Improving the clinical workflow of a MR-Linac by dosimetric evaluation of synthetic CT

Abstract

Adaptive radiotherapy performed on the daily magnetic resonance imaging (MRI) is an option to improve the treatment quality. In the adapt-to-shape workflow of 1.5-T MR-Linac, the contours of structures are adjusted on the basis of patient daily MRI, and the adapted plan is recalculated on the MRI-based synthetic computed tomography (syCT) generated by bulk density assignment. Because dosimetric accuracy of this strategy is a priority and requires evaluation, this study aims to explore the usefulness of adding an assessment of dosimetric errors associated with recalculation on syCT to the clinical workflow. Sixty-one patients, with various tumor sites, treated using a 1.5-T MR-Linac were included in this study. In Monaco V5.4, the target and organs at risk (OARs) were contoured, and a reference CT plan that contains information about the outlined contours, their average electron density (ED), and the priority of ED assignment was generated. To evaluate the dosimetric error of syCT caused by the inherent approximation within bulk density assignment, the reference CT plan was recalculated on the syCT obtained from the reference CT by forcing all contoured structures to their mean ED defined on the reference plan. The dose–volume histogram (DVH) and dose distribution of the CT and syCT plan were compared. The causes of dosimetric discrepancies were investigated, and the reference plan was reworked to minimize errors if needed. For 54 patients, gamma analysis of the dose distribution on syCT and CT show a median pass rate of 99.7% and 98.5% with the criteria of 3%/3 mm and 2%/2 mm, respectively. DVH difference of targets and OARs remained less than 1.5% or 1 Gy. For the remaining patients, factors (i.e., inappropriate ED assignments) influenced the dosimetric agreement of the syCT vs. CT reference DVH by up to 21%. The causes of the errors were promptly identified, and the DVH dosimetry was realigned except for two lung treatments for which a significant discrepancy remained. The recalculation on the syCT obtained from the planning CT is a powerful tool to assess and decrease the minimal error committed during the adaptive plan on the MRI-based syCT.

Potential dosimetric error in the adaptive workflow of a 1.5 T MR-Linac from patient movement relative to immobilisation systems

“…Once the problem reported here was recognised, subsequent rectum cancer patients were immobilised without masks to avoid the issue, using Wing-STEP, KneeSTEP, and FeetSTEP supports (IT-V, Innsbruck, Austria) instead. A bladder catheter was used at simulation and at each treatment fraction to ensure consistent filling [ 11 , 12 ]. Simulation imaging was acquired on a Brilliance Big Bore CT (Philips Medical System, Cleveland, OH, USA) and the MR-linac’s scanner, consecutively on the same day, with the same immobilization devices.…”

“…The CT reference plan includes all the information needed to generate the sCT during the online MR-based adaptive workflow, including average EDs and a given layering priority for each contour and ppd component. The accuracy of the calculation using the sCT generated by bulk density assignment is assessed for each patient within the clinical workflow [ 11 ].…”

“…During the online adapt-to-shape (ATS) procedure, the daily acquired MRI is registered to the simulation CT or MRI from a previous session and the contour information, including average EDs and their layering (prioritisation), is propagated to MRI to generate a sCT by ED assignment on the MRI. The adapted plan is reoptimized and recalculated on the sCT, so the dosimetric accuracy of this strategy needs accurate evaluation [ 1 , 9 – 11 ].…”

Towards simulation-free MR-linac treatment: utilizing male pelvis PSMA-PET/CT and population-based electron density assignments

“…Previous literature has described the key benefits and adaptive radiotherapy (ART) workflows of these systems (Winkel et al 2019, de Leon et al 2022, Pham et al 2022, Rammohan et al 2022. The bulk RED override step of this workflow occurs when the voxelised CT-numbers, provided by the individual patient's simulation-CT images, are converted to an average RED value for each contoured organ (Tang et al 2022). These values are often the primary driver of dose calculation differences between tissue types and are propagated throughout the ART workflow, which includes contour adjustment or re-generation based on daily MR-images.…”

“…Recent studies have reported on the dosimetric impact of using bulk RED overrides on MRL systems (Ding et al 2022, Green et al 2019, Pham et al 2022, Tang et al 2022. Due to the presence of the magnetic field, investigating these effects in such systems are of particular interest, especially near tissue-air boundaries which are subject to the electron return effect by the Lorentz force (Uilkema et al 2015, Green et al 2019, Pham et al 2022, Rammohan et al 2022.…”