Field Flatness Research Articles

SU‐E‐T‐778: Use of the 2D MatriXX Detector for Measuring Scanned Ion Beam Parameters

Purpose:The quality assurance (QA) procedure has to check the most relevant beam parameters to ensure the delivery of the correct dose to patients. Film dosimetry, which is commonly used for scanned ion beam QA, does not provide immediate results. The purpose of this work is to answer whether, for scanned ion beam therapy, film dosimetry can be replaced with the 2D MatriXX detector as a real‐time tool.Methods:MatriXX, equipped with 32×32 parallel plate ion‐chambers, is a commercial device intended for pre‐treatment verification of conventional radiation therapy.The MatriXX, placed at the isocenter, and GAFCHROMIC films, positioned on the MatriXX entrance, were exposed to 131.44 MeV proton and 221.45 MeV/u Carbon‐ion beams.The OmniPro‐I'mRT software, applied for the data taking of MatriXX, gives the possibility of acquiring consecutive snapshots. Using the NI LabVIEW, the data from snapshots were logged as text files for further analysis. Radiochromic films were scanned with EPSON scanner and analyzed using software programs developed in‐house for comparative purposes.Results:The field dose uniformity, flatness, beam position and beam width were investigated. The field flatness for the region covering 6×6 cm2 square field was found to be better than 2%. The relative standard deviations, expected to be constant over 2×2, 4×4 and 6×6 pixels from MatriXX measurement gives a uniformity of 1.5% in good agreement with the film results.The beam center position is determined with a resolution better than 200 µm for Carbon and less than 100 µm for proton beam.The FWHM determination for a beam wider than 10 mm is satisfactory, whilst for smaller beams the determination is uncertain.Conclusion:Precise beam position and fast 2D dose distribution can be determined in real‐time using MatriXX detector. The results show that MatriXX is quick and accurate enough to be used in charged‐particle therapy QA.

A Monte Carlo simulation framework for electron beam dose calculations using Varian phase space files for TrueBeam Linacs.

To develop a framework for accurate electron Monte Carlo dose calculation. In this study, comprehensive validations of vendor provided electron beam phase space files for Varian TrueBeam Linacs against measurement data are presented. In this framework, the Monte Carlo generated phase space files were provided by the vendor and used as input to the downstream plan-specific simulations including jaws, electron applicators, and water phantom computed in the EGSnrc environment. The phase space files were generated based on open field commissioning data. A subset of electron energies of 6, 9, 12, 16, and 20 MeV and open and collimated field sizes 3 × 3, 4 × 4, 5 × 5, 6 × 6, 10 × 10, 15 × 15, 20 × 20, and 25 × 25 cm(2) were evaluated. Measurements acquired with a CC13 cylindrical ionization chamber and electron diode detector and simulations from this framework were compared for a water phantom geometry. The evaluation metrics include percent depth dose, orthogonal and diagonal profiles at depths R100, R50, Rp, and Rp+ for standard and extended source-to-surface distances (SSD), as well as cone and cut-out output factors. Agreement for the percent depth dose and orthogonal profiles between measurement and Monte Carlo was generally within 2% or 1 mm. The largest discrepancies were observed within depths of 5 mm from phantom surface. Differences in field size, penumbra, and flatness for the orthogonal profiles at depths R100, R50, and Rp were within 1 mm, 1 mm, and 2%, respectively. Orthogonal profiles at SSDs of 100 and 120 cm showed the same level of agreement. Cone and cut-out output factors agreed well with maximum differences within 2.5% for 6 MeV and 1% for all other energies. Cone output factors at extended SSDs of 105, 110, 115, and 120 cm exhibited similar levels of agreement. We have presented a Monte Carlo simulation framework for electron beam dose calculations for Varian TrueBeam Linacs. Electron beam energies of 6 to 20 MeV for open and collimated field sizes from 3 × 3 to 25 × 25 cm(2) were studied and results were compared to the measurement data with excellent agreement. Application of this framework can thus be used as the platform for treatment planning of dynamic electron arc radiotherapy and other advanced dynamic techniques with electron beams.

Development of High Frequency Resonators for Proton Acceleration in the Energy Range from 2.3 Till 5 MeV

High frequency optimization of multigap IH-/CH-resonators requires solving the range of problems, particularly receiving high values of effective shunt impedance and realization of the uniform accelerating field distribution on its axes. H-mode resonators design contains flat vanes (pylons)to reach both of these goals. This article presents the results of electrodynamic modeling of CH- and IH-resonators in case of zero gap between the vanes and resonator end wall. The values of the optimized shunt impedances, Q-factor and field flatness for IH- and CH-designs are shown.

