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RESEARCH PRODUCT

TH-C-AUD A-08: Evaluation of Electronic Equilibrium Conditions Near Brachytherapy Sources

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Purpose: For high‐energy photon‐emitting brachytherapysources such as 60 Co , 137 Cs , 192 Ir , and 169 Yb , the main contribution of the systematic uncertainty in the dose distributions near the sources is understanding of electronic equilibrium and the contribution of β‐rays due to radioactive disintegration. Thus, it is important to study these effects in detail to accurately depict dose distributions near these brachytherapysources. This work studies the relative importance of β‐ray contributions to total dose (β + γ + x‐ray), and feasibility of using the approximation “collision kerma equals dose in electronic equilibrium conditions.” Method and Materials:Characteristics of kerma and dose distributions were studied for spherical 60 Co , 137 Cs , and 192 Ir sources with composition, encapsulation, and dimensions similar to those existing in the literature. Dose contribution of β‐rays and γ+x‐rays were individually examined using the GEANT4 Monte Carlo radiation transport code. Results: The comparison of kerma and dose rate distributions indicate ∼ 20% electronic disequilibrium within 1 mm of sources, with 60 Co have the most pronounced effect. When examining the dose contribution of β‐rays, 60 Co again had the most pronounced effect out to 5 mm beyond the capsule, with β‐rays contributions for 192 Ir and 137 Cs 1.5 and 0.5 mm beyond the capsule, respectively. Conclusion: The dosimetriceffect of β‐rays for high‐energy photon‐emitting radionuclides and the influence of the electronic disequilibrium near of the sources were studied using Monte Carlo methods. For 60 Co and 137 Cs brachytherapysources, electronic disequilibrium has an important role near the source. For 192 Ir the main perturbing dosimetriceffect near the source is from β‐ray contributions and not electronic disequilibrium.