**6. Target antigens**

In recent years, there has been a significant improvement in the understanding of molecular events and critical pathways involved in breast cancer. The studies have confirmed the feasibility of using radiolabeled antibodies for imaging and therapy of primary and metastatic breast cancer and that a diverse array of molecules can serve as targets for localizing antibodies (Carlos et al., 2006). Theoretically, an ideal target for radionuclide detection and therapy of metastatic breast cancer would be tumor-specific, generously expressed on all the breast cancer cells, not expressed by normal tissues and not released into the blood circulation (Mohammadnejad et al., 2010). For antibody screening of breast cancer, it is essential that the antibody is well characterized with little cross-reactivity to other antigens. The antibodies show cross-reactivity with, e.g. leucocytes, potentially yielding false-positive images. The majority of breast cancer targeting antibody studies have used antibodies against MUC1, CEA, TAG-72 and L6 antigens (Carlos et al., 2006).