Development of an expanded-field irradiation technique using a gimbaled x-ray head.

The Vero4DRT has a maximum field size of 150.0 × 150.0 mm. The purpose of the present study was to develop expanded-field irradiation techniques using the unique gimbaled x-ray head of the Vero4DRT and to evaluate the dosimetric characteristics thereof. Two techniques were developed. One features gimbal swing irradiation and multiple static segments consisting of four separate fields exhibiting 2.39° gimbal rotation around two orthogonal axes. The central beam axis for each piecewise-field is shifted 40 mm from the isocenters of the left-right (LR) and superior-inferior (SI) directions, and, thus, the irradiation field size is expanded to 230.8 × 230.8 mm. Adjacent regions were created at the isocenter (a center-adjacent expandedfield) and 20 mm from the isocenter (an off-adjacent expandedfield). The field gaps or overlaps of combined piecewise-fields were established by adjustment of gimbal rotation and movement of the multileaf collimator (MLC). Another technique features dynamic segment irradiation in which the beam is delivered while rotating the gimbal. The dose profile is controlled by a combination of gimbal swing motion and opening and closing of the MLC. This enabled the authors to expand the irradiation field on the LR axis because the direction of MLC motion is parallel to that axis. A field 220.6 × 150.0 mm in dimensions was configured and examined. To evaluate the dosimetric characteristics of the expandedfields, films inserted into water-equivalent phantoms at depths of 50, 100, and 150 mm were irradiated and field sizes, penumbrae, flatness, and symmetry analyzed. In addition, the expanded-field irradiation techniques were applied to intensity-modulated radiation therapy (IMRT). A head-and-neck IMRT field, created using a conventional Linac (the Varian Clinac iX), was reproduced employing an expanded-field of the Vero4DRT. The simulated dose distribution for the expanded-IMRT field was compared to the measured dose distribution. The field sizes, penumbrae, flatness, and symmetry of the center- and off-adjacent expanded-fields were 230.2-232.1 mm, 6.8-10.7 mm, 2.3%-5.1%, and -0.5% to -0.4%, respectively, at a depth of 100 mm. Similarly, the field sizes, penumbrae, flatness, and symmetry of dynamic segment irradiation on the LR axis were 219.2 mm, 6.0-6.2 mm, 3.4%, and -0.1%, respectively, at a depth of 100 mm. In the area of expanded-IMRT dose distribution, the passing rate of 5% dose difference was 85.8% between measurements and simulation, and the 3%/3 mm gamma passing rate was 96.4%. Expanded-field irradiation techniques were developed using a gimbaled x-ray head. The techniques effectively extend target areas, as required when whole-breast irradiation or head-and-neck IMRT is contemplated.

Design and multiphysics analysis of a 176 MHz continuous-wave radio-frequency quadrupole

We have developed a new design for a 176 MHz cw radio-frequency quadrupole (RFQ) for the SARAF upgrade project. At this frequency, the proposed design is a conventional four-vane structure. The main design goals are to provide the highest possible shunt impedance while limiting the required rf power to about 120 kW for reliable cw operation, and the length to about 4 meters. If built as designed, the proposed RFQ will be the first four-vane cw RFQ built as a single cavity (no resonant coupling required) that does not require π -mode stabilizing loops or dipole rods. For this, we rely on very detailed 3D simulations of all aspects of the structure and the level of machining precision achieved on the recently developed ATLAS upgrade RFQ. A full 3D model of the structure including vane modulation was developed. The design was optimized using electromagnetic and multiphysics simulations. Following the choice of the vane type and geometry, the vane undercuts were optimized to produce a flat field along the structure. The final design has good mode separation and should not need dipole rods if built as designed, but their effect was studied in the case of manufacturing errors. The tuners were also designed and optimized to tune the main mode without affecting the field flatness. Following the electromagnetic (EM) design optimization, a multiphysics engineering analysis of the structure was performed. The multiphysics analysis is a coupled electromagnetic, thermal and mechanical analysis. The cooling channels, including their paths and sizes, were optimized based on the limiting temperature and deformation requirements. The frequency sensitivity to the RFQ body and vane cooling water temperatures was carefully studied in order to use it for frequency fine-tuning. Finally, an inductive rf power coupler design based on the ATLAS RFQ coupler was developed and simulated. The EM design optimization was performed using cst Microwave Studio and the results were verified using both hfss and ansys . The engineering analysis was performed using hfss and ansys and most of the results were verified using the newly developed cst Multiphysics package.

