bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of Fatigue Strength of Similar and Dissimilar Welded Joints of Modified 9Cr–1Mo Steel
Authors: Takaya Shigeru
Published: June 2015

This paper presents an evaluation method for the fatigue strength of similar and dissimilar welded joints of modified 9Cr–1Mo steel, which is a candidate structural material for demonstration of fast-breeder reactor being developed in Japan. The discontinuity of the mechanical properties across a welded joint causes a nonhomogeneous strain distribution, and this effect should be considered in the evaluation of the fatigue strength of welded joints. In this study, a “two-element model,” which comprises base metal and welded metal, was employed. First, the strain ranges of each element are calculated, and second, the fatigue lives of each element are evaluated. Finally, the shorter fatigue life is selected as the fatigue life of the welded joint. The failure position can be also estimated by this model. The evaluation results were compared with experimental data obtained at elevated temperature, and the results were in good agreement.
Control Rod Withdrawal Analysis of the Supercritical Water Reactor-Fuel Qualification Test Facility in the LVR-15 Research Reactor
Authors: Maráczy Csaba , Hegyi György , Trosztel István , Temesvári Emese
Published: June 2015

The aim of the supercritical water reactor-fuel qualification test (SCWR-FQT) Euratom-China collaborative project is to design an experimental facility for qualification of fuel for the supercritical water-cooled reactor. The facility is intended to be operated in the LVR-15 research reactor in the Czech Republic. The pressure tube of the FQT facility encloses four fuel rods that will operate in similar conditions to the evaporator of the HPLWR reactor. This article deals with the three-dimensional (3D) coupled neutronic-thermohydraulic steady-state and transient analysis of LVR-15 with the fueled loop. Conservatively calculated enveloping parameters (e.g., reactivity coefficients) were determined for the safety analysis. The control rod withdrawal analysis of the FQT facility with and without reactor SCRAM was carried out with the KIKO3D-ATHLET-coupled dynamic code.
Motion Analysis of a Vibrational Microrobot With Two Perpendicular Harmonic Actuators and Deriving the Design Parameters in Stick–Slip Mode
Authors: Jalili Hadi , Vossoughi Gholamreza , Salarieh Hassan
Published: June 2015

In this paper, the stick–slip motion of a microrobot with two perpendicular vibrational actuators is studied. This motion is based on the friction drive principle. To determine the effective parameters in the motion of microrobot, the equations of motion of the microrobot are derived. To simplify the equations for determining the design parameters, the vibrational actuators are modeled with two perpendicular harmonic forces. To study the motion dynamics of the microrobot, its equation of motion is derived in a nondimensional expression by defining the nondimensional effective parameters. The Fourier expansion (F.E.) method is used to analyze the numerical results and it showed some differences between the obtained results and the studies performed by the harmonic balance (H.B.) method. The discussion about motion characteristics of microrobot is done by defining the mean velocity and performance coefficient of the stick–slip motion. Finally, a practical model of this microrobot is designed and fabricated with two piezo-electric actuators, and then, the motion capability of the microrobot is verified by test.
Extracting Time-Accurate Acceleration Vectors From Nontrivial Accelerometer Arrangements
Authors: Franck Jennifer A. , Blume Janet , Crisco Joseph J. , Franck Christian
Published: June 2015

Sports-related concussions are of significant concern in many impact sports, and their detection relies on accurate measurements of the head kinematics during impact. Among the most prevalent recording technologies are videography, and more recently, the use of single-axis accelerometers mounted in a helmet, such as the HIT system. Successful extraction of the linear and angular impact accelerations depends on an accurate analysis methodology governed by the equations of motion. Current algorithms are able to estimate the magnitude of acceleration and hit location, but make assumptions about the hit orientation and are often limited in the position and/or orientation of the accelerometers. The newly formulated algorithm presented in this manuscript accurately extracts the full linear and rotational acceleration vectors from a broad arrangement of six single-axis accelerometers directly from the governing set of kinematic equations. The new formulation linearizes the nonlinear centripetal acceleration term with a finite-difference approximation and provides a fast and accurate solution for all six components of acceleration over long time periods (>250 ms). The approximation of the nonlinear centripetal acceleration term provides an accurate computation of the rotational velocity as a function of time and allows for reconstruction of a multiple-impact signal. Furthermore, the algorithm determines the impact location and orientation and can distinguish between glancing, high rotational velocity impacts, or direct impacts through the center of mass. Results are shown for ten simulated impact locations on a headform geometry computed with three different accelerometer configurations in varying degrees of signal noise. Since the algorithm does not require simplifications of the actual impacted geometry, the impact vector, or a specific arrangement of accelerometer orientations, it can be easily applied to many impact investigations in which accurate kinematics need to be extracted from single-axis accelerometer data.
Experimental demonstration of passive acoustic imaging in the human skull cavity using CT‐based aberration corrections
Authors: Jones Ryan M. , O'Reilly Meaghan A. , Hynynen Kullervo
Published: June 2015

Purpose:

Experimentally verify a previously described technique for performing passive acoustic imaging through an intact human skull using noninvasive, computed tomography (CT)‐based aberration corrections Jones et al. [Phys. Med. Biol. 58, 4981–5005 (2013)].

