EGSnrc-based Monte Carlo dosimetry of CSA1 and CSA2 137Cs brachytherapy source models.
OBJECTIVE
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, water-kerma 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) x 10(-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 > or = 2 mm from source capsules as opposed to > or = 1 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.
Selvam
T Palani
TP
Radiological Physics and Advisory Division, Health, Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai 400094, India. pselvam66@yahoo.co.in
Sahoo
S
S
Vishwakarma
R S
RS
D016880:Anisotropy; D001918:Brachytherapy / Q000295:instrumentation / Q000379:methods; D002588:Cesium Radioisotopes / Q000737:chemistry / Q000627:therapeutic use; D003198:Computer Simulation; D016208:Databases, Factual; D008962:Models, Theoretical; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D012984:Software; D014618:Vacuum; D014867:Water / Q000737:chemistry
D002588:Cesium Radioisotopes; D014867:Water