MOSFET Detectors Research Articles

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

The magnetic field of a transverse MR-linac alters electron trajectories as the photon beam transits through materials, causing lower doses at flat entry surfaces and increased doses at flat beam-exiting surfaces. This study investigated the response of a MOSFET detector, known as the MOSkin™, for high-resolution surface and near-surface percentage depth dose measurements on an Elekta Unity. Simulations with Geant4 and the Monaco treatment planning system (TPS), and EBT-3 film measurements, were also performed for comparison. Measured MOSkin™ entry surface doses, relative to Dmax, were (9.9 ± 0.2)%, (10.1 ± 0.3)%, (11.3 ± 0.6)%, (12.9 ± 1.0)%, and (13.4 ± 1.0)% for 1 × 1 cm2, 3 × 3 cm2, 5 × 5 cm2, 10 × 10 cm2, and 22 × 22 cm2 fields, respectively. For the investigated fields, the maximum percent differences of Geant4, TPS, and film doses extrapolated and interpolated to a depth suitable for skin dose assessment at the beam entry, relative to MOSkin™ measurements at an equivalent depth were 1.0%, 2.8%, and 14.3%, respectively, and at a WED of 199.67 mm at the beam exit, 3.2%, 3.7% and 5.7%, respectively. The largest measured increase in exit dose, due to the electron return effect, was 15.4% for the 10 × 10 cm2 field size using the MOSkin™ and 17.9% for the 22 × 22 cm2 field size, using Geant4 calculations. The results presented in the study validate the suitability of the MOSkin™ detector for transverse MR-linac surface dosimetry.

Validation of an individualized home-made superficial brachytherapy mold applied for deep nonmelanoma skin cancer.

This study was conducted to evaluate the effect of brachytherapy (BT) customized mold [Condensation silicone elastomer (ProtesilTM)] and its thickness on the dose distribution pattern of deep nonmelanoma skin cancers (NMSC). Four blocks of mold material were constructed in 5, 10, 15, and 20 mm thickness and 100 × 100 mm2 area by a plastic cast. The high dose rate (HDR) plus treatment planning system (TPS) (Version 3, Eckert & Ziegler BEBIG Gmbh, Berlin, Germany) with a 60Co source (model: Co0.A86, EZAG BEBIG, Berlin, Germany) as an high dose rate brachytherapy (HDR-BT) source was used. Solid phantom and MOSFETTM and GAFCHROMICTM EBT3 film dosimeters were used for experimental dosimetry of the different thicknesses (up to 20 mm) of BT customized mold. Skin dose and dose to different depths were evaluated. The TPS overestimated the calculated dose to the surface. Skin dose can be reduced from 250% to 150% of the prescription dose by increasing mold thickness from 5 mm to 20 mm. There was a 7.7% difference in the calculated dose by TPS and the measured dose by MOSFET. There was a good agreement between film dosimetry, MOSFET detector, and TPS' results in depths less than 5 mm. Each BT department should validate any individualized material chosen to construct the customized surface BT mold. Increasing the mold thickness can treat lesions without overexposing the skin surface. Superficial BT can be recommended as an appropriate treatment option for some deep NMSC lesions (up to 20 mm) with pre-planning considerations employing thicker molds.

Responsivity enhancement techniques for CMOS source‐driven terahertz detectors

AbstractThe size and asymmetry effects of MOSFET detectors operating in a source‐driven configuration on responsivity are discussed in this paper. Seven MOSFET detectors with different sizes and structures are fabricated in a standard 55 nm CMOS technology. At 2.58 THz, the measurement results show that for the symmetric device, the minimum size MOSFET can achieve a higher responsivity, and for the asymmetric device, when the ratio of the source channel width to the drain channel width reaches 4:1 or higher, the asymmetric detector can achieve a higher responsivity than the detector based on the minimum size device available in the process. The best responsivity of the 2.03 kV/W is achieved by the asymmetry structure MOSFET detector with the ratio of the source channel width to the drain channel width reaching 4:1.

