Mathematical formulation of 125 I seed dosimetry parameters and heterogeneity correction in lung permanent implant brachytherapy.

OBJECTIVES: Precise determination of dose distribution around low-energy brachytherapy sources as well as considering tissue heterogeneity is crucial for optimized treatment planning. This study is aimed at determination and mathematically formulation of American Association of Physicists in Medicine Task Group No. 43 (AAPM TG-43) dosimetry parameters of 125I seed (model 6711) and calculation of dose difference caused by neglecting lung heterogeneity in permanent implant brachytherapy.

MATERIALS AND METHODS: Using MCNPX 2.6.0 code, 125I seed (model 6711) was simulated in a cubic water environment, and its dosimetry parameters mentioned in AAPM TG-43 protocol were obtained. After benchmarking of parameters and comparison with prior studies, mathematical equations were fitted to the data, and a specific set of 125I seeds was simulated on a plane in simulated lung and water environments. Appropriate photon histories were considered to achieve data with maximum accuracy (max error 1%). In the end, isodose curves, profiles, depth dose, and dose difference between lung and water environments were obtained.

RESULTS: For 125I seed (model 6711), radial dose function and anisotropy functions were obtained precisely with R2 > 0.99, all in good agreement with previous studies and protocol. In addition, percentage dose difference between inhomogeneous lung and homogenous water environments in a 5 cm distance was calculated and presented as D (r) function with R2 > 0.99.

CONCLUSIONS: Considering practical difficulties in dose calculations, 125I seed dosimetry parameters and lung heterogeneity corrections can be obtained precisely by MCNPX. Equations presented in this study are recommended to be considered in future studies based on lung permanent implantation.