Cylindrical Diode Research Articles

SU-GG-T-239: Initial Experience of Patient Specific Rotational Quality Assurance for VMAT Using a Cylindrical Diodes Array Detector System

Purpose: Our recent success in commissioning volumetric modulated arc therapy (VMAT) on an Elekta Synergy accelerator calls for a convenient and suitable system for patient specific quality assurance. Traditional 2D diode array devices exhibit significant angular dependence under rotational beams. The newly available cylindrical diode array device ‐ ArcCHECK (Sun Nuclear Corp., FL) was designed specifically for rotational dosimetry by arranging diodes on a cylindrical plane. The purpose of this study is to evaluate the performance of the ArcCHECK device for VMAT. Results are compared with TomoTherapy QA. Method and Material: Both ArcCHECK and MapPHAN were commissioned on a Monaco treatment planning system (Version 2.0.3, CMS Inc., MO). Initial testing of the system involved four representative VMAT cases at various tumor sites: lung,liver, spine and prostate. The planned dose map on the diode plane was extracted from the QA plans for comparison with ArcCHECK measurements. Absolute differences between measured and planned doses were analyzed using 3% dose deviation and 3mm distance‐to‐agreement (DTA). For comparison, the selected plans were also calculated and delivered to a 2D diode array device (at 5cm effective depth) using MapCHECK (Sun Nuclear Corp., FL) integrated with a square solid water phantom (MapPHAN). Results from VMAT were compared with results from 14 QA plans from TomoTherapy. Results: Both diode array devices provide reasonable QA results for all cases. Doses measured with ArcCHECK agreed better with plans, with average passing rates and standard deviations of 98.3 ± 0.5% and 97.0 ± 1.8% for VMAT and Tomo respectively at all tumor sites. QA plans utilizing the 2D MapPHAN had average passing rates of 92.2 ± 2.4% (VMAT) and 90.0 ± 4.7% (Tomo). Conclusion: Compared to the 2D MapPHAN, ArcCHECK provided improved agreement between measured and calculated doses for rotational QA, and is recommended for rotational IMRT QA.

Experimental Study on Axial Dependence of Anode Current Distribution in an Oven Magnetron

The axial dependence of the anode current distribution in a 2.45-GHz oven magnetron was investigated. We previously demonstrated that installing a cathode shield is an effective way to reduce magnetron noise. The objective of the present study is to determine why installing a cathode shield reduces the noise. We measured the anode current of a test magnetron with an axially segmented anode block. When no magnetic field was applied, the test magnetron behaved like a cylindrical vacuum diode, and the axial electron distribution corresponded to the axial distribution of the cathode temperature. When a magnetic field was applied, the cathode shield increased the normalized anode current in the central region, which contributes to noise reduction.

Electron beam current in high power cylindrical diode

Intense electron beam generation studies were carried out in high power cylindrical diode to investigate the effect of the accelerating gap and diode voltage on the electron beam current. The diode voltage has been varied from 130 to 356 kV, whereas the current density has been varied from 87 to 391 A/cm2 with 100 ns pulse duration. The experimentally obtained electron beam current in the cylindrical diode has been compared with the Langmuir–Blodgett law. It was found that the diode current can be explained by a model of anode and cathode plasma expanding toward each other. However, the diode voltage and current do not follow the bipolar space-charge limited flow model. It was also found that initially only a part of the cathode take part in the emission process. The plasma expands at 4.2 cm/μs for 1.7 cm anode-cathode gap and the plasma velocity decreases for smaller gaps. The electrode plasma expansion velocity of the cylindrical diode is much smaller as compared with the planar diode for the same accelerating gap and diode voltage. Therefore, much higher voltage can be obtained for the cylindrical diodes as compared with the planar diodes for the same accelerating gap.

