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Characterization and Validation of TLD and EBT3 Film in MR/CT Visible Phantoms Under the Presence of 0T, 0.35T and 1.5T

A Steinmann1*, D O'Brien2 , K Mittauer3 , J Bayouth3 , G Sawakuchi1 , Z Wen1 , L Court1 , R Stafford1 , C Fuller1 , D Followill1 , (1) UT MD Anderson Cancer Center, Houston, TX,(2) Elekta, London, UK ,(3) University of Wisconsin-Madison, Madison, WI


(Tuesday, 7/31/2018) 10:00 AM - 10:30 AM

Room: Exhibit Hall | Forum 2

Purpose: To characterize and validate TLD and radiochromic EBT3 film dosimeters inside MR/CT visible geometric head and lung phantoms in the presence of: 0T, 0.35T and 1.5T magnetic fields.

Methods: TLD reproducibility studies were examined by irradiating IROC’s TLD block 5 times under 0T and 1.5T configurations of Elekta’s Unity system and 3 times under 0T and 0.35T configurations of ViewRay’s MRIdian Co-60 system. Both systems were irradiated with an equivalent 10x10cm² field size, and prescribed dose of 3Gy to dmax. EBT3 film and TLDs were characterized using two geometrical MRI guided radiotherapy (MRgRT) head and lung phantoms. Each geometrical phantom had eight quadrants that combined to create a centrally located tumor (3x3x5cm³) surrounded by tissue to form a 15x15x15cm³ phantom. Liquid PVC plastic and Superflab were used to simulate the tumor and surrounding tissue in the head phantom, respectively. Synthetic ballistic gel and in-house petroleum jelly/styrofoam ball mixture was used to construct the tumor and surrounding tissue in the lung phantom, respectively. EBT3 and TLDs were used in the phantoms to compare beam profiles and point dose measurements with and without magnetic fields. GEANT4 Monte Carlo simulations were also performed to validate the detectors for both Unity 0T/1.5T and MRIdian 0T/0.35T configurations.

Results: Average TLD block measurements comparing magnetic field effects (magnetic field vs. 0T) on the Unity and MRIdian systems were 2.5% and 1.2%, respectively. Average ratio between magnetic fields for the lung and head configurations under the Unity system were -0.7% and -3.2% and under the MRIdian system were -0.3% and 0.2%, respectively. Beam profiles generated with both systems agreed with Monte Carlo measurements and previous literature findings.

Conclusion: TLDs and EBT3 film dosimeters can be implemented in the developed MR/CT visible QA phantoms for MRgRT delivery systems.

Funding Support, Disclosures, and Conflict of Interest: IROC Houston grant # CA180803


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