CTLA4 is a receptor expressed on the surface of activated T cells and regulatory T cells. CTLA-blockade has been tested in clinical trials using humanized monoclonal antibodies, and some biological responses have been reported. The anti-tumor immune response may be further augmented by the combination of CTLA4-blockade with radiation, with the potential to mediate regression of metastases outside of the field of radiation. The primary goal of this study therefore is to establish the safety of CTLA4-blockade using the antibody CP-675, 206 in combination with radiation. This trial is being conducted as a prelude to a planned phase II trial of CP-675,206 in combination with radiation in metastatic breast cancer.