bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐380: Evaluation of Interfraction Motion of the Strut‐Adjusted Volume Implant (SAVI) Using 3D Reconstruction from CT Scout Images
Authors: Park S , DeMarco J , Kamrava M , Demanes D , Low D
Published: November 2011

Purpose: The Strut‐Adjusted Volume Implant (SAVI) is a partial breast irradiation applicator. We developed a method to reconstruct the 3D device location using scout images to provide applicator position and proper expansion verification. We also used this technique to evaluate interfraction motion. Methods: The SAVI device was implanted in a lumpectomy cavity. The patient was aligned by CT lasers and skin tattoos to ensure reproducible setup. A post‐operative CT scan was performed for treatment planning. The patient was treated in 10 fractions over the course of 5 days. Daily CT scans and anterior and lateral scout scans were acquired prior to each fraction. Radio‐opaque markers located on three of the struts were localized using a peak detection filter. The location of each marker on the 2D scout image was backprojected towards the CT x‐ray source. Each 3D marker position was reconstructed at the backprojection intersection. The 3D marker position was compared to the location in the 3D CT image. The interfractional displacement of the device was assessed from the reconstructed marker locations. Results: The average distance (standard deviation) between the marker positions reconstructed using the scout images and the CT images was 0.76 (0.28) mm. Using the scout image data, the average interfractional device movement (standard deviation) in the SI, AP, and LR directions, and 3D was 0.51 (0.46) mm, 0.95 (0.81) mm, 0.73 (0.61) mm, and 1.56 (0.68) mm. Conclusion: SAVI interfraction motion can be accurately measured using scout images. The patient setup for partial breast brachytherapy can be improved by correcting the applicator displacement. This proposed technique eliminates the need for CT verifications, and therefore additional dose to the patient, while still accurately identifying applicator displacement.
SU‐E‐T‐382: Revision of the Dosimetric Parameters of the CSM11 LDR Cs‐137 Source
Authors: Granero D , Otal A , Fernandez J Martinez , Vijande J , Perez‐Calatayud J , Ballester F
Published: November 2011

Purpose: The clinical use of brachytherapy sources requires the existence of dosimetric data with enough quality for the proper application of treatments in clinical practice. It has been found that the published data for the low dose rate CSM11 Cs‐137 source [Ballester et al. Med Phys 2000;27:2182–2189] lacks of smoothness in some regions because data are too noisy. The purpose of this study is to calculate the dosimetric data for this source to improve the quality dosimetric data of Ballester et al. Methods: To obtain the dose rate distributions Monte Carlo simulations were done using the GEANT4 code. A spherical phantom 40 cm in radius with the Cs‐137 source located at the centre of the phantom was used. Results: The results from Monte Carlo simulations were used to derive AAPM Task Group 43 dosimetric parameters: Anisotropy function, radial dose function, air kerma strength, and dose rate constant. The dose rate constant obtained has been 1.094±0.002 cGy ĥ‐1 Û‐1. Conclusions: The new calculated data agrees within statistical uncertainties with the old data of Ballester et al. being the present data much less nosisy. The present data fulfils now all the requirements of the TG‐43U1 and thus it can be used in clinical practice.
SU‐E‐T‐383: Using a Force Gauge to Detect the Treatable Length of a HDR Catheter
Authors: Liu L , Li W
Published: November 2011

Purpose: Complex interstitial implant using flexible catheters often introduces tight curvatures or kinks in the catheters. The purpose of this study is to utilize a force gauge to determine potential blockage in HDR catheters, thus helping to create optimal deliverable treatment plans and to avoid potential treatment emergencies. Method and Materials: A VariSource iX HDR machine was used in this study. A compact digital force gauge from Wagner Instruments (Greenwich, CT) was used to measure the friction encountered in the catheters. A stainless steel telescoping‐type tube, about 5 cm long when retracted, was used to guide the check wire while it is being pushed into the catheter.

