Commissioning and performance evaluation of RadCalc for the Elekta unity MRI‐linac

Abstract

Recent availability of MRI-guided linear accelerators has introduced a number of clinical challenges, particularly in the context of online plan adaptation. Paramount among these is verification of plan quality prior to patient treatment. Currently, there are no commercial products available for monitor unit verification that fully support the newly FDA cleared Elekta Unity 1.5 T MRI-linac. In this work, we investigate the accuracy and precision of RadCalc for this purpose, which is a software package that uses a Clarkson integration algorithm for point dose calculation. To this end, 18 IMRT patient plans (186 individual beams) were created and used for Rad-Calc point dose calculations. In comparison with the primary treatment planning system (Monaco), mean point dose deviations of 0.0 ± 1.0% (n = 18) and 1.7 ± 12.4% (n = 186) were obtained on a per-plan and per-beam basis, respectively. The dose plane comparison functionality within RadCalc was found to be highly inaccurate, however, modest improvements could be made by artificially shifting jaws and multi leaf collimator positions to account for the dosimetric shift due to the magnetic field (67.3% vs 96.5% mean 5%/5 mm gamma pass rate). K E Y W O R D S dose calculation, Elekta Unity, MRI-Linac, RadCalc The first 1.5 T MRI-equipped linear accelerator (Elekta Unity, Elekta AB, Stockholm, Sweden) was cleared by the U.S. Food and Drug Administration in December of 2018. Coupling a high-magnetic field MRI with a linear accelerator introduces a number of commissioning and routine quality assurance challenges that are not associated with conventional linear accelerators. Notably, all equipment ---

Commissioning of a 1.5T Elekta Unity MR‐linac: A single institution experience

“…The nonstandard geometry of the Unity, including the extended SSD, introduces additional complexity in the reference dosimetry calibration of the machine. In this work a beam specifier of TPR 20 10 was used and converted into a % dd (10) x such that the remainder of the procedure would follow the AAPM TG-51 guidelines. The measured T A B L E 5 Output versus Gantry with Incorporation of MR-Linac Couch.…”

“…Due to the extended SSD of the Unity, the beam quality specifier measured was TPR 20 10 . 27 The TPR 20 10 was then converted into a %dd (10) x using the formalism provided by Kalach et al 28 The remainder of the reference dosimetry calibration procedure followed the recommendations provided by AAPM Task Group Report 51 addendum. 29…”

“…The TPR 20 10 beam quality specifier value measured was 0.704. Using the formalism provided by Kalach et al 28 this equates to a %dd (10) x The difference between the reported delivered dose and the independently measured dose was −0.9%. Figure 7 shows representative PDDs and profiles measured during commissioning.…”

“…Collimator rotation is disabled in this design, and the MLC leaves move in the superior/inferior direction with respect to patient anatomy with a leaf width of 7.175 mm at the isocenter plane. 10 The Unity has a source to isocenter distance of 143.5 cm and an inner-bore diameter of 70 cm. The system is currently only capable of delivering Intensity Modulated Radiotherapy (IMRT) and 3D conformal treatments.…”

“…While the Unity shows promise for improving patient care, it also poses significant commissioning challenges which are not present for standard linacs. Some of these challenges include performing reference dosimetry in nonstandard geometry, 11 accounting for dosimetric effects from the Lorentz force on secondary electrons, 10,12,13 and ensuring coincidence between the imaging and radiation coordinate systems. As the MRI is used directly for treatment planning in some adaptive workflows, the geometric accuracy, effect of the linac and gantry components on the MRI, and general image quality of the MRI requires evaluation.…”