Factors influencing MMP-9 level of serum-derived sEVs. (a) Relationship between age and MMP-9 levels in the control group. (nCTRL = 36). (b) MMP-9 levels of the two genders in the control group (boxplots show the median with interquartile range, mean with dotted line, error bars range from the 5th–95th percentiles; nCTRL male = 16, nCTRL female = 20). (c,d) Changes in individual patient’s MMP-9 levels before and after surgical resection for the GBM and BM groups, respectively (nGBM preop-postop = 14, nBM preop-postop = 6). (e) MMP-9 levels of serum-derived sEVs regarding the original tumour and the recurrence in GBM group before surgical resection (nGBM original = 52, nGBM recurrence = 14). (f) MMP-9 levels in GBM group to treatment status (nGBM before therapy = 52, nGBM therapy = 15).