SU-F-303-15: Ion Chamber Dose Response in Magnetic Fields as a Function of Incident Photon Energy
Purpose: In considering the continued development of synergetic MRI-radiation therapy machines, we seek to quantify the variability of ion chamber response per unit dose in the presence of magnetic fields of varying strength as a function of incident photon beam quality and geometric configuration. Methods: To account for the effect of magnetic fields on the trajectory of charged particles a new algorithm was introduced into the EGSnrc Monte Carlo code. In the egs_chamber user code the dose to the cavity of an NE2571 ion chamber is calculated in two configurations, in 0 to 2 T magnetic fields, with an incoming parallel 10×10 cm2 photon beam with energies ranging between 0.5 MeV and 8 MeV. In the first, the photon beam is incident on the long-axis of the ion chamber (config-1), and in the second the beam is parallel to the long-axis and incident from the conical end of the chamber (config-2). For both, the magnetic field is perpendicular to the direction of the beam and the long axis of the chamber. Results: The ion chamber response per unit dose to water at the same point is determined as a function of magnetic field and is normalized to the 0T case for each of incoming photon energies. For both configurations, accurate modeling of the ion chamber yielded closer agreement with the experimental results obtained by Meijsing et. al (2009). Config-1 yields a gradual increase in response with increasing field strength to a maximum of 13.4% and 1.4% for 1 MeV and 8 MeV photon beams, respectively. Config-2 produced a decrease in response of up to 6% and 13% for 0.5 MeV and 8 MeV beams, respectively. Conclusion: These results provide further support for ion chamber calibration in MRI-radiotherapy coupled systems and demonstrates noticeable energy dependence for clinically relevant fields.