A flexible catheter wrapped around a cylinder was used to find out the maximum push force of the VariSource HDR machine. The farthest position that the HDR machineˈs check cable advanced before reporting a blockage was marked, and the force gauge was then used to push a check wire to the same position and the gaugeˈs reading was recorded. Results: The maximum push force of the VariSource HDR machine as measured using the catheter‐around‐cylinder method was found to be about 1.4 lb. For a patient implant, once the problematic catheter(s) were identified during CT simulation, a treatment plan was developed using only unblocked catheters and reachable dwell positions. Conclusions: By using a force gauge to identify potential blockage in catheters a prior, optimal HDR treatment plan is developed using only unblocked catheters and treatable dwell positions. This avoids the need for modifying the plan at the time of treatment and reduces potential treatment emergencies.
SU‐E‐T‐384: Dosimetry Studies for Radioactive Gold Nanoparticles
Authors: Kulkarni R , Lattimer J , Boote E , Kannan R , Katti K
Published: November 2011

Purpose: This investigation was initiated to determine the absorbed dose and radioactivity distribution of radioactive gold nanoparticles.Methods: Polymer gel dosimeter was prepared using about 12% gelatin (Sigma Aldrich, 300 bloom type A), 9% methacrylic acid(Sigma Aldrich), 10mM THPC (Sigma Aldrich), 18mM hydroquinone (Sigma Aldrich) in deionized water. These dosimeters are used for calibration purpose and meant to estimate the absorbed dose in the medium from the radiation emitted by the radioactive gold nanoparticles. The high energy gamma dose, activity distribution were measured using SPECT imaging and the phantom used was the ball of play‐doh made by Hasbro Toys. A previously constructed acrylic cast of beagle dogˈs head was used in this study to place the phantom. The radioactive gold nanoparticles were synthesized at the University owned nuclear reactor with the high neutron flux of ∼10e14 neutrons/cm2/sec.Results: Carbohydrate suspended radioactive nanoparticles with approximately 200 microCuries were injected in a play doh ball, depicting the prostate gland of beagle dog, approximately 1 inch diameter (8–10ml) in size. The phantom was placed on the SPECT imaging table near the isocenter of the imaging plane. The Gamma camera was peaked for the 411keV photon and equipped with a high energy (Iodine collimator). The phantom was then imaged using SPECT imaging protocol. The activity imaged at various planes and count rates was evenly distributed as predicted Conclusions: The activity distribution as seen in the SPECT imaging indicated active hot and cold spots of radioactive gold nanoparticles. The use of play‐doh determines the activity distribution but uneven diffusion. To compensate for that polymer gel dosimeters, equivalent to the tissue densities may be considered, besides there is also an advantage of estimating the absorbed doses.
SU‐E‐T‐386: Improved Dose‐Response Relationship for Focal Cancer Therapy Using Liposome‐Encapsulated 186Re Radionuclides
Authors: Hrycushko B , Ware S , Li S , Goins B , Bao A
Published: November 2011

Purpose: Vital to all cancer therapy modalities, is an effective relationship between treatment and effect. This work evaluated this relationship for intratumoral infusions of liposome‐encapsulated, 186Re for the treatment of human head and neck squamous cell carcinoma xenografts in nude rats. Methods: Rats were separated into three treatment groups receiving intratumoral infusions of, 186Re‐liposomes (5mCi/cm3 tumor; infused volume = 45% tumor volume): one group received a single infusion location, one group received multiple (3) infusion locations, and one group received multiple (3) infusion locations following two systemically injected bevacizumab treatments (1mg/rat). Planar gamma camera imaging was used to obtain tumor clearance kinetics. SPECT/CT images were used to obtain intratumoral activity distributions. Tumor sizes were periodically measured up to 43 days post‐treatment. Intratumoral dose calculations were performed using a dose‐point‐kernel convolution technique with co‐registered SPECT/CT images. Average dose and EUD values were calculated for individual tumors in attempt to correlate values with relative tumor shrinkage by comparing day 43 tumor volumes with day 0.Results: Intratumoral dose calculations showed significant intratumoral dose heterogeneity. The use of three separate tumor infusion locations improved dose homogeneity compared to a single infusion location as a result of a more uniform radioactivity distribution. An excellent dose‐response correlation was seen with EUD calculations (R, 2 = 0.84) compared with average tumor doses (R, 2 = 0.22). Varying radiobiological parameters over commonly used values showed little effect on relative EUD calculations among rats and suggests individualized parameter use is of little significance as long as the intratumoral dose heterogeneity is taken into consideration in the dose‐response relationship. Conclusions: This work showed a much improved treatment‐effect relationship for intratumoral delivery of, 186Re‐liposomes when using tumor EUD calculations. This is useful for treatment planning and evaluation with this radiation delivery technique.
SU‐E‐T‐388: Dosimetric Study of a New‐Generation Iodine‐125 Source in Polymer: Dosimetric Impact of Fluorescence X‐Rays
Authors: Abboud F , Hollows M , Scalliet P , Vynckier S
Published: November 2011

