Background: The present study aimed to investigate the equivalent dose in vital organs, including heart and lung, due to secondary particles produced during breast proton therapy. Materials and Methods: The numerical ORNL female-phantom was improved and simulated using the Monte Carlo MCNPX code. The depth-dose profile of proton beams with different energies was simulated. The proper energy range of incident proton beams has been estimated in order to have the Bragg peaks inside the breast tissue. The equivalent dose of secondary particles, including neutron and photon in vital organs, were evaluated. The TALYS code was used to investigate the neutron and photon particles’ production cross-sections. Results: The results showed that for the proton energy range of 60-70 MeV, the Bragg peaks positioned inside the breast. The maximum dose of 0.65 mSv/nA-p was in Heart-Left Ventricle due to neutrons production by incident 70 MeV protons. However, the maximum absorbed dose, due to the secondary particles, was less than 0.0004% of proton equivalent dose at the Bragg peak. The maximum photons dose and the protons dose into the Heart-Left Ventricle were 8.42 µSv/nA-p and 68.08 µSv/nA-p, respectively, which were negligible compared to the proton equivalent dose at the Bragg peak. Conclusion: The results confirmed a noticeable lower dose in the heart and lungs for breast proton therapy, compared with the previously reported dose for breast radiotherapy using photon. Most of the dose absorbed by the organs is due to the secondary neutrons, but those are low enough to be neglected.