This prospective study investigated radiation dose and volume changes during breathing cycle. Ten patients with left breast carcinoma receiving radiotherapy were included. Treatment planning images were obtained as three different sets of series taken: without breath control (F), deep inspiration (I), and end of expiration (E), with 3-mm intervals. As such, whole breath cycle was simulated. CT images taken during I and E were registered to F, according to DICOM coordinates. Each patient's target and organ at risk volumes were contoured by the primary radiation oncologist except heart components which were contoured by radiologist on F, I and E series. Radiotherapy planning was done on F series, then planning and beam data were transferred from F to I and E image series. Target and organs at risk (OAR) dose distributions for E and I image series were obtained. Dose changes between F, E, and I phases for whole heart and components, namely, left ventricle (LV), right ventricle (RV), left auricle (LA), right auricle (RA), and left anterior descendent artery (LAD) were examined. Furthermore, the issue of any compartment representing the maximum heart dose was investigated. Volume and dose variations for heart, LV, RV, LA, RA, and LAD were observed during breath cycle. Exposured dose was more than defined tolerance level for LV, RV, and LAD in some patients. However, dose differences between F-I and F-E were not statistically significant. Radiotherapy planning without breath control is not capable of compensating for whole intra-fraction heart and its components' volumes and dose changes.