Characterizing magnetically focused contamination electrons by off-axis irradiation on an inline MRI-Linac

Abstract

Purpose: The aim of this study is to investigate off-axis irradiation on the Australian MRI-Linac using experiments and Monte Carlo simulations. Simulations are used to verify experimental measurements and to determine the minimum offset distance required to separate electron contamination from the photon field.

Methods: Dosimetric measurements were performed using a microDiamond detector, Gafchromic^® EBT3 film, and MOSkin^TM . Three field sizes were investigated including 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm² . Each field was offset a maximum distance, approximately 10 cm, from the central magnetic axis (isocenter). Percentage depth doses (PDDs) were collected at a source-to-surface distance (SSD) of 1.8 m for fields collimated centrally and off-axis. PDD measurements were also acquired at isocenter for each off-axis field to measure electron contamination. Monte Carlo simulations were used to verify experimental measurements, determine the minimum field offset distance, and demonstrate the use of a spoiler to absorb electron contamination.

Results: Off-axis irradiation separates the majority of electron contamination from an x-ray beam and was found to significantly reduce in-field surface dose. For the 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm² field, surface dose was reduced from 120.9% to 24.9%, 229.7% to 39.2%, and 355.3% to 47.3%, respectively. Monte Carlo simulations generally were within experimental error to MOSkin^TM and microDiamond, and used to determine the minimum offset distance, 2.1 cm, from the field edge to isocenter. A water spoiler 2 cm thick was shown to reduce electron contamination dose to near zero.

Conclusions: Experimental and simulation data were acquired for a range of field sizes to investigate off-axis irradiation on an inline MRI-Linac. The skin sparing effect was observed with off-axis irradiation, a feature that cannot be achieved to the same extent with other methods, such as bolusing, for beams at isocenter.

Keywords: dosimetry; electron contamination; inline MRI-Linac; magnetic field; radiotherapy; skin dose.

FIGURE 1

Schematic of the Australian MRI‐Linac system. Top: Side view showing a split‐bore MRI, Linac, MLC, and solid water phantom. Bottom: Top view of the offset phantom where the overlay grid indicates the dimensions of the system (mm). A color map ranging 0–1 T has been included to show the relative intensity of the magnetic field. The x‐ray beam and electron tracks are represented by a green outline and red tracks, respectively

FIGURE 2

Experimental setup: (a) Solid water phantom with microDiamond detector at surface, (b) microDiamond phantom, and (c) MOSkin ^TM solid water phantom

FIGURE 3

Labeled Monte Carlo 2D dose maps for field size 5.8 × 5.8 cm² at the surface to indicate locations of PDD measurements. (a) 2D dose map at the surface of the water phantom that indicates the central axis for a central beam (black vertical line), denoted as “1 T” for PDD plots. The horizontal red line indicates the position of cross plane profiles. (b) 2D dose map at the surface of the water phantom that indicates the center of an off‐axis field (white vertical line) and magnetic isocenter axis that coincides to the leakage of adjoining MLC leaf pairs (black vertical line)

FIGURE 4

Depth dose curves for 1.9× 1.9 cm² field measured with MOSkin ^TM(× ), microDiamond (*), EBT3 film (+), and Monte Carlo simulations (–). (a) PDD of the off‐axis field and centered field, denoted as 1 T within the figure legend, with a surface dose of 24.9% and 120.9%, respectively. (b) First 10 mm PDD for off‐axis field measurements. (c) First 10 mm PDD at the central magnetic axis during off‐axis irradiation with a maximum surface dose of 27.7%. Shaded gray error bars in (b) and (c) represent MOSkin ^TM uncertainties and conventional error bars for microDiamond. For field size 1.9× 1.9 cm², maximum uncertainty of MOSkin ^TM, microDiamond, and film was 7.0%, 3.2%, and 4.0%

