Background:
In the clinical application of brachytherapy, the relevant quantities of brachytherapy seed strength must be converted into absorbed dose at a reference depth of 1 cm in water. The current method of obtaining the absorbed dose in water is based on the air kerma strength and dose rate constant, which has an uncertainty of more than 10% (k=2), potentially affecting cancer treatment outcomes.
Purpose:
To ensure accurate dosimetry for 125I brachytherapy seeds, an extrapolation chamber embedded in the water-equivalent material was designed and manufactured to measure the absorbed dose in water directly.
Methods:
The mathematical model for determining the absorbed dose in water is based on radiation transport theory, where the key term conversion factor is determined using the Monte Carlo (MC) methods. In this paper, the basic structure, the measurement method, and the MC simulation of the extrapolation chamber are described. The dose rate constant of the model 6711 125I brachytherapy seed was obtained using three methods (experimental measurement, MC simulation, and AAPM recommended values), and the results was compared and analyzed.
Results:
The absorbed dose in water of the model 6711 125I brachytherapy seed was determined, and after repeated measurements and uncertainty evaluation, the result was 12.39 mGy/h, with an uncertainty of 3.5% (k=2). In addition, the brachytherapy seed was calibrated using an absolute measurement device for the air kerma strength, and its dose rate constant was calculated, which was in good agreement with both the AAPM-recommended values and MC simulated values.
Conclusions:
We successfully developed an absolute measurement device for the absorbed dose in water, which reduced the measurement uncertainty for 125I brachytherapy seeds and achieved dose accuracy for external radiotherapy. This study contributes to the establishment of primary standards for the absorbed dose in water of 125I brachytherapy seeds.
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