Dimensional Diode Research Articles

SU‐E‐T‐52: A New Device for Quality Assurance of a Single Isocenter Technique for the Simultaneous Treatment of Multiple Brain Metastases

Purpose:Comprehensive quality assurance (QA) of a single isocenter technique for the simultaneous treatment of multiple brain metastases is presently impractical due to the time consuming nature of measuring each lesion's dose on film or with a micro‐chamber. Three dimensional diode array and full field film measurements are sometimes used to evaluate these plans, but gamma analysis may not reveal local errors that have significant effects on one or a few of several targets. This work aimed to design, build and test a phantom to simplify comprehensive measurement and evaluation.Methods:A phantom was designed with 28 stackable slabs. The top and bottom slabs are 1.5 centimeters (cm) in thickness, and central 26 slabs are 0.5 cm thick. When assembled with radiochromic film in all 27 gaps, the phantom measures 16.5 × 15 × 19 cm. Etchings were designed to aide in identification of specific film planes on computed tomography (CT) images and correlation of individual PTVs with closest bisecting planes. Patient verification plans with a total of 16 PTVs were calculated on the phantom CT, and test deliveries both with and without couch kicks were performed to test the ability to identify correct film placements and subsequent PTV specific dose distributions on the films.Results:Bisecting planes corresponding to PTV locations were easily identified, and PTV specific dose distributions were clear for all 16 targets. For deliveries with couch kicks, the phantom PTV dose distributions closely approximated those calculated on the patient's CT. For deliveries without couch kicks, PTV specific dosimetry was also possible, although the distributions had ‘ghosts’ equaling the number of couch kicks, with distance between ghosts increasing with distance from the isocenter.Conclusion:A new phantom facilitates fast comprehensive commissioning validation and PTV specific dosimetry for a single isocenter technique for treating multiple brain metastases.This work was partially funded by CIRS, Inc.

Quantitative Analysis of Head Scatter and Jaw Transmission Correction Factor for Enhanced Dynamic Wedge Fields Using a 2-dimensional Diode Array

Quantitative Analysis of Head Scatter and Jaw Transmission Correction Factor for Enhanced Dynamic Wedge Fields Using a 2-dimensional Diode Array

Poster - Thur Eve - 17: Control point analysis comparison of three different treatment planning and delivery complexity levels using a commercial three dimensional diode array.

To use Control Point Analysis (Sun Nuclear Corporation, Melbourne, Florida, USA) to analyze and compare delivered VMAT plans for three different treatment planning complexity levels. Nineteen patients were chosen and fully anonymized for the purpose of this study. Ten SBRT, six H&N, one breast and two prostate VMAT plans were generated on Pinnacle3 and delivered on a Varian LINAC. The delivered dose was measured using ArcCHECK™. Each plan was analyzed using SNC Patient 6 and Control Point Analysis. Gamma passing percentage was used to assess the differences between the measured and planned dose distributions and to assess the role of various control point binning scenarios. The prostate cases reported the highest gamma passing percentages for SNC Patient 6 (99.3%-99.5%,3%/3mm) and Control Point Analysis (99.1--99.3%,3%/3mm). The mean percentage of passing control point sectors for the prostate cases increased from 48.9±3.1% for individual control points to 69.5 ± 3.9% for 5 control points binned together to 100±0% for 10 control points binned together. Over all, there was a trend in the percentage of sectors passing gamma analysis increasing with the increase of the number of control points binned together in one sector for both passing criteria considered (48.9±3.1% for individual control points to 69.5±3.9% for 5 control points binned together in one sector to 100±0% for 10 control points binned together in one sector for the prostate). The delivery accuracy per control point depends on the MU/control point (SBRT) and the plan degree of modulation (H&N).