Purpose: A dosimetric characterization of IBt‐Bebig iodine‐125 interstitial seed, the first iodine seed in biocompatible polymer, recommended by AAPM (American Association of Physicists in Medicine) was carefully determined for medical applications by measurements and two independent Monte Carlo calculations. Methods: Measurements were obtained with LiF thermoluminescent dosimeters (1mm3 microcubes) in solid water phantoms (WT1). Measured data were then compared to Monte Carlo calculated data in solid water utilizing the MCNP5 code with using the standard photoatomic data library, mcplib04. Results: Four seeds were calibrated according to NIST 1999 standard to calculate the dose rate constant, Λ. A comparison of anisotropy function with previous studies of metallic seed in literature indicates a clear improvement in isotropy for studied seed especially near the seed axis (−40° < angle < 40°); for example, at 5° it has a value of about 92%, whereas for Amersham model 6702 is in the order of 59% . Therefore, the new seed is more isotropic and penetrating. The impact of fluorescence X‐rays of 12 keV generated by marker on the dosimetry was evaluated. The lead X‐rays at 12 keV contribute 8.7% of the dose at 0.1 cm and rapidly fall to 2.7% at 0.5 cm. Its contribution becomes negligible at 0.7 cm however; they are not excluded from NIST measurements. This will result an overestimation of dose rate constant about of 14%. Conclusions: Dosimetric characteristics, following TG43U1 formalism, are presented for a new iodine seed made with a biocompatible polymeric shell with internal centrally located leaded glass marker. Studied seed doesnˈt have a large anisotropy effect and the use of polymer shell may eliminate the artifacts that occur in traditional medical imaging due to the metallic shell. All these effects, when taken together, make this seed potentially useful source for permanent brachytherapy.
SU‐E‐T‐390: Real‐Time Measurement of Urethral Dose and Position Using a RADPOS Array during Permanent Seed Implantation for Prostate Brachytherapy
Authors: Cherpak A , Cygler J , E C , Perry G
Published: November 2011

Purpose: The new in vivo dosimetry tool, RADPOS, combines MOSFET dosimetry with an electromagnetic positioning sensor. It has recently been modified to include a MOSFET array rather than a single MOSFET for dose monitoring at five points along the detector axis. The detector is in use as part of a clinical trial which is the first to measure both urethral dose and internal motion concurrently during permanent seed implantation for prostate brachytherapy using a single probe. Methods: The RADPOS detector was secured inside a Foley catheter inside the patientˈs urethra. Spatial coordinates of the RADPOS detector were read every 0.5 s and the timing of events such as needle insertion was noted. MOSFET readings were taken over two ten minute periods once all seeds had been implanted both before and after the TRUS probe was removed. Results: Locations of the dosimeters could be inferred using a marker located at the end of the detector wire which is visible on fluoroscopy images. Although detector position varied slightly between patients, on average one dose point was inside the bladder, two/three dose points were within the prostatic urethra, and one/two dose points were at or inferior to the apex of the prostate. Maximum integral dose in the prostatic urethral ranged from 110–195 Gy, and it was found that the dose can change up to 63 cGy depending on whether the rectal probe is in place. The average change in displacement from the beginning of the procedure to the end for all patients was Δr=(5.5 ± 2.2) mm, with changes in individual coordinates of Δx (right/left) = (0.1–2.6) mm, Δy (superior/inferior) = (0.3–8.7) mm, and Δz (anterior/posterior) = (1.2–5.0) mm. Conclusions: The modified RADPOS is a powerful tool that can provide accurate real time dose and position information for use during prostate brachytherapy.

This project has been supported by grants from The Health Technology Exchange, Ontario Research Funds RE‐04‐026, The Ottawa Hospital Cancer Centre Foundation and by financial and technical support from Best Medical Canada and Ascension Technology Corporation.
SU‐E‐T‐396: Heterogeneity Effect on Dose Distributions for COMS Eye Plaques with 125I Seeds (Model I25.S16): A Radiochromic EBT Film Dosimetry Study
Authors: Acar H , Chiu‐Tsao S
Published: November 2011

