A novel simulation model for the dose distribution of moving targets for high-energy photons was analyzed using the EGSnrc Monte Carlo simulation. We provide here a fundamental numerical framework for the calculation of doses delivered to moving tissues in respiratory systems with improved accuracy. A spherical object with periodic motions inside a water phantom irradiated with incident photons was taken into consideration. The dose distributions of the target and its surrounding region were calculated for a variety of radiation conditions such as photon energy, beam numbers, and beam orientations as well as the target motions determined by realistic respiratory patterns. To determine the optimal dose, two parameters, the average absorbed dose ratio and dose deviation, were newly defined for the moving targets in the phantom. Optimal conditions were examined for treatment planning on tumors in motions based on the defined parameters. We found that the actual doses delivered to the tumor generally were not correlated to the respiratory patterns. Our quantitative assessment suggests useful guidelines for improved clinical radiotherapy to escalate dose concentration in the tumors by using multiple photon beams.