Characterization of a novel two dimensional diode array the “magic plate” as a radiation detector for radiation therapy treatment

Intensity modulated radiation therapy (IMRT) utilizes the technology of multileaf collimators to deliver highly modulated and complex radiation treatment. Dosimetric verification of the IMRT treatment requires the verification of the delivered dose distribution. Two dimensional ion chamber or diode arrays are gaining popularity as a dosimeter of choice due to their real time feedback compared to film dosimetry. This paper describes the characterization of a novel 2D diode array, which has been named the "magic plate" (MP). It was designed to function as a 2D transmission detector as well as a planar detector for dose distribution measurements in a solid water phantom for the dosimetric verification of IMRT treatment delivery. The prototype MP is an 11 × 11 detector array based on thin (50 μm) epitaxial diode technology mounted on a 0.6 mm thick Kapton substrate using a proprietary "drop-in" technology developed by the Centre for Medical Radiation Physics, University of Wollongong. A full characterization of the detector was performed, including radiation damage study, dose per pulse effect, percent depth dose comparison with CC13 ion chamber and build up characteristics with a parallel plane ion chamber measurements, dose linearity, energy response and angular response. Postirradiated magic plate diodes showed a reproducibility of 2.1%. The MP dose per pulse response decreased at higher dose rates while at lower dose rates the MP appears to be dose rate independent. The depth dose measurement of the MP agrees with ion chamber depth dose measurements to within 0.7% while dose linearity was excellent. MP showed angular response dependency due to the anisotropy of the silicon diode with the maximum variation in angular response of 10.8% at gantry angle 180°. Angular dependence was within 3.5% for the gantry angles ± 75°. The field size dependence of the MP at isocenter agrees with ion chamber measurement to within 1.1%. In the beam perturbation study, the surface dose increased by 12.1% for a 30 × 30 cm(2) field size at the source to detector distance (SDD) of 80 cm whilst the transmission for the MP was 99%. The radiation response of the magic plate was successfully characterized. The array of epitaxial silicon based detectors with "drop-in" packaging showed properties suitable to be used as a simplified multipurpose and nonperturbing 2D radiation detector for radiation therapy dosimetric verification.

SU-GG-T-217: Can Two Dimensional Diode Arrays Serve as an Effective Tool for VMAT Quality Assurance?

Purpose: Two dimensional diode arrays have been widely adopted in the verification of fixed field IMRT deliveries. With increasing adoption of volume modulated arc therapy (VMAT), an immersed question is whether these diode arrays can be used for quality assurance for rotational therapy. The major concerns are the angular dependence of diode arrays and the detector to detector variance within the array. In this work, we studied the angular dependence of each detector and investigated the tools for correcting this dependency. Material and Methods: A MapCheck-I (Sun Nuclear Inc.) was used as the diode array for this study. The two dimensional (2D) array was inserted in a multi-cube solid water phantom and was irradiated on an Elekta Synergy Linac using open square fields from different gantry angles. The measured 2D intensity map was then compared to theoretical map generated in the treatment planning system (TPS), Pinnacle3. A 2D correction ratio map for each gantry angle was produced. Correction was implemented to static beams and VMAT plans of prostate cases. Results: The correction maps for all gantry angles were obtained and the angular response of each individual detector was determined. The correction map was found be dependent on the field size that was used to irradiate the diode array. The correction ratio, as defined as measure/planned, is higher for larger field size. The correction to prostate VMAT cases has no significant improvement. The verification of individual segment reveals that the irregular MLC shapes has impact on the diode response. Conclusion: Cautious should be taken to use 2D diode array for rotational arc therapy quality assurance. More studies need to be done to correct the complex dependence of the correction ratio on field size and aperture shape. Research supported by Elekta.

SU-FF-T-204: The Determination of Optimal Hounsfield Unit Assignment of Materials in a 2 Dimensional Diode Array Used in the Calculation of Rotational Therapy Verification Plans

