MOSFET dosimeter characterization in MR‐guided radiation therapy (MRgRT) Linac

Abstract

Purpose: With the increasing use of MR-guided radiation therapy (MRgRT), it becomes important to understand and explore accuracy of medical dosimeters in the presence of magnetic field. The purpose of this work is to characterize metal-oxide-semiconductor field-effect transistors (MOSFETs) in MRgRT systems at 0.345 T magnetic field strength.Methods: A MOSFET dosimetry system, developed by Best Medical Canada for invivo patient dosimetry, was used to study various commissioning tests performed on a MRgRT system, MRIdian ® Linac. We characterized the MOSFET dosimeter with different cable lengths by determining its calibration factor, monitor unit linearity, angular dependence, field size dependence, percentage depth dose (PDD) variation, output factor change, and intensity modulated radiation therapy quality assurance (IMRT QA) verification for several plans. MOSFET results were analyzed and compared with commissioning data and Monte Carlo calculations.Results: MOSFET measurements were not found to be affected by the presence of 0.345 T magnetic field. Calibration factors were similar for different cable length dosimeters either placed at the parallel or perpendicular direction to the magnetic field, with variations of less than 2%. The detector showed good linearity (R 2 = 0.999) for 100-600 MUs range. Output factor measurements were consistent with ionization chamber data within 2.2%. MOSFET PDD measurements were found to be within 1% for 1-15 cm depth range in comparison to ionization chamber.MOSFET normalized angular response matched thermoluminescent detector (TLD) response within 5.5%. The IMRT QA verification data for the MRgRT linac showed that the percentage difference between ionization chamber and MOSFET was 0.91%, 2.05%, and 2.63%, respectively for liver, spine, and mediastinum. Conclusion: MOSFET dosimeters are not affected by the 0.345 T magnetic field in MRgRT system. They showed physics parameters and performance comparable to TLD and ionization chamber; thus, they constitute an alternative to TLD for realtime in-vivo dosimetry in MRgRT procedures. Magnetic resonance image-guided radiation therapy (MRgRT) has gained impetus recently. ViewRay ® (ViewRay Inc., Oakwood, OH) is one of the first MRgRT treatment modalities with an onboard MR scanner attached to a teletherapy 60 Co beam. 1 The MRIdian ® Linac system which combines MR imaging and a 6 MV beam is the latest system developed by ViewRay. MRgRT offers many advantages including real-time imaging, accuracy in treatment delivery, higher soft tissue contrast, and no additional imaging dose. 2 Though there are significant advantages of these hybrid treatment systems with onboard MR scanners, these present a unique challenge to dosimetric measurements with some of the conventional radiation detectors, due to possible magnetic field susceptibility. Several dosimeters are used for quality assurance in radiotherapy; these include ionization chambers (IC), thermoluminescent dosimeters (TLD), film, and active dosimeters such as diodes and metal-oxide-semic...

ICRU REPORT 97: MRI-Guided Radiation Therapy Using MRI-Linear Accelerators

“…In addition, the performance of many electronic detectors is affected. This has led to the development of magnetic field compatible versions for 2D and 3D electronic detectors (de Vries et al , 2018; Desai et al , 2021; Houweling et al , 2016; Mönnich et al , 2020; Yadav et al , 2020). With these tools, similar QA measurements as for conventional linacs can be performed for commissioning and acceptance procedures (Roberts et al , 2021).…”

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].…”