Evaluation of Sine Spin flat detector CT imaging compared with multidetector CT

BackgroundFlat detector computed tomography (FDCT) is widely used for periprocedural imaging in the angiography suite. Sine Spin FDCT (SFDCT) is the latest generation of cone beam CT using a double...

PO-1782 Doses from 2.5 MV and 6 MV 2D-imaging in IGRT, measured with MOSFET detectors

PO-1782 Doses from 2.5 MV and 6 MV 2D-imaging in IGRT, measured with MOSFET detectors

A new phantom developed to test the ATCM performance of chest CT scanners

In this study, a new ATCM phantom was developed to test the performance of the automatic tube current modulation (ATCM) of computed tomography (CT) scanners.. Based on the Chinese reference man and Monte Carlo simulations of x-ray attenuation, a more realistic ATCM phantom made of polymethyl methacrylate was developed. The phantom has a length of 20 cm, and it can be used to measure the dose profile along the central axis using 19 real-time MOSFET detectors. The image noise can be calculated slice by slice in the phantom’s center. Test experiments showed that the phantom could initiate tube current modulation under different modulation levels of CT scans, and the actual effects of ATCM could be evaluated with the aid of the dose profile measurements. Using the measured dose profiles and image noise, the preferred dose can easily be identified from a choice of different modulation levels. The new phantom developed in this study can be used to test the ATCM performance of CT scanners, and is useful for further studies of the optimization of CT scan protocols with ATCM.

A novel add-on collimator for preclinical radiotherapy applications using a standard cell irradiator: design, construction, and validation.

Preclinical radiotherapy applications require dedicated irradiation systems which are expensive and not widely available. In this work, a clinical dual source 137 Cs cell irradiator was adapted to deliver 1-cm diameter preclinical treatment beams using a lead and stainless steel custom-made collimator to treat one or two mice at a time. The dosimetric characteristics of all the different components of the system (including collimator, phantoms, and radiation sources) have been simulated with EGSnrc Monte Carlo methods. The collimator was constructed based on these simulations and the calculated results were verified against dosimetric measurements with MOSKin detectors, GAFchromic films, and dosimetric gels. The comparisons showed an agreement, in terms of full width half maximum values, between the simulated and the measured dose cross profiles at the midline within 4% for both gel dosimetry and GAFchromic films. Out of beam dose, measured in air at the collimator midplane with MOSFET detectors was between 6% and 10% of the beam axis dose. The dimensions of the beam are constant along the vertical axis of the collimator and also the simulated and measured Percentage Depth Dose (PDD) curves show an agreement within 1%. The collimator design developed in this work allows the creation of a beam with the necessary characteristics for ablative radiotherapy treatments on small animals using a standard clinical cell irradiator. This collimator design will make advanced preclinical studies with ablative beams possible for all those institutions which do not have collimated preclinical irradiators available.

Design and demonstration of antenna-coupled Schottky diodes in a foundry complementary metal-oxide semiconductor technology for electronic detection of far-infrared radiation

Electronic detection of far-infrared (FIR) radiation up to 9.74 THz is reported in a foundry complementary metal-oxide semiconductor (CMOS) technology. The detectors were fabricated with Schottky-barrier diodes (SBDs) formed in 130-nm CMOS without any process modifications. Direct-antenna matched detectors achieve a measured peak optical responsivity (RV) of 383 and 25 V/W at 4.92 and 9.74 THz, respectively, near the 5 and 10 THz fundamental frequency of the antennas. A significantly improved RV at 9.74 THz (25× compared to the MOSFET detectors and ∼2× compared to the SBD) ensures negligible impact on the system noise-equivalent power (NEP) due to the input-referred noise of the amplifier following the detector. This work also demonstrated that by incorporating the effects of plasma resonance, transit time, and FIR absorption behavior of SiO2, as well as the 3D electromagnetic simulations into the SBD model, good agreement between the measurements and simulations can be attained. The detector designed for a 10-THz operation achieves an optical NEP of 1.1 nW/√Hz at 9.74 THz in the shot-noise limit, which is comparable to that of commercially available pyro-detectors that are 50 000× larger.

