| Summary: | 碩士 === 國立清華大學 === 生醫工程與環境科學系 === 94 === 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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