Purpose: To study the heterogeneity effect on dose distributions in eye phantom for COMS eye plaques with 125I seeds using radiochromic EBT film dosimetry. Methods: COMS eye plaques (14,16,18 and 20mm in size) uniformly loaded with 125I seeds (model I25.S16) were studied. EBT film (lot #36076‐003AL) was positioned in a polystyrene eye phantom above the eye plaque, in two configurations: (1) in the plaqueˈs central plane, (2) perpendicular to the central axis at depth=5 or 12mm (from inner sclera) in the eye phantom. With the seed air‐kerma‐strengths of 2.4–4U, the exposure times (3.5–8.7h) were adjusted to deliver ∼400cGy at 5mm depth. Calibration films were irradiated by a single 125I seed at ∼5mm distance, one at a time, for doses up to 1200cGy. All films were scanned using Epson 10000XL scanner and data were analyzed using Mephysto FilmCal Mc2 software. With the established calibration curve, dose conversion was performed. Central axis depth doses and off‐axis profiles were determined. Results: The film dose data were compared with the calculated doses using Plaque Simulator v5.3.9 with homogeneous assumption (Homo) and heterogeneity correction (Hetero). The dose ratio (film/Homo) and (film/Hetero) values were obtained. The dose ratio (film/Homo) values are substantially lower than unity (mostly between 0.8 and 0.9) for all plaque sizes studied, indicating dose reduction by COMS plaque compared with homogeneous assumption. The film dose data agree with the Hetero values within the uncertainty of measurement data. Conclusions: We found significant heterogeneity effect on the 125I (model I25.S16) dose distributions in an eye for COMS plaques using radiochromic EBT film dosimetry. The film data agree with the calculated doses using Plaque Simulator with heterogeneity correction. The dose reduction effect for COMS plaques uniformly loaded with 125I seed (model I25.S16) found in this study is similar to that for other 125I seed models.
SU‐E‐T‐399: CT Image Artifacts from Brachytherapy Seed Implants: The Impact of Seed Size and Motion
Authors: Wells D , Basran P , Robertson A
Published: November 2011

Purpose: To determine the impact of seed thickness on post‐operative CT image quality, seed detection and dosimetry, with and without breathing motion. Methods: An acrylic prostate phantom mounted to a computer controlled motion platform was created for both standard (5 × 1 mm)‐ and small (5 × 0.5 mm)‐diameter non‐radioactive I‐125 seeds (Oncura Inc., Arlington Heights, IL). Seed arrangement within the phantom was based on a patient brachytherapy plan. Ten low‐contrast inserts, 2, 5, and 10 mm in diameter with contrast differences of 2, 12 and 29 HU, respectively, were embedded in the phantom. Volumetric CT scans were performed while the phantom exhibited vertical, longitudinal and elliptical motion cycles of amplitudes 0, 2, and 5mm. Streak artifacts were quantified by sampling the standard deviation (SD) of pixel values over regions of interest (ROI) within the phantom volume. Soft tissue contrast was quantified by calculating the contrast to noise ratio (CNR) for each low‐contrast insert. Seed localization and dosimetry were assessed using VariSeed v8.0 (Varian Medical Systems, Palo Alto, CA) with prostate, rectal and urethral volumes copied from the patient plan onto the phantom. Results: Image noise was reduced with the small seeds for no motion cases (SD 49 vs. 32HU) and for 5mm amplitude cases (SD 60 vs. 37HU). CNR with the small seeds was higher by a median value of 55% for the low‐contrast ROIs. Although VariSeed identified more erroneous seed positions for the small seeds, there were no clinically significant dosimetric differences to the prostate, rectal or urethral volumes. Conclusions: The use of smaller seeds for prostate brachytherapy results in improved image quality of post‐operative CT scans, without hindering the ability in localizing the seeds or affecting the dose to the prostate, rectum or urethra, irrespective of motion parameters.

Seeds used in this study were donated by GE Healthcare.
SU‐E‐T‐400: Evaluation of a Pencil Beam Algorithm and Pencil Beam Redefinition Algorithm for Bolus Electron Conformal Therapy
Authors: Kavanaugh J , Carver R , Chu C , Mancuso G , Hogstrom K
Published: November 2011