Rectal and Bladder Dose Measurements in the Intracavitary Applications of Cervical Cancer Treatment with HDR Afterloading System: Comparison of TPS Data with MOSFET Detector.

Background: Intracavitary brachytherapy plays a major role in management of cervical carcinoma. Assessment of dose received by OAR’s therefore becomes crucial for the estimation of radiation toxicities in HDR brachytherapy.Objective: Purpose of this study is to evaluate the role of in vivo dosimetry in HDR brachytherapy and to compare actual doses delivered to OAR’ s with those calculated during treatment planning.Material and Methods: In this retrospective study, 50 patients of cervical carcinoma were treated by Microselectron HDR. Out of 50 patients, 26 were treated with a dose of 7 Gy and 24 with a dose of 9 Gy, prescribed to point A. Brachytherapy planning and evaluation of dose to bladder and rectum was done on TPS & in vivo dosimetry was performed using portable MOSFET.Results: Calibration factors calculated for both dosimeters are almost equal and are 0.984 cGy/mV and 1.0895 cGy/mV. For bladder, dose deviation was found to be within ± 5% in 28 patients, ± 5-10% in 14 patients, ± 10-15% in 4 patients. Deviation between TPS-calculated dose and dose measured by MOSFET for rectum was within ± 5% in 31 patients, ± 5–10% in 8 patients, and ± 10–15% in 7 patients.Conclusion: TPS calculated doses were slightly higher than that measured by MOSFET. The use of small size MOSFET dosimeter is an efficient method for accurately measuring doses in high-dose gradient fields typically seen in brachytherapy. Therefore, to reduce the risk of large errors in dose delivery, in vivo dosimetry can be done in addition to TPS computations.

Development and validation of an open source Monte Carlo dosimetry model for wide-beam CT scanners using Fluka.

PurposeDevelopment and validation of an open source Fluka‐based Monte Carlo source model for diagnostic patient dose calculations.MethodsA framework to simulate a computed tomography (CT) scanner using Fluka Monte Carlo particle transport code was developed. The General Electric (GE) Revolution scanner with the large body filter and 120 kV tube potential was characterized using measurements. The model was validated on benchmark CT test problems and on dose measurements in computed tomography dose index (CTDI) and anthropomorphic phantoms. Axial and helical operation modes with provision for tube current modulation (TCM) were implemented. The particle simulations in Fluka were accelerated by executing them on a high‐performance computing cluster.ResultsThe simulation results agreed to better than an average of 4% of the reference simulation results from the AAPM Report 195 test scenarios, namely: better than 2% for both test problems in case 4 using the PMMA phantom, and better than 5% of the reference result for 14 of 17 organs in case 5, and within 10% for the three remaining organs. The Fluka simulation results agreed to better than 2% of the air kerma measured in‐air at isocenter of the GE Revolution scanner. The simulated air kerma in the center of the CTDI phantom overestimated the measurement by 7.5% and a correction factor was introduced to account for this. The simulated mean absorbed doses for a chest scan of the pediatric anthropomorphic phantom was completed in ~9 min and agreed to within the 95% CI for bone, soft tissue, and lung measurements made using MOSFET detectors for fixed current axial and helical scans as well as helical scan with TCM.ConclusionA Fluka‐based Monte Carlo simulation model of axial and helical acquisition techniques using a wide‐beam collimation CT scanner demonstrated good agreement between measured and simulated results for both fixed current and TCM in complex and simple geometries. Code and dataset will be made available at https://github.com/chezhia/FLUKA_CT.

