Investigation of TLD and EBT3 performance under the presence of 1.5T, 0.35T, and 0T magnetic field strengths in MR/CT visible materials.

PURPOSE: The aim of this study was to investigate thermoluminescent dosimeters (TLD) and radiochromic EBT3 film inside MR/CT visible geometric head and thorax phantoms in the presence of: 0, 0.35, and 1.5 T magnetic fields. METHODS: Thermoluminescent Dosimeters reproducibility studies were examined by irradiating IROC-Houston's TLD acrylic block five times under 0 and 1.5 T configurations of Elekta's Unity system and three times under 0 and 0.35 T configurations of ViewRay's MRIdian Cobalt-60 (60 Co) system. Both systems were irradiated with an equivalent 10 × 10 cm2 field size, and a prescribed dose of 3 Gy to the maximum depth deposition (dmax). EBT3 film and TLDs were investigated using two geometrical Magnetic Resonance (MR)-guided Radiation Therapy (MRgRT) head and thorax phantoms. Each geometrical phantom had eight quadrants that combined to create a centrally located rectangular tumor (3 × 3 × 5 cm3 ) surrounded by tissue to form a 15 × 15 × 15 cm3 cubic phantom. Liquid polyvinyl chloride plastic and Superflab were used to simulate the tumor and surrounding tissue in the head phantom, respectively. Synthetic ballistic gel and a heterogeneous in-house mixture were used to construct the tumor and surrounding tissue in the thorax phantom, respectively. EBT3 and double-loaded TLDs were used in the phantoms to compare beam profiles and point dose measurements with and without magnetic fields. GEANT4 Monte Carlo simulations were performed to validate the detectors for both Unity 0 T/1.5 T and MRIdian 0 T/0.35 T configurations. RESULTS: Average TLD block measurements which, compared the magnetic field effects (magnetic field vs 0 T) on the Unity and MRIdian systems, were 0.5% and 0.6%, respectively. The average ratios between magnetic field effects for the geometric thorax and head phantoms under the Unity system were -0.2% and 1.6% and for the MRIdian system were 0.2% and -0.3%, respectively. Beam profiles generated with both systems agreed with Monte Carlo measurements and previous literature findings. CONCLUSIONS: TLDs and EBT3 film dosimeters could potentially be used in MR/CT visible tissue equivalent phantoms that will experience a magnetic field environment.