Author: Brankica Andelic, Young-Bin Cho, Tara M. Gray, Andrew Robert Hadfield, Hakan Nordstrom 👨🔬
Affiliation: Elekta, Cleveland Clinic 🌍
Purpose: Spatially Fractionated Radiotherapy (SFRT) requires highly modulated dose distribution inside a tumor volume optimized using sets of planning structure of spheres in a lattice arrangement. To overcome the previously reported inconsistent dosimetry in static lattice design, a new dynamic lattice algorithm is presented for consistent dose modulation regardless of tumor size and sphere dimension.
Methods: 20 GK patients with large brain metastasis enrolled in IRB-approved study were retrospectively planned for SFRT with Gamma Knife. Plans were manually optimized for the coverage of V70% > 50% to the group of spheres placed in the arrangement of face-centered-cubic (FCC) inside of target. The distance between the spheres in each cell of lattice was set to 4 times the sphere diameter for each 8mm and 4mm collimator in the static lattice. The valley dose is known to increase with the number of spheres or tumor volume. For consistent dose modulation regardless of tumor size, the number of spheres and the choice of shot size, 3D dose profiles of 4mm and 8mm shots were collected. The optimal lattice size was iteratively calculated in the dynamic lattice design. Optimal placement of spheres is determined by maximizing the correlation between 3D tumor volume and sets of lattice spheres. The number of spheres, valley-to-peak ratio (VPR), average target dose, and average sphere dose were compared.
Results: Average (±STD) number of spheres, VPR, target dose [Gy] and mean dose to spheres [Gy] for 4mm shot in static design are 29.2±8.2, 0.32±0.04, 6.59±0.69, and 18.6±0.62. Those in dynamic design are 18.6±2.7, 0.33±0.03, 5.04±0.32, and 19.4±0.13 showing a significant improvement in consistency. The improvements are similar in 8mm shot plans.
Conclusion: Dynamic lattice design was developed to produce the consistent dosimetry regardless of tumor size, the number of spheres and the choice of collimator in GK SFRT.