ESTRO 2021 Abstract Book

S288

ESTRO 2021

Purpose or Objective MR-linacs create the opportunity for daily treatment response monitoring using quantitative MRI without an increase in patient burden. Imaging tumor perfusion is a promising candidate for treatment response assessment as low tumor perfusion is linked to poor patient outcome. Typically, dynamic contrast-enhanced MRI (DCE) is used for perfusion imaging. However, daily use of contrast agent, required for DCE, is not desirable. A potential alternative for non-invasive perfusion measurements is intravoxel incoherent motion (IVIM) MRI. This is an extension to diffusion MRI, where perfusion information is extracted from a bi- exponential model. The aim of this study is to determine if changes in IVIM parameters during treatment correlate with changes in DCE parameters to reveal if IVIM could be used as a substitute for perfusion imaging. Materials and Methods 18 prostate cancer patients were imaged daily during radiation treatment (20 x 3 Gy). Daily IVIM and weekly DCE scans were acquired on a 1.5T MR-linac (Unity, Elekta AB, Sweden). The Tofts model was used to derive tracer kinetic parameters K trans , k ep , and v e from the DCE images. A bi-exponential IVIM model was fit to the DWI data to derive the D, f, and D* parameters. Healthy prostate and tumor ROIs were delineated on T2- weighted scans of the first treatment fraction. T2-weighted scans of the other fractions were registered to the first fraction. This registration was applied to the DCE and IVIM images and the delineations were propagated. Median values from the voxels within the delineations were used for further analysis. ANOVA was used to test if there were significant changes (p < 0.05) in the parameters compared to the first fraction. The Pearson correlation coefficient was calculated for the mean values of the parameters. Results Figure 1 shows the changes in the DCE and IVIM parameters during treatment. In the prostate, ANOVA revealed a significant increase in all parameters except for D. f, K trans , and k ep increased significantly after one week, and D* and v e after two weeks. In the tumor, the D was only significantly different at two weeks, and f after three weeks. The other parameters did not show a significant change. Correlations between the parameters on a group level are shown in Figure 2. The significant correlations between DCE and IVIM parameters are especially high in the prostate, as is the correlation between f and v e in the tumor.