Methods:

A sparse hemispherical receiver array (30 cm diameter) consisting of 128 piezoceramic discs (2.5 mm diameter, 612 kHz center frequency) was used to passively listen through ex vivo human skullcaps (n = 4) to acoustic emissions from a narrow‐band fixed source (1 mm diameter, 516 kHz center frequency) and from ultrasound‐stimulated (5 cycle bursts, 1 Hz pulse repetition frequency, estimated in situ peak negative pressure 0.11–0.33 MPa, 306 kHz driving frequency) Definity™ microbubbles flowing through a thin‐walled tube phantom. Initial in vivo feasibility testing of the method was performed. The performance of the method was assessed through comparisons to images generated without skull corrections, with invasive source‐based corrections, and with water‐path control images.

Results:

For source locations at least 25 mm from the inner skull surface, the modified reconstruction algorithm successfully restored a single focus within the skull cavity at a location within 1.25 mm from the true position of the narrow‐band source. The results obtained from imaging single bubbles are in good agreement with numerical simulations of point source emitters and the authors’ previous experimental measurements using source‐based skull corrections O'Reilly et al. [IEEE Trans. Biomed. Eng. 61, 1285–1294 (2014)]. In a rat model, microbubble activity was mapped through an intact human skull at pressure levels below and above the threshold for focused ultrasound‐induced blood–brain barrier opening. During bursts that led to coherent bubble activity, the location of maximum intensity in images generated with CT‐based skull corrections was found to deviate by less than 1 mm, on average, from the position obtained using source‐based corrections.

Conclusions:

Taken together, these results demonstrate the feasibility of using the method to guide bubble‐mediated ultrasound therapies in the brain. The technique may also have application in ultrasound‐based cerebral angiography.
SU‐B‐213‐07: Panel Discussion
Authors: Starkschall G.
Published: June 2015

The North American medical physics community validates the education received by medical physicists and the clinical qualifications for medical physicists through accreditation of educational programs and certification of medical physicists. Medical physics educational programs (graduate education and residency education) are accredited by the Commission on Accreditation of Medical Physics Education Programs (CAMPEP), whereas medical physicists are certified by several organizations, the most familiar of which is the American Board of Radiology (ABR). In order for an educational program to become accredited or a medical physicist to become certified, the applicant must meet certain specified standards set by the appropriate organization.

In this Symposium, representatives from both CAMPEP and the ABR will describe the process by which standards are established as well as the process by which qualifications of candidates for accreditation or certification are shown to be compliant with these standards.

The Symposium will conclude with a panel discussion.

Learning Objectives:

Recognize the difference between accreditation of an educational program and certification of an individual

Identify the two organizations primarily responsible for these tasks

Describe the development of educational standards

Describe the process by which examination questions are developed

GS is Executive Secretary of CAMPEP.
SU‐C‐213‐02: Characterizing 3D Printing in the Fabrication of Variable Density Phantoms
Authors: Madamesila J , McGeachy P , Villarreal‐Barajas J , Khan R
Published: June 2015

Purpose:

In this work, we present characterization, process flow, quality control and application of 3D fabricated low density phantoms for radiotherapy quality assurance.

Methods:

A Rostock delta 3D printer using polystyrene filament of diameter 1.75 mm was used to print geometric volumes of 2×2×1 cm3 of varying densities. The variable densities of 0.1 to 0.75 g/cm 3 were created by modulating the infill. A computed tomography (CT) scan was performed to establish an infill‐density calibration curve as well as characterize the quality of the print such as uniformity and the infill pattern. The time required to print these volumes was also recorded. Using the calibration, two low density cones (0.19, 0.52 g/cm3) were printed and benchmarked against commercially available phantoms. The dosimetric validation of the low density scaling of Anisotropic Analytical Algorithm (AAA) was performed by using a 0.5 g/cm3 slab of 10×10×2.4 cm3 with EBT3 GafChromic film. The gamma analysis at 3%/3mm criteria were compared for the measured and computed dose planes.

Results:

Analysis of the volume of air pockets in the infill resulted in a reasonable uniformity for densities 0.4 to 0.75 g/cm3. Printed phantoms with densities below 0.4 g/cm3 exhibited a higher ratio of air to polystyrene resulting in large non‐uniformity. Compared to the commercial inserts, good agreement was observed only for the printed 0.52 g/cm3 cone. Dosimetric comparison for a printed low density volume placed in‐between layers of solid water resulted in >95% gamma agreement between AAA calculated dose planes and measured EBT3 films for a 6MV 5×5 cm2 clinical beam. The comparison showed disagreement in the penumbra region.

Conclusion:

In conclusion, 3D printing technology opens the door to desktop fabrication of variable density phantoms at economical prices in an efficient manner for the quality assurance needs of a small clinic.
SU‐C‐213‐05: Evaluation of a Composite Copper‐Plastic Material for a 3D Printed Radiation Therapy Bolus
Authors: Vitzthum L , Ehler E , Sterling D , Reynolds T , Higgins P , Dusenbery K
Published: June 2015

Purpose:

To evaluate a novel 3D printed bolus fabricated from a copper‐plastic composite as a thin flexible, custom fitting device that can replicate doses achieved with conventional bolus techniques.

Methods:

Two models of bolus were created on a 3D printer using a composite copper‐PLA/PHA. Firstly, boluses were constructed at thicknesses of 0.4, 0.6 and 0.8 mm. Relative dose measurements were performed under the bolus with an Attix Chamber as well as with radiochromic film. Results were compared to superficial Attix Chamber measurements in a water equivalent material to determine the dosimetric water equivalence of the copper‐PLA/PHA plastic. Secondly, CT images of a RANDO phantom were used to create a custom fitting bolus across the anterolateral scalp. Surface dose with the bolus placed on the RANDO phantom was measured with radiochromic film at tangential angles with 6, 10, 10 flattening filter free (FFF) and 18 MV photon beams.

