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Small Field Output Factor (FOF) Measurements During Commissioning of a 0.35T MR-Linac System

M Alghamwa1*, J Sohn1,2, R Hawranko1, S Kim1, P Yadav2, I Das2, W Song1, (1) Virginia Commonwealth University, Richmond, VA, (2) Northwestern University Feinberg School of Medicine, Chicago, IL

Presentations

SU-H430-IePD-F4-6 (Sunday, 7/10/2022) 4:30 PM - 5:00 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 4

Purpose: To study field output factors (FOF) on an MRIdian MR-Linac system using four different detectors in a 0.35T magnetic field.

Methods: The FOFs were measured for fields from 0.83×0.83 cm² to 24.1×27.4 cm² at SSD 85 cm and 5 cm depth and normalized with the field size of 9.96×9.96 cm². Small field sizes considered in this study are less than 4×4 cm². The FOFs were obtained with the following detectors: PTW Semiflex 3D, Standard Imaging Exradin A28 MR, PTW MicroDiamond MR, and Sun Nuclear Edge. The FOF baselines were calculated using the MRIdian Monte Carlo (MC) dose calculation engine. Measurements and calculations were compared to determine the percentage difference.

Results: The FOFs measured by the Edge exhibits the closest agreement compared to the MRIdian TPS for field sizes of 0.83×0.83 cm2 and 1.66×1.66 cm2 with a percentage difference of 2.67% and 0.12%, respectively, while for the MicroDiamond the percentage difference was 3.81% and 0.20%, respectively. Furthermore, the field sizes of 2.49×2.49 cm² and 3.32×3.32 cm², MicroDiamond shows good agreement with 1.77%, and 0.84% compared to Edge with 1.47%, and 1.36% respectively. On the other hand, thimble chambers such as the Semiflex 3D presents large differences of 22.80%, 0.20%, 0.07%, and 0.84% for fields of 0.83×0.83 cm², 1.33×1.33 cm², 2.49×2.49 cm², and 3.32×3.32 cm², respectively. The Exradin A28 MR demonstrates the largest differences in this study, varying by 34.68%, 3.15%, 1.03%, and 1.03% for field sizes of 0.83×0.83 cm², 1.66×1.66 cm², 2.49×2.49 cm², and 3.32×3.32 cm², respectively.

Conclusion: For the smallest field sizes, the Edge and MicroDiamond MR can be used with the closest agreement between the MRIdian MC algorithm. On the contrary, thimble chambers such as Semiflex 3D MR and Exradin A28 MR are not suitable for small field dosimetry of 0.35T MR-Linac systems.

Keywords

Small Fields, Commissioning, MR

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined: experimental dosimetry (other than ion chamber)

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