| Summary: | 博士 === 國立清華大學 === 核子工程與科學研究所 === 103 === This study included two parts: (1) Dose distributions of an 192Ir brachytherapy source in different media ; (2) Influence of metal of the applicator on the dose distribution during brachytherapy. The AAPM TG-43 report provides dose calculation formula and dose parameters for brachytherapy. Although it can be used to evaluate the radiation dose received by the soft tissue, the human organs such as nasopharynx, esophagus, bronchi, lungs and bones are of different densities. AAPM TG-43 does not provide the corresponding dose parameters, therefore, the dose in these tissues can’t be assessed accurately. This may result in tumor recurrence or severe side effects in normal tissues.
The MCNPX code is used to investigate the 192Ir dose distribution in water, bone and lung tissue. The glass dosimeter measurement was performed to verify the calculation results. It is found that dose rate constant, radial dose function and anisotropy function in water agreed well with previous literatures. The lung dose near the source, however, would be overestimated by up to 12% if water was used as the lung material. The result implies that if tumor is located in lung, the tumor dose will be overestimated if the difference in material density is not taken into consideration. The calculated results from this study could offer as a clinical reference for improving the accuracy of dose delivered for brachytherapy within the patient of lung cancer.
The 2nd part explores how the metal materials of the applicator influence the dose distribution when performing brachytherapy for cervical cancer. (1) 192Ir source located at a single position: For dose distribution in water with the presence of the tandem, differences among measurement, MCNPX calculation and treatment planning system results are < 5%. For dose distribution in water with the presence of the ovoid, the MCNPX result agrees with the measurement. But the doses calculated from treatment planning system were overestimated by up to a factor of 4. This is due to the shielding effect of the metal materials in the applicator not being considered in the treatment planning system. (2) Multiple 192Ir source dwell positions: When the applicator was used in treatment, the absolute dose difference between the TLD results and the MCNPX simulation results agreed within ~ 6 %. Compared with the MCNPX results, the TPS overestimated the ICRU rectum and bladder reference dose point by 58% and 50%, respectively. This result shows that the dose distribution calculated by TPS would be affected due to the use of applicator containing metal material, which suggests that the TPS result should be modified to take into account the shielding effect of the applicator to ensure the accuracy of the dose delivery.
|
|---|
