The commercially available microMOSFET dosimeter was characterized for its dosimetric properties in radiotherapy treatments. The MOSFET exhibited excellent correlation with the dose and was linear in the range of 5-500 cGy. No measurable effect in response was observed in the temperature range of 20-40 degrees C. No significant change in response was observed by changing the dose rate between 100 and 600 monitor units (MU) min(-1) or change in the dose per pulse. A 3% post-irradiation fading was observed within the first 5 h of exposure and thereafter it remained stable up to 60 h. A uniform energy response was observed in the therapy range between 4 MV and 18 MV. However, below 0.6 MeV (Cs-132), the MOSFET response increased with the decrease in energy. The MOSFET also had a uniform dose response in 6-20 MeV electron beams. The directional dependence of MOSFET was within +/-2% for all the energies studied. The inherent build-up of the MOSFET was evaluated dosimetrically and found to have varying water equivalent thickness, depending on the energy and the side of the beam entry. At depth, a single calibration factor obtained by averaging the MOSFET response over different field sizes, energies, orientation and depths reproduced the ion chamber measured dose to within 5%. The stereotactic and the penumbral measurements demonstrated that the MOSFET could be used in a high gradient field such as IMRT. The study showed that the microMOSFET dosimeter could be used as an in vivo dosimeter to verify the dose delivery to the patient to within +/-5%.

A prototype of a new 4D in vivo dosimetry system capable of simultaneous real-time position monitoring and dose measurement has been developed. The radiation positioning system (RADPOS) is controlled by a computer and combines two technologies: MOSFET radiation detector coupled with an electromagnetic positioning device. Special software has been developed that allows sampling position and dose either manually or automatically in user-defined time intervals. Preliminary tests of the new device include a dosimetric evaluation of the detector in 60Co, 6 MV, and 18 MV beams and measurements of spatial position stability and accuracy. In addition, the effect of metals and other materials on the performance of the positioning system has been investigated. Results show that the RADPOS system can measure in-air dose profiles that agree, on average, within 3%-5% of diode measurements for the energies tested. The response of the detector is isotropic within 1.6% (1 SD) with a maximum deviation of +/- 4.0% over 360 degrees. The maximum variation in the calibration coefficient over field sizes from 6 x 6 to 25 x 25 cm2 was 2.3% for RADPOS probe with the high sensitivity MOSFET and 4.6% for the probe with the standard sensitivity MOSFET. Of the materials tested, only aluminum, lead, and brass caused shifts in the RADPOS read position. The magnitude of the shift varied between materials and size of the material sample. Nonmagnetic stainless steel (Grade 304) caused a distortion of less than 2 mm when placed within 10 mm of the detector; therefore, it can provide a reasonable alternative to other metals if required. The results of the preliminary tests indicate that the device can be used for in vivo dosimetry in 60Co and high-energy beams from linear accelerators.

In this study, the authors proposed a dosimetry procedure, based on the novel RADPOS system, to accurately determine the position of the radiation dosimeter with respect to the applicator. The authors found that it is possible to monitor the delivered dose in a point and compare it to the predetermined dose. This allows in principle the detection of problems such as bladder motion/filling or source mispositioning. Further clinical investigation is warranted.

In the present work we systematically study the ion energy distribution functions (IEDFs) in argon discharges produced by a combination of pulsed (1–2 kHz) microwave (MW) and continuous wave (cw) radio frequency (rf) excitations. We show that the IEDFs for the pulsed MW discharges are structured, with individual features originating from different periods of the pulse. In the dual-mode MW/rf discharge, significant modulation of the self-bias voltage, Vb, during the MW pulse cycle is observed, which we attribute to changes in the overall plasma impedance: We demonstrate that in the pulsed-MW/cw-rf mode the impedance is highly resistive when the MW signal is on, while it is predominantly capacitive during the period between individual pulses. Using the measured time evolution of Vb in combination with time-resolved measurements of individual ion species, IEDFs at the rf-powered electrode at each instant of the MW pulse have been obtained. This approach is then used to reconstruct the total IEDF in pulsed-MW/cw-rf plasma in order to determine the total energy delivered by the impacting ions.

The control of plasma-surface interactions in terms of synergistic effects of ions, photons, and chemically active species is important for the optimization of plasma enhanced chemical vapor deposition of thin films and for plasma-induced surface modification. In the present work, we use a dual-mode microwave/radio frequency (MW/rf) plasma system, in which we investigate the effect of plasma parameters (gas type and pressure, self-bias voltage, for example) on the energy and flux of ionic species arriving at the specimen surface. We determine the ion energy distribution functions (IEDFs) using a mass spectrometer/energy analyzer, in Ar and N2 discharges, excited at different frequencies. The results for Ar+, N2+, and N+ ions show structured IEDFs at the rf-powered electrode in the single- and dual-frequency modes, while a single peak is observed in the continuous MW plasma. The MW/rf plasma presents substantially higher ion flux and plasma density, and a much thinner sheath than the rf case. Changes in plasma impedance, measured by a rf current–voltage probe, support the results on plasma density and sheath thickness, determined from the IEDFs. The MW/rf discharge impedance displays a resistive behavior in contrast to rf plasma, where the impedance is capacitive.

No 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...

No abstract