Dosimetric Optimization and Commissioning of a High Field Inline MRI-Linac

Abstract

Purpose: Unique characteristics of MRI-linac systems and mutual interactions between their components pose specific challenges for their commissioning and quality assurance. The Australian MRI-linac is a prototype system which explores the inline orientation, with radiation beam parallel to the main magnetic field. The aim of this work was to commission the radiation-related aspects of this system for its application in clinical treatments. Methods: Physical alignment of the radiation beam to the magnetic field was fine-tuned and magnetic shielding of the radiation head was designed to achieve optimal beam characteristics. These steps were guided by investigative measurements of the beam properties. Subsequently, machine performance was benchmarked against the requirements of the IEC60976/77 standards. Finally, the geometric and dosimetric data was acquired, following the AAPM Task Group 106 recommendations, to characterize the beam for modeling in the treatment planning system and with Monte Carlo simulations. The magnetic field effects on the dose deposition and on the detector response have been taken into account and issues specific to the inline design have been highlighted. Results: Alignment of the radiation beam axis and the imaging isocentre within 2 mm tolerance was obtained. The system was commissioned at two source-to-isocentre distances (SIDs): 2.4 and 1.8 m. Reproducibility and proportionality of the dose monitoring system met IEC criteria at the larger SID but slightly exceeded it at the shorter SID. Profile symmetry remained under 103% for the fields up to ~34 × 34 and 21 × 21 cm² at the larger and shorter SID, respectively. No penumbra asymmetry, characteristic for transverse systems, was observed. The electron focusing effect, which results in high entrance doses on central axis, was quantified and methods to minimize it have been investigated. Conclusion: Methods were developed and employed to investigate and quantify the dosimetric properties of an inline MRI-Linac system. The Australian MRI-linac system has been fine-tuned in terms of beam properties and commissioned, constituting a key step toward the application of inline MRI-linacs for patient treatments.

Keywords: MRI-linac; beam characterization; commissioning; dosimetry; magnetic field.

Figure 1

Layout of the Australian MRI-linac with a coordinate system originating at the system's isocentre overlaid.

Figure 2

Dedicated phantoms and setups used in this work: (A) MRI phantom and (B) MV phantom used for system alignment, (C) stand for vertical positioning of the solid water slabs, (D) setup used for MOSkin™ measurements and a close-up od one of the MOSkin™ detectors, (E) setup used for microDiamond measurements, and (F) solid water pieces used for the measurements with microDiamond.

Figure 3

(A) Horizontal (x) and (B) vertical (y) profiles of a nominal 11 × 11 cm² field acquired at different SIDs with only external magnetic shielding (dotted lines) and with optimized internal magnetic shielding (solid lines).

Figure 4

(A) Schematic representation of the phantom setup used for geometrical alignment of the system (B) MR scans of the alignment phantom in x-z plane (left) and x-y plane (right) showing the bore holes aligned to the imaging isocentre (indicated by the superimposed grid) (C) example composite portal images showing the projections of the fiducial markers (aligned to the in-room lasers) and of the edges of half blocked fields formed by the MLC for SID of 2.4 m (left) and SID of 1.8 m (right). Composite images are created as: |image_{negativexblocked} – image_{positivexblocked}| + |image_{negativeyblocked} – image_{positiveyblocked}| allowing visualization of the MLC axes.

Figure 5

System alignment for all SIDs (A) in the horizontal (x) and (B) in the vertical (y) direction.

Figure 6

Dose output linearity (A) at SID of 2.4 m and (B) at SID of 1.8 m. Different symbols indicate regions of applicability of the absolute and the relative deviation criterion according to IEC60976 (39).

Figure 7

The dose distributions acquired in the region of ±10 cm around the beam CAX, normalized at depth of 10 cm, (A,B) at SID of 2.4 m for field sizes of 10.6 × 10.5 and 34.3 × 34.0 cm² and (C,D) at SID of 1.8 m for field sizes 9.7 × 9.6 and 21.4 × 21.2 cm².

Figure 8

Investigations of the entrance dose at SID of 2.4 m with varying bolus placement (A) with microDiamond for a 10.6 × 10.5 cm² field at SID of 2.4 m (normalized at depth of 5 cm) and (B) with MOSkin™ and microDiamond detector for a 7.9 × 7.8 cm² field (normalized at depth of 2 cm).

Figure 9

PDDs measured (A) without (B) with the bolus at SID of 2.4 m for field sizes 2.6 × 2.6, 10.6 × 10.5, 18.5 × 18.3, and 34.3 × 34.0 cm² (normalized to the 10.6 × 10.5 cm² field at a depth of 10 cm) and (C) without and (D) with the bolus at SID of 1.8 m for field sizes 1.9 × 1.9, 9.7 × 9.6, 17.5 × 17.3, and 21.4 × 21.2 cm² (normalized to the 9.7 × 9.6 cm² field at a depth of 10 cm).

Figure 10

Profiles in the horizontal (x) direction (solid lines) and in the vertical (y) direction (dotted lines) measured without the bolus at the surface and at depths of 1, 5, 10, and 20 cm (A–C) at SID of 2.4 m for field sizes 2.6 × 2.6, 10.6 × 10.5, and 18.5 × 18.3 cm² (normalized to CAX value of the 10.6 × 10.5 cm² field at a depth of 10 cm) and (D–F) at SID of 1.8 m for field sizes 1.9 × 1.9, 9.7 × 9.6, and 17.5 × 17.3 cm² (normalized to CAX value of the 9.7 × 9.6 cm² field at a depth of 10 cm). Note: the secondary y-axis was used for surface profiles and primary y-axis was used for all remaining profiles.

Figure 11

Total (S_CP) and collimator (S_C) scatter factors (A) at SID of 2.4 m and (B) at SID of 1.8 m.

Figure 12

Leakage dose (relative to an open field dose) through a closed MLC (A) at SID of 2.4 m and (B) at SID of 1.8 m.