TH‐C‐12A‐03: Development of Expanded Field Irradiation Technique with Gimbaled X‐Ray Head

Purpose:The Vero4DRT has a maximum field size of 150×150 mm2. The purposes of this study were to develop an expanded field irradiation technique using a unique gimbaled x‐ray head of Vero4DRT and to evaluate its dosimetric characteristic.Methods:The expanded field irradiation consisted of four separate fields with 2.39 degree gimbal rotation around orthogonal two axes. The central beam axis for each field shifted 40 mm from the isocenter for longitudinal and lateral directions, and thus, the field size was expanded up to 230×230 mm2. Adjacent region were created at the isocenter (center‐adjacent expanded‐field) and 20 mm from isocenter (offadjacent expanded‐field). To create flat dose distribution in the combined piecewise‐fields, the overlapping and gaps regions on the isocenter plane were adjusted with the gimbal rotating and the MLC. To evaluate dosimetric characteristic of the expanded‐field, films inserted in water‐equivalent phantoms at 50, 100 and 150 mm depth were irradiated and the field size, penumbra, flatness and symmetry were analyzed.In addition, the expandedfield irradiation technique was applied to IMRT. A head and neck IMRT field, which was planned for the conventional linac (Varian Clinac iX), was reproduced with the expanded‐field of the Vero4DRT. The simulated dose distribution for the expanded IMRT field was compared to the measured dose distribution.Results:The field size, penumbra, flatness and symmetry of center‐ and off‐ adjacent expanded‐fields were 230.2–232.1 mm, 7.8–10.7 mm, 2.3–6.5% and –0.5–0.4% at 100 mm depth. The 82.1% area of the expanded IMRT dose distribution was within 5% difference between measurement and simulation, which was analyzed upper 50% dose area, and the 3%/3 mm gamma pass rate was 98.4%.Conclusions:The expandedfield technique was developed using the gimbaled x‐ray head. To extend applied targets, such as whole breast irradiations or head and neck IMRT, the expanded‐field technique would be effective.

SU‐E‐T‐52: Beam Data Comparison for 20 Linear Accelerators in One Network

Purpose:To compare photon beam data for the 20 Varian linear accelerators (TrueBeam, iX, and EX models) in use at five centers in the same network with the intent to model with one set of beam data in Eclipsec.Methods:Varian linear accelerators, TrueBeam (3), 21 EX, iX, and Trilogy (14), and 6 EX (3), installed between 1999 and 2014 have their 6 MV and 15 MV x‐ray beams reevaluated. Full commissioning, including output factors (St), percent depth doses (PDD), and off‐axis profiles, was recently performed for a TrueBeam with a cc04 ion chamber in an IBA Blue phantom. Similarly, a subset of beam data for each of the other accelerators was measured recently as follows: for 3×3, 10×10, and 30×30 cm2 field sizes, flatness and penumbra (80–20%) were measured at dmax and 10 cm depths, PDD were measured at 10 and 20 cm depths, and St were measured at 5 cm depth. Measurement results for all machines were compared.Results:For 15 high‐energy (6 and 15 MV) and 3 low‐energy machines (6MV only): 1) PDD agreed within 1.4% at 10 and 20 cm depths; 2) penumbra agreed within 1.0 mm at dmax and 10 cm depths; 3) flatness was within 1.3% at dmax and 10 cm depths; and 4) with exception of the three low energy machines, output factors were within 1.1% and 0.5% for 3×3 and 30×30 cm2, respectively. Measurement uncertainty, not quantified here, accounts for some of these differences.Conclusion:Measured beam data from 15 high‐energy Varian linacs are consistent enough that they can be classified using one beam data set in Eclipse. Two additional high‐energy machines are removed from this group until their data are further confirmed. Three low‐energy machines will be in a separate class based upon differences in output factors (St).

