The difference in organ at risks (OAR) dose during breast cancer radiotherapy after radical vs. partial breast cancer surgery Objectives: The objective of this study was to investigate whether there are differences in the radiation dose received by organs at risk following partial and radical breast surgery. Study design: Cross-sectional study Participants and methods: The study included patients who underwent radiotherapy following partial or radical breast surgery, treated at the Oncology and Radiotherapy Clinic of the Clinical Hospital Center in Osijek. Data from the radiotherapy archive of the same clinic were obtained, including age and gender, type of surgery, and tumor localization. Additionally, data were obtained on the mean heart dose, the volume of the lung receiving 20 Gy, the maximum dose to the spinal cord, and the coverage of the radiation volume. Subsequently, the radiation dose to organs at risk was compared based on tumor localization and type of surgical procedure. Results: The study was conducted od 435 patients, the majority of whom were women (97 %). The result showed a significantly higher mean dose to the heart, the volume of the ipsilateral lung receiving 20 Gy, and the maximum dose to the spinal cord during radiotherapy of the left side compared to the right side, and after radical surgery compared to partial surgery. Additionally, the percentage od CTV receiving 98 % of the dose, and the percentage of PTV receiving 95 % of the dose, were higher during radiotherapy after breast-conserving surgery compared to radical surgery. Conclusion: There is a correlation between left-sided breast cancer and a higher risk of side effects during radiotherapy and irradiation of surrounding organs at risk.