eess.IVMar 9, 2026

Rectified flow-based prediction of post-treatment brain MRI from pre-radiotherapy priors for patients with glioma

Authors: Selena Huisman, Nordin Belkacemi, Vera C. Keil, Joost Verhoeff, Szabolcs David

Organizations: Amsterdam UMC, Department of Radiation Oncology, De Boelelaan 1117, 1081 HV Amsterdam, The Netherlands · Cancer Center Amsterdam, Imaging and Biomarkers, De Boelelaan 1117, 1081 HV Amsterdam, The Netherlands · Department of Radiology and Nuclear Medicine, De Boelelaan 1117, 1081 HV Amsterdam, The Netherlands · Amsterdam UMC, Vrije Universiteit Amsterdam, Amsterdam Neuroscience, Brain Imaging De Boelelaan 1117, 1081 HV Amsterdam, the Netherlands

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Abstract

Brain tumors result in 20 years of lost life on average. Standard therapies induce complex structural changes in the brain that are monitored through MRI. Recent developments in artificial intelligence (AI) enable conditional multimodal image generation from clinical data. In this study, we investigate AI-driven generation of follow-up MRI in patients with intracranial tumors through conditional image generation. This approach enables realistic modeling of post-radiotherapy changes, allowing for treatment optimization. The public SAILOR dataset of 25 patients was used to create a 2D rectified flow model conditioned on axial slices of pre-treatment MRI and RT dose maps. Cross-attention conditioning was used to incorporate temporal and chemotherapy data. The resulting images were validated with structural similarity index measure (SSIM), peak signal-to-noise ratio (PSNR), Dice scores and Jacobian determinants. The resulting model generates realistic follow-up MRI for any time point, while integrating treatment information. Comparing real versus predicted images, SSIM is 0.88, and PSNR is 22.82. Tissue segmentations from real versus predicted MRI result in a mean Dice-Sørensen coefficient (DSC) of 0.91. The rectified flow (RF) model enables up to 250x faster inference than Denoising Diffusion Probabilistic Models (DDPM). The proposed model generates realistic follow-up MRI in real-time, preserving both semantic and visual fidelity as confirmed by image quality metrics and tissue segmentations. Conditional generation allows counterfactual simulations by varying treatment parameters, producing predicted morphological changes. This capability has potential to support adaptive treatment dose planning and personalized outcome prediction for patients with intracranial tumors. Code will be available upon peer-reviewed publication at: https://github.com/SelenaIHuisman/RF-GlioPREDICT

Figures & tables

Figure 1: The architecture and training process of the rectified flow diffusion U-Net. The model is trained with pre-treatment information as conditioning, and post treatment information for the L1 loss function.
Figure 2: The architecture and inference process of the rectified flow diffusion U-Net. Follow-up MRIs are generated for any timepoint and treatment combination based on the input information.
Figure 3: An example slice from the middle of the brain, with the radiotherapy dose, real baseline MRI, real follow-up MRI and a predicted follow-up MRI. Zoomed-in sections of the ventricles are shown below the images to highlight local morphological changes predicted by the model. For the RT dose, warmer colors mean a higher dose. For each image, right is on the right.
Figure 4: The Jacobian determinants between baseline and real follow-up and baseline and predicted follow-up, with the baseline image underlaid. Red means that a pixel expands during non-linear registration, while blue means that the pixel contracts. Values between -0.1 and 0.1 are excluded. For each image, right is on the right.
Figure 5: Counterfactual simulations showing how different treatment combinations affect the brain. The baseline, real followup and the true treatment (1x RT dose and temozolomide (TMZ) are shown in the top row as reference. The bottom 3 x 3 MRI slices show the prediction outcomes at 224 days, each for a different treatment combination. The predicted images are all subtracted from the predicted image based on the true treatment (the middle one), resulting in an overlay that shows pixel-intensity differences. Red means positive values, while blue means negative values. The baseline, real follow-up and RT dose are exactly the same as figure 3 . For each image, right means right. Values between -10 and 10 are excluded for clarity.
Figure 6: Eight counterfactual generations for every 60 days, varying only in the temporal conditioning. To highlight tumor and tissue morphological evolution, each image is subtracted from the previous one, therefore a difference map for each interval is created. Blue indicates an intensity decrease from one image to the next, while red indicates an intensity increase. The same baseline MRI slice from Figure 3 is included, and right is on the right. Values between -10 and 10 are excluded for clarity.

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