Pattern of recurrence with 1.0 cm CTV in brain glioblastoma treated with radiotherapy.

Purpose: Historically, clinical target volume (CTV) expansions for glioblastoma (GBM) have ranged from 2 to 3 cm. Since 2017, our institution has adopted a 1 cm CTV expansion beyond the gross tumor volume (GTV) and postoperative cavity on contrast-enhanced T1 Magnetic Resonance (MR) scan. Our study evaluates recurrence patterns using this approach.

Methods: We identified 50 patients who developed tumor recurrence following treatment between 01/01/2017-01/31/2024 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-54 Gy were permitted to meet organ-at-risk constraints. All but three patients received concurrent temozolomide. Radiotherapy target volumes were defined as follows: GTV included gross tumor and postoperative cavity on contrast-enhanced T1 MR, CTV included GTV plus a 1 cm expansion (edited for anatomical barriers), and the planning target volume included a 2 mm expansion beyond the CTV. Recurrences were classified based on the recurrence volume within the 100% isodose: in-field (> 80%), marginal (20-80%), and distant (< 20%).

Results: Recurrence patterns were as follows: 80% (40/50) in-field alone, 10% (5/50) in-field and distant, 8% (4/50) distant alone, and 2% (1/50) marginal alone. The single marginal recurrence had 52% of its volume within the 100% isodose.

Conclusions: In GBM patients treated with a 1 cm CTV margin on the gross tumor and postoperative cavity on contrast-enhanced T1 MR, we observed a very low marginal recurrence (2%). These findings support the continued use of a 1 cm CTV expansion at our institution.