SU‐E‐T‐227: Could the Alpha/Beta Ratio Change in a Strong Magnetic Field?

Abstract

Purpose:Magnetic resonance imaging (MRI) is being integrated into radiotherapy delivery for MRI‐guided radiotherapy. The presence of a strong magnetic field from a MRI machine during radiotherapy delivery presents a new challenge since the trajectories of electrons liberated by ionizing radiation in patients are strongly dependent on the applied magnetic field. The purpose of this work is to explore the potential effect of a strong magnetic field on the α/β ratio, an important radiobiological parameter in radiotherapy.Methods:Based on the theory of dual radiation action, the α/β ratio can be expressed by an integral of the product of two microdosimetry quantities γ(x) and t(x), where γ(x) is the probability that two energy transfers, a distance × apart, results in a lesion, and t(x) is the proximity function, which is the energy‐weighted point‐pair distribution of distances between energy transfer points in a track. The quantity t(x) depends on the applied magnetic field. An analytical approach has been used to derive a formula that can be used to calculate the α/β ratio in an extremely strong magnetic field.Results:The α/β ratio has been evaluated in the special case when the applied magnetic field approaches infinity, which gives the upper limit of the potential change of the α/β ratio due to the presence of a strong magnetic field. For V79 Chinese hamster cells it has been shown that the α/β ratio could be increased by 2.90 times for Pd‐103, 2.97 times for I‐125 and about 2.3 times for Co‐60 sources when the applied magnetic field approaches infinity.Conclusion:It has been shown theoretically that the α/β ratio can change in a strong magnetic field, and there could be up to a nearly three‐fold increase in the α/β ratio, depending on the strength of the applied magnetic field, the cell type and the radiation used.