cGy). On the other hand, MBMC simulation predicted a higher dose distribution to the optic nerves, lens, eyeball, spinal cord, temporomandibular joints, parotid glands, and middle ears than AAA. The difference in the mean doses between MBMC and AAA suggests that critical organ doses should be confirmed in order to avoid serious complication from overdose. HR-MBMC simulation of three NPC patients revealed that in PTV1 the mean D95% for HR-MBMC (6763.3 cGy) was less than that for AAA (6847.1 cGy). Small volume organs (volume < 1 cm3) such as the eighth cranial nerve, semicircular canal, and cochlea showed a mean dose increase of 287.5 cGy when compared with AAA. HR phantom simulation of the skull base tumor also showed higher dose to the ear structures than AAA. The mean doses predicted by the HR-MBMC for the right 8th cranial nerve, right cochlea, and right semicircular canal compared with AAA were 751.5 cGy vs. 732.2 cGy, 532.5 cGy vs.468.2 cGy, and 870.7 cGy vs 817.0 cGy, respectively. It suggests that HR-MBMC has the potential to assist in detailed dose analysis for small critical organs (<1 cm3) in high dose gradient skull base area. The MBMC dose simulation method can serve as a good dose evaluation reference for IMRT plans having complex tissue composition and small critical structures since it applies EPID measured efficiency maps with very fine spatial resolution. The HR-MBMC method is well suitable for detailed evaluation of dose distribution for small volume organs such as the ears.

並列關鍵字

Monte Carlo simulation ； Nasopharyngeal Cancer ； Intensity Modulated Radiation Therapy ； Measurement-based Monte Carlo ； Ear ； Anisotropic Analytical Algorithm ； electronic portal imaging device

參考文獻

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應用高解析測量式蒙地卡羅評估強度調控放射治療於鼻咽癌之耳朵劑量

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