Investigation of Dosimetric Characteristics of a MOSFET Detector for Clinical Electron Beams

Investigation of Dosimetric Characteristics of a MOSFET Detector for Clinical Electron Beams

Effects of Parasitic Source/Drain Junction Area on Terahertz Responsivity of MOSFET Detector

This paper reports the effect of the source/drain junction area on the responsivity of a metal-oxide field-effect transistor (MOSFET)-based terahertz (THz) detector. From the numerical analysis based on the distributed-channel model for the plasma-wave detection mechanism, it is predicted that both the responsivity and noise-equivalent power (NEP) are improved with a relatively larger source junction area than drain junction area. For experimental verification, three types of MOSFET detectors with different source/drain junction areas are fabricated with 65-nm CMOS technology. From on-wafer measurement at 0.3 THz, 2.57 times enhanced responsivity and 63% reduced NEP have been obtained in the sample with the smallest drain junction area and the largest source junction area.

EFFECTIVE DOSE ESTIMATION FROM ORGAN DOSE MEASUREMENTS IN FAST-kV SWITCH DUAL ENERGY COMPUTED TOMOGRAPHY.

The purpose of this study was to validate a novel approach to estimating effective dose (E) in 'fast-kV switch dual energy computed tomography' using MOSFET detectors. The effective energy of the combined dual energy environment was characterized with the dual energy CT scanner and then MOSFETs were calibrated matching to the effective energy of the dual energy CT beam with a conventional CT beam. The calibration method was then experimentally validated by comparing the dose between MOSFET and an ion chamber (IC) using a standard CTDI body phantom. The measured doses of the MOSFET and IC were 17.1 mGy ± 3.8% and 17.1 mGy ± 0.4%, respectively. To measure organ doses, an adult anthropomorphic phantom loaded with 18 MOSFET detectors was scanned using a standard fast-kV switch dual energy abdomen/pelvis CT protocol. E was calculated by applying ICRP 103 tissue weighting factors as well as partial volume correction factors for organs that were not completely covered by the protocol field-of-view. E from the dual energy abdomen/pelvis CT was calculated to be 17.8 mSv ± 11.6%. This calculation was then compared to E from dose length product method, which yielded 14.62 mSv.

Interplay effect on a 6-MV flattening-filter-free linear accelerator with high dose rate and fast multi-leaf collimator motion treating breast and lung phantoms.

Using a new linear accelerator with high dose rate (800MU/min), fast MLC motions (5.0cm/s), fast gantry rotation (15s/rotation), and 1cm wide MLCs, we aimed to quantify the effects of complexity, arc number, and fractionation on interplay for breast and lung treatments under target motion. To study lung interplay, eight VMAT plans (1-6 arcs) and four-nine-field sliding-window IMRT plans varying in complexity were created. For the breast plans, four-four-field sliding-window IMRT plans were created. Using the Halcyon 1.0 linear accelerator, each plan was delivered five times each under sinusoidal breathing motion to a phantom with 20 implanted MOSFET detectors; MOSFET dose (cGy), delivery time, and MU/cGy values were recorded. Maximum and mean dose deviations were calculated from MOSFET data. The number of MOSFETs with at least 19 of 20 detectors agreeing with their expected dose within 5% per fraction was calculated across 106 iterations to model dose deviation as function of fraction number for all plan variants. To put interplay plans into clinical context, additional IMRT and VMAT plans were created and delivered for the sites of head and neck, prostate, whole brain, breast, pelvis, and lung. Average modulation and interplay effect were compared to those from conventional linear accelerators, as reported from previous studies. The mean beam modulation for plans created for the Halcyon 1.0 linear accelerator was 2.9MU/cGy (two- to four-field IMRT breast plans), 6.2MU/cGy (at least five-field IMRT), and 3.6MU/cGy (four-arc VMAT). To achieve treatment plan objectives, Halcyon 1.0 VMAT plans require more arcs and modulation than VMAT on conventional linear accelerators. Maximum and mean dose deviations increased with increasing plan complexity under tumor motion for breast and lung treatments. Concerning VMAT plans under motion, maximum, and mean dose deviations were higher for one arc than for two arcs regardless of plan complexity. For plan variants with maximum dose deviations greater than 3.7%, dose deviation as a function of fraction number was protracted. For treatments on the Halcyon 1.0 linear accelerator, the convergence of dose deviation with fraction number happened more slowly than reported for conventional linear accelerators. However, if plan complexity is reduced for IMRT and if tumor motion is less than ~10-mm, interplay is greatly reduced. To minimize dose deviations across multiple fractions for dynamic targets, we recommend limiting treatment plan complexity and avoiding one-arc VMAT on the Halcyon 1.0 linear accelerator when interplay is a concern.