Results:

Mean surface doses for 6, 10, 10FFF and 18 MV were measured as a percent of Dmax for the flat bolus devices of each thickness. The 0.4 mm thickness bolus was determined to be near equivalent to 2.5 mm depth in water for all four energies. Surface doses ranged from 59–63% without bolus and 85–90% with the custom 0.4 mm copper‐plastic bolus relative to the prescribed dose for an oblique tangential beam arrangement on the RANDO phantom.

Conclusion:

Sub‐millimeter thickness, 3D printed composite copper‐PLA/PHA bolus can provide a build‐up effect equivalent to conventional bolus. At this thickness, the 3D printed bolus allows a level of flexure that may provide more patient comfort than current 3D printing materials used in bolus fabrication while still retaining the CT based custom patient shape.

Funding provided by an intra‐department grant of the University of Minnesota Department of Radiation Oncology
SU‐C‐304‐01: Investigation of Various Detector Response Functions and Their Geometry Dependence in a Novel Method to Address Ion Chamber Volume Averaging Effect
Authors: Barraclough B , Li J , Lebron S , Fan Qiyong , Liu C , Yan G
Published: June 2015

Purpose:

A novel convolution‐based approach has been proposed to address ion chamber (IC) volume averaging effect (VAE) for the commissioning of commercial treatment planning systems (TPS). We investigate the use of various convolution kernels and its impact on the accuracy of beam models.

Methods:

Our approach simulates the VAE by iteratively convolving the calculated beam profiles with a detector response function (DRF) while optimizing the beam model. At convergence, the convolved profiles match the measured profiles, indicating the calculated profiles match the “true” beam profiles. To validate the approach, beam profiles of an Elekta LINAC were repeatedly collected with ICs of various volumes (CC04, CC13 and SNC 125) to obtain clinically acceptable beam models. The TPS‐calculated profiles were convolved externally with the DRF of respective IC. The beam model parameters were reoptimized using Nelder‐Mead method by forcing the convolved profiles to match the measured profiles. We evaluated three types of DRFs (Gaussian, Lorentzian, and parabolic) and the impact of kernel dependence on field geometry (depth and field size). The profiles calculated with beam models were compared with SNC EDGE diode‐measured profiles.

Results:

The method was successfully implemented with Pinnacle Scripting and Matlab. The reoptimization converged in ∼10 minutes. For all tested ICs and DRFs, penumbra widths of the TPS‐calculated profiles and diode‐measured profiles were within 1.0 mm. Gaussian function had the best performance with mean penumbra width difference within 0.5 mm. The use of geometry dependent DRFs showed marginal improvement, reducing the penumbra width differences to less than 0.3 mm. Significant increase in IMRT QA passing rates was achieved with the optimized beam model.

Conclusion:

The proposed approach significantly improved the accuracy of the TPS beam model. Gaussian functions as the convolution kernel performed consistently better than Lorentzian and parabolic functions. Further improvement of using geometry‐dependent kernels was marginal.
SU‐C‐304‐03: Experimental Investigation On the Accuracy of Plastic Scintillation Dosimeters in Small Fields
Authors: Papaconstadopoulos P , Archambault L , Seuntjens J
Published: June 2015

Purpose:

To investigate the accuracy of the Exradin W1 (SI) and of an “in‐house” plastic scintillation dosimeter (CHUQ PSD) in small radiation fields.

Methods:

Output factor (OF) measurements with the W1 and CHUQ PSD were performed for field sizes of 0.5 × 0.5, 1 × 1 and 2 × 2 cm2. Both detectors were placed parallel to the central axis (CAX) in water. The spectrum discrimination calibration method was performed in each set‐up to account for the Cerenkov (CRV) signal created in the fiber. The OFs were compared to the expected field factors in water derived using i) Monte Carlo (MC) simulations of an accurate accelerator model and ii) microLion (PTW) and D1V diode (SI) OFs. MC‐derived correction factors were applied to both the microLion and D1V OFs. For the CHUQ PSD the calibration was repeated in water (// CAX), solid water (perpendicular to CAX) and under a shielded configuration. The signal was collected using a spectrometer (wavelength range = 185–1100 nm). Spectral analysis was performed to evaluate potential changes of the spectral distributions under the various calibration set‐up configurations.

Results:

The W1 OFs presented an over‐response for the 0.5 × 0.5 cm2 in the range of 3 – 4.1% relative to the expected field factor. The CHUQ PSD presented an under‐response in the range of 1.5 – 2.7%, without accounting for volume averaging. The CRV spectra under the various calibration procedures appeared similar to each other and only minor changes were observed to the respective OFs.

Conclusion:

The W1 and CHUQ PSD can be used in small fields down to a 1 × 1 cm2 field size. Discrepancies were encountered between the two detectors for the smallest field size of 0.5 × 0.5 cm2 with the CHUQ PSD exhibiting a closer agreement to the expected field factor.

