(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: the geometry necessary to position TLD capsules and OSLD nanodot for output check verification of photon beams from an MR Linac.
Methods: 7 FFF MV photon beam from an Elekta Unity with a magnetic field of 1.5 T was used for the irradiations of two types of dosimeters, OSLD nanodot and TLD-100 cylindrical capsules. The capsules were single-loaded (air inside capsule per supplier), single-loaded compressed (air removed in house), and double-loaded (double amount of powder, no air, per supplier). Detectors (two OSLD nanodots or three TLD capsules) were irradiated in the center of an acrylic miniphantom block, using an anterior-posterior beam with field size 10 cm x 10 cm at isocenter (SAD=143.5 cm). Doses delivered were 300 cGy for TLD and 100 cGy for OSLD. The OSLD nanodots were irradiated with the magnetic field in the same plane as the face of the carrier. TLD were oriented in two directions, with the capsule axis either parallel or perpendicular to the magnetic field.
Results: ratio between OSLD dose and delivered dose was 1.004 +/- 0.011. The TLD capsules showed a 2.5 % difference in the signal between the two orientations, with the capsules in the perpendicular direction overestimating the output. The single-loaded compressed capsules and the double-loaded capsules showed the same results, regardless of orientation. Results from single-loaded capsules with air were inconsistent with delivered dose (further investigation required).
Conclusion: the geometry used, the OSLD showed no change in the response with the presence of the magnetic field. For output checks based on TLD double-loaded or single-loaded compressed capsules, the TLD should be oriented parallel to the magnetic field.
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