•5 patients with pelvic lymph node metastases received SBRT using a 1.5 T MR-linac.•Session time was <60 min for all 25 treatment fractions.•All quality assurance tests were passed (dose calculations ...& film measurements).
Online adaptive radiotherapy using the 1.5 Tesla MR-linac is feasible for SBRT (5 × 7 Gy) of pelvic lymph node oligometastases. The workflow allows full online planning based on daily anatomy. Session duration is less than 60 min. Quality assurance tests, including independent 3D dose calculations and film measurements were passed.
For commissioning and quality assurance for adaptive workflows on the MR‐linac, a dosimeter which can measure time‐resolved dose during MR image acquisition is desired. The Blue Physics model 10 ...scintillation dosimeter is potentially an ideal detector for such measurements. However, some detectors can be influenced by the magnetic field of the MR‐linac. To assess the calibration methods and magnetic field dependency of the Blue Physics scintillator in the 1.5 T MR‐linac. Several calibration methods were assessed for robustness. Detector characteristics and the influence of the calibration methods were assessed based on dose reproducibility, dose linearity, dose rate dependency, relative output factor (ROF), percentage depth dose profile, axial rotation and the radial detector orientation with respect to the magnetic field. The potential application of time‐resolved dynamic dose measurements during MRI acquisition was assessed. A variation of calibration factors was observed for different calibration methods. Dose reproducibility, dose linearity and dose rate stability were all found to be within tolerance and were not significantly affected by different calibration methods. Measurements with the detector showed good correspondence with reference chambers. The ROF and radial orientation dependence measurements were influenced by the calibration method used. Axial detector dependence was assessed and relative readout differences of up to 2.5% were observed. A maximum readout difference of 10.8% was obtained when rotating the detector with respect to the magnetic field. Importantly, measurements with and without MR image acquisition were consistent for both static and dynamic situations. The Blue Physics scintillation detector is suitable for relative dosimetry in the 1.5 T MR‐linac when measurements are within or close to calibration conditions.
Purpose
To develop and implement an acceptance procedure for the new Elekta Unity 1.5 T MRI‐linac.
Methods
Tests were adopted and, where necessary adapted, from AAPM TG106 and TG142, IEC 60976 and ...NCS 9 and NCS 22 guidelines. Adaptations were necessary because of the atypical maximum field size (57.4 × 22 cm), FFF beam, the non‐rotating collimator, the absence of a light field, the presence of the 1.5 T magnetic field, restricted access to equipment within the bore, fixed vertical and lateral table position, and the need for MR image to MV treatment alignment. The performance specifications were set for stereotactic body radiotherapy (SBRT).
Results
The new procedure was performed similarly to that of a conventional kilovoltage x‐ray (kV) image guided radiation therapy (IGRT) linac. Results were acquired for the first Unity system.
Conclusions
A comprehensive set of tests was developed, described and implemented for the MRI‐linac. The MRI‐linac met safety requirements for patients and operators. The system delivered radiation very accurately with, for example a gantry rotation locus of isocenter of radius 0.38 mm and an average MLC absolute positional error of 0.29 mm, consistent with use for SBRT. Specifications for clinical introduction were met.
A volumetric modulated arc therapy (VMAT) approach to total body irradiation (TBI) has recently been introduced at our institution. The planning target volume (PTV) is divided into separate ...sub‐volumes, each being treated with 2 arcs with their own isocentre. Pre‐treatment quality assurance of beams is performed on a Sun Nuclear ArcCHECK diode array. Measurement of junction regions between VMAT arcs with separate isocentres has previously been performed with point dose ionization chamber measurements, or with films. Translations of the ArcCHECK with respect to a known distance between the adjacent isocentres of two arcs, which are repeated with the ArcCHECK in an inverted position, allows the recording of a junction dose map. A 3%/3 mm global gamma analysis (10% threshold) pass rate for arc junctions were comparable to their component arcs. Dose maps of junction regions between adjacent arcs with different isocentres can be readily measured on a Sun Nuclear ArcCHECK diode array.
Abstract
A volumetric modulated arc therapy (
VMAT
) approach to total body irradiation (
TBI
) has recently been introduced at our institution. The planning target volume (
PTV
) is divided into ...separate sub‐volumes, each being treated with 2 arcs with their own isocentre. Pre‐treatment quality assurance of beams is performed on a Sun Nuclear ArcCHECK diode array. Measurement of junction regions between
VMAT
arcs with separate isocentres has previously been performed with point dose ionization chamber measurements, or with films. Translations of the Arc
CHECK
with respect to a known distance between the adjacent isocentres of two arcs, which are repeated with the Arc
CHECK
in an inverted position, allows the recording of a junction dose map. A 3%/3 mm global gamma analysis (10% threshold) pass rate for arc junctions were comparable to their component arcs. Dose maps of junction regions between adjacent arcs with different isocentres can be readily measured on a Sun Nuclear ArcCHECK diode array.
Preliminary photometric data for SN 1998A are presented, showing that it increased in brightness by 1.0 ± 0.1 mag in R and I over the 80-d period following discovery. Maximum brightness occurred ...approximately 70 d after discovery, centred within a broad plateau-like peak lasting about 30 d. The shapes of the light curves during the entire 170-d period are similar to those of SNe 1987A and 1909A, suggesting that SNe 1998A and 1909A may also have had blue supergiant progenitors. Spectroscopic and further photometric study of SN 1998A will allow more detailed comparisons between this and other unusual Type II supernovae.
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