Funding sources: 1) Alexander S. Onassis Public Benefit Foundation in Greece and 2) CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council, Grant number: 432290, RGPIN‐2014‐06475
SU‐C‐304‐07: Are Small Field Detector Correction Factors Strongly Dependent On Machine‐Specific Characteristics?
Authors: Mathew D , Tanny S , Parsai E , Sperling N
Published: June 2015

Purpose:

The current small field dosimetry formalism utilizes quality correction factors to compensate for the difference in detector response relative to dose deposited in water. The correction factors are defined on a machine‐specific basis for each beam quality and detector combination. Some research has suggested that the correction factors may only be weakly dependent on machine‐to‐machine variations, allowing for determinations of class‐specific correction factors for various accelerator models. This research examines the differences in small field correction factors for three detectors across two Varian Truebeam accelerators to determine the correction factor dependence on machine‐specific characteristics.

Methods:

Output factors were measured on two Varian Truebeam accelerators for equivalently tuned 6 MV and 6 FFF beams. Measurements were obtained using a commercial plastic scintillation detector (PSD), two ion chambers, and a diode detector. Measurements were made at a depth of 10 cm with an SSD of 100 cm for jaw‐defined field sizes ranging from 3×3 cm2 to 0.6×0.6 cm2, normalized to values at 5×5cm2. Correction factors for each field on each machine were calculated as the ratio of the detector response to the PSD response. Percent change of correction factors for the chambers are presented relative to the primary machine.

Results:

The Exradin A26 demonstrates a difference of 9% for 6×6mm2 fields in both the 6FFF and 6MV beams. The A16 chamber demonstrates a 5%, and 3% difference in 6FFF and 6MV fields at the same field size respectively. The Edge diode exhibits less than 1.5% difference across both evaluated energies. Field sizes larger than 1.4×1.4cm2 demonstrated less than 1% difference for all detectors.

Conclusion:

Preliminary results suggest that class‐specific correction may not be appropriate for micro‐ionization chamber. For diode systems, the correction factor was substantially similar and may be useful for class‐specific reference conditions.
SU‐C‐BRD‐01: Multi‐Centre Collaborative Quality Assurance Program for IMRT Planning and Delivery: Year 3 Results
Authors: McNiven A , Jaffray D , Letourneau D
Published: June 2015

Purpose:

A multi‐centre quality assurance program was developed to enable quality improvement by coupling measurement of intensity modulated radiotherapy (IMRT) planning and delivery performance for site‐specific planning exercises with diagnostic testing. The third year of the program specifically assessed the quality of spine stereotactic body radiotherapy (SBRT) planning and delivery amongst the participating centres.

Methods:

A spine SBRT planning exercise (24 Gy in 2 fractions) was created and completed by participants prior to an on‐site visit. The delivery portion of the on‐site visit included spine SBRT plan delivery and diagnostic testing, which included portal image acquisition for quantification of phantom positioning error and multi‐leaf collimator (MLC) calibration accuracy. The measured dose was compared to that calculated in the treatment planning system (TPS) using 3%/2mm composite analysis and 3%/3mm gamma analysis.

Results:

Fourteen institutions participated, creating 17 spine SBRT plans (15 VMAT and 2 IMRT). Three different TPS, two beam energies (6 MV and 6 MV FFF), and four MLC designs from two linac vendors were tested. Large variation in total monitor units (MU) per plan (2494–6462 MU) and dose‐volume parameters was observed. The maximum point dose in the plans ranged from 116–149% and was dependent upon the TPS used. Pass rates for measured to planned dose comparison ranged from 89.4–100% and 97.3–100% for 3%/2mm and 3%/3mm criteria respectively. The largest measured MLC error did Result in one of the poorer pass rates. No direct correlation between phantom positioning error and pass rates overall.

Conclusion:

Significant differences were observed in the planning exercise for some plan and dose‐volume parameters based on the TPS used. Standard evaluation criteria showed good agreement between planned and measured dose for all participants, however on an individual plan basis, diagnostic tests were able to identify contributing factors for poorer results, when using more stringent evaluation criteria.

Funding from Cancer Care Ontario is acknowledged.
SU‐C‐BRD‐02: Monte Carlo Based VMAT Dose Re‐Optimization for Patient Specific Quality Assurance and Failed Plan Recovery Using Re‐Weighted Aperture MUs
Authors: Folkerts M , Modiri A , Ungun B , Jia X , Jiang S , Sawant A
Published: June 2015

Purpose:

To offer an answer to the question: “What do we do if a treatment plan is shown to be sub‐optimal or unsafe after patient specific VMAT QA based on Monte Carlo (MC) dose calculations?” We attempt to disprove the null hypothesis that re‐weighting the MUs delivered through each aperture of a VMAT plan will not improve the MC delivered dose distribution.

Methods:

We interface with a GPU‐based patient specific MC quality assurance (QA) system to generate dose distributions for apertures of a multiple arc VMAT head‐and‐neck treatment plan. The distributions were combined into a dose‐per‐aperture matrix. We tested and compared the results of two optimization schemes. To solve the convex problem of re‐weighting the MC dose matrix to match the dose provided by the treatment planning system (TPS), the least‐squares (LSQ) algorithm was used. To meet or improve our DVH goals, a particle swarm optimization (PSO) algorithm was implemented. After normalizing each Result to match the 66.0Gy D95 prescription requirement, we used QUANTEC guidelines to evaluate the clinical impact of each approach.

