Evaluation of the Biological Effectiveness of Mixed Sphere Size Spatially Fractionated Radiation Therapy Using Gamma-Knife Vs. VMAT
Abstract
Purpose
To evaluate the biological effect of Spatially Fractionated Radiation Therapy (SFRT) using Gamma Knife (GK) or volumetric-modulated arc therapy (VMAT) with multiple sphere sizes in a lattice arrangement.
Methods
CT images and tumor contours were obtained from the 20 patients with the largest brain metastases from a group previously enrolled in an IRB-approved study. Spherical contours were generated by in-house scripts and placed inside of the tumor, with a mix of 4 mm and 8 mm sphere sizes. For VMAT plans, 3 full arcs were used to generate a dose of D50% ≥ 18 Gy. For GK plans, shots were manually placed to cover sphere targets with 18 Gy to the 70% isodose line. The sphere arrangement for both VMAT and GK was a face-centered-cubic. The valley-to-peak ratio (VPR), average target dose, EUD, and therapeutic ratio (TR) are compared.
Results
On average, VPR and tumor mean dose was higher in VMAT plans than for GK plans with mixed shot sphere sizes. The sphere mean dose was slightly higher in GK plans than for VMAT. GK plans showed more consistency in EUD and TR as compared with VMAT. On average, EUD was 36% higher for the VMAT plan as compared with the GK plan, and VMAT TR was higher for semi-resistant (96%) and resistant tumors (254%), while GK TR was higher for sensitive tumors (22%). GK plans showed more consistency in EUD and TR across all 20 patients as compared with VMAT, with ranges in values of 2.7 and 4.4, respectively.
Conclusion
On average, VMAT plans had higher VPR, average target dose, EUD and TR as compared with GK. This could be because VMAT plans had higher tumor doses on average. Improvements were especially high for radio-resistant tumor TR. GK plans showed improvement in TR for radio-sensitive tumors.