SU-G-BRB-08: Investigation On the Magnetic Field Effect On TLDs, OSLDs, and Gafchromic Films Using An MR-Linac.
PURPOSE
To investigate whether a strong magnetic field (B=1.5 T) can affect dose responses of thermoluminescent dosimeters (TLDs), optically stimulated luminescence dosimeters (OSLDs) and Gafchromic films using an MR-Linac (Elekta) before and after the magnet was ramped down from 1.5 T to 0 T.


METHODS
Three types of dosimeters (TLDs, OSLDs, EBT3 films) were divided into two groups. Group 1 was first irradiated in a phantom of Solid Water slabs (Standard Imaging) inside a B=1.5 T field with a 7 MV beam from an MR-Linac system. The radiation output at the location of the dosimeters (isocenter at 10 cm depth) was measured using an ion chamber (NE2571, Phoenix Dosimetry). Three doses (150, 300, 600 MU, corresponding to 1.18, 2.36, and 4.74 Gy) were delivered to the dosimeters. A week later the MR magnet was ramped down to zero field and dosimeters in Group 2 were irradiated with the same MUs. Dosimeters of each type were read out during the same session (about 4 weeks post irradiation in the B field, and 3 weeks with no B field). The ratios of signals between Group 1 and Group 2 were calculated.


RESULTS
Radiation output measured with the chamber was within 1% before and after ramping down the MR magnet. For TLDs, the ratio of signals with B field to signals without B field averaged over three dose levels was 1.003±0.016; for OSLDs, the ratio was 0.994±0.022; for films, the ratios of two batches (different manufacturing dates) were 0.997 and 0.985.


CONCLUSION
Dose responses of all three dosimeters seem not affected by the presence of a 1.5 T magnetic field within uncertainty of ∼2%. More measurements will be conducted to test reproducibility. We acknowledge research support from Elekta AB.