SU-E-T-450: An Evaluation Study of Dose Calculation Algorithms for Stereotactic Body Radiation Therapy in Heterogeneous Phantom.

Abstract

In Stereotactic body radiation therapy (SBRT) higher dose per fraction is delivered to patients and thus the need for an accurate dose computation is further elevated. However under such conditions of small field geometries, the electronic equilibrium can be lost, making it challenging for the dose-calculation algorithm to accurately predict the dose, especially in the presence of tissue heterogeneities. Thus the aim of this study is to evaluate the performance of the algorithms implemented in our treatment planning system. In this study different phantoms were designed to model the lung with a small tumor volume of sizes that are usually encountered in SBRT. Solid water and cork slabs were used in constructing the phantom body. Perspex pieces of different diameters were created to be inserted in precut holes in the cork to simulate the tumor. The various dose calculation algorithms that are implemented in our CMS XIO planning system were then used for dose calculation inside the target. A significant discrepancies between maximum, minimum and mean dose for PTV was found between Clarkson, FFT Convolution, Fast superposition and superposition algorithms in dose calculation for the lung tumor. The FFT Convolution and Clarkson algorithms have higher calculated minimum dose than that predicted by the superposition and the fast superposition algorithm. A percentage difference as high as 62.5% was obtained between Clarkson and the superposition algorithm in the estimated minimum dose. A percentage difference up to 13.4% was obtained between the FFT Convolution and the superposition algorithm for the estimated maximum dose. Superposition and fast superposition showed little discrepancies for maximum, minimum and mean dose for the PTV. It is very critical to address the accuracy of dose computation for SBRT as a significant dose overestimation can occur inside the target due to the heterogeneous geometries.