This grant application examines the hypothesis that radiolabeled antibody heterogeneity is the principal source of failure of radioimmunotherapy (RIT). It proposes that this limitation can be overcome by the combination of RIT with external beam radiotherapy (XRT). This study will investigate the interaction between XRT with RIT, and determine the optimum combination of these two modalities for maximum tumor efficacy. The hypothesis will be tested with the murine SW1222 colorectal xenograft model system using intact A33 IgG monoclonal antibody as the targeting agent and its single chain hypervariable sFv fragment. There are four specific aims: (1) To determine the 3D microdistribution of radiolabeled A33 IgG within the SW1222 tumor xenograft system using phosphor plate autoradiography. The dependence of the distribution will be investigated with respect of the antibody/antigen molar ratio, the effect of molecular weight, by concomitant administration of the single chain hypervariable fragment sFv A33 with the parent A33 IgG, and following pre-external beam irradiation. (2) To determine how radiation tissue damage facilitates/impedes the penetration of antibody into the tumor. This will be achieved by conventional histological and immunohistochemical stains designed to ascertain the effects of radiation damage on tumor blood supply, tumor cell density, as well as antigen density and inflammatory response. These changes will be used to provide a scientific rationale for the measured radiolabeled antibody distribution from specific aim 1. In addition, this aim will attempt to quantitate changes in tumor cell response by determining the fraction of cells undergoing apoptotic cell death, (by the tunel assay), as well as changes in the fraction of cycling cells, (determined by IUdR incorporation and Ki67 and PCNA immunohistochemistry), as a surrogate of mitotic cell death. (3) To determine the therapeutic efficacy by tumor growth delay and cure after treatment with XRT and RIT alone and then in combination. Experiments will be performed in which tumors are treated with XRT followed by RIT, XRT and RIT simultaneously, and RIT prior to XRT, in order to determine the optimum combination therapy. Variations in the tumor response will be correlated with the results from aim 1 and 2. (4) To develop a radiobiological model, which determines the therapeutic effectiveness from a heterogeneous activity distribution. The model will be tested using information from the source distribution from autoradiographic data (aim 1) to predict overall tumor response measured by specific aim 3.