Purpose: To determine the optimal Hounsfield unit (HU) assignments for the materials in a two dimensional diode detector array used for the quality assurance of rotational radiotherapy.Method and Materials: A 2 dimensional diode array (MapCheck, Sun Nuclear Corporation) with a surrounding uniform density phantom (MapPhan) was imaged with 2.5 mm CT slices. The array contains a plane of 445 solid state detectors in contact on both the top and bottom with a fiberglass pc board and polycarbonate plate. The attenuating properties of this entire detector assembly were modeled in the eclipse planning system by assigning different HU to the materials and comparing calculated (AAA 8.2.23 algorithm, Varian Medical Systems) and measured dose. The diodes were assigned HU of −50, 0, 50 or 3000, and the polycarbonate plates were assigned HU of −50, 0, 50, 150, 250 or 350. Rectangular fields of length 25cm and widths of 5cm, 10cm and 30cm, were planned with different HU assignments and measured in a 358° arc. Results: Comparisons were made using the agreement rate between calculated and measured results within 2% dose. The phantom defined by uniform density had 99.1%, 95.7% and 92.4% agreement for HU values of −50, 0, and 50 respectively. Definitions of the polycarbonate plates of HU of 50, 150, 250, and 350 yielded calculation agreements with measurement of 96.4%. 93.3%, 90.3% and 84.5%. When each of the 445 diodes is assigned 3000 HU, the 2% dose agreement rate is 94.2% and pixel density correction yielded an agreement of 60.4%. Conclusion: The best results were obtain with uniform HU assignments of 0 or −50 HU or by defining the polycarbonate plates with a HU of 50 and the remainder of the assembly 0 HU. Unacceptable results are obtained when using pixel based density corrections.

SU-EE-A2-04: Comparison of a MLC Calibration Method Using a Diode Array and Electronic Portal Imaging Device

Purpose: Current methods of MLC calibration and quality assurance include the use of graph paper, scanning water tanks, ion chambers, film and EPIDs; these methods can be both laborious and/or non-quantitative. We propose a convenient and quantitative method for calibrating and performing routine quality assurance for MLCs. Method and Materials: The Profiler 2 (Sun Nuclear Corp., Melbourne, FL), a two dimensional diode array, was visually aligned to the backup jaw and irradiated with an Elekta linac. The backup jaw served as a radiation reference line and was used to determine the positional diode offsets. The MLC was stepped over the Profiler 2 and the 50% position for each diode was determined. Leaf offsets were calculated by offsetting each diodes 50% position by the 50% position of the diode corresponding to leaf pair 20 and subtracting the corresponding positional diode offset. This method was repeated with an EPID for comparison purposes. Results: Linear dose changes of approximately 15%/mm and 13.5%/mm were observed in the penumbras of the backup jaw and MLC, respectively. Leaf positions measured by the EPID and Profiler 2 differed on average by ± 0.12 mm per leaf. Leaf reproducibility of 0.36 mm or better was observed and minimal reproducibility changes were seen with the backup jaw. Conclusion: This work demonstrates the potential of the Profiler 2 as a tool for calibrating and performing quantitative QA for MLC leaf positioning. The Profiler 2 methods are fast, taking an average of 20 minutes to perform and accurately represent leaf positions and reproducibility. Conflict of Interest: This work was supported in part by SBIR Contract No. HHSN261200522014C, ADB Contract No. N43-CM-52214, the University of Florida and Sun Nuclear Corporation.

A time dependent solution to the relativistic one-dimensional diode

Work in understanding electron flow across a simple potential gap, a diode, has a long history: The Child-Langmuir law [C. Child, Phys. Rev. 32 (1911) and I. Langmuir, Phys. Rev. 2 (1913) 450] describes the current density achievable in a one dimensional diode in the steady state. Pierce [Theory and Design of Electron Beams (Van Nostrand, Princeton and London, 1949)] extended the work by demonstrating that a beam of charged particles of finite transverse extent will exactly obey the one dimensional law as long as appropriate boundary conditions exterior to the beam are satisfied. Jory and Trivelpiece extended the solution to relativistic energies [J. Appl. Phys. 40 (1969) 3924], and Lampel and Tiefenback presented a solution for a time dependent non-relativistic step function current [Appl. Phys. Lett. 43 (1983) 57]. This paper presents an analytic solution for a voltage waveform across a fully relativistic one-dimensional diode which produces a constant electron current exiting the diode after a time equal to an electron transit time under steady state space charge limited operation. This result is compared to the previous result obtained for the non-relativistic case. Relevance of this solution to single cell RF-guns is discussed.