SU-E-T-623: Delivery of 3D Conformal Proton-Therapy Fields at Extended Source- To-Axis Distances

Purpose: To evaluate the dosimetric properties of proton dose distributions delivered at extended source-to-skin distances (SSD). Methods: Radiation was delivered with a gantry-mounted proton double scattering system (Proteus 230, IBA). This system has a maximum field diameter of 24 cm at isocenter and a nominal source-to-axis distance of 230 cm. Dose was measured at nominal SSD as well as at -10, +10, +25, +50, +75, and +100 cm for several range and modulation width combinations. Depth dose distributions were measured with a multi-layer ionization chamber (MLIC), and lateral dose distributions with a 2D ionization chamber array as well as with a diode in a water phantom. Results: The maximum field diameter was found to increase from 24.0 cm at nominal SSD to 29.1 cm and 33.3 cm at +50 cm and +100 cm respectively. Field flatness remained below 3% for all SSD. Tilt in the spread-out Bragg peak depth dose distribution increased with SSD up to 0.4 %-per-g/cm2 at +100 cm. The measured range decreased with 1.1×10-3 g/cm2 per centimeter shift in SSD due to proton energy loss in air, very close to the theoretically calculated value of 1.06×10-3 g/cm3. The output and dose rate decrease with the inverse of the SSD squared as expected. Conclusion: Extending the SSD up to 100 cm increases the maximum field diameter from 24.0 cm to 33.3 cm while the dose uniformity remains acceptable.

Open-loop correction for an eddy current dominated beam-switching magnet.

A beam-switching magnet and the pulsed power supply it requires have been developed for the Japan Proton Accelerator Research Complex. To switch bunched proton beams, the dipole magnetic field must reach its maximum value within 40 ms. In addition, the field flatness should be less than 5 × 10(-4) to guide each bunched beam to the designed orbit. From a magnetic field measurement by using a long search coil, it was found that an eddy current in the thick endplates and laminated core disturbs the rise of the magnetic field. The eddy current also deteriorates the field flatness over the required flat-top period. The measured field flatness was 5 × 10(-3). By using a double-exponential equation to approximate the measured magnetic field, a compensation pattern for the eddy current was calculated. The integrated magnetic field was measured while using the newly developed open-loop compensation system. A field flatness of less than 5 × 10(-4), which is an acceptable value, was achieved.

A novel high-throughput irradiator for in vitro radiation sensitivity bioassays

This paper describes the development and characterization of a fully automated in vitro cell irradiator using an electronic brachytherapy source to perform radiation sensitivity bioassays. This novel irradiator allows complex variable dose and dose rate schemes to be delivered to multiple wells of 96-well culture plates used in standard biological assays. The Xoft Axxent® eBx™ was chosen as the x-ray source due to its ability to vary tube current up to 300 µA for a 50 kVp spectrum using clinical surface applicators. Translation of the multiwell plate across the fixed radiation field is achieved using a precision motor driven computer controlled positioning system. A series of measurements was performed to characterize dosimetric performance of the system. Measurements have shown that the radiation output measured with an end window ionization chamber is stable between operating currents of 50–300 µA. In addition, radiochromic film was used to characterize the field flatness and symmetry. The average field flatness in the in-plane and cross-plane direction was 2.9 ±1.0% and 4.0 ±1.7%, respectively. The average symmetry in the in-plane and cross-plane direction was 1.8 ±0.9% and 1.6 ±0.5%, respectively. The optimal focal spot resolution at the cellular plane was determined by measuring sequential irradiations on radiochromic film for three different well spacing schemes. It was determined that the current system can irradiate every other well with negligible impact on the radiation field characteristics. Finally, a performance comparison between this system and a common cabinet irradiator is presented.

Preliminary study on the RF tuning of CSNS DTL

In the R&D of the CSNS Drift Tube Linac (DTL), the first unit tank with 28 drift tubes has been developed. The axial accelerating field is ramped from 2.2 MV/m to 3.1 MV/m in this tank. The required field flatness is less than ±2% with the standard deviation of 1% for the beam dynamics; the field stability should be less than 1% for machine stable operation. After successful alignment, RF tuning was carried out focusing on the field profile measurement. Four slug tuners and eleven post couplers were applied in this procedure. The ramped field and required stability had been achieved by fine adjustment of the slug tuners and post couplers. In this paper, the preliminary tuning results are presented and discussed.

