Development of geometrically ideal dose distribution as a reference for treatment planning in VMAT using filtered back-projection method.

To determine optimal dose distribution in the treatment planning of volumetric modulated arc therapy (VMAT), a virtually ideal dose distribution was developed as a reference by applying filtered back-projection method. Delineated structures in patient CT scans were identified using a treatment planning system. The projection of the planning target volume (PTV) was calculated along the X-ray direction for each angle of rotation. Each projection was Fourier transformed to the frequency space; a Shepp-Logan filter was applied, then an inverse Fourier transformation was performed. As the dose irradiation cannot assume a negative value, the filtered projections were shifted using the minimum value inside of the PTV. All values outside of the PTV were set to zero. The corrected filtered projections were then multiplied by the tissue-maximum ratio according to each voxel depth from the surface of the body to simulate X-ray attenuation. Finally, the distributions of multiple rotational angles were convolved to simulate the dose distribution of the VMAT. Ideal dose distributions were generated with sufficient uniformity inside of the PTV. Dose spreading except for the PTV due to external irradiation was reproduced in the case of a brain tumor. A reference dose distribution including OAR sparing was produced. The efficacy of this process as a target for optimum planning was confirmed. Using applied filtered back-projection, the ideal dose distribution, which excluded some device-oriented restrictions, was generated. This application will provide support for the determination of VMAT planning quality by providing reference dose distributions.

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Copyright © 2018 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.