Purpose: AAPM TG-56 recommends the use of a specific dosimetric dataset for each brachytherapy source model. In this study, a full dosimetric dataset for indigenously developed ¹³⁷Cs source models, namely, the CSA1 and CSA2, in accordance with the AAPM TG-43U1 formalism is presented. The study includes calculation of dose-to-kerma ratio D/K in water around these sources including stainless steel encapsulated ¹³⁷Cs sources such as RTR, 3M, and selectron/LDR ¹³⁷Cs.
Methods: The Monte Carlo–based EGSnrcMP code system is employed for modeling the sources in vacuum and in water. Calculations of air-kerma strength, S_K for the investigated sources and collision kerma in water along the transverse axis of the RTR source are based on the FLURZnrc code. Simulations of water- kerma and dose in water for the CSA1, CSA2, RTR, 3M, and selectron/LDR ¹³⁷Cs sources are carried out using the DOSRZnrc code. In DOSRZnrc calculations, waterkerma and dose are scored in a cylindrical water phantom having dimensions of 80 cm diameter x 80 cm height.
Results: The calculated dose-rate constants for the CSA1 and CSA2 sources are 0.945
1 and 1.023(1) cGy/(h U), respectively. The calculated value of S_K per unit source activity, S_K/A for the CSA1 and CSA2 sources is 7.393(7) x10⁻⁸ cGy cm² /(h Bq). The EGSnrcMP-based collision kerma rates for the RTR source along the transverse axis (0.25–10 cm) agree with the corresponding GEANT4- based published values within 0.5%. Anisotropy profiles of the CSA1 and CSA2 sources are significantly different from those of other sources. For the selectron/ LDR single pellet ¹³⁷Cs spherical source (modeled as a cylindrical pellet with dimensions similar to the seed selectron), the values of D/_K at 1 and 1.25 mm from the capsule are 1.023(1) and 1.029(1), respectively. The value of D/K at 1 mm from the CSA1, CSA2, RTR, and 3M ¹³⁷Cs source capsules (all sources have an external radius of 1.5 mm) is 1.017(1) and this ratio is applicable to axial positions z = 0 to z = ± L/2. This is in contrast to a published GEANT4-based Monte Carlo dosimetric study on RTR and 3M ¹³⁷Cs sources wherein the authors have assumed that collision kerma is approximately equal to absorbed dose at 1 mm from the source capsules. Collision kerma is approximately equal to absorbed dose for distances ³2 mm from source capsules as opposed to ³1 mm reported in
published studies. A detailed electron transport is necessary up to 2 mm from source capsules.
Conclusions: The Monte Carlo–calculated dose-rate data for the CSA1 and CSA2 sources can be used as input data for treatment planning or to verify the calculations by radiotherapy treatment planning system.