Dose Calculation Due to a 125I Source Model 6711 and Determination of Its Dosimetry Parameters in Water and Soft Tissue Phantom

In the present research, by the use of Monte Carlo method the TG-43 dosimetry parameters of 125I model 6711 have been determined and its relative dose has also been calculated. 125I model 6711 brachytherapy source which is manufactured by Amersham Company and has been approved by the American Association of physicists in Medicine (AAPM) was used in this simulation. The Source was considered in the center of 30 cm×30 cm × 30 cm water and soft tissue phantom and the relative dose variation along the parallel and perpendicular direction to the source axis were calculated using MCNP 4C code. Water having a density of 1 gram per cubic centimeter composed of two hydrogen atoms and one oxygen atom and soft tissue with density of 1.04 gram per cubic centimeter with accurate composition was used in this simulation. In this research, the percentage depth dose (PDD) variation along the different axis parallel and perpendicular to the source has been calculated using F6: p tally with less than 5% relative error. These data were then used to derive the isodose curves for PDD= 125%, 100%, 75%, 50% and 25%. Also, anisotropy function F(r,θ) and radial dose function g(r) have been calculated for the source.Discussion and The percentage dose variation and dosimetry parameters have been calculated in this research are in good agreement with the results of others. This finding can be used to improve brachytherapy result with this source. Since the absorbed dose near a source is high and its gradient is significant, only Monte Carlo simulation can be used for an accurate calculation. The results demonstrate a useful approach using MCNP code in dose calculation that can be applied in other fields of medical physics.