| Summary: | 博士 === 國立清華大學 === 原子科學系 === 90 === Convolution/superposition algorithm is the most accurate deterministic dose calculation algorithm at present time. However, its accuracy has to be re-examined due to the more complex treatment condition and more inhomogeneous media it will encounter in clinics. This thesis considers two limited conditions, the existence of hip prosthesis and that of air cavity in media, and examines the dose distribution difference between the results of Monte Carlo simulation and convolution/superposition algorithm. In the condition of existing hip prosthesis in media, we focus on discussing the effects that high Z, high density material caused in convolution/superposition algorithm. With existing air cavity in media, we are mainly interested in the accuracy of convolution/superposition algorithm in the condition of electron disequilibrium.
In the condition of existing high Z, high density material in media, significant dose differences between the results of Monte Carlo simulation and convolution/superposition algorithm were found around the interface. In re-buildup region and even deeper sites, convolution/superposition algorithm cannot accurately predict the dose as well. Two percent and 4-5% dose underestimation were found in deep part of PDDTi and PDDCo. For the dose profiles, underdosage of 1-2% was also found at off-axis distance 1.0-2.0 cm. Insufficient accounts for scatter and back-scatter radiation while photon beam passing through high Z, high density material in convolution/superposition algorithm lead to these results. Adding dose kernels in different media and improving density scaling method in treating dose in heterogeneous media in convolution/superposition algorithm to account for scatter and back-scatter radiation would be a possible solution.
Underestimating the expansion of dose kernel in air cavity in convolution/superposition algorithm was found at the condition of electron disequilibrium. This leads to the over-estimated dose in air cavity and in following re-buildup region. The region including top 0.5 cm in acrylic phantom was effected.
Adding the consideration of over expansion of dose kernel in air cavity should be done in convolution/superposition algorithm by improving density scaling method for accurate dose estimation.
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