Out‐of‐field dose assessment for a 1.5 T MR‐Linac with optically stimulated luminescence dosimeters

Abstract

To assess the out-of-field surface and internal dose of the 1.5T MR-Linac compared to the conventional external beam linac using optically stimulated luminescence dosimeters (OSLDs), and evaluate the out-of-field dose calculation accuracy of the Monaco treatment planning system of the 1.5T MR-Linac. Methods: A cubic solid water phantom, with OSLDs on the surface, was vertically irradiated by MR-Linac square fields with different sizes. In addition, OSLDs were arranged out of the beam edges in four directions. An anthropomorphic adult phantom, with 125 cm 3 simulated volume, was irradiated in 4 orthogonal directions by both MR-Linac and conventional linac at the head, thoracic, and pelvic sites. Out-of-field doses were measured by OSLDs on both the surface and internal emulational organs at risk (OARs). The results were compared to the simulated dose from Monaco treatment planning system (TPS). Results: At different field sizes (5 × 5 to 20 × 20 cm 2) and distances (1 to 10 cm) to beam edge, the out-of-field surface dose measured on MR-Linac varied from 0.16 % (10 cm to 5 × 5 cm 2 edge) to 7.02 % (1 cm to 20 × 20 cm 2 edge) of the maximum dose laterally and from 0.14 % (10 cm to 5 × 5 cm 2 edge) to 8.56 % (1 cm to 20 × 20 cm 2 edge) of the maximum dose longitudinally. Compared to the OSLDs measured data, the Monaco TPS presented an overestimate of the out-of-field dose of OARs at 0-2 % isodose area on both surface and internal check points, and the overestimation gets greater as the distance increases. The underestimation was found to be 0-35% at 2%-5% Accepted Article This article is protected by copyright. All rights reserved isodose area on both surface and internal check points. Compared to the conventional linac, MR-Linac delivered higher average values of out-of-field dose on surface check points (20 %, 19 %, 21 %) and internal simulated OARs (42 %, 37 %, 9 %) of the anthropomorphic phantom at head, thoracic and pelvic irradiations, respectively. Conclusions: Compared to the conventional linac, MR-Linac has the same out-of-field dose distribution. However, considering the absolute dose values, MR-Linac delivered relatively higher out-of-field doses on both surface and internal OARs. Additional radiation shielding to patients undergoing MR-Linac may provide protection from out-of-field exposure.

Compact bunker shielding assessment for 1.5 T MR-Linac

“…In addition, secondary electrons generated via the interaction between photons and air, are influenced by the static magnetic field and exhibit spiral movement. Owing to the movement of these electrons, the scattered beam can be formed wider than the expected area compared to the conventional Linac 20 . In particular, even if the magnetic field strength is small, it can occur from the leakage radiation passing through the duct.…”

High-resolution entry and exit surface dosimetry in a 1.5 T MR-linac

“…Previous investigations of near-surface dose in a transverse MR-linac have used dosimeters with varying effective point of measurement (EPOM). These include radiochromic film [6,14,15], PTW 31,021 Semiflex 3D [16], optically stimulated luminescence dosimeters (OSLDs) [17][18][19], thermoluminescent dosimeters (TLDs) [3], metal-oxide-semiconductor field-effect transistor (MOSFET) [20], gel [21], PTW 60019 microDiamond [6,14,16], and a PTW 34045 Advanced Markus chamber [6,12]. Parallel-plate chambers, such as the Advanced Markus chamber, are commonly used for dose measurements in the build-up region on conventional linacs.…”

“…Similarly for small-field dosimetry, the size of the SV should be considerably smaller than the field size as a loss of CPE impacts the detectors readings [26]. With skin dose specified at 0.07 mm, previous near-surface dose investigations on a transverse MR-linac are lacking as dosimeters with larger EPOMs were used [6,[14][15][16][17][18][19][20][21]27]. The dose averaged across the SV of an OSLD corresponds to a near-skin water equivalent depth (WED) of 0.16 mm, with the external casing of the OSLD removed [17].…”

High-resolution entry and exit surface dosimetry in a 1.5 T MR-linac

“…Previous investigations of near-surface dose in a transverse MR-linac have used dosimeters with varying effective point of measurement (EPOM). These include radiochromic lm [6], [14], [15], PTW 31021 Semi ex 3D [16], optically stimulated luminescence dosimeters (OSLDs) [17]- [19], thermoluminescent dosimeters (TLDs) [20], metal-oxide-semiconductor eld-effect transistor (MOSFET) [21], gel [22], PTW 60019 microDiamond [6], [14], [16], and a PTW 34045 Advanced Markus chamber [6], [12]. Parallel-plate chambers, such as the Advanced Markus chamber, are commonly used for dose measurements in the build-up region on conventional linacs.…”

“…Similarly for small-eld dosimetry, the size of the SV should be considerably smaller than the eld size as a loss of CPE impacts the detectors readings [27]. With skin dose speci ed at 0.07 mm, previous nearsurface dose investigations are lacking as dosimeters with larger EPOMs were used [6], [14]- [19], [21], [22], [28]. The dose averaged across the SV of an OSLD corresponds to a near-skin water equivalent depth (WED) of 0.16 mm, with the external casing of the OSLD removed [17].…”