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Beam Orientation Optimization in 4pi Space for Liver SBRT

P Dong

P Dong1*, T Long2, P Lee1, D Ruan1, E Romeijn2, D Low1, K Sheng1, (1) UCLA School of Medicine, LOS ANGELES, CA,(2) University of Michigan, Ann Arbor, MI,

TH-A-213AB-7 Thursday 8:00:00 AM - 9:55:00 AM Room: 213AB

Non-coplanar beam geometry has been shown to improve plan conformity but manual selection of non-coplanar beams is difficult. Previous beam orientation optimization algorithms did not include dose distribution explicitly in the objective function. Therefore, their ability to achieve high quality plans efficiently and automatically was limited. The purpose of this study is to implement a novel non-coplanar 4pi optimization algorithm that overcomes these difficulties.
A two stage inverse optimization was performed to select the beams and optimize fluences. For both stages, a pricing and column generation algorithm was used to iteratively optimize the inner product of the derivative of the objective function and the beam or beam aperture. The iterative procedure continued until a certain number of beams or beam apertures are selected or until the incremental inner product becomes negative. Ten liver stereotactic body radiation therapy (SBRT) patients previously treated with volumetric modulated arc therapy (VMAT) were replanned using the 4pi technique and compared with the actual delivered plan. 14 noncoplanar fields were selected by the 4pi algorithm to cover 95% of the PTV with the prescription dose.
With the same number of beams, the objective function value and the 50% isodose volume (V50) of the 4pi plan converge to a lesser value than the coplanar plan. For the liver cases, an average reduction of 14% in V50 was observed. The average liver dose and the volume of normal liver receiving 15 Gy or higher were reduced for 14% and 19%, respectively. Improved OAR sparing and target dose homogeneity were achieved simultaneously. Greater dosimetric gains were observed on lesions off patient center axis.
Significantly improved dosimetry was achieved using the novel 4pi planning method compared with state-of-the-art coplanar technique. 4pi technique is well suited for liver SBRT due to the organ laterality.

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