Purpose: MRI-guided radiation therapy systems (MRIgRT) are being developed to improve onlineimaging during treatment delivery. At present, the operation of single point dosimeters and anionization chamber array have been characterized in such systems. This work investigates a novel 2Ddiode array, named magic plate, for both single point calibration and 2D positional performance, thelatter being a key element of modern radiotherapy techniques that will be delivered by these systems.Methods: GEANT 4 Monte Carlo methods have been employed to study the dose response of a silicondiode array to 6 MV photon beams, in the presence of in-line and perpendicularly aligned uniformmagnetic fields. The array consists of 121 silicon diodes (dimensions 1.5×1.5×0.38 mm3) embeddedin kapton substrate with 1 cm pitch, spanning a 10×10 cm2 area in total. A geometrically identical,water equivalent volume was simulated concurrently for comparison. The dose response of the silicondiode array was assessed for various photon beam field shapes and sizes, including an IMRT field, at1 T. The dose response was further investigated at larger magnetic field strengths (1.5 and 3 T) for a4×4 cm2 photon field size.Results: The magic plate diode array shows excellent correspondence (<±1%) to water dose in thein-line orientation, for all beam arrangements and magnetic field strengths investigated. The perpendicularorientation, however, exhibits a dose shift with respect to water at the high-dose-gradientbeam edge of jaw-defined fields [maximum (4.3 ±0.8)% over-response, maximum (1.8 ±0.8)%under-response on opposing side for 1 T, uncertainty 1?]. The trend is not evident in areas within-field dose gradients typical of IMRT dose maps.Conclusions: A novel 121 pixel silicon diode array detector has been characterized by MonteCarlo simulation for its performance inside magnetic fields representative of current prototype andproposed MRIlinear accelerator systems. In the in-line orientation, the silicon dose is directlyproportional to the water dose. In the perpendicular orientation, there is a shift in dose responserelative to water in the highest dose gradient regions, at the edge of jaw-defined and single-segmentMLC fields. The trend was not observed in-field for an IMRT beam. The array is expected to bea valuable tool in MRIgRT dosimetry.