Abstract: To explore the effects of changes in the physical properties of lung caused by breathing exercise on the absorbed dose of lung and tumor, Monte Carlo method for simulation calculation using EGSnrc was used to convert the collected lung data of different densities into corresponding phantom materials, to establish tissue models under different breathing states, and to analyze the differences between PDD (percentage depth dose) and Profile under different radiation fields. Results showed that, the smaller the radiation field size, the more obvious the difference in tissue absorbed dose under different breathing states. There is no significant difference in lung absorbed dose under the 10 cm×10 cm radiation field, and the tumor absorbed dose is reduced by 3.86%. When the radiation field is smaller than 5 cm×5 cm, the absorbed dose of upstream lung increases first and then decreases with the increase of depth. When radiation field is 1 cm×1 cm, the maximum difference of lung absorbed dose is 46.87%, the tumor absorbed dose is reduced by 6.97%. At the same time, there is an obvious electron imbalance in low-density tissue under small radiation field. The difference in tissue dose caused by breathing exercise under 3D-CRT technology is smaller than IMRT and SBRT. Under breathing gating technology, clinicians can combine the TPS algorithm to delineate the target area at the end of inspiration, which can reduce the normal lung tissue dose and increase the target area dose.

Key words: breathe, lung, density, dose, Monte Carlo simulation