- 學位論文
利用蒙地卡羅方法建立靜態/旋轉強度調控放射治療劑量模擬系統
Monte Carlo simulation for static and rotational intensity modulated radiation therapy
摘要
放射治療目的為給予腫瘤高度劑量，同時抑低正常組織之劑量，以治癒癌病。靜態強度調控放射治療與旋轉強度調控放射治療皆能給予高度順形之劑量分布，此類治療技術之劑量給予方式複雜，更需要有效且完整的病患治療驗證系統確保治療之正確性，本研究將利用蒙地卡羅方法建立靜態以及旋轉治療之劑量模擬系統並應用於治療劑量驗證。 對於靜態強度調控放射治療劑量模擬(MBMC系統)，本研究利用電子式影像擷取裝置(Electronic portal imaging device, EPID)測量得到之通量分布在蒙地卡羅模擬系統中修改相空間射源粒子(phase space particle)之權重以重建強度調控放射治療之通量分布，並傳播至病患/假體中，得到病患體內劑量以及穿透劑量，用以進行治療前劑量驗證及線上劑量驗證，如此一來，多葉式準直儀造成之劑量效應(例如：rounded leaf end leakage, tongue and groove effect)便可被包含在模擬中；而對於旋轉強度調控放射治療，本研究則利用DICOM-RT檔案所攜之輻射劑量給予資訊去計算強度調控通量分布，並利用數值方式修正多葉式準直儀之劑量效應。 假體實驗結果顯示MBMC系統與膠片測量之假體劑量與穿透劑量皆十分吻合，且多葉式準直儀之劑量效應忠實地被呈現在模擬之劑量分布中，而電腦治療計畫系統計算結果與膠片測量及MBMC模擬略有差異，顯示該電腦治療計畫系統對於多葉式準直儀之劑量效應並未完善地考量；而在MC-Arc系統驗證研究中所使用之電腦治療計畫系統已將多葉式準直儀之劑量效應考量在劑量計算當中，假體研究顯示，本系統所採用之多葉式準直儀之劑量效應修正方式可有效修正該效應，MC-Arc系統所計算之劑量分布與電腦治療計畫系統有良好的吻合度；以上假體實驗結果皆證實本研究建立之劑量模擬系統之正確性。為提升臨床應用之可行性，MC-Arc系統與高效能之MCSIM蒙地卡羅模擬程式進一步結合，運用在旋轉強度調控放射治療之病患劑量驗證，研究中挑選了十位病患，針對六種不同患部進行蒙地卡羅模擬與電腦治療計畫計算之病患劑量比較，結果顯示，對於組織不均質之患部，以及複雜的治療計畫 (例如:極小的腫瘤，高度不規則照野)，利用蒙地卡羅模擬病患劑量是必要的。 鑑於合理的運算時間以及準確性，本研究所建立之系統可做為病患驗證系統、治療計畫劑量之獨立驗證工具以及做為臨床研究之平台。
並列摘要
The goal of radiation therapy is to deliver a dose as high as possible to the target volume, while limiting radiation damage to the surrounding normal tissues. Advanced radiation therapy techniques, such as static and rotational intensity modulated radiotherapy (IMRT) are proposed to improve the dose conformity of the tumor while sparing the dose to normal tissue. Along with the rising interest in the highly conformal treatments comes the need for appropriate and efficient quality assurance (QA) solutions to ensure the accuracy of plan dose calculation and treatment delivery. This study developed MC-based dose simulation systems for static and rotational IMRT dose verification. For the static IMRT, the measurement based fluence reconstruction approach was proposed. The measurement based Monte Carlo system (MBMC) performs, within one systematic calculation, both pretreatment and on-line transit dose verifications for static intensity-modulated radiotherapy dose verification. An EPID-measured efficiency map was used to reconstruct the IM fluence in MC simulation. For the rotational IMRT, the calculation based fluence reconstruction approach was adopted. The MC-Arc system reconstructs the fluence distribution from the information provided by the DICOM-RT file. The dosimetric features of the MLC, rounded leaf end transmission, intra-leaf transmission, and tongue and groove effect, were included in this system. The reliability and clinical applicability of the built systems were validated via phantom study and patient study. The results showed that the MBMC system was able to preserve multileaf collimator delivery effects such as the tongue-and-groove effect and interleaf leakage. The perfect agreement between measurements and MC simulations supported the reliability of the MBMC system. For the MC-Arc system, the DVH comparison of the phantom study indicated that with all three MLC corrections, the mean doses of the PTV by the MC-Arc and TPS agreed to within 0.4%. The system was combined with an efficient MC code (MCSIM) to dosimetry validation for rotational IMRT treatment plans. The results indicated that MC simulation was necessary especially for the heterogeneous region and complex treatment. With the reasonable CPU time and superior accuracy, we conclude that this system can be clinical used for dose verification, serve as an independent plan check tool and research platform.