OBJECTIVE: The purpose of this study was to evaluate the radiobiological implications of clinical use of respiratory-gated techniques for postoperative radiation therapy of early-stage left-sided breast cancer after breast-conserving surgery. MATERIAL AND METHODS: Radiation therapy treatment plans of 80 patients with early-stage breast cancer (stage I-II), receiving whole breast irradiation after breast-conserving therapy, were analyzed. The control group consisting of 47 patients received standard radiation therapy, and the respiratory-gated group consisting of 33 patients received deep inspiration-gated radiation therapy. Normal tissue complication probabilities (NTCP) for cardiac mortality and for clinical radiation-induced pneumonitis were calculated for all patients included in present study, using relative seriality model. NTCP data were analyzed for 113 radiation therapy plans, which included free breathing plans for the respiratory-gated groups. RESULTS: Pneumonitis probability was 0.6% (range 0.0-2.8%) and 0.3% (0.0-1.2%) for control and respiratory-gated group, respectively. Cardiac mortality was 1.3% (0.0-5.0%) and 0.2% (0.0-2.8%) for control and respiratory-gated group, respectively. Using respiratory-gated radiation therapy, NTCP was reduced in comparison with the control group by 83% (P<0.00001) and by 55% (P=0.01270) for cardiac mortality and for clinical radiation-induced pneumonitis, respectively. CONCLUSIONS: Use of respiratory-gated radiation therapy, for postoperative treatment of early-stage breast cancer, significantly reduces excessive cardiac mortality probability and pulmonary complication probability, as compared to standard radiation therapy techniques. This is especially important from heart complication probability point of view, as cardiac mortality remains one of the important issues of postoperative breast irradiation in patients with early stage breast cancer.