Introduction The breast cancer incidence during pregnancy is low, but according to the FIGO 2018 report, there is an increase in the incidence of malignancies during a pregnancy. Due to the potential deterministic effects on the developing fetus, which occur above 100 mGy, there is no consensus for the use of radiotherapy in the treatment of breast cancer in pregnant women in the first two trimesters, which is the time of greatest radiosensitivity of the fetus. So far, the physical phantom of a pregnant woman does not yet exist. Objectives The goals of this research are to examine whether a low-budget, physical phantom of a pregnant woman can be used in fetal dose measurements during breast radiotherapy in order to experimentally measure fetal dose during breast radiotherapy on a physical phantom and, depending on the origin of fetal dose, to propose optimization of radiotherapy plans for breast cancer during pregnancy. Study Design A prospective study of the type of non-randomized controlled trial. Material and Methods From the diagnostic imaging data of the pregnant woman, CAD 3D models of 31 structures were created and merged into a BREP phantom, from which the voxelized phantom and DICOM data were developed. Further computer 3D modeling was used to create CAD 3D models of molds for casting substitution tissues, made from today's available materials with a detailed description of their preparation. MC simulations of radiotherapy of the left breast were performed on a voxelized phantom and, using the same radiotherapy plan, a physical phantom was experimentally irradiated and the level of fetal dose was measured using RPL dosimeters. Results The physical phantom of a pregnant woman which was named Tena comes as “ready to print” .STL files of molds, as a computer phantom and in DICOM format. The developed substitution tissues simulate human tissues according to their characteristics. According to the MC simulation, 90.5% of fetal dose comes as scattered radiation from the mother's body with average photon energies of 0.2 MeV, while 9.5% is "leakage" radiation from LINAC with average photon energies of 0.43 MeV. The measured fetal doses by MC simulation are 84.9 mGy, and experimentally 74.13 mGy. Conclusion According to the results of the MC simulations on materials of substitution tissues and considering the small difference between MC and experimental dosimetry results, the developed physical phantom can be used for further dosimetry purposes. Fetal dose from experimental measurements and MC simulations indicates that the fetus will receive doses below the threshold of 100 mGy recommended by the ICRP and that the main contribution to that dose is scattered radiation from the mother. Keywords: breast neoplasms; dosimetry; method, Monte Carlo; phantom; pregnancy; radiotherapy