Calibration of a novel four‐dimensional diode array

The aim of this work is to develop effective calibration methods for a novel fourdimensional (4D) diode array for pretreatment verification of intensity-modulated radiation therapy (IMRT) and rotational therapy. A novel 4D diode array (ArcCHECK, Sun Nuclear, Melbourne, FL) was developed to meet the needs of appropriate and efficient quality assurance for IMRT and especially rotational radiotherapy. The diode array presents a consistent detector image in beam's eye view at arbitrary gantry angles due to isotropic arrangement of diodes in a three-dimensional (3D) cylindrical phantom. The 50 ms simultaneous update of all diodes on the detector array (fourth dimension) makes it capable of time-resolved beam delivery analysis with any rotational delivery techniques. The calibration procedure consisted of delivering and measuring a series of calibration beams with 5.8 degrees angular spacing surrounding the cylindrical diode array. Correction factors for diode intrinsic sensitivity and directional response dependence were derived from these measurements. A real-time algorithm to derive gantry angles based on the detector signal was developed to interpolate and apply the corresponding angular correction factors. The calibration was validated with ion chamber scanned beam profiles in a 3D water tank. Excellent agreement was observed between diode array measurement and treatment planning system calculation. The accuracy of the gantry angle derivation algorithm was within 1 degree which caused a less than 0.2% dosimetric uncertainty. With the proposed calibration method and the automatic gantry angle derivation algorithm, the 4D diode array achieved isotropic detector response and is suitable for both IMRT and rotational therapy pretreatment verification.

Volumetric Modulated Arc Therapy for Stereotactic Body Radiation Therapy: Initial Clinical Experience

Volumetric modulated arc therapy (VMAT) is a relatively new treatment modality characterized by variable angular dose rate dynamic arc delivery. Our institution is one of the first to apply this technique clinically for treatment of stereotactic body radiation therapy (SBRT). The purpose of this work is to present our initial clinical experience with VMAT for SBRT of the lung and spine. Six SBRT patients (3 lung, 3 spine) have been treated with VMAT under IRB approved institutional protocols. Each patient had a conformal or IMRT plan created for comparison purposes. Patients were selected based on expected tolerable treatment time, quality of the initial IMRT or 3DCRT plan, time available between simulation and treatment, and physician preference. Lung SBRT was prescribed as 48-60 Gy in 4-5 Fx, and spine SBRT was prescribed as 15-18 Gy in 1 Fx. Planning time, plan quality, QA procedure and results, and treatment efficiency for VMAT vs. conventional techniques were evaluated. Seven courses of SBRT were delivered to 6 patients (1 lung patient had 2 isocenters) using VMAT. Planning time was increased by approximately 30% and plan quality was equivalent or superior for VMAT in all patients compared to 3DCRT or IMRT. In 2 lung cases, VMAT reduced the lung and/or heart dose low enough to allow SBRT treatment that would not have been possible due to DVH constraints. PTV D95 was increased by 0.8-3.1%, and GTV mean dose was increased by an average of 3.0% for the 4 lesions treated with VMAT vs. 3DCRT. Lung V20, V12.5, and mean dose were lower in all patients with max reductions in V20 and V12.5 of 2.6% and 5.8%, respectively. In 3 spine patients, OAR doses met protocol guidelines for both techniques, but PTV D90 was improved by 1.8-3.4%, and GTV and PTV mean doses increased by 1.1-4.3% when using VMAT vs. IMRT. QA on a cylindrical diode phantom showed excellent results for all techniques (> 95% passing, 3% / 2 mm gamma). QA time was increased with VMAT compared to our standard planar dose QA for IMRT. MU were increased slightly with VMAT vs. 3DCRT in lung SBRT (0.4-5.3%), but treatment times were reduced by 44-59% (mean = 7.9 min). MU were reduced with spine VMAT vs. IMRT by up to 28%, and treatment times were reduced by 37-52% (mean = 9 min). We have treated 6 lung or spine SBRT patients with VMAT. Compared with alternative IMRT or conformal plans, VMAT plans had equivalent or better plan quality, and the treatment times were reduced up to 13 min or 59%. Despite an increase in time required for planning and QA, VMAT is an excellent option for treatment using SBRT. In addition to the positive impact on patient comfort and clinic workload, treatment time reductions also reduce the potential for intrafraction motion and may make it possible for patients to better tolerate SBRT times.

