Temporal analysis of prostate MR-Linac data.

Abstract

There is a strong desire to exploit daily MR-Linac (MRL) imaging for biomarker extraction and outcome prediction. Current prediction models are based on pretreatment and follow-up scans. If MRL data are to provide additional value, this would need to originate from the dynamic nature of MRL intratreatment signals. Demonstrating the existence of such dynamics and how to discover them is the purpose of this work. We analysed T₂-weighted and diffusion MRL data of prostate cancer patients undergoing hypofractionated radiation treatment. Next to standard mask-based analyses, we introduce exploratory analyses using classic fMRI neuroimaging techniques, designed to determine both the existence and spatial location of temporal signal changes. Finally, we link dose deposition to the slope of observed T₂w signal changes in pelvic bone tissue, chosen for its wide range of dose values received (0-26 Gy; 0-47 Gy EQD2). Mask-based analyses of the MRI data of all fractions reveal T₂w signal decreases for prostate tissue whereas the GTV signal stays more constant throughout the treatment. The diffusion coefficient of the prostate gland remains constant while the GTV shows an increase. Prostate volume increases by 10% in the first week after which it slowly decreases again. The fMRI-inspired analysis shows prostate T₂w signal change to be confined to the peripheral zone. Furthermore, it revealed signal changes in the rectum, bladder, pelvic and femoral bones, the zona orbicularis, and the penis (despite the latter only accumulating 3 Gy). The dose-effect relationship of pelvic bone shows a clear discontinuity with a sharp transition from 0% to 1.5% signal change per fraction within a very short dose interval between 0-1 Gy (0.55 Gy EQD2). Exploratory fMRI-inspired analyses were able to demonstrate many temporal, non-linear trends in MRL data. This is promising for future biomarker research once MRL outcome data become more prevalent. The pelvic bone response shows signs of dose-threshold behaviour with a sudden dose-response starting at 1 Gy.