Magnetic Resonance Imaging in Radiation Oncology

Dose painting for prostate cancer

The research in prostate cancer focuses on improvement of biochemical disease-free survival after radiotherapy by focal dose escalation to the tumors inside the prostate gland. We initiated the FLAME trial, a multi-center phase III randomized trial. In this study, a focal boost to the visible tumor inside the prostate to a dose of 95 Gy was given and compared to the standard treatment of 77 Gy to the prostate gland. In total 571 patients have been randomized in four institutes. The underlying hypothesis of the trial is that at a standard dose patients with high-risk disease may experience metastatic failure because of undertreatment of the primary tumor.

Toxicity results were published recently and showed no increase in toxicity in the study arm.Results of the primary endpoint, 5 year biochemical disease-free survival, will be available later in 2020.

For translation of the FLAME technique to routine clinical practice, guidelines for tumor delineation and for treatment planning are necessary. We investigated the high interobserver variability in tumor delineation and developed therefore a logistical regression model predicting the likelihood of cancer presence in a voxel-by-voxel manner. To allow rapid estimation of the achievable boost dose for individual patients, we developed a knowledge-based planning model based on the location of the tumor in the prostate relative to the surrounding anatomy.

To enable a pattern-of-failure analysis, we image patients with recurrent prostate cancer (Pca) with PET and/or multi-parametric (mp-)MRI to delineate local recurrences. We trained a tumor probability model for recurrent cancer based on mp-MRI on a cohort of patients with recurrent PCa. The model was tested on another cohort of patients who received salvage prostatectomy, allowing for whole-mount section histological validation of the model predictions. The model achieved an accuracy of automatically derived tumor delineations that was comparable to delinations by trained radiologists.

Quantitative MRI for radiotherapy

While many imaging biomarker studies have been conducted to establish their prognostic value after radiotherapy, the vast majority of studies is small and the methodology between studies varies widely. On the Unity MR-linac system patients receive an MRI as part of each treatment fraction. Acquisition of quantitative MRI sequences is feasible without prolonging the treatment time. This makes the Unity system uniquely suitable for MRI biomarker studies for response assessment. Within the MR-linac consortium, we established the feasibility of multi-parametric MRI on the Unity system, applying relaxometry, diffusion-weighted MRI and dynamic contrast-enhanced MRI during a radiation treatment session. The repeatability of these quantitative techniques was measured in a study with 4 centers and showed results consistent with diagnostic MRI scanners.

Within the MR-linac consortium we recently formed a collaborative working group with the aim of initiating MRI biomarker studies using consistent methods for quantitative MRI.