FIGURE 5

Depth dose curves for 5.8× 5.8 cm² field measured with MOSkin ^TM(× ), microDiamond (*), EBT3 film (+), and Monte Carlo simulations (–). (a) PDD of the off‐axis field and centered field, denoted as 1 T within the figure legend, with a surface dose of 39.2% and 229.7%, respectively. (b) First 10 mm PDD for off‐axis field measurements. (c) First 10 mm PDD at the central magnetic axis during off‐axis irradiation with a maximum surface dose of 66.1%. Shaded gray error bars in (b) and (c) represent MOSkin ^TM uncertainty and conventional error bars for microDiamond. For field size 5.8× 5.8 cm², maximum uncertainty of MOSkin ^TM, microDiamond, and film was 6.0%, 3.3%, and 4.0%

FIGURE 6

Depth dose curves for 9.7× 9.6 cm² field measured with MOSkin ^TM(× ), microDiamond (*), EBT3 film (+), and Monte Carlo simulations (–). (a) PDD of the off‐axis field and centered field, denoted as 1 T within the figure legend, with a surface dose of 47.3% and 355.3%, respectively. (b) First 10 mm PDD for off‐axis field measurements. (c) First 10 mm PDD at the central magnetic axis during off‐axis irradiation with a maximum surface dose of 137.2%. Shaded gray error bars in (b) and (c) represent MOSkin ^TM uncertainties and conventional error bars for microDiamond. For field size 9.7× 9.6 cm², maximum uncertainty of MOSkin ^TM, microDiamond, and film was 7.2%, 3.7%, and 4.0%

FIGURE 7

Film profiles at depths 1, 15, 20, and 50 mm for field sizes (a) 1.9× 1.9 cm², (b) 5.8× 5.8 cm², and (c) 9.7× 9.6 cm². Above each profile graph is a raw film scan prior to image processing. All film data were normalized at the center of the off‐axis field at a depth of 1.5 cm. The vertical black line overlaid on each plot indicates the central magnetic axis (isocenter)

FIGURE 8

Monte Carlo simulations for a 1.9× 1.9 cm² field. (a) B = 1 T, 2D dose map and crossplane profiles for 10 cm off‐axis field at 1 mm depth for B = 1 T and B = 0 T. (b) B = 1 T, 2D dose map and crossplane profiles for a minimum off‐axis distance of 3.1 cm from the center of the field to the central axis at 1 mm depth for B = 1 T and B = 0 T. (c) B = 1 T, 2D dose map and crossplane profiles at 1 mm depth for a minimum offset distance of 3.1 cm from the center of the field to the central axis with the addition of a water spoiler, outlined in white, for B = 1 T and B = 0 T

FIGURE 9

Monte Carlo simulations for a 5.8× 5.8 cm² field. (a) B = 1 T, 2D dose map and crossplane profiles for 10 cm off‐axis field at 1 mm depth for B = 1 T and B = 0 T . (b) B = 1 T, 2D dose map and crossplane profiles for a minimum off‐axis distance of 4.8 cm from the center of the field to the central axis at 1 mm depth for B = 1 T and B = 0 T. (c) B = 1 T, 2D dose map and crossplane profile at 1 mm depth for a minimum offset distance of 4.8 cm from the center of the field to the central axis with the addition of a water spoiler, outlined in white, for B = 1 T and B = 0 T

FIGURE 10

Monte Carlo simulations for a 9.7× 9.6 cm² field. (a) B = 1 T, 2D dose map for B = 1 T and crossplane profiles for the 10 cm off‐axis field at 1 mm depth for B = 1 T and B = 0 T. (b) B = 1 T, 2D dose map and crossplane profiles for a minimum off‐axis distance of 6.5 cm from the center of the field to the central axis at 1 mm depth for B = 1 T and B = 0 T. (c) B = 1 T, 2D dose map and crossplane profiles at 1 mm depth for a minimum offset distance of 6.5 cm from the center of the field to the central axis with the addition of a water spoiler, outlined in white, for B = 1 T and B = 0 T