A review article compared the clinical evidence including the parameters of the dose distribution in organs at risk, such as the heart, blood vessels, and lungs, using irradiation techniques including whole breast irradiation and accelerated partial breast irradiation; as well as outcomes data focusing on toxicity. This review concludes that the use of a proton beam is an appropriate approach in the treatment of difficult-to-reach tumor locations. Studies showed a significant reduction in side effects related to cardiac risk events with the use of proton therapy.