RADT-40. EVALUATION OF SMALLER CLINICAL TARGET VOLUMES AND PATTERNS OF PROGRESSION IN PATIENTS WITH NEWLY DIAGNOSED GLIOBLASTOMA TO INFORM FUTURE DIRECTIONS IN RADIATION TREATMENT PLANNING

Abstract BACKGROUND NRG Oncology Group guidelines recommend a clinical target volume (CTV) including generous expansions and incorporation of T2/FLAIR to account for potential tumor infiltration for glioblastoma patients, though smaller CTV expansions may be feasible. We retrospectively reviewed a large cohort of GBM patients to characterize patterns of recurrence and to assess the potential for smaller CTV margins. METHODS 312 consecutive adult patients with histologically confirmed diagnosis of glioblastoma treated with radiation therapy and temozolomide at a single institution were included in this study. All relevant clinical and pathological data was retrieved from local electronic medical records. MRIs from four timepoints 1) Pre-operative, 2) post-operative, 3) post-radiation and 4) time of progression were reviewed and fused with each other with rigid registration. We created a gross tumor volume including the resection cavity and residual enhancing tumor (GTV-GAD) and a GTV which included the GAD-GTV and pre-radiation T2/FLAIR signal (GTV-FLAIR). Multiple CTV expansions were applied to the GAD-GTV and FLAIR-GTV to evaluate coverage of subsequent progressive disease. RESULTS We evaluated 312 patients with a median age of 59.8 (range 20-93), 59% male, 61% KPS ≥ 90, 49% underwent gross total resection (51% subtotal resection/biopsy), 43% methylated and 57% unmethylated. CTV expansions of 2 cm, 1.5 cm, 1 cm, 0.5 cm, and 0 cm from GTV-GAD would have covered the epicenter of recurrent tumor in 90.7%, 88.1%, 84.6%, 77.5%, and 53.5% of cases respectively. CTV expansions from GTV-FLAIR of 2 cm, 1.5 cm, 1 cm, 0.5 cm, and 0 cm would have covered the epicenter of recurrent tumor in 93.5%, 92.6%, 91.3%, 88.1%, and 78.8% of cases respectively. On multivariable analysis, GAD-GTV and FLAIR-GTV volume were not associated with progression-free or overall survival(P>0.05). CONCLUSION Based upon our analysis, smaller treatment volumes could represent an opportunity to reduce treatment-related morbidity without compromising clinical outcomes.