**10.2 Antigen**

Tumors produce a chaotic vascular system in which blood flow is slow and can be interrupted or even reversed. The ratio of tissue cells to vascular support is lower than most normal tissues. These effects create areas of tumor hypoxia which are relatively resistant to radiation therapy and therefore reduce the efficacy of RIT (Sergides et al., 1999). Some antigens of breast cancer are epithelial surface antigens lying on the inner surfaces of cells, thus being exposed to the circulation only by neoplastic architectural disruption. Other factors influencing the suitability of antigens for tumor targeting are internalization and shedding into the bloodstream. Unfortunately, the majority of identified antigens in human tumors represent tumor-associated antigens, not only present on the tumor tissues, but also detectable on normal tissues (Sergides et al., 1999).
