Tumor microenvironment modifications recorded with IVIM perfusion analysis after radiotherapy.

Purpose or Objective
Neoadjuvant radiotherapy (NeoRT) improves tumor local control and facilitates tumor resection in many cancers. The timing between the end of the NeoRT and surgery is driven by the occurrence of side effects or the tumor downsizing. Some clinical studies demonstrated that the timing of surgery and the RT schedule influence tumor dissemination and subsequently patient overall survival (Acta Oncol 2006). Previously, we developed a pre-clinical model demonstrating an impact of NeoRT schedule and the timing of surgery on metastatic spreading (Oncotarget 2015). Here, we used functional MRI (fMRI) to record tumor microenvironment modifications after NeoRT. We aim to get non-invasive markers to establish the best timing to perform surgery and avoiding tumor spreading.
Material and Methods
Based on our NeoRT model, MDA-MB 231 and 4T1 cells were implanted in the flank of SCID and BalbC mice, respectively. We locally irradiated (PXI, X-Rad SmART) tumors with 2x5Gy and then surgically removed at different time points after RT. We acquired fMRI (9,4T Agilent) before and after RT. Diffusion Weighted (DW) - MRI was performed every 2 days between RT and surgery. For each tumor, 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 performed IVIM (IntraVoxel Incoherent Motion) analysis to obtain information on intravascular diffusion, related to perfusion (F: perfusion factor) and subsequently tumor vessels perfusion.
Results
With the MDA-MB 231, we observed a significant and transient increase (60% of the basal value (n=6, p<0,05)) of F and D* parameters related to perfusion. The other parameters of the DW-MRI, ADC and D presented no modification. We observed similar results with 4T1 cells, where F increased at day 3 (55% of the basal value, n=10, p<0,05) then returned to initial level. The difference in timing for the peak of F (day 6 vs day 3) could be related to the difference in tumor growth according to the cell line (four weeks for MDA-MB 231 cells vs one week for 4T1 cells). We also observed a decrease of hypoxia (pimonidazole staining) when surgery was performed on the peak but vascular architecture was not affected. Moreover, performing surgery during F and D* peak, in the MDA-MB 231model, is associated with an increase of lung metastases: 115% and 187% compared to a surgery performed before or after the peak.
Conclusion
We demonstrated the feasibility of repetitive fMRI imaging in preclinical models after NeoRT. We showed a significant difference in perfusion-related parameters (D* and F) at a specific time point depending of tumor cells correlated with tumor metastases. We demonstrated the feasibility of Image Guided Surgery for decreasing tumor metastases after NeoRT.