LOCL-16 IMPACT OF MGMT PROMOTER METHYLATION STATUS ON TUMOR DYNAMICS DURING WEEKLY ADAPTIVE RADIOTHERAPY FOR GLIOBLASTOMA

PURPOSE: Adaptive MRI-guided radiotherapy (RT) on a 1.5T-MR-Linac using reduced clinical target volumes (CTV) of 5mm instead of the 15mm standard for glioblastoma (GBM) is currently being evaluated on the UNITED clinical trial (NCT04726397). We explored the morphological changes that occur during adaptive RT with concurrent temozolomide between tumors with MGMT promotor methylation (MGMT-m) vs. unmethylation (MGMT-um). METHODS: Thirty patients with IDH-wildtype GBMs were treated with 60Gy in 30 (n=12) or 40Gy in 15 fractions (n=18) (Fx). The CTV included a 5mm expansion on the gross tumor volume (GTV) +/- FLAIR hyperintense areas-at-risk and a 3mm planning target volume. Planning was performed on a pre-treatment reference MRI (FxRef) followed by weekly on-line adaptive re-planning at Fx1, Fx6, etc. acquired on the MR-Linac. Interim fractions were image-guided by pre-beam-on onboard MRI. The GTV/CTVs were quantified by their absolute volumes, volumes relative to the FxRef and the maximum linear distance from the edges of the reference contour to the weekly adapted contours (migration distance, d(mig)). MGMT promoter methylation status was explored as a fixed effect in a linear mixed statistical model. RESULTS: Weekly median changes in GTV relative to FxRef in MGMT-um tumors (n=12) were 10.3%, 9.2%, 10.6%, 14.5%, 18.0% and 17.3%, respectively, while for MGMT-m (n=18) were 3.4%, 0.0%, -8.6%, -11.3%, -11.3% and -5.6% (p=0.021). Between FxRef and Fx1, the GTV increased by over 10% in 58% of MGMT-um tumors vs. 33% of MGMT-m tumors. Similar significant trends were observed with the CTVs. MGMT-um tumors had significantly larger d(mig) compared to tumors with MGMT-m (median 9.6mm vs. 5.8mm, respectively (p=0.018)). CONCLUSIONS: MGMT-um GBM exhibited significant changes in morphology and migration distance between the time of treatment planning to the first treatment fraction, as well as throughout a course of RT. In this population, our results support a greater frequency of imaging and plan adaptation when applying personalized reduced CTV margins.