title: Monte Carlo simulation of the dose response of a novel 2D silicon diode array for use in hybrid MRI-LINAC systems.
abstract: OBJECTIVE
  MRI-guided radiation therapy systems (MRIgRT) are being developed to improve online imaging during treatment delivery. At present, the operation of single point dosimeters and an ionization chamber array have been characterized in such systems. This work investigates a novel 2D diode array, named "magic plate," for both single point calibration and 2D positional performance, the latter being a key element of modern radiotherapy techniques that will be delivered by these systems.
  
  
  METHODS
  geant4 Monte Carlo methods have been employed to study the dose response of a silicon diode array to 6 MV photon beams, in the presence of in-line and perpendicularly aligned uniform magnetic fields. The array consists of 121 silicon diodes (dimensions 1.5 × 1.5 × 0.38 mm(3)) embedded in kapton substrate with 1 cm pitch, spanning a 10 × 10 cm(2) area in total. A geometrically identical, water equivalent volume was simulated concurrently for comparison. The dose response of the silicon diode array was assessed for various photon beam field shapes and sizes, including an IMRT field, at 1 T. The dose response was further investigated at larger magnetic field strengths (1.5 and 3 T) for a 4 × 4 cm(2) photon field size.
  
  
  RESULTS
  The magic plate diode array shows excellent correspondence (< ± 1%) to water dose in the in-line orientation, for all beam arrangements and magnetic field strengths investigated. The perpendicular orientation, however, exhibits a dose shift with respect to water at the high-dose-gradient beam 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 1σ]. The trend is not evident in areas with in-field dose gradients typical of IMRT dose maps.
  
  
  CONCLUSIONS
  A novel 121 pixel silicon diode array detector has been characterized by Monte Carlo simulation for its performance inside magnetic fields representative of current prototype and proposed MRI-linear accelerator systems. In the in-line orientation, the silicon dose is directly proportional to the water dose. In the perpendicular orientation, there is a shift in dose response relative to water in the highest dose gradient regions, at the edge of jaw-defined and single-segment MLC fields. The trend was not observed in-field for an IMRT beam. The array is expected to be a valuable tool in MRIgRT dosimetry.
authors: Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.
authors: Maegan
authors: M
authors: Gargett
authors: Illawarra Cancer Care Centre, Wollongong Hospital, NSW 2500, Australia and Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.
authors: Brad
authors: B
authors: Oborn
authors: Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia and Liverpool Cancer Therapy Centre and Ingham Institute, NSW 2170, Australia.
authors: Peter
authors: P
authors: Metcalfe
authors: Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.
authors: Anatoly
authors: A
authors: Rosenfeld
mesh_terms: D008279:Magnetic Resonance Imaging; D009010:Monte Carlo Method; D010315:Particle Accelerators; D011829:Radiation Dosage; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation*; D050397:Radiotherapy, Intensity-Modulated; D012825:Silicon
chemical_list: D012825:Silicon