Space Charge Limited Currents Calculations in Coaxial Cylindrical Diodes Using Particle-in-Cell Simulations

Three dimensional particle-in-cell (PIC) simulations are conducted to measure the space charge limited (SCL) currents in coaxial cylindrical diodes. New cut-cell emission and absorption boundary algorithms implemented in the electromagnetic PIC code, VORPAL are utilized for this study. Comparisons of VORPAL SCL current results are made with i) theoretical SCL current results for the one dimensional planar diode, ii) analytical solutions of SCL current results for the one dimensional and two dimensional cylindrical diode cases, and iii) an experimental cylindrical diode operated as a virtual cathode oscillator. VORPAL SCL current results show agreement within a few percent with the analytical two dimensional cylindrical diode results for the non-relativistic operating voltages of 1-100 kV and within error bars with the experimental cylindrical diode results for the diode operating voltage of 340 kV.

SU-FF-T-216: Comparison of a 2D and 3D Array of Diodes for IMRT QA

Purpose: To compare the IMRT QA pass/fail rates of a 2D diode array system MapCHECK™ and a cylindrical 3D diode array system Delta4™, and to investigate the benefit of DVH‐based IMRT QA. Methods and Materials: Eight treatment plans totaling 62 IMRT fields were measured using both MapCHECK and Delta4. The data were compared to the treatment planning data using Gamma analysis. Passing criteria was defined as 95% of measured points had to have a gamma value ⩽1.0 using a distance to agreement of 4mm, a 5% dose window, and a 10% dose threshold. Structures, including GTV and organs at risk (OAR), and dose volume histograms (DVHs) were exported from the treatment planning system to Delta4 for comparison to measured DVHs. Results: QA with Delta4 used an average of 466 detector points per field. Using gamma analysis, 60/62 (96.8%) IMRT fields passed with an average of 98.9% of detector points within a gamma value ⩽1.0 when measured on the Delta4. QA with MapCHECK used an average of 88 detector points per field. Application of the same gamma analysis resulted in 14/62 (22.6%) IMRT fields passing with an average of 91.3% of detector points with a gamma value ⩽1.0 as measured by MapCHECK. Further analysis of IMRT fields that failed using MapCHECK QA, indicated that the measured data was within 1–2% of the treatment plan. Planned GTV DVHs corresponded with the Delta4 measured GTV DVHs, however measured OAR DVHs differed from their planned DVHs. Conclusions: This study suggests that QA results acquired with Delta4 correspond more accurately to the actual treatment plan as compared to MapCHECK. Incorporating Delta4 into routine QA will decrease the overall QA analysis time. The increased pass rate with Delta4 may result from the increased amount of detectors per treatment field.

Application of a cylindrical diode as a load with vacuum insulation in high-voltage generators

Results of experimental tests of the possibility of using cylindrical diodes in the self-magnetic insulation mode as an equivalent load with a stable impedance of about units of ohms at voltages below 450 kV are given. The diode with a radial size of ∼10 cm and an interelectrode gap of ∼1.5 cm demonstrated a stable resistance of ∼3 Ω within ∼200 ns. The analyzed results show that the current of the self-magnetic insulation in the diode is determined by the limiting current.

Cylindrical Silicon-on-Insulator Microdosimeter: Design, Fabrication and TCAD Modeling

A novel silicon-on-insulator (SOI) microdosimeter has been designed and fabricated using planar processing techniques to realise a device with a micron-scale well-defined sensitive volume. Cylindrical structures were employed to allow for an improved definition of the average chord length of the sensitive volume over that of previous elongated parallelepiped solid-state detector designs. The structures were manufactured on individual silicon mesas situated on top of a buried oxide insulating layer. The mesa design eliminated lateral charge diffusion. Two kinds of test structures were designed with sensitive region widths of 2 mum and 10 mum. In addition, an array of 900 cylindrical diodes was fabricated to increase the charge collection statistics. TCAD (Technology Computer Aided Design) modeling of the electrostatic potential and electric field profile of the cylindrical microdosimeter was carried out to obtain 3D potential and electric field profiles. The modeling revealed a radial electric field within the cylindrical-shaped sensitive volume with a 1/r dependence. While the electric field at the core of the cylindrical microdosimeter was not sufficiently high to induce avalanche signal multiplication, the higher electric field at the core should still assist in the measurement of low linear-energy transfer (LET) events.