Results:

Our non‐convex PSO scheme to re‐weight MUs was able to produce matching if not better OAR DVH results for the one plan we tested when compared to the original plan. For example, the spinal cord was prescribed a Dmax of 42.8Gy, however, the MC QA results reported 44.7Gy; the PSO was able to reduce that Result to 42.0Gy, a 6% improvement; the LSQ optimization produced 45.2Gy. Also, the PSO reduced the max dose of the PTV from 81.7Gy (MC QA) to 75.1Gy, while the LSQ algorithm produced 77.4Gy.

Conclusion:

We successfully disproved our null hypothesis and showed that it is indeed possible to improve the MC dose distribution of a VMAT plan by re‐weighting MUs delivered through each aperture.

This project was partially funded by NIH grant #R01CA169102
SU‐C‐BRD‐05: Non‐Invasive in Vivo Biodosimetry in Radiotherapy Patients Using Electron Paramagnetic Resonance (EPR) Spectroscopy
Authors: Bahar N , Roberts K , Stabile F , Mongillo N , Decker RD , Wilson LD , Husain Z , Contessa J , Williams BB , Flood AB , Swartz HM , Carlson DJ
Published: June 2015

Purpose:

Medical intervention following a major, unplanned radiation event can elevate the human whole body exposure LD50 from 3 to 7 Gy. On a large scale, intervention cannot be achieved effectively without accurate and efficient triage. Current methods of retrospective biodosimetry are restricted in capability and applicability; published human data is limited. We aim to further develop, validate, and optimize an automated field‐deployable in vivo electron paramagnetic resonance (EPR) instrument that can fill this need.

Methods:

Ionizing radiation creates highly‐stable, carbonate‐based free radicals within tooth enamel. Using a process similar to nuclear magnetic resonance, EPR directly measures the presence of radiation‐induced free radicals. We performed baseline EPR measurements on one of the upper central incisors of total body irradiation (TBI) and head and neck (H&N) radiotherapy patients before their first treatment. Additional measurements were performed between subsequent fractions to examine the EPR response with increasing radiation dose. Independent dosimetry measurements were performed with optically‐stimulated luminescent dosimeters (OSLDs) and diodes to more accurately establish the relationship between EPR signal and delivered radiation dose.

Results:

36 EPR measurements were performed over the course of four months on two TBI and four H & N radiotherapy patients. We observe a linear increase in EPR signal with increasing dose across the entirety of the tested range. A linear least squares‐weighted fit of delivered dose versus measured signal amplitude yields an adjusted R‐square of 0.966. The standard error of inverse prediction (SEIP) is 1.77 Gy. For doses up to 7 Gy, the range most relevant to triage, we calculate an SEIP of 1.29 Gy.

Conclusion:

EPR spectroscopy provides a promising method of retrospective, non‐invasive, in vivo biodosimetry. Our preliminary data show an excellent correlation between predicted signal amplitude and delivered dose. With further development, a robust means of predicting delivered radiation dose from EPR measurements is expected.

This project was funded by the Biomedical Advanced Research and Development Authority (BARDA) within the U.S. Department of Health and Human Services subcontracted through the Geisel School of Medicine at Dartmouth and by the Dartmouth Physically‐Based Biodosimetry Center for Medical Countermeasures Against Radiation (Dart‐Dose CMCR) Pilot Program.
SU‐C‐BRD‐07: Three‐Dimensional Dose Reconstruction in the Presence of Inhomogeneities Using Fast EPID‐Based Back‐Projection Method
Authors: Ren Q , Cao R , Pei X , Wang H , Hu L
Published: June 2015

Purpose:

Three‐dimensional dose verification can detect errors introduced by the treatment planning system (TPS) or differences between planned and delivered dose distribution during the treatment. The aim of the study is to extend a previous in‐house developed three‐dimensional dose reconstructed model in homogeneous phantom to situtions in which tissue inhomogeneities are present.

Methods:

The method was based on the portal grey images from an electronic portal imaging device (EPID) and the relationship between beamlets and grey‐scoring voxels at the position of the EPID. The relationship was expressed in the form of grey response matrix that was quantified using thickness‐dependence scatter kernels determined by series of experiments. From the portal grey‐value distribution information measured by the EPID the two‐dimensional incident fluence distribution was reconstructed based on the grey response matrix using a fast iterative algorithm. The accuracy of this approach was verified using a four‐field intensity‐modulated radiotherapy (IMRT) plan for the treatment of lung cancer in anthopomorphic phantom. Each field had between twenty and twenty‐eight segments and was evaluated by comparing the reconstructed dose distribution with the measured dose.

Results:

The gamma‐evaluation method was used with various evaluation criteria of dose difference and distance‐to‐agreement: 3%/3mm and 2%/2 mm. The dose comparison for all irradiated fields showed a pass rate of 100% with the criterion of 3%/3mm, and a pass rate of higher than 92% with the criterion of 2%/2mm.

Conclusion:

Our experimental results demonstrate that our method is capable of accurately reconstructing three‐dimensional dose distribution in the presence of inhomogeneities. Using the method, the combined planning and treatment delivery process is verified, offing an easy‐to‐use tool for the verification of complex treatments.
SU‐C‐BRB‐01: Development of Dynamic Gimbaled X‐Ray Head Swing Irradiation Technique
Authors: Ono T , Miyabe Y , Yokota K , Takahashi K , Akimoto M , Mukumoto N , Ishihara Y , Nakamura M , Mizowaki T , Hiraoka M
Published: June 2015

Purpose:

The Vero4DRT has a unique gimbaled x‐ray head with rotating around orthogonal two axes. The purpose of this study was to develop a new irradiation technique using the dynamic gimbaled x‐ray head swing function.

