Impact of 2D Vs 3D Cell Survivals On the RBE for Proton Therapy Using GATE

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Purpose: Proton therapy treatments are based on a constant RBE (relative biological effectiveness) of 1.1. However, it has been shown that proton RBE varies with linear energy transfer (LET), physiological and biological factors, and clinical endpoint. After many years of improvement, the micro-kinetic model (MKM) allows now to obtain a fine description of the impact of radiation on tissues by using LET distributions and experimental cell survival curves. But, these measurements are generally performed using a 2D cell configuration, which is far to be realistic, compared to the natural arrangement of cells in tissues.To address this issue, we propose the prediction of RBE using the GATE Monte Carlo platform (www.opengatecollaboration.org) for 2D and 3D melanoma cell models irradiated with a therapeutic proton beam.Methods: Cell lines in 2D and 3D configurations are irradiated using a reference X-ray beam of 250 kVp and a therapeutic proton beam of 65 MeV. For the proton beam, cell survival curves are measured at several positions of a SOBP (spread-out Bragg peak) profile. The measured survival curves are then used to set the biological MKM parameters for both configurations.The MKM method combined with the LET simulation is implemented in the GATE platform. The LET distribution is obtained using a tissue-equivalent proportional counter geometry emulating a sphere of tissue equivalent to a cell nucleus (1 µm in diameter).Finally,Results: RBE calculated along proton SOBP depth profiles using 2D and the 3D cell configurations were compared. Survival curves obtained with the reference X-ray beam for 2D and 3D configurations revealed a higher sensitivity to radiation for 3D cell configuration.Conclusion: The setting of the MKM parameters (α₀, β, rd) in GATE for melanoma cells has been performed; the same methodology will be applied for other cell lines during 2017.Funding Support, Disclosures, and Conflict of Interest: This work was supported by grants from LabEx PRIMES (Physique, Radiobiologie, Imagerie Medicale et Simulation)

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