為了提升放射治療科學的進步，本研究應用蒙地卡羅方法來模擬小照野劑量分佈，以發展出一套針對小動物的高解析度光子照射系統，使日後在研究新的照射技術或輻射生物效應時，可以對小動物作微小體積照射，最終並能設計及評估1 mm圓錐型準直儀的實用性。研究中使用2006版BEAM code來模擬Varian Clinac 2100C/D直線加速器，加上BrainLab公司的30、14、6、4及2 mm圓錐型準直儀所產生的小照野劑量分佈，並且利用四種偵檢器(XV-film、small volume ion chamber、PTW PinPoint chamber和PTW Diamond detector)驗證模擬的準確性。研究結果顯示，XV-film與PTW Diamond是較適合測量小照野劑量分佈的工具，模擬百分深度劑量分佈與實驗結果在深度1.5及20 cm之間的劑量差異約1%左右，80%照野寬度內之模擬剖面劑量分佈也與測量結果只有1.5%的劑量差異，兩側20%及80%相同劑量處的距離差異在0.45 mm以內，所以最佳化的模擬參數所得蒙地卡羅模擬結果與實驗結果是吻合的。此外，不論是模擬或實驗，都發現在深度2 cm後之劑量是隨深度呈指數衰減。最後，由模擬所預估的1 mm圓錐型準直儀之照野輸出因子在水下8 cm深為0.185，在1.5 cm深為0.230，亦表示使用此圓錐型準直儀來照射動物的可行性。

PURPOSES:
This study tries to develop a Monte Carlo (MC) based dose simulation system for small conical collimators (ex. 2 mm) where results of clinical measurements are far from certain due to finite detector sizes and lack of lateral electronic equilibrium. This system also has the potential to be modified for small animal irradiation studies.
MATERIALS AND METHODS:
BEAM06 Monte Carlo codes were adopted for this study. Accuracy of our simulation was verified with a small volume ion chamber, a PTW PinPoint chamber, a PTW Diamond detector (in vertical and horizontal directions) and XV-films for cone fields (BrainLab 30, 14, 6, 4 and 2 mm on Varian Clinac 2100C/D). A virtual 1 mm cone which may play a role in irradiation for small animals was also created for MC simulation. Simulation results of PDDs, profiles and output factors at 1.5 and 8 cm depths were compared to measurements. All the simulations/measurements were set for SAD setting (SAD=100 cm). Disagreements between simulation and measurement were evaluated through dose differences of PDD between 1.5 and 20 cm depths, dose differences within 80% field width and distance to agreement (DTA) at lateral positions of 20% and 80% doses of the penumbra region.
RESULTS:
Dose differences for the PDDs and profiles within 80% field width obtained from Monte Carlo simulation show the best agreement to measurements with films and a PTW Diamond detector positioned horizontally (< 1.5%). Disagreement of DTA in all cases was less than 0.45 mm. MC calculation predicts a output factor of 0.185 for 1 mm cone at 8 cm depth and 0.230 at 1.5 cm depth.
CONCLUSIONS:
This work has demonstrated that a small animal irradiation system can be reliably modeled with BEAM/EGS4 Monte Carlo codes. Based on MC simulation and measurement results, films and the PTW Diamond detector positioned horizontally are more suitable for small field dosimetry.

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