3010 - Investigation and Optimization of the Biological Effectiveness of Spatially Fractionated Radiation Therapy for VMAT SRS vs. Gamma Knife on Brain Metastases Using an Intermediate Lattice Sphere Size
Presenter(s)
T. Gray1, C. McGrath1, A. Hadfield2, H. Nordstroem3, B. Andelic4, B. Guo5, N. Yoon6, J. G. Scott7, J. H. Suh5, and Y. B. Cho5; 1Department of Radiation Oncology, Taussig Cancer Institute, Cleveland Clinic, Cleveland, OH, 2Cleveland Clinic, Cleveland, OH, 3Elekta Instrument AB, Stockholm, Sweden, 4Elekta, Stockholem, Sweden, 5Department of Radiation Oncology, Cleveland Clinic Foundation, Cleveland, OH, 6Adelphi University, Garden City, NY, 7Department of Radiation Oncology, Taussig Cancer Institute, Cleveland Clinic Foundation, Cleveland, OH
Purpose/Objective(s): To investigate and compare the biological effectiveness of spatially fractionated radiation therapy (SFRT) using VMAT SRS with an intermediate, 6 mm lattice sphere size, against standard Gamma Knife (GK) and VMAT SRS using 4 mm and 8 mm lattice sphere sizes.
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 (4 mm, 6 mm and 8 mm diameter) were manually placed inside of the tumor using a face-centered cubic lattice arrangement for VMAT SRS plans and given a dose of 18 Gy in a single fraction. Three full arcs were used to generate a dose of D50%=18 Gy. For GK SFRT plans, shots of either 4 mm or 8 mm diameter, were manually placed to cover sphere contours inside of the tumor with 18 Gy 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 and VMAT SRS planning systems. Sphere arrangement was a face-centered-cubic (FCC) structure for both 4 mm and 8 mm GK 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, Therapeutic ratio (TR) and Equivalent Uniform Dose (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. VPR for VMAT 6mm was most comparable to GK 4mm. EUD improved for VMAT 6mm for smaller tumor sizes, less than 80cc, by 2-5 Gy for radioresistant tumors compared to all other lattice types. TR also improved significantly for VMAT 6 mm when compared to all other lattice types except for VMAT 4mm, which showed superior TR over all lattice types for radioresistant tumors. All parameters compared were statistically significant, with p<0.05.
Conclusion: The VMAT 6mm sphere lattice size serves as a good potential alternative to GK 4mm and GK 8mm sphere lattice sizes due to improved biological effectiveness and comparable VPR. It can also be beneficial for treating smaller radioresistant tumors due to improved EUD and TR for smaller tumor sizes.
Table 1: Dosimetric characterization of GK vs VMAT SRS SFRT for varying lattice sphere sizes| Lattice Type | # spheres | VPR | Target mean dose (Gy) | Sphere mean dose (Gy) |
| GK 4 mm | 29±8.04 [17-46] | 0.33±0.03 [0.4-2.5] | 5.04±0.32 [4.2-5.7] | 19.35±0.13 [19.05-19.55] |
| GK 8 mm | 4.6±1.46 [3-9] | 0.26±0.03 [0.3-0.4] | 6.35±0.57 [4.8-7.2] | 20.64±0.25 [20.17-21.07] |
| VMAT 4 mm | 29±8.04 [17-46] | 0.50±0.05 [0.3-0.6] | 12.99±0.73 [11.7-14.7] | 18.0±0.07 [17.92-18.12] |
| VMAT 6 mm | 14.10±3.14 [10-22] | 0.33±0.11 [0.1-0.5] | 9.79±0.89 [7.6-11.6] | 17.94±0.10 [17.8-18.1] |
| VMAT 8 mm | 4.6±1.46 [3-9] | 0.28±0.05 [0.1-0.4] | 7.27±0.09 [5.49-10.12] | 18.18±0.16 [17.92-18.62] |