Performance and Modeling of the MWIR HgCdTe Electron Avalanche Photodiode

The operation of the mid-wave infrared (MWIR) HgCdTe cylindrical electron injection avalanche photodiode (e-APD) is described. The measured gain and excess noise factor are related to the collection region fill factor. A two-dimensional diffusion model calculates the time-dependent response and steady-state pixel point spread function for cylindrical diodes, and predicts bandwidths near 1 GHz for small geometries. A 2 μm diameter spot scan system was developed for point spread function and crosstalk measurements at 80 K. An electron diffusion length of 13.4 μm was extracted from spot scan data. Bandwidth data are shown that indicate bandwidths in excess of 300 MHz for small unit cells geometries. Dark current data, at high gain levels, indicate an effective gain normalized dark density count as low as 1000 counts/μs/cm2 at an APD gain of 444. A junction doping profile was determined from capacitance–voltage data. Spectral response data shows a gain-independent characteristic.

Influence of Insulator Length on the Downstream Electron Temperature and Density in the Coaxial Plasma Focus Device

We have generated the Ar plasma in the diode chamber based on the established coaxial electrode type and investigated the emitted visible light for emission spectroscopy. The optical emission spectrum data have been obtained for the focused plasma in the cylindrical diode chamber under the input voltage of 4.5 kV and pressure of 40 mtorr. We observed the nine emission lines of Ar II: 407.3, 410.6, 422.4, 426.8, 427.9, 435.0, 438.1, 444.5, and 488.0 nm. The downstream electron temperature and density have been measured by the Boltzmann plot and Stark broadening, respectively, from the assumption of local thermodynamic equilibrium. The length of the acrylic insulator has been changed to 30, 40, and 45 mm, respectively, which is covered on the inner cathode surface, to investigate the influence of insulator length on the downstream electron temperature and density in this experiment. Our results show that the downstream electron temperature and density of focused plasma are 2.5 eV and 1.6times1016 cm-3, respectively, for the coaxial plasma focus with cylindrical electrodes at the insulator length of 40 mm.

Dosimetric verification of RapidArc treatment delivery

Purpose. Recently, Varian Medical Systems have announced the introduction of a new treatment technique, in which dose is delivered over a single gantry rotation with variable MLC positions, dose rate and gantry speed. In February 2008, a preclinical installation of the RapidArc™ beam delivery approach was carried out on a Varian Clinac at Rigshospitalet in Copenhagen. The purpose of the installation was to perform measurements to verify the correctness of doses delivered with the RapidArc technique. In May 2008, the clinical release of the RapidArc application was installed at Rigshospitalet. Methods and materials. Nine treatment plans were generated in the Eclipse version 8.5 including the RapidArc optimizer for H&N and prostate cases. The plans were delivered to the Scandidos Delta4® cylindrical diode array phantom. First, the measured dose distributions were compared with the calculated doses. All plans were then delivered several times to verify consistency of the delivery. Gamma analysis was used to verify the correspondence between dose distributions. The temporal resolution of the delivery was analysed by investigating the arc segments between control points separately. Results. Overall, good agreement was observed between measured and calculated doses in most cases with gamma values above 1 in >95% of measured points. The reproducibility of delivery was also very high. Gamma analysis between two consecutive runs of the same delivery plan generally showed gamma values above 1 in none of the measured points, and dose deviation less than 1%. Temporal analysis showed small discrepancies between doses delivered between control points (∼2 degrees of the rotation) in consecutive runs of a plan, however these were cancelled out in the accumulated dose. Conclusion. The delivery of RapidArc beam delivery has been verified to correspond well with calculated dose distributions for a number of different cases. The delivery was very reproducible, and was carried out with high stability of the accelerator performance.

