Analysis of patient‐specific quality assurance for Elekta Unity adaptive plans using statistical process control methodology

Abstract

The Elekta Unity MR-linac utilizes daily magnetic resonance imaging (MRI) for online plan adaptation. In the Unity workflow, adapt to position (ATP) and adapt to shape (ATS) treatment planning options are available which represent a virtual shift or full re-plan with contour adjustments respectively. Both techniques generate a new intensity modulated radiation therapy (IMRT) treatment plan while the patient lies on the treatment table and thus adapted plans cannot be measured prior to treatment delivery. A statistical process control methodology was used to analyze 512 patient-specific IMRT QA measurements performed on the MR-compatible SunNuclear ArcCheck with a gamma criterion of 3%/2 mm using global normalization and a 10% low dose threshold. The lower control limit (LCL) was determined from 68 IMRT reference plan measurements, and a one-sided process capability ratio ðC p,l Þ was used to assess the pass rates from 432 measured ATP and 80 measured ATS plans. Further analysis was performed to assess differences between SBRT or conventional fractionation pass rates and to determine whether there was any correlation between the pass rates and plan complexity. The LCL of the reference plans was determined to be a gamma pass rate of 0.958, and the C p,l of the measured ATP plans and measured ATS plans were determined to be 1.403 and 0.940 for ATP and ATS plans, respectively, while a C p,l of 0.902 and 1.383 was found for SBRT and conventional fractionations respectively. For plan complexity, no correlation was found between modulation degree and gamma pass rate, but a statistically significant correlation was observed between the beam-averaged aperture area and gamma pass rate. All adaptive plans passed the TG-218 guidelines, but the ATS and SBRT plans tended to have a smaller beam-averaged aperture area with slightly lower gamma pass rates.

Correlation between patient-specific quality assurance in volumetric modulated arc therapy and 2D dose image features

“…In fact, the process control charts could be implemented without a large database. They can be created with only 20-30 data points 6,23 . In addition, the magnitude in C pk index was detected to be proportional to the distance between LCL and CL.…”

“…The SPC technique has been widely utilized to monitor the QA process of treatment planning, by which process-based tolerances and action limits were set according to the protocol of TG-218 21 . It also determines the effects of changes in methods or conditions during the course of a radiotherapy experiment, such as dosimetric changes to a treatment plan when using alternative techniques [22][23][24] . In general, statistical quality control consists of comparing a process's current performance with its historical performance 25 .…”

Online adaptive radiotherapy for head and neck cancers on the MR linear Accelerator: Introducing a novel modified Adapt-to-Shape approach

“…For this study’s assessments, simulated online plans for each ATP variant were retrospectively calculated (AS) or optimised (OW or OS) using the default Monaco TPS optimisation parameters for every fraction. ATS-Lite uses the standard Monaco TPS inverse planning optimiser, whereas ATP utilises a simplified optimisation algorithm specifically developed for this planning strategy [7] . Daily plans were calculated to 30 fractions to allow the use of local standard clinical goals to assess plan acceptability.…”

Clinical Implementational and Site-Specific Workflows for a 1.5T MR-Linac

“…For both ATP and ATS treatment options, data-transfer integrity and secondary dosimetric calculations are performed using RadCalc for each fraction [ 9 ]. Additionally, ArcCheck measurements were performed for the first 512 fractions until a statistical evaluation was performed demonstrating confidence that adaptive plans are expected to be acceptable if the reference-plan patient-specific QA measurement was also acceptable [ 10 ].…”

“…The workflows presented are specific to a single institution but can provide a useful foundation for any center building a new program through the presentation of case studies and discussion of clinical rationale. It is not the purpose of this work to discuss commissioning of a MRgART program; for that discussion, readers are referred to other works [ 8 , 9 , 10 , 11 , 12 , 13 ].…”