Characterization and longitudinal assessment of daily quality assurance for an MR‐guided radiotherapy (MRgRT) linac

Abstract

PurposeTo describe and characterize daily machine quality assurance (QA) for an MR‐guided radiotherapy (MRgRT) linac system, in addition to reporting a longitudinal assessment of the dosimetric and mechanical stability over a 7‐month period of clinical operation.MethodsQuality assurance procedures were developed to evaluate MR imaging/radiation isocenter, imaging and patient handling system, and linear accelerator stability. A longitudinal assessment was characterized for safety interlocks, laser and imaging isocenter coincidence, imaging and radiation (RT) isocentricity, radiation dose rate and output, couch motion, and MLC positioning. A cylindrical water phantom and an MR‐compatible A1SL detector were utilized. MR and RT isocentricity and MLC positional accuracy was quantified through dose measured with a 0.40 cm2 x 0.83 cm2 field at each cardinal angle. The relationship between detector response to MR/RT isocentricity and MLC positioning was established through introducing known errors in phantom position.ResultsCorrelation was found between detector response and introduced positional error (N = 27) with coefficients of determination of 0.9996 (IEC‐X), 0.9967 (IEC‐Y), 0.9968 (IEC‐Z) in each respective shift direction. The relationship between dose (DoseMR/RT+MLC) and the vector magnitude of MLC and MR/RT positional error (Errormag) was calculated to be a nonlinear response and resembled a quadratic function: DoseMR/RT+MLC[%] = −0.0253 Errormag [mm]2 − 0.0195 Errormag [mm]. For the temporal assessment (N = 7 months), safety interlocks were functional. Laser coincidence to MR was within ±2.0 mm (99.6%) and ±1.0 mm (86.8%) over the 7‐month assessment. IGRT position–reposition shifts were within ±2.0 mm (99.4%) and ±1.0 mm (92.4%). Output was within ±3% (99.4%). Mean MLC and MR/RT isocenter accuracy was 1.6 mm, averaged across cardinal angles for the 7‐month period.ConclusionsThe linac and IGRT accuracy of an MR‐guided radiotherapy system has been validated and monitored over seven months for daily QA. Longitudinal assessment demonstrated a drift in dose rate, but temporal assessment of output, MLC position, and isocentricity has been stable.

Characterization of radiotherapy component impact on MR imaging quality for an MRgRT system

“…In addition to increased spatial integrity mean error, the SNR was significantly reduced when including studies using an identical phantom to that used in this study. 16,[19][20][21][22] In addition, the phantom used in this study cannot…”

“…Many components of the LINAC and beam delivery systems have the potential to further reduce the accuracy of the integrated MRI system. The impact of gantry position on radiation‐MR isocenter coincidence has been investigated on MR‐ 60 Co and MR‐LINAC systems, and shown variations in the MR isocenter depending on gantry position 15,16 . Utilizing an MLC for beam shaping introduces mobile metallic material, which is often the RT component closest to the magnet, that may also impact the magnetic field.…”

“…The impact of gantry position on radiation-MR isocenter coincidence has been investigated on MR- 60 Co and MR-LINAC systems, and shown variations in the MR isocenter depending on gantry position. 15,16 Utilizing an MLC for beam shaping introduces mobile metallic material, which is often the RT component closest to the magnet, that may also impact the magnetic field. A computational study by Kolling et al investigated MLC impact on field homogeneity including source-to-isocenter distance, field size, magnetic field strength, and other properties and found that MLC field size caused dynamic changes in field distribution.…”

The quantification and potential impact of dark current on treatments with an MR‐guided radiotherapy (MRgRT) system

“…There was a consistent decrease in the beamon dose rate up until February 2019 as noted by daily QA measurements. 10 A significant decrease in the beam-on dose rate was observed during the February 2019 measurement and was likely the reason for the corresponding spike in relative dark current dose rate displayed in Fig. 2b.…”

STAT-ART: The Promise and Practice of a Rapid Palliative Single Session of MR-Guided Online Adaptive Radiotherapy (ART)

“…For the pre-plan generation, segmentation performed by the radiation oncologist is kept to a minimum with only two regions of interest requiring contouring: external or skin and a target volume. Initial planning of the pre-plan or template plan is performed by the medical physicist or dosimetrist with a single isocenter and 3D conformal beams defined by the MLC (19, 20). 3D conformal planning of the pre-plan or template plan involves defining an isocenter point of interest, inputting beams, setting gantry angles for ideal geometry, defining the MLC aperture, optimization of beam weights, and a Monte Carlo dose calculation with magnetic field corrections.…”