Electron streaming dose measurements and calculations on a 1.5 T MR-Linac

Abstract

Purpose: To evaluate the accuracy of different dosimeters and the treatment planning system (TPS) for assessing the skin dose due to the electron streaming effect (ESE) on a 1.5 T magnetic resonance (MR)-linac.

Method: Skin dose due to the ESE on an MR-linac (Unity, Elekta) was investigated using a solid water phantom rotated 45° in the x-y plane (IEC61217) and centered at the isocenter. The phantom was irradiated with 1 × 1, 3 × 3, 5 × 5, 10 × 10, and 22 × 22 cm² fields, gantry at 90°. Out-of-field doses (OFDs) deposited by electron streams generated at the entry and exit surface of the angled phantom were measured on the surface of solid water slabs placed ±20.0 cm from the isocenter along the x-direction. A high-resolution MOSkin™ detector served as a benchmark due to its shallower depth of measurement that matches the International Commission on Radiological Protection (ICRP) recommended depth for skin dose assessment (0.07 mm). MOSkin™ doses were compared to EBT3 film, OSLDs, a diamond detector, and the TPS where the experimental setup was modeled using two separate calculation parameters settings: a 0.1 cm dose grid with 0.2% statistical uncertainty (0.1 cm, 0.2%) and a 0.2 cm dose grid with 3.0% statistical uncertainty (0.2 cm, 3.0%).

Results: OSLD, film, the 0.1 cm, 0.2%, and 0.2 cm, 3.0% TPS ESE doses, underestimated skin doses measured by the MOSkin™ by as much as -75.3%, -7.0%, -24.7%, and -41.9%, respectively. Film results were most similar to MOSkin™ skin dose measurements.

Conclusions: These results show that electron streams can deposit significant doses outside the primary field and that dosimeter choice and TPS calculation settings greatly influence the reported readings. Due to the steep dose gradient of the ESE, EBT3 film remains the choice for accurate skin dose assessment in this challenging environment.

Keywords: Elekta Unity; MRgRT; MR‐linac; Monaco Unity; electron streaming effect; out‐of‐field dose; skin dose.

FIGURE 1

Schematic of the setup used to generate backscattered and ejected ESE.

FIGURE 2

(a) Photograph of the experimental setup with the MOSkin™ positioned in the holder placed 20.0 cm from the isocenter to measure ejected ESE. (b) Photograph of the microDiamond positioned using a customized phantom to measure ejected ESE.

FIGURE 3

(a) A coronal slice of the calculated dose distribution for a G90, 1000 MU 10 × 10 cm² beam irradiating the angled solid water. (b) Ejected ESE transverse dose profiles in the x‐direction for a 1 × 1 cm² and 10 × 10 cm² beam at the surface of the +y positioned solid water phantom. Similarly in (c) for backscattered ESE at the ‐y positioned block. The dashed vertical line in (b) and (c) indicates the position used for comparison between experimental and TPS computed datasets. All data is normalized to D_max of a 10 × 10 cm² field.

FIGURE 4

Measured and TPS predicted backscattered ESE doses for (a) 1 × 1 cm², (b) 3 × 3 cm², (c) 5 × 5 cm², (d) 10 × 10 cm², and (e) 22 × 22 cm² fields. All data is normalized to Dmax of a 10 × 10 cm² field. MOSkin™ (×), OSLD (○), film (□), microDiamond (+), TPS (0.1 cm, 0.2%) (△), and TPS (0.2 cm, 3.0%) (◇) point doses are denoted by their respective marker whilst the dashed line represents curve‐fit MOSkin™ doses. For each dosimeter measurement, the estimated error is given by an error bar.

FIGURE 5

Measured and TPS predicted ejected ESE doses for (a) 1 × 1 cm², (b) 3 × 3 cm², (c) 5 × 5 cm², (d) 10 × 10 cm², and (e) 22 × 22 cm² fields. All data is normalized to D_max of a 10 × 10 cm² field. MOSkin™ (×), OSLD (○), film (□), microDiamond (+), TPS (0.1 cm, 0.2%) (△), and TPS (0.2 cm, 3.0%) (◇) point doses are denoted by their respective marker while the dashed line represents curve‐fit MOSkin™ doses. For each dosimeter measurement, the estimated error is given by an error bar.

FIGURE 6

Box plot of the relative difference, ΔD, between the 0.07 mm WED MOSkin™ recorded dose values and OSLDs, film, and planning system values for (a) backscattered ESE, and (b) ejected ESE doses across all five field sizes investigated. For each box plot the red line indicates the median value, the box indicates the 25th–75th percentiles, the bar indicates extreme data points, and the red crosses indicate outliers.