Purpose: To perform a dosimetric comparison between clinically deliverable (treatment time <20 minutes) Gamma Knife® Icon (GK) and linac-based FSRT plans on the basis of normal brain dose sparing for recurrent glioblastomas (GBM) with target volumes <14 cc.
Methods: Twelve patients with recurrent GBM who were treated using re-irradiation via linac-based FSRT with a dose of 35 Gy in 10 fractions or 60 Gy in 5 fractions were selected for this study. New clinically deliverable plans were created in the Leksell GammaPlan® V11 TPS with identical image sets, structures and prescription doses to the original linac-based plans for two of the twelve patients. Planning objectives for both modalities included target coverage of PTV V100 ≥95%, and maximum dose to cranial organs-at-risk (optic nerves, chiasm, brainstem) D0.03cc <15 Gy. Plan comparison was performed in MIM. Dosimetric indices for comparison were target coverage (TC), Conformity Index (CI), Gradient Index (GI), normal brain (Brain-PTV) dose (V4, V12, V20, and mean dose) and treatment time. Statistical significance was measured using a two-tailed, paired t-test with p<0.05.
Results: For all patients replanned at this time, comparable target coverage and CI were seen between the linac and GK plans. GK plans showed lower GI and lower normal brain in all quantities evaluated. As expected, treatment times for GK plans were longer than linac treatment times.
Conclusion: GK treatment plans resulted in lower volume of normal brain dose and mean brain dose compared to linac-based FSRT, with improvements on all dosimetric indices while maintaining clinical deliverability.