Purpose: To evaluate the accuracy of the pencil beam algorithmˈs (PBA) and pencil beam redefinition algorithmˈs (PBRA) calculated electron conformal therapy (ECT) dose distributions by comparing with phantom measured dose distributions. Method: The planning target volumes (PTV) from a parotid and a chest wall patient were modified to fit the CT image set of a cylindrical polystyrene phantom. A bolus ECT plan for both PTV sets was developed using .decimal p.d software to design a custom bolus, with the distal bolus surface fitting to the phantom and the proximal bolus surface conforming the 90% isodose surface to the distal PTV edge. PBA and PBRA dose distributions were calculated using the Pinnacle treatment planning system and in‐house software, respectively. In‐phantom film measurements were taken for each plan in five transverse slices and one sagittal slice. Accuracy of both the PBA and PBRA calculations was tested by comparing the measured 2D film data to corresponding planar dose files from both the PBA and PBRA treatment plans, respectively. Passing criteria of 5% dose difference or 2mm distance to agreement were applied to each pixel (0.5mm pixel size) to quantify the accuracy of each dose calculation. Results: Comparison between measured and PBA calculated pixels for the post‐mastectomy chest wall and parotid cases for all 6 measured planes resulted in 93.3% and 95.3% pass rates, respectively. Using the same criteria, the PBRA had pass rates of 96.6% and 95.6%, respectively. In a phantom with no heterogeneities, the PBA and PBRA are both expected to have excellent agreement. Conclusions: The PBA and PBRA produced highly accurate calculations for bolus ECT dose distributions. As expected, the PBRA is slightly more accurate than the PBA, however this difference may not have any clinical significance unless significant heterogeneities are present, currently under study.

Support: This work was supported in part by a research grant from .decimal, Inc.
SU‐E‐T‐401: The Development of Intensity and Energy Modulated Electron Radiotherapy; An Alternative to Photon Volumetric Modulated Arc Therapy
Authors: Alexander A , Soisson E , Sarfehnia A , Hijal T , Deblois F , Seuntjens J
Published: November 2011

Purpose: This work explains the development of an advanced treatment planning system for the generation of intensity and energy modulated electron radiotherapy (MERT) plans. The quality of MERT plans for the treatment of tumour bed boost in breast cancer is compared to direct electrons (DE) and volumetric modulated photon arc therapy (MAT). Method and Materials: The MERT treatment planning and delivery system at McGill University consists of MMCTP, an inverse optimization toolkit and the few leaf electron collimator. The stepwise planning process consists of: 1) generating a series of field openings, 2) Monte Carlo dose calculation for each field, 3) planning constraints, 4) iterative direct‐aperture optimization. For evaluation purposes, fourteen patients with breast cancer treated by lumpectomy and requiring post‐operative whole breast radiotherapy with tumour bed boost were planned using conventional DE, MAT and MERT. The planning goal was to deliver 10 Gy to at least 95% of the target volume. Dosimetry parameters for all techniques were compared. Results: Target coverage and homogeneity was best for MERT (D98=9.77 Gy, D2=11.03 Gy) followed by MAT (D98=9.56 Gy, D2=11.07 Gy) and DE (D98=9.81 Gy, D2=11.52 Gy). Relative to the DE plans, the MERT plans predicted a reduction of 35% in mean breast dose (p<0.05), 54% in mean lung dose (p<0.05) and 46% in mean body dose (p<0.05). Relative to the MAT plans, the MERT plans predicted a reduction of 24%, 36% and 39% in mean breast dose, heart dose and body dose respectively (p<0.05). Conclusions: MERT was a considerable improvement in dosimetry over DE. In some cases, there was a dosimetric advantage in using MERT over MAT for increased target conformity and low‐dose sparing of healthy tissue. Based on the favorable comparisons shown in this work it is reasonable to suggest that MERT could play a more significant role in breast radiotherapy.
SU‐E‐T‐404: The Effect of Field/target Size on Quality Assurance of TomoTherapy IMRT Treatments When Using a Diode Array for Absolute and Relative Dose Measurements
Authors: Foottit C , La Russa D , Studinski R
Published: November 2011

Purpose: Despite the known energy sensitivity of diodes due to an over‐ response to low‐energy photons, the presence of which depends on field size, there is trend toward using diode arrays for performing dose measurements in IMRT QA. The purpose of this investigation is to quantify the dependence of diode array dose measurements on field/target size, assess machine output tracking abilities, and improve the accuracy of diode arrays measurements for helical TomoTherapy treatments. Methods: A Delta, 4 diode array was calibrated for TomoTherapy patient QA. Using the calibration set‐up, the dose from static beams was measured with both the Delta, 4 and an A1SL chamber as a function of field size. The dose tracking abilities of the Delta, 4 was then assessed with daily helical exposures of uniform dose to a cylindrical target in the phantom, followed each time by a second measurement with an A1SL chamber in a cheese phantom. Additional measurements of this type were obtained as a function of target diameter to derive field/target size scaling factors for the dose measured by Delta, 4. A1SL and Delta, 4 measurements were also obtained for series of patient DQAs. Results: Delta, 4 dose measurements with field size diverged from A1SL results by up to 2% as the field size diverged from the reference. Clinically acceptable machine output tracking was exhibited by the Delta, 4 based on daily measurements of the helical exposures to a target with fixed diameter. However, the scaling factors for the Delta, 4 based on these measurements are as high as 4% for large target sizes. When applying the scaling corrections to patient DQAs, agreement between the Delta, 4 and A1SL is improved to within 1% in most cases. Conclusions: Dose measurements with the Delta, 4 diode array are sensitive to changes in field/target size. Accuracy can be improved with the application of measured correction factors.
SU‐E‐T‐410: Clinical Application of Dose Reconstruction Based on Full‐Scope Monte Carlo Calculations
Authors: Yeo I , Xu Q , Chen Y , Jung J , Kim J
Published: November 2011