Simple lumped circuit model applied to field flatness tuning of four-rod radio frequency quadrupoles

The field flatness of any radio frequency quadrupole (RFQ) is an important parameter that needs to be carefully tuned because it can affect beam transmission efficiency. In four-rod RFQs, the heights of a set of tuning plates determine the quality of the field flatness. The goals of this paper are (a) to show that by using a lumped circuit model of a four-rod RFQ, the field flatness profile for any tuning plate height distribution can be quickly calculated, (b) to derive a perturbative solution of the model so that insights into the physics of the tuning process and its effects can be understood, and (c) to compare the predicted field profiles to measurements.

Full three-dimensional approach to the design and simulation of a radio-frequency quadrupole

We have developed a new full 3D approach for the electromagnetic and beam dynamics design and simulation of a radio-frequency quadrupole (RFQ). A detailed full 3D model including vane modulation was simulated, which was made possible by the ever advancing computing capabilities. The electromagnetic (EM) design approach was first validated using experimental measurements on an existing prototype RFQ and more recently on the actual full size RFQ. Two design options have been studied, the original with standard sinusoidal modulation over the full length of the RFQ; in the second design, a trapezoidal modulation was used in the accelerating section of the RFQ to achieve a higher energy gain for the same power and length. A detailed comparison of both options is presented supporting our decision to select the trapezoidal design. The trapezoidal modulation increased the shunt impedance of the RFQ by 34%, the output energy by 15% with a similar increase in the peak surface electric field, but practically no change in the dynamics of the accelerated beam. The beam dynamics simulations were performed using three different field methods. The first uses the standard eight-term potential to derive the fields, the second uses 3D fields from individual cell-by-cell models, and the third uses the 3D fields for the whole RFQ as a single cavity. A detailed comparison of the results from TRACK shows a very good agreement, validating the 3D fields approach used for the beam dynamics studies. The EM simulations were mainly performed using the CST Microwave-Studio with the final results verified using other software. Detailed segment-by-segment and full RFQ frequency calculations were performed and compared to the measured data. The maximum frequency deviation is about 100 kHz. The frequencies of higher-order modes have also been calculated and finally the modulation and tuners effects on both the frequency and field flatness have been studied. We believe that with this new full 3D approach, the enhanced computing capabilities and the calculation precision the electromagnetic design software offer, we may be able to skip the prototyping phase and build the final product at once, although we recognize that prototyping is still needed to establish and validate the fabrication procedure.

Experimental validation of the TOPAS Monte Carlo system for passive scattering proton therapy

TOPAS (TOol for PArticle Simulation) is a particle simulation code recently developed with the specific aim of making Monte Carlo simulations user-friendly for research and clinical physicists in the particle therapy community. The authors present a thorough and extensive experimental validation of Monte Carlo simulations performed with TOPAS in a variety of setups relevant for proton therapy applications. The set of validation measurements performed in this work represents an overall end-to-end testing strategy recommended for all clinical centers planning to rely on TOPAS for quality assurance or patient dose calculation and, more generally, for all the institutions using passive-scattering proton therapy systems. The authors systematically compared TOPAS simulations with measurements that are performed routinely within the quality assurance (QA) program in our institution as well as experiments specifically designed for this validation study. First, the authors compared TOPAS simulations with measurements of depth-dose curves for spread-out Bragg peak (SOBP) fields. Second, absolute dosimetry simulations were benchmarked against measured machine output factors (OFs). Third, the authors simulated and measured 2D dose profiles and analyzed the differences in terms of field flatness and symmetry and usable field size. Fourth, the authors designed a simple experiment using a half-beam shifter to assess the effects of multiple Coulomb scattering, beam divergence, and inverse square attenuation on lateral and longitudinal dose profiles measured and simulated in a water phantom. Fifth, TOPAS' capabilities to simulate time dependent beam delivery was benchmarked against dose rate functions (i.e., dose per unit time vs time) measured at different depths inside an SOBP field. Sixth, simulations of the charge deposited by protons fully stopping in two different types of multilayer Faraday cups (MLFCs) were compared with measurements to benchmark the nuclear interaction models used in the simulations. SOBPs' range and modulation width were reproduced, on average, with an accuracy of +1, -2 and ±3 mm, respectively. OF simulations reproduced measured data within ±3%. Simulated 2D dose-profiles show field flatness and average field radius within ±3% of measured profiles. The field symmetry resulted, on average in ±3% agreement with commissioned profiles. TOPAS accuracy in reproducing measured dose profiles downstream the half beam shifter is better than 2%. Dose rate function simulation reproduced the measurements within ∼2% showing that the four-dimensional modeling of the passively modulation system was implement correctly and millimeter accuracy can be achieved in reproducing measured data. For MLFCs simulations, 2% agreement was found between TOPAS and both sets of experimental measurements. The overall results show that TOPAS simulations are within the clinical accepted tolerances for all QA measurements performed at our institution. Our Monte Carlo simulations reproduced accurately the experimental data acquired through all the measurements performed in this study. Thus, TOPAS can reliably be applied to quality assurance for proton therapy and also as an input for commissioning of commercial treatment planning systems. This work also provides the basis for routine clinical dose calculations in patients for all passive scattering proton therapy centers using TOPAS.

