Introduction: In brachytherapy treatments, high-dose sources are used in interstitial placement. In this paper, the modeling of a common breast cancer treatment applicator called MammoSite is done by the Monte Carlo simulation code. Then, to study and calculate the amount of doses in the breast and the dose rate of the organs at risk of radiation, including: lungs, ribs and skin, Then the absorbed dose ratio is calculated in the target volume to the treated chest.
Materials and Methods: The MCNPX code: is the based on the Monte Carlo method. The code, with input file information and the use of the cross-section library, solves the problem and produces results in an output file.
MIRD phantom: To calculate the dose, a model of the human body is required as a phantom. All organs of the human body are characterized by a detailed description for the calculation of the absorbed dose in this phantom.
Mammosite applicator: This device has a catheter that can be attached to the high dose rate after loading machine that connect to the source. The end of catheter is surrounded by a balloon. The most common size of this balloon is 4-5 cm and 5-6 cm, which has a volume between 34-113 cm cubic meters.
Results: As expected, the absorbed dose in the target volume and the treated breast was the highest and after that the lung received the same significant dose. That is, the highest doses of the target or tumor volume and the lower doses of the surrounding organs are reached. Healthy organs have received around high dose ratios. When the right breast was treated, the right lungs, ribs, and skin had the highest dose ratios relative to the dose received by the tumor and when the left breast was treated, the left lung, ribs and skin had the highest dose ratios relative to the dose received by the tumor.
Conclusion: Given that in Iran, not only the mammosite applicator but also the treatment planning system for brachytherapy is not available. The inability of the MammoSite applicator in the asymmetric dose distribution cannot be shown. However, due to the absorption dose ratio of breast tissue to absorbed dose by the target volume in the right breast, which is numerically equal to 1.035, And the absorbed dose rate of the left breast tissue to the absorbed dose by the target volume, which is a number equal to 1.033, both of which are approximately equal, It can be concluded that the same dose that reached the tumor tissue is about the same size as the healthy tissue of the treated breast. This represents a symmetric dose and proves that the MammoSite applicator is incapable of asymmetric dispensing.