Calculation of absorbed dose rate in the lungs, ribs and skin by the mammosite applicator in breast cancer brachytherapy with MCNP

Introduction: Breast cancer is the most common cancer among women. Brachytherapy is one of the ways to treat breast cancer. In brachytherapy treatments, high-dose sources are used in interstitial placement. Here, Iridium-192 is used. This source emits gamma rays with energies ranging from 136 kV to 1060 kV electron volts. The activity of the used spring is 10 Ci. The simulated source here is a cylinder with a length of 0.5 and a radius of 0.017 cm.  In this paper, the modeling of a common breast cancer treatment applicator called MammoSite is done by the Monte Carlo simulation code. Then, calculate the amount of doses in the brachytherapy of the breast, target volume (tumor) and the dose rate of the organs at risk of radiation, including lungs, ribs and skin. Materials and Methods: MCNPX: 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 afterloading machine that connect to the source. The end of catheter is surrounded by a balloon that can be filled with normal saline or water. The balloon is placed in the lumpectomy cavity where the 34Gy can reach the treatment site. 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: The absorbed dose by the organs of the skin, ribs and target volume (tumor) is constant when the breast is treated right or left, dose rate of: target volume= 0/7, ribs=0/026, skin=0/009 Gy/min, but when the breast is treated right, the absorbed dose in: right lung=0/045, right breast=0/726, left lung=0/005, left breast=0/006 Gy/min and when the breast is treated left, the absorbed dose in: left lung=0/044, left breast=0/726, right lung=0/006, right breast=0/0068 Gy/min. Conclusion: As expected, absorbed dose in the target volume and the treated breast was the highest and after that the lung received the same significant dose. Indeed, the higher the organ absorbed dose to the target dose rate is better. When the right breast was treated, the right lungs, ribs, and skin had the highest absorbed dose to the dose received by the tumor And when the left breast was treated, the left lung, ribs and skin had the highest absorbed dose to the dose received by the target valume or tumor.