Methods:

The Vero4DRT has maximum field size of 150Χ150 mm2. The expanded irradiation field (expanded‐field) for the longitudinal direction which is vertical to the MLC sliding direction, was created by the MLC motion and the gimbaled x‐ray head rotation. The gimbaled x‐ray head was rotated ± 35 mm, and the expanded‐field size was set as 150Χ220 mm2. To irradiate uniform dose distribution, the diamond‐shaped radiation field was created and continuously moved for the longitudinal direction. It was achieved by combination of opening and closing of the MLC and gimbal swing rotation. To evaluate dosimetric characteristic of the expanded‐field, films inserted in water‐equivalent phantoms at 100 mm depth were irradiated and the field size, penumbra, flatness and symmetry were analyzed.In addition, the expanded‐field irradiation technique was applied to virtual wedge irradiation. Wedged beam was acquired with the delta–shaped radiation field. 150Χ 220 mm2 fields with 15, 30, 45, and 60 degree wedge were examined. The wedge angles were measured with irradiated film and compared with assumed wedge angles.

Results:

The field size, penumbra, flatness and symmetry of the expanded‐field were 150.0 mm, 8.1–8.4 mm, 2.8% and −0.8% for the lateral direction and 220.1 mm, 6.3–6.4 mm, 3.2% and −0.4% for the longitudinal direction at 100 mm depth. The measured wedge angles were 15.1, 30.2, 45.2 and 60.2 degrees. The differences between assumed and measured angles were within 0.2 degrees.

Conclusion:

A new technique of the gimbal swing irradiation was developed. To extend applied targets, especially for whole breast irradiation, the expanded‐field and virtual wedge irradiations would be effective.
SU‐C‐BRB‐02: Symmetric and Asymmetric MLC Based Lung Shielding and Dose Optimization During Translating Bed TBI
Authors: Ahmed S , Kakakhel MB , Ahmed SBS , Hussain A
Published: June 2015

Purpose:

The primary aim was to introduce a dose optimization method for translating bed total body irradiation technique that ensures lung shielding dynamically. Symmetric and asymmetric dynamic MLC apertures were employed for this purpose.

Methods:

The MLC aperture sizes were defined based on the radiological depth values along the divergent ray lines passing through the individual CT slices. Based on these RD values, asymmetrically shaped MLC apertures were defined every 9 mm of the phantom in superior‐inferior direction. Individual MLC files were created with MATLAB™ and were imported into Eclipse™ treatment planning system for dose calculations. Lungs can be shielded to an optimum level by reducing the MLC aperture width over the lungs. The process was repeated with symmetrically shaped apertures.

Results:

Dose‐volume histogram (DVH) analysis shows that the asymmetric MLC based technique provides better dose coverage to the body and optimum shielding of the lungs compared to symmetrically shaped beam apertures. Midline dose homogeneity is within ±3% with asymmetric MLC apertures whereas it remains within ±4.5% with symmetric ones (except head region where it drops down to −7%). The substantial over and under dosage of ±5% at tissue interfaces has been reduced to ±2% with asymmetric MLC technique. Lungs dose can be reduced to any desired limit. In this experiment lungs dose was reduced to 80% of the prescribed dose, as was desired.

Conclusion:

The novel asymmetric MLC based technique assures optimum shielding of OARs (e.g. lungs) and better 3‐D dose homogeneity and body‐dose coverage in comparison with the symmetric MLC aperture optimization.

The authors acknowledge the financial and infrastructural support provided by Pakistan Institute of Engineering & Applied Sciences (PIEAS), Islamabad and Aga Khan University Hospital (AKUH), Karachi during the course of this research project. Authors have no conflict of interest with any national / international body for the presented work.
SU‐C‐BRB‐06: Utilizing 3D Scanner and Printer for Dummy Eye‐Shield: Artifact‐Free CT Images of Tungsten Eye‐Shield for Accurate Dose Calculation
Authors: Park J , Lee J , Kim H , Kim I , Ye S
Published: June 2015

Purpose:

To evaluate the effect of a tungsten eye‐shield on the dose distribution of a patient.

Methods:

A 3D scanner was used to extract the dimension and shape of a tungsten eye‐shield in the STL format. Scanned data was transferred into a 3D printer. A dummy eye shield was then produced using bio‐resin (3D systems, VisiJet M3 Proplast). For a patient with mucinous carcinoma, the planning CT was obtained with the dummy eye‐shield placed on the patient's right eye. Field shaping of 6 MeV was performed using a patient‐specific cerrobend block on the 15 × 15 cm2 applicator. The gantry angle was 330° to cover the planning target volume near by the lens. EGS4/BEAMnrc was commissioned from our measurement data from a Varian 21EX. For the CT‐based dose calculation using EGS4/DOSXYZnrc, the CT images were converted to a phantom file through the ctcreate program. The phantom file had the same resolution as the planning CT images. By assigning the CT numbers of the dummy eye‐shield region to 17000, the real dose distributions below the tungsten eye‐shield were calculated in EGS4/DOSXYZnrc. In the TPS, the CT number of the dummy eye‐shield region was assigned to the maximum allowable CT number (3000).

Results:

As compared to the maximum dose, the MC dose on the right lens or below the eye shield area was less than 2%, while the corresponding RTP calculated dose was an unrealistic value of approximately 50%.

