| Summary: | <p><b>Purpose:</b> The purpose of this study was to evaluate the capability of u-SPECT-II/CT to monitor the progression of breast cancer lung metastasis using <sup>99m</sup>Tc-3P-RGD<sub>2</sub> as a radiotracer.</p><p><b>Methods: </b>The breast cancer lung metastasis model was established by tail-vein injection of 2 x 10<sup>5</sup> - 1.5 x 10<sup>6</sup> MDA-MB-231 cells into each athymic nude mouse. SPECT/CT studies were performed at a specified time after inoculation of MDA-MB-231 cells. Histological staining was used to further confirm the presence of lung metastases.</p><p><b>Results: </b>We found that both inoculation time and tumor cell load had significant influence on the extent of lung metastasis. For example, if animals were injected with 2 x 10<sup>5</sup> MDA-MB-231 cells, there were no detectable metastatic breast tumors in the lungs after 8 weeks. If animals were injected with 1 x 10<sup>6</sup> MDA-MB-231 cells, there were many tumors in both lungs at week 8. When 1.5 x 10<sup>6</sup> MDA-MB-231 cells were injected, the animal became very weak by week 7. We also found a rare example of breast cancer metastasis in the muscle and mediastinal lymph nodes. The tumor necrotic regions were clearly delineated by u-SPECT-II/CT.</p><p><b>Conclusion: </b>This study clearly demonstrated that <sup>99m</sup>Tc-3P-RGD<sub>2</sub> is an excellent radiotracer for noninvasive imaging of metastatic breast tumors in the lungs, mediastinal lymph nodes and muscles. <sup>99m</sup>Tc-3P-RGD<sub>2</sub> SPECT/CT is an outstanding platform for monitoring the progression of breast cancer lung metastases, semi-quantification of breast tumor load in the lungs and delineation of tumor necrosis in small animals.</p>
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