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Measurement of Output Factors with Various Detectors in the Presence of the ViewRay MRIdian Magnetic Field

B Barraclough*, D Dunkerley , K Vredevoogd , B Paliwal , K Mittauer , P Yadav , School of Medicine and Public Health UW-Madison, Madison, WI


(Saturday, 3/30/2019)  

Room: Exhibit Hall

Purpose: Many detectors have been already been examined for their ability to accurately measure OFs in standard and small fields. However, the introduction of magnetic fields into the treatment field necessitates a re-examination of these detectors. In this work, the ability of a variety detectors to measure OFs in the presence of the magnetic field of a ViewRay MRIdian Linac was examined.

Methods: OFs were measured at 5cm depth in either a 1D water phantom or a solid water phantom. The phantom was placed at 75cm SSD with field sizes ranging from 0.415x0.415 cm2 to 25.5x25.5cm2 and readings normalized to the 9.96x9.96cm2 field. The magnetic field strength was 0.35T. The detectors examined were the SunNuclear Edge diode, Standard Imaging Exradin W2 scintillator, and Best Medical MOSFET. These were compared to Gafchromic EBT3 radiochromic film. Additionally, all detectors were compared with Monte Carlo (MC) calculated OFs from ViewRay's TPS.

Results: OFs measured by all detectors were within 2% of the film OFs. The largest discrepancies were for the diode and MOSFET at the smallest field size measured by film, 0.83x0.83cm2. They were 1.9% and 1.4% larger than the radiochromic film, respectively. The W2 at this field size was within 0.6%. It’s greatest difference from film was in the 3.32x3.32cm2 field, where it was 1.0% lower. The MC calculation’s largest deviation was 1.0% higher than film at the 0.83x0.83cm2 field size. Comparing each detector against the MC calculation, the film, diode, and W2 were all within 1.6%. The MOSFET had a 2.2% difference at the 12.45x12.45cm2 field.

Conclusion: The detectors examined accurately measured output factors for ViewRay’s MRIdian Linac. The 0.35T field has a minimal effect on the measurement of OFs. The W2 scintillator measured OFs most similar to the film measurements. Agreement of all detectors with MC calculations was within 2.2%.


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