Conclusion:

Utilizing a 3D scanner and a 3D printer, a dummy eye‐shield for electron treatment can be easily produced. The artifact‐free CT images were successfully incorporated into the CT‐based Monte Carlo simulations. The developed method was useful in predicting the realistic dose distributions around the lens blocked with the tungsten shield.
SU‐C‐BRB‐07: Optimizing Fiducial Visibility On Periodically Acquired Megavoltage and Kilovoltage Image Pairs During Volumetric Modulated Arc Therapy
Authors: Zhang P , Wang P , Laura H , Ravindranath B , Mageras G , Hunt M
Published: June 2015

Purpose:

Robust detection of implanted fiducials is essential for monitoring intra‐fractional motion during hypo‐fractionated treatment. We developed a plan optimization strategy to ensure clear visibility of implanted fiducials, and facilitate 3D localization during volumetric modulated arc therapy (VMAT).

Method:

Periodic kilovoltage (kV) images were acquired at 20° gantry intervals, and paired with simultaneously acquired 4.4° short arc megavoltage digital tomosynthesis (MV‐DTS) to localize three fiducials during VMAT delivery for prostate cancer. Beginning with the original optimized plan, control point segments where fiducials (with 1cm margin to account for potential motion) were consistently blocked by MLC within each 4.4° MV‐DTS interval were first identified. For each segment, MLC apertures were edited to expose the fiducial that led to the least increase in a dosimetric cost function. Subsequently, MLC apertures of all control points not involved with fiducial visualization were re‐optimized to compensate for plan quality losses and match the original DVH. MV dose for each MV‐DTS was also kept above 0.4 MU to ensure acceptable image quality. Different imaging (gantry) intervals and visibility margins around fiducials were also evaluated.

Result:

Fiducials were consistently blocked by the MLC for, on average, 38% of the imaging control points for five hypofractionated prostate VMAT plans but properly exposed after re‐optimization. Optimization resulted in negligible dosimetric differences compared with original plans, and outperformed simple aperture editing: on average, PTV D98 recovered from 87% to 94% of prescription, and PTV dose homogeneity improved from 9% to 7%. Without violating plan objectives and compromising delivery efficiency, the highest imaging frequency and largest margin that can be achieved are a 10° gantry interval, and 15mm, respectively.

Conclusion:

VMAT plans can be made to accommodate MV‐kV imaging of fiducials. Fiducial visualization rate and workflow efficiency are significantly improved with an automatic modification and re‐optimization approach.

Research supported by Varian Medical System
SU‐C‐BRA‐03: Prediction of Laser Induced Thermal Therapy: Results of Model Training and Cross Validation
Authors: Fahrenholtz SJ , Madankan R , Hazle JD , Stafford RJ , Fuentes D
Published: June 2015

Purpose:

MR‐guided laser induced thermal therapy (MRgLITT) is a minimally invasive surgery with applications in the brain, among other sites. In especially precise interventions, like neurosurgery, accurate planning may behoove surgical planning by aiding in the decision of where and how many laser ablations are required. Previous models of tissue heating have relied on literature values extrapolated primarily from normal brain animal research and ex vivo data. In this abstract, an inverse problem provides model parameter data from retrospective analysis of MR temperature imaging data in patient tumor tissue, which represent a training cohort. Within the same cohort, leave‐one‐out cross validation (LOOCV) estimates the predictive accuracy of the trained model.

Methods:

The training has three parts: MR temperature datasets (n=20), a relatively simple steady state bioheat model, and a global optimization algorithm maximizing the Dice similarity coefficient (DSC). DSC ranges from 0 to one; >0.7 is considered a ‘successful’ prediction. DSC compares the regions exceeding 57 °C—i.e. ablated tissue regions—and measures accuracy. Blood perfusion and optical parameters are optimized according to DSC, creating a library of 20 pairs of parameters that are used in prediction. The predictive accuracy is estimated by using LOOCV. LOOCV begins by dropping an optimal parameter pair from the library and makes a model prediction, given the average from the remaining 19 pairs. This procedure is permuted so that every dataset is predicted using the other parameter pairs.

Results:

The distribution of DSC during predictive LOOCV is described by: mean=0.822; median=0.849; standard deviation=0.0872; minimum=0.583; maximum=0.930; 19/20 datasets pass (i.e. DSC>0.7). Best available literature values perform comparably worse: mean=0.668; median=0.656; standard deviation=0.115; minimum=0.455; maximum=0.857; 8/20 pass.

Conclusion:

Data strongly indicate population‐based parameter optimization is potentially useful in treatment planning, as the model far outperforms available literature values in this preliminary study.

Research reported in this publication was supported by the National Center for Advancing Translational Sciences of the National Institutes of Health under Award Number TL1TR000369. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
SU‐C‐BRA‐05: Fast Generation of Respiratory Gated CT Images at User Selected Breathing Phases On a Graphics Processing Unit
Authors: O'Connell D. P. , Thomas D. H. , Dou T. H. , Lamb J. M. , Yang L. , Low D. A.
Published: June 2015

Purpose:

The previously published 5DCT respiratory gated image acquisition and analysis technique enables generation of images at any user selected breathing phase. This work describes acceleration of the image generation process using a graphics processing unit (GPU) and its application to internal target volume (ITV) definition and the creation of simulated cine scans.