On the Electromagnetic Design of a <formula formulatype="inline"><tex Notation="TeX">$\beta_{g}=0.61$ </tex></formula> 650-MHz Superconducting Radio Frequency Cavity

We present calculations for an electromagnetic design of a β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> = 0.61 multicell superconducting radio frequency cavity for the Indian Spallation Neutron Source project. The geometry parameters of the midcells are optimized using a step-by-step 1-D optimization technique. This is followed by the optimization of end cells, which is done to achieve the required field flatness and to avoid trapping of higher order modes. Calculations of the threshold beam current for the excitation of regenerative beam break-up instability excited by the dipole modes are also presented, which is followed by wakefield calculations and estimation of related effects. Specific aspects in these calculations, which are relevant for medium β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> cavities, are highlighted. Finally, studies are performed for the static and dynamic Lorentz force detuning, based on which the stiffness design of the cavity is optimized.

Characteristics of a normoxic polymethacrylic acid gel dosimeter for a 72-MeV proton beam

The characteristics of a normoxic polymethacrylic acid gel dosimeter for a 72-MeV proton beam were evaluated. A polymer gel dosimeter was synthesized using gelatin, methacrylic acid, hydroquinone, tetrakis(hydroxymethyl) phosphonium chloride, and highly purified distilled water. The dosimeter was manufactured by placement in a polyethylene (PE) container. Irradiated dosimeters were analyzed to determine the transverse relaxation time (T2) using a 1.5-T MRI. A calibration curve was obtained as a function of the absorbed dose.A Bragg curve made by irradiating the gel with mono-energy was compared with the results for a parallel plate ionization chamber. The spread-out Bragg peak (SOBP) range and distal dose fall-off (DDF) were comparatively analyzed by comparing the irradiated gel with a spread-out Bragg peak against with the ion chamber.Lastly, the gel's usefulness as a dosimeter for therapeutic radiation quality assurance was evaluated by obtaining its practical field size, flatness, and symmetry, through comparison of the profiles of the gel and ion chamber.

Quality assurance of electron beams using a Varian electronic portal imaging device

The feasibility of utilizing an electronic portal imaging device (EPID) for the quality assurance of electron beams was investigated. This work was conducted on a Varian 2100iX machine equipped with an amorphous silicon (aS1000) portal imager. The linearity of the imager pixel response as a function of exposed dose was first confirmed. The short-term reproducibility of the EPID response to electron beams was verified. Low (6 MeV), medium (12 MeV) and high (20 MeV) energies were tested, each along with small (6 × 6 cm2), medium (10 × 10 cm2) and large (20 × 20 cm2) applicators. Acquired EPID images were analyzed using an in-house MATLAB code for radiation field size, penumbra, symmetry and flatness. Field sizes and penumbra values agreed with those from film dosimetry to within 1 mm. Field symmetry and flatness constancies were measured over a period of three weeks. The results indicate that EPID can be used for routine quality assurance of electron beams.

TH‐F‐105‐04: Development of a Novel High‐Throughput Variable Dose Rate Irradiator for in Vitro Radiobiology Research

