Investigating the dosimetric effects of grid size on dose calculation accuracy using volumetric modulated arc therapy in spine stereotactic radiosurgery

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J Radiosurg SBRT


Purpose: Sharp dose gradients between the target and the spinal cord are critical to achieve dose constraints in spine stereotactic radiosurgery (SRS), however the accuracy of the doses to the spinal cord at these high dose gradients is sensitive to the how the dose is sampled across the structure using a discretized isotropic calculation grid. In this study, the effect of the grid size (GS) on the dosimetric accuracy of volumetric modulated arc therapy (VMAT) spine SRS plans was investigated.

Methods: The Eclipse v11.0 Anisotropic Analytical Algorithm (AAA) algorithm was used for dose calculation. Plan qualities of fifty treatment plans were evaluated with a GS of 2.5 (AAA's default value), 1.5 and 1mm. All plans were prescribed to the 90% isodose line in 1 fraction. Parameters used for plan comparison included the distance-to-fall-off (DTF) between the 90% and 50% isodose levels in the axial plane, planning tumor volume (PTV) coverage to 99%, 95%, 5% and 0.03cc, dose to 10% (Cord_D10%) and 0.03cc (Cord_D0.03cc) of the spinal cord sub volume. The dosimetric accuracy was evaluated based on film dosimetry percent gamma pass rate, line profile through the cord. Calculation times between different grid sizes as well as DVH algorithm differences between two treatment planning systems (Eclipse vs Velocity) were compared. Paired t-test was used to investigate the statistical significance.

Results: The DTF decreased for all plans with 1mm compared to 1.5mm and 2.5mm GS (2.52±0.54mm, 2.83±0.58mm, 3.30±0.64, p

Conclusion: Plans calculated with a 1mm grid size resulted in the most accurate representation of the dose delivered to the cord, however resulted in less uniform dose distributions in the high dose region of the PTV. The use of a 1.5mm grid size may balance accurate cord dose and PTV coverage, while also being more practical with respect to computation time.

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