The Australian MRI-Linac Program: measuring profiles and PDD in a horizontal beam

Abstract

A back-projection algorithm in the presence of an extra attenuating medium: towards EPID dosimetry for the MR-Linac I Torres-Xirau, I Olaciregui-Ruiz, R A Rozendaal et al. Abstract. The Australian MRI-Linac consists of a fixed horizontal photon beam combined with a MRI. Commissioning required PDD and profiles measured in a horizontal set-up using a combination of water tank measurements and gafchromic film. To validate the methodology, measurements were performed comparing PDD and profiles measured with the gantry angle set to 0 and 90° on a conventional linac. Results showed agreement to within 2.0% for PDD measured using both film and the water tank at gantry 90° relative to PDD acquired using gantry 0°. Profiles acquired using a water tank at both gantry 0 and 90° showed agreement in FWHM to within 1 mm. The agreement for both PDD and profiles measured at gantry 90° relative to gantry 0° curves indicates that the methodology described can be used to acquire the necessary beam data for horizontal beam lines and in particular, commissioning the Australian MRI-linac.

Commissioning and evaluation of a radiochromic EBT3 film dosimetry system

“…The acceptance tolerance of the beam symmetry and flatness should not exceed 3%. The beam is more symmetric and flattened at higher energies, [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] which is well in agreement with our results, as our symmetry and flatness value did not exceed 3% for red and green channels, and also symmetry and flatness were better at high energies except in some cases. The results are in agreement with those of Sahani et al 33 and define the acceptance criteria of photon beam characteristics such as for symmetry, flatness and penumbra for pre-machine commissioning and for QC purposes at different energies and field sizes using International Electro Technical Commission (IEC 60976, 2008), which ensures that there is only a small negligible difference between calculated and measured data using two detectors.…”

Technical Note: Experimental characterization of the dose deposition in parallel MRI‐linacs at various magnetic field strengths

“…Setting up beam characterization measurements for a horizontal beam presented some unique challenges not normally observed during conventional linac commissioning. Comparison between horizontal film measurements and vertical water tank setups on a convention linear accelerator ensured the methodology used to collect the Linatron PDDs and profiles was accurate. Due to the metallic components and size restrictions, water tank equipment could not be used on the MR‐linac.…”

First measurements with a plastic scintillation dosimeter at the Australian MRI-LINAC

“…For dosimetry with MRI-LINACs, the presence of the MRIs magnetic field alters the trajectories of charged particles traversing through the magnetic field and consequently the dose distributions delivered [2,3,4,5]. The trajectory taken, and hence dose deposited by a charged particle in a volume of material is dependent on a materials density [2,3].…”

“…MRI-Linear accelerators (MRI-LINACs) combine an MRIs optimal soft-tissue contrast and a modern LINACs highly conformal dose distributions to deliver image guided radiation therapy with non-invasive tumour tracking (Liney et al 2018). For dosimetry with MRI-LINACs, the presence of the MRIs magnetic field alters the trajectories of charged particles traversing through the magnetic field and consequently the dose distributions delivered (Raaymakers et al 2004, Raaijmakers et al 2005, Liney et al 2016, Begg et al 2017. The trajectory taken, and hence dose deposited by a charged particle in a volume of material is dependent on a materials density (Raaymakers et al 2004, Raaijmakers et al 2005.…”