Purpose: To reconstruct dose delivered to a deformable phantom from images acquired by an electronic portal imaging device (EPID) using a full‐scope Monte Carlo based non‐iterative reconstruction method we have developed and to evaluate the delivered dose to the deformed organs using the resulting reconstructed dose. Methods: A deformable prostate phantom was embedded into a 20cm‐deep and 40cm‐wide water phantom. The phantom was CT scanned and the anatomical models of prostate, seminal vesicles, and rectum were contoured. A coplanar 4‐field IMRT plan was used for this study. The XVMC Monte Carlo code, XVMC, was used for verification of planned dose in phantom and calculation of dose response by EPID. Organ deformation was simulated by inserting a “transrectal” balloon containing 20ml of water and air. A new CT scan was obtained and the deformed structures were contoured. The IMRT plan was delivered to the two phantoms and integrated EPID images were respectively acquired. Dose reconstruction was performed using these images with calculated EPID responses. The deformed phantom was registered to the original phantom using an in‐house developed software based on the Demons algorithm. The transfer matrix for each voxel was obtained and used to correlate the two sets of the reconstructed dose to generate a cumulative reconstructed dose on the original phantom. Results: Forwardly calculated planning dose in the original phantom was compared to the cumulative reconstructed dose from EPID in the original phantom. The prescribed 200 cGy isodose lines showed little difference with respect to the “prostate” and “seminal vesicles”, but appreciable difference (3%) was observed at 210cGy dose level. Conclusions: The results validate our dose reconstruction method for EPID dosimetry. Organ deformation resulted small but observable dose changes in the target and critical structure.

This study was in part supported by Varian Medical Systems, Inc.
SU‐E‐T‐428: 1D Direct Dosimetric Leaf Separation Measurements as An M Quality Control Tool
Authors: Nygren I , Mei X , Villarreal‐Barajas J E
Published: November 2011

Purpose: To demonstrate the use of a diode array for the direct measure of the dosimetric leaf separation (DLS) associated with a multi collimator (MLC). Methods: Using the Profiler(TM) diode array, the has been directly measured at multiple points (diode positions). These measurements were obtained by collecting the dose rate spectra as a gap‐sliding window is delivered over the diode array. The DLS is de from the full width at half maximum (FWHM) determined by fitting sigmoid functions to the dose rate spectra and using the MLC spee calculate an effective gap. Taking the difference between the effective nominal gap provides a directly measured DLS for every diode position. In the present study, 49 measurements were obtained with a single sliding window delivery. Using three gap widths, this results in 147 DLS values for each evaluated MLC. Results: The DLS for three MLCs associated with three Varian(TM) linear accelerators has been measured using three sliding window gap sizes (5mm, 10mm and 15mm) and 49 detectors for each measurement. The results are averaged for each MLC along with associated standard deviations. A more detailed characterization of the DLS‐MLC can be obtained by plotting the DLS as a function of position along the main axis perpendicular to the direction of leaf motion. This 1‐D DLS distribution contains information associated with the effect of inter‐leaf and intra leaf transmission on the DLS. Conclusions: Using a diode array for the direct measurement of the DLS provides a quick and simple way to determine the DLS in 1‐D. The process can be repeated on a routine basis as part of a Quality Assurance program to monitor the behavior of the MLC system over the course of its use.
SU‐E‐T‐430: Quantifying Alterations in Flattened and Flattening‐Filter‐Free Beam Characteristics from a Gantry Mounted, in Vivo Beam Delivery Verification System on a TrueBeam Linear Accelerator
Authors: Riegel AC , Cao Y , Kapur A , Jamshidi A
Published: November 2011

