3D analysis and evolution of prostate and tumor oxygenation using MRI-derived relative oxygen extraction fraction...

Tumor hypoxia is a major cause of radioresistance, motivating the development of biomarkers related to tissue oxygen physiology that can be monitored throughout treatment. MRI-derived relative Oxygen Extraction Fraction (rOEF) is a non-invasive imaging biomarker related to tissue oxygen extraction. This study characterized the temporal evolution of rOEF during MR-guided stereotactic body radiotherapy (MRgSBRT) for prostate cancer.

Twenty-three prostate cancer patients treated with five-fraction MRgSBRT were analyzed. rOEF maps were acquired at treatment planning and at each treatment fraction. Average rOEF was extracted from the healthy prostate, gross tumor volume (GTV), and two gluteal muscle regions receiving either residual (in-field) or negligible (out-of-field) irradiation. Longitudinal changes in average rOEF and in the fraction of GTV voxels with rOEF >70% were quantified using patient-specific linear fits. Slopes of average rOEF in the healthy prostate were also calculated between successive imaging sessions.

Average rOEF decreased significantly in all irradiated regions (healthy prostate, GTV and in-field gluteal muscle), with no significant change in the out-of-field gluteal muscle. The fraction of GTV voxels with rOEF >70% also declined significantly. In the healthy prostate, rOEF decreased predominantly during the first four fractions, with the steepest median slope observed between planning and the first treatment fraction (-3.2).

MRgSBRT was associated with a progressive reduction in rOEF confined to irradiated tissues, with the largest decrease in healthy prostate rOEF occurring after the first irradiation. These findings suggest rOEF could be a potential imaging biomarker of physiological changes associated with radiation exposure and warrant further investigation of its relationship with physiological processes and tissue hypoxia.