Investigation of TLD and EBT 3 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 EBT 3 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 MRI dian Cobalt‐60 ( 60 Co) system. Both systems were irradiated with an equivalent 10 × 10 cm 2 field size, and a prescribed dose of 3 Gy to the maximum depth deposition (dmax). EBT 3 film and TLD s were investigated using two geometrical Magnetic Resonance ( MR )‐guided Radiation Therapy ( MR g RT ) head and thorax phantoms. Each geometrical phantom had eight quadrants that combined to create a centrally located rectangular tumor (3 × 3 × 5 cm 3 ) surrounded by tissue to form a 15 × 15 × 15 cm 3 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. EBT 3 and double‐loaded TLD s were used in the phantoms to compare beam profiles and point dose measurements with and without magnetic fields. GEANT 4 Monte Carlo simulations were performed to validate the detectors for both Unity 0 T/1.5 T and MRI dian 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 MRI dian 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 MRI dian system were 0.2% and −0.3%, respectively. Beam profiles generated with both systems agreed with Monte Carlo measurements and previous literature findings. Conclusions TLD s and EBT 3 film dosimeters could potentially be used in MR / CT visible tissue equivalent phantoms that will experience a magnetic field environment.