Purpose: Planning and delivery of radiation therapy are becoming more complex with the wide use of intensity‐modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). In vivo quality assurance of field modulation is possible using a novel multiwire “harp” ionization chamber array DAVID (PTW, Freiburg, Germany) positioned at the treatment head. The purpose of this work is to pre‐validate its use in vivo by characterizing potential dosimetric changes in the beam introduced by the device.Methods: Beam profiles for 6, 10, 15 MV with flattening filter and 6 MV flattening‐filter‐free (FFF) were acquired for square field sizes of 5, 10, 20, and 35 cm at 5.8 cm depth on a TrueBeam linear accelerator (Varian, Palo Alto, CA). Profiles were acquired in the target‐gun, left‐right, and diagonal directions using the StarCheck Maxi ion chamber array (PTW, Freiburg, Germany) with and without DAVID in place. Ion chamber spacing was 3 mm. Attenuation was measured by calculating the transmission factor at the central chamber with and without DAVID. Results: For beams with a flattening filter, transmission factors ranged from 95–96% with 15 MV to 92–94% with 6 MV. For the 6 MV FFF, the transmission factor was 91–92%. Beam profiles were minimally affected by DAVID, demonstrating at most 1–2% difference at 35 cm field size, for the higher energy beams. For 6 MV and 6 MV FFF and field sizes less than 20 cm, profiles agreed within 1% even in penumbral regions Conclusions: Except for a correctable transmission factor, the presence of DAVID minimally influences dosimetric profiles. Transmission factors range from 92%–96% for conventional beams, but can be lower for FFF beams. In future work, percent depth dose and electron contamination will be measured to examine spectral influences of DAVID on FFF beams
SU‐E‐T‐432: Independent Calculation of Modulation Factors Based on Comprehensive MLC and Head Scatter Modeling to Streamline IMRT 2nd Check Process
Authors: Steciw S , Warkentin B , Rathee S , Fallone B
Published: November 2011

Purpose: To extend a previously described technique of modeling 2‐D fluence from any MLC leaf sequence, and develop an independent and accurate calculation of modulation factors required for pre‐treatment QA of IMRT plans. This calculation is intended to replace ion chamber measurements and thus offers a quicker and more stream‐lined verification process. Methods: In‐house software developed in MATLAB was used to calculate each IMRT fieldˈs modulation factor, defined as the ratio of the dose with MLC to the corresponding open field dose (without MLC) at a given reference point. For IMRT fields, the program uses the leaf sequence file for each dynamic field to calculate 2‐D maps of the primary fluence modulation and the timings of leaf openings. Fluence modeling includes beam horn characterization and head scatter effects. Commissioning values of leaf transmission and the dynamic leaf gap for our Varian Millenium MLCs are used to incorporate the effects of transmission and rounded leaf‐end design on the fluence modulation. Interleaf leakage and tongue‐and‐groove effects are modeled based on the timing map and the shape (including penumbra) of the interleaf leakage and tongue‐and‐groove characterized from 2‐D EPID images. Once calculated, the IMRT fluence is convolved with a Monte‐Carlo (EGSnrcMP) pencil‐beam kernel to generate a dose at the reference point depth, and divided by the open field dose to obtain the calculated modulation factor. Results: Results from 18 IMRT plans (121 fields), including treatments from pelvis, prostate, head and neck, and anal canal cancer, show excellent agreement between calculated and ion‐chamber measured modulation factors. The mean discrepancy is −0.3%, with standard deviation of 1.2%, where calculation times are 55 seconds per field on one PC core. Conclusions: An accurate and streamlined calculation method for determining modulation factors has been developed to replace tedious ion chamber measurements needed for pre‐treatment IMRT QA.
SU‐E‐T‐439: Implementation of an Iterative Deconvolution Method in a Daily In‐Vivo Verification Procedure with the DAVID System
Authors: Poppe B , Looe H , Luellau T , Willborn K , Harder D
Published: November 2011

