ESTRO 35 2016 S967 ________________________________________________________________________________ timing of surgery and the RT schedule could influence tumor dissemination and subsequently patient overall survival. We demonstrated the impact of NeoRT on metastatic spreading in a Scid mice model. After an irradiation of 2x5gy, we show more metastasis in the lung when the mice are operated at day 4 compared to day 11 (1). Here, our aim is to evaluate with functional MRI (fMRI) the impact of the

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... radiation treatment on the tumor microenvironment and subsequently to identify non-invasive markers helping to determine the best timing to perform surgery for avoiding tumor spreading. Material and Methods: We used two models of NeoRT in mice we have previously developed: MDA-MB 231 and 4T1 cells implanted in the flank of mice (1). When tumors reached the planned volume, they are irradiated with 2x5 Gy and then surgically removed at different time points after RT. Diffusion Weighted (DW) -MRI was performed every 2 days between RT and surgery. For each tumors we acquired 8 slices of 1 mm thickness and 0.5 mm gap with an "in plane voxel resolution" of 0.5 mm. For DW-MRI, we performed FSEMS (Fast Spin Echo MultiSlice) sequences, with 9 different B-value (from 40 to 1000) and B0, in the 3 main directions. We also performed IVIM (IntraVoxel Incoherent Motion) analysis, in the aim to obtain information on intravascular diffusion, related to perfusion (F: perfusion factor) and subsequently tumor vessels perfusion. Results: With the MBA-MB 231 we observed a significant increase of F at day 6 after irradiation than a decrease and stabilization until surgery. No other modifications of the MRI signal, ADC, D or D* were observed. We observed similar results with 4T1 cells, F increased at day 3 than returned to initial signal (fig 1) . The difference in the peak of F can be related to the difference in tumor growth between MBA-MB 231 in four weeks and 4T1 in one week.