2077 - Evaluation of the Biological Effectiveness of Spatially Fractionated Radiation Therapy Using Gamma-Knife vs. VMAT SRS on Brain Metastases
Presenter(s)
T. Gray1, A. Hadfield2, H. Nordstroem3, B. Andelic4, B. Guo1, N. Yoon5, J. G. Scott6, J. H. Suh7, and Y. B. Cho7; 1Department of Radiation Oncology, Taussig Cancer Institute, Cleveland Clinic, Cleveland, OH, 2Cleveland Clinic, Cleveland, OH, 3Elekta Instrument AB, Stockholm, Sweden, 4Elekta, Stockholem, Sweden, 5Adelphi University, Garden City, NY, 6Department of Radiation Oncology, Taussig Cancer Center, Cleveland Clinic, Cleveland, OH, 7Department of Radiation Oncology, Cleveland Clinic Foundation, Cleveland, OH
Purpose/Objective(s): To investigate and compare the biological effect of Spatially Fractionated Radiation Therapy (SFRT) using volumetric-modulated arc therapy stereotactic radiosurgery (VMAT SRS) vs Gamma Knife (GK) SFRT with a face-centered cubic lattice for multiple brain metastases. This retrospective study presents Equivalent Uniform Dose (EUD) and Therapeutic Ratio (TR) as well as a direct dosimetric characterization between VMAT SRS SFRT and GK SFRT techniques using two different sphere sizes (4mm and 8mm diameters).
Materials/Methods:
CT images and target contours were obtained from 20 patients with the largest brain metastases and enrolled in an IRB-approved study over the past 10 years. Spherical target contours (8mm and 4mm diameter) were manually placed inside of the tumor using a face-centered cubic lattice arrangement and given a dose of 18Gy in a single fraction. For VMAT SRS SFRT plans, 3 full arcs were used to generate a dose of D50%=18Gy. For the GK SFRT plans, shots were manually placed to cover sphere contours inside of the tumor with 18Gy to the 70% isodose line in a single fraction. Sphere contours for the lattice structure were generated by in-house scripts and imported to the GK planning system. Sphere arrangement was a face-centered-cubic (FCC) structure for both 4mm and 8mm collimator sizes. Shot placement and weights were tuned to deliver adequate coverage to the spheres of V18Gy>50%. The number of spheres, valley-to-peak ratio (VPR), average target dose, average sphere dose, TR and EUD are compared.Results: Average number of spheres, valley-to-peak ratio, target mean dose (Gy) and sphere mean dose (Gy) are shown in Table 1. Mean EUD changed by only 0.5 Gy between 4mm and 8mm sphere sizes for GK, compared with 3.6 Gy for VMAT. GK TR is consistent, with only a 0.4 difference between 4mm and 8mm sphere sizes for radio-resistant tumors. Treatment time is much improved for VMAT over GK. All parameters were statistically significant, with p<0.05.
Conclusion: GK showed more consistency in EUD and TR as well as better EUD and TR for radio-resistant tumors. It also shows a decreased VPR and Target mean dose difference between 4mm and 8mm lattice sizes. However, sphere mean dose was generally larger for GK and treatment delivery times for the 4 mm lattice size are not ideal for GK as compared with VMAT.
Abstract 2077 - Table 1: Dosimetric characterization of GK vs VMAT SRS SFRT for 4mm and 8mm lattice sizes| Metric | |||||
| Lattice Type | # spheres | VPR | Target mean dose (Gy) | Sphere mean dose (Gy) | Treatment time (min) |
| GK 4mm | 29±8.04 [17-46] | 0.33±0.03 [2.5-0.37] | 5.04±0.32 [4.16-5.66] | 19.35±0.13 [19.05-19.55] | 202.96±49.11 [124.22-311.02] |
| GK 8mm | 4.6±1.46 [3-9] | 0.26±0.03 [0.25-0.37] | 6.35±0.57 [4.82-7.19] | 20.64±0.25 [20.17-21.07] | 33.76±7.93 [23.35-53.24] |
| VMAT 4mm | 29±8.04 [17-46] | 0.50±0.05 [0.33-0.60] | 12.99±0.73 [11.72-14.66] | 18.0±0.07 [17.92-18.12] | 4.35±0.56 [3.42-5.70] |
| VMAT 8mm | 4.6±1.46 [3-9] | 0.28±0.05 [0.09-0.43] | 7.27±0.09 [5.49-10.12] | 18.18±0.16 [17.92-18.62] | 4.03±0.54[3.28-5.59] |