EP-1204: Limited margin radiotherapy and temozolomide for glioblastoma multiforme: pattern of failure

Purpose or ObjectiveRadiation therapy (RT) is a mainstay of treatment for primary and metastatic brain tumours.Various toxicities, ranging from asymptomatic to fatal, occur in the central nervous system after RT to the brain.MR imaging is useful in detecting these toxicities and complications.White matter changes in the brain after RT is well-known and is seen on T2-weighted or FLAIR MR images.The purpose of this study was to evaluate post-RT imaging changes using FLAIR T2-weighted MR images in adult primary brain tumour patients previously treated with RT.We aimed to see if any correlation existed between isodose of the delivered doses and subsequent T2 changes post-RT. Material and MethodsBetween Sept.2012 and Dec.2016, we retrospectively analysed records of all newly diagnosed low grade brain tumours who received RT to brain (with/without concurrent Temozolomide-TMZ).Fourteen patients who had MR images consisting of T2/FLAIR sequences, available from 6-24 months after RT completion were taken for the final analysis.In all patients RT was initiated 4-6 weeks after surgery/biopsy.Two patients were treated using IMRT while 12 patients were treated using partial arc VMAT.Concurrent TMZ to a dose of 75mg/m2 was given in 6 patients.Imaging datasets 6-24 months post-RT were analysed.The MR sequences included T2-FLAIR weighted images with a slice thickness of 2-3mm, which was reviewed by a radiologist and a radiation oncologist.These images were then fused with RT planning MRI on the BrainLab (BrainLab AG, Feldkirchen, Germany).The identified white matter changes were contoured on the FLAIR sequences.The MR images prior to RT were also reviewed to exclude preexisting hyperintensity related to peritumoral edema, tumour infiltration or post-operative changes. ResultsFinal analysis included 14 patients.Median age was 48.5yrs (3-63yrs) at time of RT.Histological diagnoses were astrocytoma (n=10), oligodendroglioma(n=3) and ependymoma(n=1).One patient had grade 1 histology while all others had grade 2 histology.Median dose was 55.9Gy (45 -60Gy) at 1.8-2Gy per fraction, 5 days a week.Mean PTV volume (vol.) was 305.6±358.6cc(32.5 -1461.4cc).Mean vol. of the contoured T2 changes was 60.9±54.9cc(0.93 -145cc).Percentage isodose encompassing 100% of T2 change vol.was 63.3±28.5% (19 -98.7%) but there was no significant correlation between the delivered dose to the PTV and the percent isodose (p=0.61).There was however, a significant correlation between the PTV and the T2 change vol.(p=0.02). ConclusionThis retrospective analysis showed that though there was significant post-RT T2 changes in the brain parenchyma, these changes were not confined to any specific isodose but were distributed from low to high dose regions.It will be imperative to study further if post-RT changes on T2-FLAIR carry significance in terms of white matter changes, which can bear correlation to toxicities vis-a-vis residual disease progression or recurrence.