Evaluating Physiological MRI Parameters in Patients with Brain Metastases Undergoing Stereotactic Radiosurgery—A Preliminary Analysis and Case Report

SIMPLE SUMMARY: Brain metastases affect up to thirty percent of the adult cancer population. One of the treatment options is stereotactic radiosurgery (SRS). To better understand SRS-related side-effects, we used advanced imaging techniques to study both the vascular and metabolic reserve capacity of the brain in nine patients with brain metastases before and three months after SRS. We observed larger declines in both vascular and metabolic markers in healthy brain regions that received a higher dose of SRS, but also saw substantial variation within- and between individuals. Our case analysis showed that the markers used in this study were able to pick up differences between patients who had either tumor growth or tumor shrinkage three months after SRS. These preliminary findings suggest there are no immediate detrimental effects on healthy brain tissue following SRS, but larger groups with a longer follow-up are needed to investigate potential late side-effects. ABSTRACT: Brain metastases occur in ten to thirty percent of the adult cancer population. Treatment consists of different (palliative) options, including stereotactic radiosurgery (SRS). Sensitive MRI biomarkers are needed to better understand radiotherapy-related effects on cerebral physiology and the subsequent effects on neurocognitive functioning. In the current study, we used physiological imaging techniques to assess cerebral blood flow (CBF), oxygen extraction fraction (OEF), cerebral metabolic rate of oxygen (CMRO(2)) and cerebrovascular reactivity (CVR) before and three months after SRS in nine patients with brain metastases. The results showed improvement in OEF, CBF and CMRO(2) within brain tissue that recovered from edema (all p ≤ 0.04), while CVR remained impacted. We observed a global post-radiotherapy increase in CBF in healthy-appearing brain tissue (p = 0.02). A repeated measures correlation analysis showed larger reductions within regions exposed to higher radiotherapy doses in CBF (r(rm) = −0.286, p < 0.001), CMRO(2) (r(rm) = −0.254, p < 0.001), and CVR (r(rm) = −0.346, p < 0.001), but not in OEF (r(rm) = −0.004, p = 0.954). Case analyses illustrated the impact of brain metastases progression on the post-radiotherapy changes in both physiological MRI measures and cognitive performance. Our preliminary findings suggest no radiotherapy effects on physiological parameters occurred in healthy-appearing brain tissue within 3-months post-radiotherapy. Nevertheless, as radiotherapy can have late side effects, larger patient samples allowing meaningful grouping of patients and longer follow-ups are needed.