Low temperature refrigeration by using thermal-field electron emission in a coaxial cylindrical diode

This paper explores possibility of refrigeration by using thermal-field emission of electrons in a coaxial cylindrical diode with a nanometer scale inner electrode (or cathode). Our calculation shows that it is possible to provide cooling at temperatures down to 200 K if the work function of the cathode is about 1 eV. The limitation on using a low work function cathode can be improved by applying an external axial magnetic field to create an additional potential barrier near the anode, more efficiently restricting the emission of low energy electrons (below the Fermi energy level). Because of this extra filtering process, emitters of arbitrary work functions can be used to provide an improved cooling capability from 300 to 10 K. The optimal conditions to maximize cooling power density are determined both numerically and analytically. The space charge effects of the emitted electrons in the gap are included self-consistently.

Intense electron beam generation in a high power cylindrical diode with a bipolar pulse

Our previously reported results demonstrated that an intense gigawatt relativistic electron beam can be generated in a cylindrical diode in the presence of prepulse. This article reports intense electron beam generation in a cylindrical electron beam diode when subjected to a high voltage bipolar pulse. The experimental results are compared to the Langmuir–Blodgett law. The diode perveance in the positive voltage pulse linearly increases with time due to the increase in the emission area. It was found that the electron beam can be generated from the diode during the negative voltage pulse due to a very little gap closure during the positive voltage pulse.

Butt-end forming electrodes for dense electron beams and axisymmetric Brillouin formations

Exact solutions are constructed for the butt-end region of electron beams in the form of finite-length cylinders; truncated cones; cylindrical and conical rings; toroidal, spherical, and ellipsoidal Brillouin formations; a cylindrical magnetron in the subcritical mode; and a cylindrical diode. Problems for finite-length cylinders with elliptic and circular cross sections are investigated. The cylinders are considered in the presence of a uniform longitudinal magnetic field for the case when the longitudinal velocity in a flow exhibits a cycloidal variation and the Cauchy conditions are exactly fulfilled on the flow’s boundary. Relativistic and nonrelativistic flows are analyzed. In the latter case, both the electrostatic and magnetostatic problems are solved. Solution of the magnetostatic problem provides for the continuous transition from the eigenfield to the external magnetic field on a beam’s boundary.

Intense relativistic electron beam generation and prepulse effect in high power cylindrical diode

Intense gigawatt relativistic electron beam has been generated in a high power cylindrical diode in the presence of prepulse. A bipolar prepulse voltage, recorded at the diode, varies both in amplitude and time duration with the Marx generator voltage. It was found that only at the accelerating gap ⩽1.65cm there is some shot to shot variation in the diode voltage and current for the same Marx generator voltage. The anode and cathode plasma expansion velocities were calculated using the perveance data. The plasma expands at 5cm∕μs for 1.85cm radial anode-cathode gap and the plasma velocity decreases for smaller gap. It was found that the effect of the prepulse is less pronounced in the cylindrical diode as compared to planar diode that allows one operation of the cylindrical diode with the gap ⩽1.85cm.

Multidimensional short-pulse space-charge-limited flow

The two-dimensional models of the space-charge-limited (SCL) current density at the short pulse limit for which the electron pulse length is comparable or smaller than the electron transit time across the gap (i.e., XCL⩽1) have been developed. In particular, the scaling laws for short-pulse SCL electron emission in a planar diode with a circular emitting strip and a cylindrical diode with a finite length have been obtained and verified with particle-in-cell simulation. It is found that the enhancement (in terms of the long-pulse SCL current density) is proportional to XCL−1 for small XCL for both planar and cylindrical cases. The enhancement of the cylindrical short-pulse SCL current density is also found to be larger for the convergent flow (cathode outside) than divergent flow (cathode inside). Multidimensional effects are important only for small emitting strips with size comparable to the effective penetration distance (into the gap) of the short-pulse electron beam. Smooth transition between the short-pulse regime and the long pulse (steady-state) regime is demonstrated.