Methods:

25 fast helical, free breathing CT scans of 7 lung cancer patients were acquired using a low dose protocol with simultaneous breathing surrogate monitoring. For each patient, the first scan was deformably registered to the following 24. Deformation vectors were used to determine voxel‐specific parameters of a motion model. A single, low noise reference image in the geometry of the first scan was created using image averaging. The motion model was used to predict the deformation from the reference image to selected breathing phases. Internal target volumes were generated by deforming a single contour of the gross tumor volume (GTV) to the most common breathing phases accounting for 90% of observed respiration. Simulated cines were created by generating volumetric images at 0.25 second intervals along the measured breathing trace and taking slices at desired positions. Computations were performed on an NVIDIA Tesla K40.

Results:

Calculation of motion model parameters took approximately 3 seconds per dataset. Image generation took approximately 0.25 seconds total for a 450 × 450 × 300 image with isotropic 1 mm3 resolution.

Conclusion:

GPU acceleration enabled rapid generation of breathing gated CT images using the 5DCT technique and facilitated use of a novel method for defining customized lung tumor ITVs that account for a specified percentage of observed respiration, and the creation of simulated cine images in a clinically acceptable time frame. Investigation of the differences between ITVs generated using the technique described here and ITVs defined on conventional 4DCT datasets is ongoing.
SU‐C‐BRA‐06: Developing Clinical and Quantitative Guidelines for a 4DCT‐Ventilation Functional Avoidance Clinical Trial
Authors: Vinogradskiy Y , Waxweiler T , Diot Q , Castillo R , Guerrero T , Castillo E , Kavanagh B , Schubert L , Miften M
Published: June 2015

Purpose:

4DCT‐ventilation is an exciting new imaging modality that uses 4DCTs to calculate lung ventilation. Because 4DCTs are acquired as part of routine care, calculating 4DCT‐ventilation allows for lung function evaluation without additional cost or inconvenience to the patient. Development of a clinical trial is underway at our institution to use 4DCT‐ventilation for thoracic functional avoidance with the idea that preferential sparing of functional lung regions can decrease pulmonary toxicity. The purpose of our work was to develop the practical aspects of a 4DCT‐ventilation functional avoidance clinical trial including: 1.assessing patient eligibility 2.developing trial inclusion criteria and 3.developing treatment planning and dose‐function evaluation strategies.

Methods:

96 stage III lung cancer patients from 2 institutions were retrospectively reviewed. 4DCT‐ventilation maps were calculated using the patient's 4DCTs, deformable image registrations, and a density‐change‐based algorithm. To assess patient eligibility and develop trial inclusion criteria we used an observer‐based binary end point noting the presence or absence of a ventilation defect and developed an algorithm based on the percent ventilation in each lung third. Functional avoidance planning integrating 4DCT‐ventilation was performed using rapid‐arc and compared to the patient's clinically used plan.

Results:

Investigator‐determined clinical ventilation defects were present in 69% of patients. Our regional/lung‐thirds ventilation algorithm identified that 59% of patients have lung functional profiles suitable for functional avoidance. Compared to the clinical plan, functional avoidance planning was able to reduce the mean dose to functional lung by 2 Gy while delivering comparable target coverage and cord/heart doses.

Conclusions:

4DCT‐ventilation functional avoidance clinical trials have great potential to reduce toxicity, and our data suggest that 59% of lung cancer patients have lung function profiles suitable for functional avoidance. Our study used a retrospective evaluation of a large lung cancer patient database to develop the practical aspects of a 4DCT‐ventilation functional avoidance clinical trial.

(R.C., E.C., T.G.), NIH Research Scientist Development Award K01‐CA181292 (R.C.), and State of Colorado Advanced Industries Accelerator Grant (Y.V.).
SU‐C‐303‐05: Photosensitizer Determination for PDT Using Interstitial and Surface Measurements of Fluorescence
Authors: Kim M , Finlay J , Zhu T
Published: June 2015

Purpose:

Photosensitizer concentration during photodynamic therapy (PDT) is an important parameter for accurate dosimetry. Fluorescence signal can be used as a measure of photosensitizer concentration. Two methods of data acquisition were compared to an ex vivo study both for in vivo and phantom models.

Methods:

Fluorescence signal of commonly used photosensitizer benzoporphyrin derivative monoacid ring A (BPD) was obtained in phantoms and mouse tumors using an excitation light of 405 nm. Interstitial fluorescence signal was obtained using a side‐cut fiber inserted into the tumor tissue of interest. Using a previously developed multi‐fiber probe, tumor surface fluorescence measurements were also collected. Signals were calibrated according to optical phantoms with known sensitizer fluorescence. Optical properties for each sample were determined and the influence of different absorption and scattering properties on the fluorescence signals was investigated. Using single value decomposition of the spectra, the sensitizer concentration was determined using the two different measurement geometries. An ex vivo analysis was also performed for tumor samples to determine the sensitizer concentration.

Results:

The two fluorescence signals obtained from the surface multi‐fiber probe and the interstitial measurements were compared and were corresponding for both phantoms and mouse models. The values obtained were comparable to the ex vivo measurements as well. Despite the difference in geometry, the surface probe measurements can still be used as a metric for determining the presence of sensitizer in small volume tumors.

Conclusion:

The multi‐fiber contact probe can be used as a tool to measure fluorescence at the surface of the treatment area for PDT and predict sensitizer concentration throughout the tumor. This is advantageous in that the measurement does not damage any tissue. Future work will include investigating the dependence of these results on intratumor sensitizer distribution.