SU‐E‐T‐494: A MOSFET‐Based In‐Vivo Dosimetry System for MR Image‐Guided Radiation Therapy (MR‐IGRT)

Abstract

Purpose:To determine if a MOSFET based in‐vivo dosimetry system can be used for patients undergoing MR‐IGRT.Methods:Standard and high sensitivity MOSFET detectors were used for in‐field and out‐of‐field measurements respectively. The systems were benchmarked and calibrated against a calibrated ionization chamber on a standard 6 MV linear accelerator, and then on the MR‐IGRT system. Known doses were delivered to a water phantom with the MOSFETs placed between the top of the phantom and underneath a layer of bolus and water equivalent plastic, using a 6 MV beam and a 6 0Co MR‐IGRT beam. The latter was performed with and without real‐time MRI‐guidance during the beam delivery (MRIGRT).Results:The in‐field dosimeter response was linear from 50‐500 cGy with little evidence of energy dependence or change in response due to the permanent static magnetic field of the MR‐IGRT system. The detector response varied by < 2% between 6 MV and 6 0Co without image guided delivery. The out‐of‐field dosimeter response was linear from 1‐50 cGy; however the detectors did display dose rate and energy dependence as the response varied by > 20% depending on distance from isocenter used during calibration. Therefore, to use the dosimeters for out‐of‐field measurements they must be calibrated out‐of‐field. Regardless of the detector orientation in the coronal plan, the response of the MOSFETs during MRI‐guided delivery increased by 5% due to induced currents from the dynamic magnetic field present with image guidance. During the MRI‐guided delivery, some loss in image quality was seen when the MOSFETs were present in the imaging plane. This was mitigated by using a handheld reader without a transmitting wireless receiver.Conclusion:A MOSFET‐based in‐vivo dosimetry system can be used for patients receiving MR‐IGRT; however the change in detector response due to the dynamic magnetic field requires a special calibration.