TU‐E‐ValB‐01: Helical Tomotherapy Targeting Total Bone Marrow ‐ Initial Clinical Experience at the University of Minnesota

Purpose: We report here the successful use of Tomotherapy at delivering intensity modulated radiotherapy to the bone and bone marrow spaces along the entire axis of a patient and describe a dosimetric analysis of the total marrow irradiation (TMI) treatment. This is part of a dose escalation trial to determine the maximum tolerated dose (MTD) of TMI when given prior to an alkylator‐intensive conditioning regimen for the treatment of high risk or relapsed solid tumors. Method and Materials: A patient enrolled in a dose escalation study trial received 600 cGy in 3 fractions. Two independent CT image sets (upper and lower part of the body) were obtained. A helical tomotherapy treatment plan was created from this CT image sets. The quality assurance was evaluated with the use of (a) ion chamber and (b) extended dose range film. The isorad‐p cylindrical diodes were used for in‐vivo dosimetry. Results: The patient showed neutrophil engraftment on day 11 and platelet engraftment by day 58. He is currently well at 120 days post transplant with no evidence of disease. The patient developed nausea and vomiting after the first fraction of Tomotherapy TMI. Other than above there were no adverse effects of TMI. The planned radiation conformed to all bone marrow sites. Average doses to lungs, kidneys, heart, and eyes were 50–70% of the prescribed dose for TMI treatments. The dose delivery verifications (pretreatment and in vivo dose measurement) were within ±3–5% of the expected dose calculated from the treatment planning station. Conclusions: We show that helical tomotherapy targeting the bone marrow of the whole body is clinically feasible. The clinical implementation of intensity modulated radiation to conform the radiation dose to all active bone marrow of the whole body opened up the possibility of a dose escalation study for high risk patients.

TU-D-224C-06: Simulation of Si Diode Response in Clinical Electron Beams

Purpose: To study the response of a silicon diode in clinical electron beams by using the EGSnrc Monte Carlo code. Method and Materials: A Monte Carlo model of a cylindrical diode (from Rikner and Grusell) has been constructed. The diode is put on the central axis of a cylindrical water phantom and the dose ratios of water/silicon at various depths in 6 MeV electron beams are calculated. The dose ratio is also calculated for a bare silicon pellet of the same size as the active volume of the diode. Both calculated dose ratios are compared with the calculated water/silicon restricted stopping‐power ratio (SPR) as a function of depth. The dose ratio is calculated as well for various field sizes at the reference depth in the 6 MeV electron beam.Results: For the diode model, the dose ratio versus depth in electron beams is nearly flat: within 1% from near the phantom surface to R50 (calculation uncertainty <0.5%). For the pellet, the dose ratio versus depth behaves almost the same as the Spencer‐Attix water/silicon SPR with Δ = 100 keV. It varies more than 3% with depth. The diode dose ratio changes by less than 0.5% for field sizes ranging from 2 to 10 cm (beam radius) although it is 1.6% smaller in a 1 cm radius beam. Conclusion: The silicon pellet behaves like an ideal Spencer‐Attix cavity with Δ slightly greater than 100 keV. The Si behind the active region plays an important role in making the overall dosimeter response flat within about 1% per unit dose to water. The silicon diode is an excellent dosimeter for electron beams since its response relative to the dose to water changes little with respect to the depth in phantom or the field size of source.

Generalization of the Langmuir–Blodgett laws for a nonzero potential gradient

The Langmuir–Blodgett laws for cylindrical and spherical diodes and the Child–Langmuir law for planar diodes repose on the assumption that the electric field at the emission surface is zero. In the case of ion beam extraction from a plasma, the Langmuir–Blodgett relations are the typical tools of study, however, their use under the above assumption can lead to significant error in the beam distribution functions. This is because the potential gradient at the sheath/beam interface is nonzero and attains, in most practical ion beam extractors, some hundreds of kilovolts per meter. In this paper generalizations to the standard analysis of the spherical and cylindrical diodes to incorporate this difference in boundary condition are presented and the results are compared to the familiar Langmuir–Blodgett relation.