Abstract ID: 134 Ant colony algorithm for driving variance reduction techniques in Monte Carlo simulations

The use of variance reduction techniques (VRTs) in Monte Carlo calculations is mandatory in different situations in which low scoring statistics occurs, for example, when the dose deposited by radiation in small detectors in complex geometries is calculated. The efficiency of this kind of simulations could be extremely low because a huge amount of CPU time is spent on tracking particles that do not contribute to deposited dose. VRTs (e.g. Russian roulette and splitting) can improve the efficiency in this type of calculations, but unfortunately, these techniques are extremely problem-dependent, and general recipes to minimize the variance cannot be given. To solve this problem, we propose to use a method based on an ant colony algorithm that allows driving the application of VRTs with minimal user intervention and is able to increase the efficiency in low statistics Monte Carlo simulations independently of the geometry. These kind of algorithms are inspired by the behavior of actual ant colonies and permit to create optimization tools to solve complex problems such as those of finding global minima. The ant colony optimization algorithm (ACOA) here developed “learns” where to apply VRTs to favor the increase of efficiency, obtaining information from the simulation itself and generating the so-called importance maps. ACOA has been successfully employed in calculations regarding the study of dose deposition in different problems: electron beams from linear accelerators, dose deposition calculations on MOSFET detectors [1] , radiosurgery photon beams [2] , improvement of dose distribution on monoisocentric beam split technique [3] and calculations of specific absorbed fractions in non-sealed radioisotope treatments [4] . The efficiency improvements range from 10 to 100 times or even more in case of high energy photon beams.

EP-1797: Dosimetric characterization of MOSFET detectors for Ir-192 and feasibility for in vivo dosimetry

EP-1797: Dosimetric characterization of MOSFET detectors for Ir-192 and feasibility for in vivo dosimetry

P17. Dose map variations over time of a blood irradiator

Introduction The Molsgaard Gammacell 2000 blood irradiator contains a Cs-137 source encapsulated in a stainless steel tube. The beaker containing the blood bag rotates during irradiation to give a uniform dose to the blood. Dose mapping is done to ensure that the given radiation dose to the blood is within the specified limits of minimum 25 Gy and maximum of 50 Gy to prevent graft versus host disease in bone marrow transplant patients. Materials and methods Dose map records exist for most years from 2005 to 2016, with the exception of 2006 and 2007. Until 2011 the dose maps were done using MOSFET detectors, with the detectors fitted in a custom tissue-equivalent phantom. Since 2012 the dose mapping was done using a type of gafchromic film in a cylindrical tissue-equivalent phantom. In addition, in 2013, dose mapping was also done using TLDs. Results In 2005 the dose map was fairly uniform and flat. The 2008 map is starting to show a very subtle dip, which becomes more pronounced in 2009 and has remained since. Minimum doses remain greater than 25 Gy. Conclusion The dose map dip that has become evident may be a sign of the source settling in its encapsulation. Annual checking of the dose map is very important for a blood irradiator. Changes of the dose distribution with time may result in a minimum dose of less than the recommended 25 Gy, which would require an increase in the irradiation time.

