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Reference Dosimetry for An MRI-Linac: The Magnetic Field Correction Factor

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K Smit

K Smit*, B Asselen, J Kok, J Lagendijk, B Raaymakers, Univ Medical Center Utrecht Utrecht

TH-E-BRB-9 Thursday 1:00:00 PM - 2:50:00 PM Room: Ballroom B

Purpose:

To investigate if a standard dosimetry protocol (e.g. AAPM TG51) can be adapted to perform absolute dose calibration for a 1.5 T MRI-linac system.

Methods:

Measurements were performed in an 1.5 T MRI-linac system, a combination of a 1.5 T Philips Achieva MR scanner and a 6 MV Elekta linear accelerator.
To determine the effect of a lateral 1.5 T magnetic field on the reading of a NE2571 Farmer type ionization chamber, measurements were performed with and without a 1.5 T magnetic field. The primary chamber was placed at the isocenter in a stationary water phantom (PTW, Freiburg, Germany). A second reference chamber was placed 168 cm behind the primary chamber on the central beam axis, where the magnetic field is 0 T.
To ascertain correct functioning of the chamber inside a magnetic field, measurement reproducibility was investigated, as well as the influence of chamber orientation with respect to the central beam axis and the magnetic field (discussed in the supplement).

Results:

The magnetic field increases the ionization chamber reading by 4.8% resulting in a correction factor of 0.954 for this setup.
The reproducibility of the ionization chamber did not appear to be affected by the magnetic field.
The ionization chamber orientation in the MRI-linac affects the reading with a maximum of 9.2%. However, for the standard dosimetry setup a relatively large orientation change of 10°, will influence the reading by less than 0.2%.

Conclusions:

These preliminary results show that a standard dosimetry protocols can be used to perform dosimetry in an MRI-linac system, if a correction factor is applied for the magnetic field influence on the ionization chamber reading.

Current research focuses on testing more NE2571 chambers and investigating the magnetic field influences on reading correction factors such as polarity and ion recombination.
Results are expected shortly.





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