SU‐E‐T‐250: Measurements of Surface and Interface Dose for 6 MV Photons in a Parallel Magnetic Field

Abstract

Purpose: The patients treated using Linac‐MR systems, being developed for real time MR guided radiotherapy, will be irradiated in the presence of a magnetic field. The present study presents the entrance surface, buildup and interface doses measured in a 6 MV beam in the presence of a parallel magnetic field. Methods: A set of phantoms with one angled surface, and a phantom with an adjustable air gap were constructed out of polystyrene. These phantoms were designed to measure both the percent depth dose and the surface dose at the same time using Gafchromic film. Two solenoid electromagnets, placed on top of each other, were used to generate a magnetic field along their vertical bore (26.5 cm deep x 17.5 cm diameter) and a decreasing fringe field outside. The magnetic field strength is variable from 0 T to 0.2 T at the bore's center. The phantoms were placed inside the bore and irradiated using field sizes that avoided irradiation of the magnetic coils. Results: The parallel magnetic field was found to focus the contaminating electrons produced in the linac head and in the air column above the phantom, within the beam area. The surface dose, for all the surface angles investigated, increased by ∼15% of the Dmax dose at 0.2 T magnetic field. For the air gap phantom the concentration of electrons inside the beam area by the parallel magnetic field is clearly visible inside the air gap on the depth dose film and in the percent depth dose curve. Conclusion: We have devised an experimental system to measure the radiation dose of a clinical linac in a parallel magnetic field. We are currently performing Monte Carlo simulations that include magnetic fields replicating the measurement system. The verified Monte Carlo system will make the dose calculation engine for future linac‐MR systems.