Radiation dose assessment of radiosensitive organs using mosfet detectors and monte-carlo based dosimetry software PCXMC during dental cone beam computed tomography examinations

Introduction Dental cone beam computed tomography is a new imaging modality increasingly being used in the oral and maxillofacial radiology, mainly for implant planning and assessment of general bony and dental pathological conditions. European Commission, Radiation Protection No 172, Cone beam CT for dental and maxillofacial radiology, 2012 report mentions; that Medical Physicists are required to routinely monitor radiation doses from such modality. Purpose We propose to establish an easily implemented method to assess the radiation dose delivered to the radiosensitive organs during imaging. Materials and methods The MOSFET detectors were used to measure the patient eye and thyroid gland radiation doses by placing them directly over the area covering the mentioned organs of the anthropomorphic adult female phantom. The measured doses were compared with the calculated doses using PCXMC software (version 2.0). Results Absorbed doses in (mGy) to the skin in the region of the eyes and thyroid were measured. MOSFET detectors calibration factor was obtained by direct comparison with calibrated standard ionization chamber. The uncertainty factor was 15% for the kV range used by the scanner. The most common DCBCT examination is the full arch high resolution scan conducted at our hospital using the WhiteFox CBCT scanner. The average measured eye and thyroid doses were 4.3 and 1.2 mGy respectively. The obtained results seem to agree with other published dosimetric studies. Conclusion The presented method can be used to routinely monitor radiation doses delivered to radiosensitive organs as result of imaging studies using DCBCT scanners in clinical environment. Disclosure we have nothing to disclose in relation with the presented work.

Prostate SBRT: The use of ERB and MOSFET in vivo dosimetry feasibility

Introduction Hypofractionated treatment regimens are indicated for some stages of prostate cancer. In order to provide the reproducibility of the relative position between rectum and prostate and to allow that only a small volume of the rectal wall remains close to the prostate, an endorectal balloon (ERB) may be used. Purpose This work aims to assess the dose on the rectal wall in the presence of the ERB filled with water or air and evaluate the deviation between the measured and calculated doses using two different algorithms (Eclipse AAA and iPlan PencilBeam) Materials and methods Two CT scans were obtained for a modified Rando phantom where an ERB (filled with water or air) with three MOSFET detectors was inserted. A simple 4 field in box plan and an IMRT (6MV beam) clinical plan were calculated in both CT sets, using two different algorithms. The treatment plans were delivered to the phantom using a Varian Novalis linac. Four sets of measurements were obtained and the results were compared with the calculated values. Results The difference between the calculated and measured doses around the ERB is lower when it is filled with water for both algorithms. The maximum relative differences when the ERB is filled with air are 1.8% for Eclipse and 5.5% for iPlan. Conclusion Both algorithms show a better a performance in the presence of water. Water filled ERB seems to be a suitable option. MOSFET dosimetry in association with ERBs for real-time in vivo during hypofractionated treatment of the prostate is a practical and reliable procedure. Disclosure The authors have no relevant financial or non-financial relationships to disclose.

Dosimetric impacts on skin toxicity for patients using topical agents and dressings during radiotherapy

AbstractBackground and purposeSkin care practices for radiotherapy patients are complicated by dosimetric concerns. This study measures the effect on skin dose of various topical agents and dressings.Materials and methodsSuperficial doses were measured under 17 topical agents and dressings and three clinical materials for reference. Dose was measured using a MOSFET detector under a 1 mm polymethyl methacrylate slab, with 6 MV photon beams at 100 cm source to surface distance.ResultsRelative skin dose under reference materials was 128% (thermoplastic mask), 158% (5 mm bolus) and 171% (10 mm bolus). Under a realistic application of topical agent (0·5 mm), relative skin doses were 106–111%. All dry dressings yielded relative dose of ≤111%; two wet dressings yielded higher relative doses (133 and 141%).ConclusionsUnder clinically relevant conditions, no cream, gel or dry dressing increased the skin dose beyond that seen with a thermoplastic mask. Dressings soaked with water produced less skin dose than 5 mm bolus. This may be unacceptable if wet dressings are in place for the majority of the treatment course. Our results suggest that skin care practices should not be limited by dosimetric concerns when using a 6 MV photon beam.