Abstract: Background: Breast cancer (BC) is a leading cause of morbidity and mortality in women worldwide, some experience significant adverse effects, potentially due to factors such as age, lifestyle, oxidative stress, and genetic variations. Radiation-induced damage affects DNA, leading to cell death and bone marrow suppression, which impacts blood cell production. Patients and Methods: 75 breast cancer under radiotherapy (aged 35-75), comprising 60 BC women and 15 healthy controls, recruited from Wasit City and Babylon City between October 2023 and January 2024. Blood samples were collected for haematological study, at various stages of radiotherapy: before the first dose (BFD), after the first dose (AFD), after half the dose (AHD), and after the last dose (ALD). Results: Significant reductions in white blood cell (WBC) and monocyte counts were observed, especially in the ALD group, compared to healthy controls. Lymphocyte levels also decreased, with a significant difference at AFD and ALD stages. Haemoglobin (HGB), showed reductions over the course of treatment, with the most significant changes occurring in the ALD group. The study confirms that radiation affects blood cell counts, particularly WBCs, Monocyte, Lymphocyte, HGB, impacting immune function and treatment response. Conclusion: Radiation therapy in BC patients leads to significant changes in blood cell counts, which may contribute to immune suppression and delayed healing. Monitoring blood cell levels is crucial to manage these effects and adjust treatment as needed to improve patient outcomes.