GFAP and myeloid sarcoma: Only little is known about the interplay of different volumetric magnetic resonance imaging (MRI) parameters, the serum biomarkers neurofilament light chain (sNfL) and glial fibrillary acidic protein (sGFAP), cognition, and fatigue in MS patients without signs of progression independent of relapse activity (PIRA; i.e. clinically stable).