Because of the heinous of the breast cancer and its greatest damage in human society,this book covers the importance of CA 15-3 antigen to monitor and follow up Breast cancer patients during the treatment via external radiation therapy and chemotherapy. It is hoped that this book will help researchers be better equiped to handle researches and information regarding Breast cancer, radiotherapy, chemotherapy, and tumor marker. This book falls into five chapters: Chapter one: an introduction theoretical framework; the chapter presents the statements of the problem, objectives, hypothesis, rational and importance of the study. Chapter two: shows the definition of breast cancer, classification, radiotherapy, chemotherapy, and radioimmunoassay and tumor marker. Chapter three: deals with material and method of experiments. Chapter four: deals with data presentation and description of tables and figures, and discusses the result of CA 15-3 level on patient's blood samples. Chapter five: is for conclusion and recommendations. Maximize your understanding of tumor marker (CA 15-3) with this essential experimental book.