Results: Median follow-up was 13.3 years. The actuarial 15-year overall survival and local control rates were 96% and 55%, re- spectively. Sixty-nine percent of relapses were observed within 5 years of treatment. Local control in patients \18 years old at the time of RT was 20% compared to 63% in those 18 to 30-years-old (p = 0.08). Local control rates for $55 Gy and \55 Gy were 79% and 30%, respectively (p = 0.02). No other factors were statistically associated with local control on univariate analysis. Twelve of 30 patients had NCI Common Toxicity Criteria (CTCAE v3) Grade 3–4 treatment complications. These included pathologic frac- tures, impaired range of motion, pain, and 2 in-field skin cancers. There were no Grade 5 complications. Fifty percent of patients receiving $55 Gy had Grade 3–4 complications, compared to 31% receiving \55 Gy (p = 0.46). Otherwise, no single factor cor- related with treatment complications. Conclusions: The role of RT in the management of young patients with desmoid tumors is not clearly established. Our institutional experience adds to the data suggesting that patient age is proportional to local control following RT and late failures are not un- common. In the pediatric and young adult population, doses $55 Gy were associated with improved local control, but may also lead to increased long-term complications. Author Disclosure: M.S. Rutenberg, None; D.J. Indelicato, None; J.A. Knapik, None; C.G. Morris, None; J. Kirwan, None; R.A. Zlotecki, None; M.T. Scarborough, None; C.P. Gibbs, None; R.B. Marcus, None. 88 Electron Paramagnetic Resonance Oxygen Images Correlates Spatially and Quantitatively with Increased Vascular Endothelial Growth Factor Concentrations H. J. Halpern 1 , M. Elas 2 , R. Bell 1 , D. Hleihel 1 , E. D. Barth 1 , C. R. Chaney 1 , K. Ahn 1 , C. A. Pelizzari 1 , M. Kocherginsky 1 , R. R. Weichselbaum 1 1 University of Chicago, Chicago, IL, 2 Jagiellonian University, Cracow, Poland Purpose/Objective(s): Regions of tumors with low pO 2 are important determinants of their biology and response to therapy. Elec- tron paramagnetic resonance oxygen images (EPROI) provide accurate spatially resolved pO 2 distributions in tumors and normal tissue. We explored the relationship between EPROI pO 2 distributions and registered localized tumor biopsy content of vascular endothelial growth factor (VEGF) in mouse fibrosarcomas. Materials/Methods: The EPROI of the legs of C3H mice bearing FSa fibrosarcomas were obtained using a well published con- tinuous wave (CW) technique in 48 minutes. Localized biopsies were obtained stereotactically registered with the EPROI using a #11 bone biopsy needle. This volume contained approximately 100 image voxels. The VEGF concentrations assayed using ELISA were compared with mean pO 2 , median pO 2 , and the fraction of voxels in the biopsy volume with pO 2 less than 3, 6, and 10 torr (HF10). Results: All pO 2 variables correlated with VEGF concentrations (p = 0.0049 to 0.019). The most significant correlation was with the fraction of voxels in the biopsy volume with pO 2 less than 10 torr. Conclusions: This further validates EPROI hypoxic fractions at the molecular level. The EPROI obtained in this way measures chronic hypoxia. The correlation with VEGF protein concentration is most likely with chronic hypoxia. The EPROI provide a new paradigm for the assessment of the response of tumor cells to their environment and a new direction for the evaluation of hypoxia in human tumors and planning cancer therapies. Author Disclosure: H.J. Halpern, None; M. Elas, None; R. Bell, None; D. Hleihel, None; E.D. Barth, None; C.R. Chaney, None; K. Ahn, None; C.A. Pelizzari, None; M. Kocherginsky, None; R.R. Weichselbaum, None. 89 Combining X-ray and Optical 3D Imaging to Guide Focal Irradiation of Molecular Targets in Mice J. W. Wong 1 , D. Artemov 2 , E. Tryggestad 1 , P. Winnard 2 1 Johns Hopkins University, Radiation Oncology and Molecular Radiation Sciences, Baltimore, MD, 2 Johns Hopkins University, Radiology-MR Research, Baltimore, MD Purpose/Objective(s): We investigate combining molecular optical imaging with X-ray cone-beam CT (CBCT) to guide the irradiation of tumor models in mice. Materials/Methods: Focal irradiation under CBCT guidance was delivered with our small animal radiation research platform (SARRP) with accuracy better than 0.25 mm. In the first study, 5  10 5 glioma (U87-Fluc-GFP) cells were implanted in 4–6- week-old male SCID mice by intracranial injection. These cells constitutively express green fluorescent protein and firefly lucif- erase. Focal irradiation was delivered 3 weeks post-inoculation. The MR images of the mouse brain with contrast were registered with CBCT images to robotically position the target for irradiation with the SARRP. Eighteen Gy was delivered to the tumor with 4, 5 mm  5 mm, beams at 4 equi-angle increments from vertical to horizontal. In the second study, 2 10 6 MDA-MB-231-td tomato breast cancer cells that express red fluorescence were implanted into the left mammary pad of a SCID mouse. Two-dimensional fluorescent images were acquired of the tumor 7 days after inoculation with a Xenogen IVIS 200 system. The centroid position of the optical signal was derived by 3D diffuse fluorescent tomographic reconstruction. The geometric center of the tumor was also estimated from the CBCT acquired at the time of irradiation. In the first irradiation session, a 5 mm  5 mm beam was used to deliver 8.5 Gy to the geometric tumor center from the vertical and 45  oblique directions. An additional 24 Gy was delivered 11 days later using a larger, 5 mm  10 mm beam to cover the same (now larger) tumor. A three beam arrangement was employed to minimize beam overlaps between the 2 sessions. For both studies, bioluminescent and fluorescent imaging were performed prior to and at various time points after each irradiation session. Results: For the glioma tumor model, the integrated bioluminescent intensity measured 48 hours postirradiation was reduced by threefold, indicating significant reduction in tumor burden. For the breast cancer tumor model, the estimated 3D position of the optical signal was within 2 mm of the geometric tumor center derived from CBCT. The integrated fluorescent intensity continued to increase after the initial 8.5 Gy, and was similar to that of an unirradiated control. It was reduced by twofold when measured 4 days after the second 24 Gy irradiation, although the tumor volume did not show significant changes. S42 I. J. Radiation Oncology d Biology d Physics Volume 75, Number 3, Supplement, 2009