The purpose of this research was to develop an experimental model of dosimetry using a breast phantom and evaluate the effects of the metallic port in tissue expanders on dose distribution in postmastectomy radiotherapy. Dose distribution was assessed using an experimental acrylic dosimetry simulator in the absence and presence of a metallic disc (MD), which is similar to that used in tissue expanders containing a magnetic port, by collecting coronal and sagittal radiochromic films. Dosimetry film analysis did not show any changes in dose distribution, except for a MD shadow in the sagittal film where the dose distribution rate was on average 14% lower than in other areas. This model very closely resembled actual breast reconstruction with a tissue expander containing a magnetic port. Scattering or attenuation of the irradiation dose due to interference of the MD was not evident in areas that could jeopardize the effectiveness of radiation therapy. Therefore, the presence of the MD inside the tissue expander does not likely have an impact on radiotherapy effectiveness during immediate breast reconstruction.