| Targeting cytotoxic agents to tumor angiogenesis, instead of the tumor itself, is an attractive new approach in Radiation Oncology. Unlike tumor cells, endothelial cells are less likely to develop radio-resistance. Investigations have shown that radiation can cause a definite increase in cell permeability. Permeability changes in the tumor capillary endothelium can contribute to circulatory failure and serve as a site for clot formation. Therefore, radiation could initiate platelet aggregation and blood coagulation locally within the tumor vasculature, leading to tumor cell killing through depletion of oxygen and nutrients.; In order to analyze the efficacy of a potential 90Y-labeled compound for vascular targeting radiotherapy and to evaluate the factors that may affect targets' absorbed dose, a tumor vasculature model including its angiogenesis process as a function of time and tumor growth are adopted and improved from the Liotta model. Its output is used to estimate targets' absorbed doses by Monte Carlo simulation. The results show that the effectiveness of vascular targeting therapy depends on the existence of available tumor endothelial cells and target expression. The αvβ3 antagonist model compound is less effective in the early stage tumors, which have very few vessels. Although a high administered dose, such as injecting of 2.1 mCi/kg to saturate all available binding sites, can destroy tumor endothelium network, the toxicity to bone marrow makes it impossible to inject such a dose. To a vascularized tumor, after giving one maximum allowable administered activity, 0.83 mCi/kg for the 90Y-labeled model compound, an average of 9.8%, 27.3%, 34.7% and 37.8% of endothelial cells would be killed when treatment starts at day 4, 7, 9 and 12 after tumor development, respectively. Therefore, recurrent treatment by vascular targeting therapy to well-vascularized tumor has the potential to slow down tumor growth or may even cause tumor regression at the primary site.; Using an alternative radionuclide, 177Lu (0.498 MeV) to replace high-energy 90Y (2.282 MeV) in the model compound to treat small size tumors is also discussed. The low absorbed dose to tumor endothelium from 177Lu suggests that it is not an ideal choice for vascular targeting radiotherapy. |
