2569 - Pattern of Recurrence with 1.0cm CTV in Glioblastoma Treated with Concurrent Radiotherapy and Temozolomide
Presenter(s)
E. Braschi Jordan1, O. U. Trumble2, R. J. Amdur1, A. C. Casper1, C. G. Morris1, and A. N. De Leo1; 1Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, FL, 2University of Florida College of Medicine, Gainesville, FL
Purpose/Objective(s): Historically, clinical target volume (CTV) expansions for glioblastoma (GBM) have ranged from 2–3 cm. Since 2017, our institution has adopted a 1 cm CTV expansion around the gross tumor volume (GTV) and postoperative cavity. Our study evaluates recurrence patterns with a 1 cm expansion and examines whether marginal recurrences could have been mitigated with a theoretical 2 cm CTV.
Materials/Methods: We included 50 patients who developed tumor recurrence following treatment between January 1, 2017, and January 31, 2024 at our institution. All patients underwent maximal safe resection followed by standard chemoradiation with 60 Gy in 30 fractions, though dose reductions to 59.4 Gy or 54 Gy were permitted to meet organ-at-risk constraints. All but three patients received concurrent temozolomide, and most also received tumor-treating fields and maintenance temozolomide. Radiotherapy target volumes were defined as follows: the GTV included the gross tumor and postoperative cavity on contrast-enhanced T1 MRI. The CTV was defined as the GTV plus a 1 cm expansion, edited for anatomical barriers, and the PTV included a 2 mm expansion beyond the CTV. Recurrences were classified as in-field if more than 80% of the recurrence volume was within the 100% isodose line (IDL), marginal if 20–80% was within the 100% IDL, and distant if less than 20% was within the 100% IDL. A theoretical 2 cm CTV expansion was created to assess whether marginal recurrences would have been covered.
Results: Recurrence patterns were as follows: 80% (40/50) were in-field alone, 10% (5/50) were in-field and distant, 8% (4/50) were distant alone, and 2% (1/50) were marginal. The single marginal recurrence had 52% of its volume within the 100% IDL and would have been fully encompassed by a 2 cm CTV. The two-year freedom from isolated marginal recurrence was 98% (95% CI: 92–100%) for a 1 cm CTV and 100% for the theoretical 2 cm CTV.
Conclusion: In patients with newly diagnosed GBM treated with a 1cm CTV margin, we observed a low marginal recurrence rate (2%), with only one patient who may have benefitted from standard 2 cm CTV margin. These findings support the continued use of a 1 cm CTV expansion at our institution.