Abstract
PurposeWith 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.MethodsA MOSFET dosimetry system, developed by Best Medical Canada for in‐vivo 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.ResultsMOSFET 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 (R2 = 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.ConclusionMOSFET 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 real‐time in‐vivo dosimetry in MRgRT procedures.
Highlights
Magnetic resonance image‐guided radiation therapy (MRgRT) has gained impetus recently
The long cable metal‐oxide‐semiconductor field‐effect transistors (MOSFETs) is mostly used in this study, as it is deemed suitable for dosimetry, with less disturbance from the MR magnetic field to the MOSFET reader, as it extends beyond the 5 Gauss line
Due to an insignificant difference in calibration factors for different cable lengths, MOSFETs with longer cables were used in this study
Summary
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 60Co 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‐semiconductor field‐effect transistor (MOSFET).[3,4]
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