The results of this investigation support the validity of both the MC calculation method and GRD measurement in this study as well as the TG-43U1 formalism. Also, this calculation is applicable to interseed attenuation and the geometry lacking the equilibrium radiation scatter.
The aim of this study is to demonstrate the feasibility of estimating the strength of the moving radiation source during patient implantation. The requirement for the counting time was investigated by comparing the results of the measurements for the static source with those for the source moving at 2, 5, 10 and 20 cm s−1. The utilized source was 125I with an air-kerma strength of 0.432 U (μGym2h–1). The detector utilized was a plastic scintillation detector (8 cm × 5 cm × 2 cm in thickness) set at 8 cm away from the needle to guide the source. Experiments were conducted in order to determine the most desirable counting time. Analysis using the maximum of the measured values while the source passed through the needle indicated that the results for the moving source increased more than those for the static source as the counting time decreased. The combined standard uncertainty, with the coverage factor of 1, was within 4% at the counting time of 100 ms. This investigation supported the feasibility of the method proposed for estimating the source strength during the implantation procedure, regardless of the source speed. The method proposed is a potential option for reducing the risk of accidental replacements of sources with those of incorrect strengths.
The post-implantation dosimetry for brachytherapy using Monte Carlo calculation by EGS5 code combined with the source strength regression was investigated with respect to its validity. In this method, the source strength for the EGS5 calculation was adjusted with the regression, so that the calculation would reproduce the dose monitored with the glass rod dosimeters (GRDs) on a water phantom. The experiments were performed, simulating the case where one of two 125I sources of Oncoseed 6711 was lacking strength by 4–48%. As a result, the calculation without regression was in agreement with the GRD measurement within 26–62%. In this case, the shortage in strength of a source was neglected. By the regression, in order to reflect the strength shortage, the agreement was improved up to 17–24%. This agreement was also comparable with accuracy of the dose calculation for single source geometry reported previously. These results suggest the validity of the dosimetry method proposed in this study.
Purpose: Final goal is to develop a technique to detect the 125I seed and measure its strength during implantation process. This will be a backup tool for QA by offering an additional measurement apart from the pre‐operation measurement. In actual implantation, the speed of the seed varies because it is conducted manually. In order to develop a simple and stable system usable in any speed, this study supposes to use single detector and repeat short‐time measurements. This report summarizes a feasibility test. Methods: The 125I seeds (Oncra Inc. Oncoseed 6711) was moved in speeds up to 200 mm/s by an electric actuator. Its strength was measured with a NaI detector (1″diam. × 1″) set at 170 mm from the needle guiding the seed. The requirement for the counting time was investigated to estimate the strength regardless the seed speed. Results: In the measurement where the seed was set static in needle, the change in counting rate of the NaI detector was 4 % at the 30 mm‐long region along the needle, while the counting rate rapidly decreased outside the region. It is desirable to start‐stop the measurement while the seed is within this region. Assuming 200 mm/s as the maximum speed, 7 to 8 measurements can be performed for the counting time of 20 ms, 3 measurements at 50 ms, 1 measurement at 100 ms. By using the average of the measured results while the seed was within the region, the estimated seed strength agreed within 10 % between the speeds 0 and 200 mm/s at counting time of 100 ms or less. Conclusion: This report shows the feasibility in principle to estimate the 125I seed strength during the implantation operation, regardless the seed velocity.
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