To characterize the changes in MRI FLAIR hyperintensity that occur in LGG patients following RT, to better understand the “pseudoprogression” that occurs in absence of true tumor regrowth.

Sixteen patients with histologic grade 2 gliomas were included (4 astrocytoma, 12 oligodendroglioma, 15/16 confirmed IDH mutation).  Time from histologic diagnosis to RT ranged from 2.2 to 235 months (median 6.9 months). 159 MRI scans were segmented using ITK-SNAP (median 9.5 MRIs/patient, range 4-13). Mean pre-RT tumor volume was 31.6 cm3 (std dev 29, range 5-100 cm3). Nine of 16 MRIs showed decreasing FLAIR volume immediately post-RT, while 7/16 showed increasing FLAIR volume.  After the initial post-RT MRI, 12/16 patients had MRIs with an increase in FLAIR volume sometime during the first year (pseudoprogression).  The FLAIR volume stabilized or decreased a median of 18.4 months and mean of 15.0 months post-RT. During the 2.5 year study period, median progression free survival and median overall survival were not reached (14/16 patients alive, 12/16 progression free). 

FLAIR hyperintensity changes on MRI are highly variable in the first 1.5 years post-RT in low grade glioma, but after 1.5 years, FLAIR volumes stabilize and decrease, likely indicating the inflection point where post-RT pseudoprogression stabilizes.