Purpose : Modern biology research often depends upon high‐throughput assays that are incompatible with traditional irradiation techniques due to dose inhomogeneity and dose rate limitations of the most common irradiator systems. To overcome these limitations, we sought to develop a high‐throughput, variable dose rate irradiator for in vitro radiobiology research. Methods: The Xoft Axxent eBx™ was chosen as the x‐ray source for its ability to vary current up to 300 μA, allowing delivery of a variety of doses over a range of dose rates to be applied to a 96‐well cell culture plate. This system delivers a 50kVp spectrum using standard surface applicators. Field flatness was achieved using a 10‐mm diameter surface applicator equipped with a flattening filter that is used for treating skin cancers. Additional collimation was designed to prevent dose contamination into adjacent wells. Dose uniformity was verified using EBT3 Gafchromic film and analyzed using ImageJ, while dose verification was achieved using TLD‐100 measurements and MCNP5 Monte Carlo simulations. Results : Translation of the well plate across the fixed radiation field is achieved using a precision motor‐driven computer controlled positioning system. An Exradin A20 ion chamber has been incorporated into this system to provide periodic air‐kerma rate verification during the irradiation cycle using newly proposed dosimetry formalism for this source. Ion chamber measurements of air‐kerma rate at the skin applicator exit window indicate achievable dose rates of 6–36 mGy/s while the dose uniformity profile of the 10mm Xoft skin applicator was maintained within 3.2% inside the cell culture well. Conclusions: This work indicates significant potential for the development of a high‐throughput, variable dose rate irradiator for in vitro radiobiology research using the Xoft Axxent eBx™ skin applicator system. This irradiator provides a powerful tool by increasing the efficiency and number of assay techniques available for the radiobiology researcher.

SU‐F‐500‐07: A Single Device for Mechanical and Radiation Quality Assurance (QA) Measurements of Medical Accelerators

Purpose: To develop a single unifying device that will record, document and track mechanical, optical and dosimetric measurements to reduce the number of devices used and time spent to perform the monthly Quality Assurance (QA) tasks recommended in AAPM Task Group 142 Report. Methods: This device includes a novel multi‐mirror system with rotating capabilities which directs the optical and radiation sources to a stationary camera to acquire images of all planes centered on the isocenter of the medical accelerator. A single phosphor screen is used as the receptor for optical light imaging. A plastic buildup sheet is added for radiation measurements. A computer interface allows efficient pass/fail analysis for laser alignment, light/radiation field coincidence, flatness and symmetry tests and MLC QA. Results: The system has been used to successfully acquire images that have never been recorded before, such as a combination of the light field, optical distance indicator and lasers as well as radiation field. QA of mechanical motions such as table rotation is readily captured. Radiation QA measurements accommodate both x‐ray and electron beams. QA of gantry motion can be performed by rotating the device. The time spent in Monthly QA according to TG‐142 for these tasks will be reduced in half. Conclusion: A single device has been successfully developed to unify all mechanical, optical and radiation components for required and recommended monthly QA tasks, reducing monthly machine maintenance time. In particular, the device records and documents optical measurements that are now only evaluated visually which will strengthen the confidence for safe patient treatment. Dr. John W. Wong is a founder of JPLC Associates LLC which has licensed the technology from Johns Hopkins University. Dr. Daniel J. Mirota's participation in this publication was as a paid consultant for JPLC Associates LLC. All opinions expressed and implied by Dr. Daniel J. Mirota in this publication are solely those of Dr. Daniel J. Mirota and do not represent or reflect the views of Johns Hopkins University. The work is supported in part by a grant from Technology Development Corporation (TEDCO) of Maryland.

A study of dosimetric characteristics for in vivo dosimetry with cylindrical n-type Isorad diode

AbstractBackground and purposeThe objective was to determine diode characteristics before actual dose verification on human phantom and patients.Materials and methodsThe reliability and stability of equipment, signal stability, precision, dose response linearity, field flatness, perturbation of radiation dose, plastic to water conversion factor (Kpl), ionisation chambers (ICs) and diode calibration were determined. Correction factors for tray (CFtray), wedge (CFwedge), field size (CFFS), SSD (CFSSD), angle (CFangle) and block (CFblock) were found. Patient dose monitor, Isorad diode (n-type) and IC (PTW Frieburg), Co-60 unit (Theratron), ATOM Adult male human phantom (Model 701-D, CIRS) were used.Results and conclusionGood signal stability, precise data, and linear dose response, variation of 0·500% and 5·000% in field flatness and perturbation tests, respectively, were noted. Kpl was 1·006 for IC PTW Frieburg TW30013, 0114. The diode calibration factor was 0·989. CFtray, CFFS, CFSSD, CFangle, CFblock were 1·001, 1·001, 0·997, 1·006 and 0·990, respectively. CFwedge were 1·024, 1·030 and 1·038 for 30°, 45° and 60° wedges, respectively. The verification of above correction factors (CFs) on Nasopharynx and lung of human phantom was also done.