Effect of inhomogeneities and source position on dose distribution of nucletron high dose rate Ir-192 brachytherapy source by Monte Carlo simulation

Background: The presence of least dense dry air and highly dense cortical bone in the path of radiation and the position of source, near or far from the surface of patient, affects the exact dose delivery like in breast brachytherapy.
Aim: This study aims to find out the dose difference in the presence of inhomogenieties like cortical bone and dry air as well as to find out difference of dose due to position of source in water phantom of high dose rate (HDR) ¹⁹² Ir nucletron microselectron v2 (mHDRv2) brachytherapy source using Monte Carlo (MC) simulation EGSnrc code, so that the results could be used in Treatment Planning System (TPS) for more precise brachytherapy treatment.
Settings and Design: The settings and design are done using different software of the computer.
Methods and Materials: For this study, the said source, water phantom of volume 30 x 30 x 30 cm³ , inhomogeneities each of volume 1 x 2 x 2 cm³ with their position, water of water phantom and position of source are modeled using three-dimensional MC EGSnrc code.
Statistical Analysis Used: Mean and probability are used for results and discussion.
Results : The % relative dose difference is calculated here as 5.5 to 6.5% higher and 4.5 to 5% lower in the presence of air and cortical bone respectively at transverse axis of the source, which may be due to difference of linear attenuation coefficients of the inhomogeneities. However, when the source was positioned at 1 cm distance from the surface of water phantom, the near points between 1 to 2 cm and 3 to 8 cm. from the source, at its transverse axis, were 2 to 3.5% and 4 to 16% underdose to the dose when the source was positioned at mid-point of water phantom. This may be due to lack of back scatter material when the source was positioned very near to the surface of said water phantom and overlap of the additional cause of missing scatter component with the primary dose for near points from the source. These results were found in good agreement with literature data.
Conclusion: The results can be used in TPS.

Keywords
Brachytherapy, inhomogeneity, Monte Carlo, radiation-tissue interaction