Purpose: The DAVID system (PTW‐Freiburg, Germany) is a multiwire ionization chamber placed in the accessory holder, capable of continuously monitoring treatment deliveries such as IMRT. As shown in previous work, an iterative deconvolution algorithm can correct the blurring of the signal profile by the lateral transport of secondary electrons within the chamber. First results of the implementation of the procedure into clinical routine and of the resulting enhanced error detection efficiency are presented. Methods: The measured signal profile of the wires can be regarded as resulting from a convolution of the true photon fluence profile with a lateral response function. After measuring the latter for each wire by simple slit measurements, a true signal can be approximated by an iterative deconvolution algorithm described in previous work of our group. Prior to the patient treatment, data for each field of an IMRT plan are measured during the pre‐treatment verification. The data are deconvolved on‐line and stored as reference values. During the daily treatment of the patient the measured and deconvolved readings are compared on‐line with the reference values. Results: The application of the procedure is important for DAVID chambers designed for MLCs with 5 mm leaf width. In these the profile blurring by laterally scattered secondary electrons in the chamber can be of the order of the primary signal. By applying the deconvolution procedure the error detection efficiency of the DAVID system can be enhanced by a factor of approximately 2. The deconvolution of the profile is performed on‐line with no remarkable loss of time. Conclusions: The implemented deconvolution algorithm corrects for the signal profile blurring of the DAVID chamber by laterally scattered electrons. The study has been implemented into the daily routine.
SU‐E‐T‐445: Use of MAPPHAN for Patient Specific VMAT Plan Verification
Authors: Comsa D , Barnett E , Moseley D , van Prooijen M
Published: November 2011

Purpose: Volumetric modulated arc therapy (VMAT) has been introduced in many clinics. The need for patient specific quality assurance tests for VMAT was reflected in a large number of publications on this subject. Many centers have used the Sun Nuclear Mapcheck diode array for IMRT patient specific QA, and thus the retrofitted MAPPHAN/Mapcheck may appear an economical and trustworthy solution for VMAT. Implementing this phantom was not straightforward. Following manufacturerˈs guidelines for comparing measured and calculated dose distributions, the pass rates were considerably lower than with other volumetric dosimeters. We present a simple method to improve these results to clinically acceptable values. Methods: We commissioned VMAT for two Elekta Infinity accelerators with the Pinnacle3(v9) treatment planning system. For MAPPHAN dose calculation, we used a CT scan of the phantom directly or assigned a density of 1.05 g/cm3 to the whole phantom, as recommended by the manufacturer and tested these on 5 prostate plans. To improve plan/measurement agreement we created an artificial CT scan using two density configurations as possible estimates of the actual phantom. To minimize couch effects, we placed the phantom on 7cm of Styrofoam. Validity of the plans was also verified with the Delta4 phantom (Scandidos) which gave 100% agreement under TG1 19 criteria. Results: Using MAPPHAN as suggested by the manufacturer gave suboptimal results. A literature search revealed that this problem was also identified by others (1). Using an artificial CT representation and raising the phantom improves agreement with plans from (93+/−6)% to (98+/−3)% (TG1 19), approaching Delta4 results Conclusions: Problems with using the MAPPHAN phantom can stem from insufficient knowledge of the phantom composition and treatment couch attenuation. A simple method was developed to improve the results of VMAT patient specific QA.
SU‐E‐T‐452: Dynamic Intensity Modulator
Authors: Kizuka K , Cerviño L , Evans B , Man K , Mushegan S , Park A , Rahimian I , Jiang S
Published: November 2011

Purpose: To reduce the radiotherapy treatment time per freaction in order to decrease the possibility of patient movement during treatment and increase patient comfort. Methods: A prototype of a Dynamic Intensity Modulator (DIM) was created which changes fluence profiles dynamically and autonomously between treatment grantry angles. Radiation Absorbing Material (RAM) is dynamically displaced in and out of the radiation beam path by radio‐translucent columns. A reservoir regulates the amount of RAM in the beam path. Between gantry angles columns shift to create new fluence profiles. Measurements of beam fluence were acquired using portal images for various RAM thicknesses. Results: Imaging varying RAM thicknesses shows definite changes in image intensity. Initially the columns were at the zero position, so a constant height layer of RAM was achieved. By comparing a constant height layer of RAM against a thicker constant height layer of RAM, an increase in attenuation was observed. Different columns height configurations were also tested. Again, areas with a thicker layer of RAM had a higher level of attenuation than areas with a thinner layer of RAM. In all tests the prototype columns used to set the height of RAM were visible. This will be resolved by creating columns, which are more radio‐translucent. Also, a significant amount of noise was visible in the attenuation profiles. This was most likely due to the material choice of RAM, a homogenous mixture of 2 different diameter lead BBˈs. Conclusions: Despite the non‐uniformity within intensity profiles, this study demonstrates the fundamental concept of the DIM. The prototype was able to displace deformable RAM in different locations to create distinct fluence profiles.
SU‐E‐T‐463: Utilizing the TG119 Assessment to Quantify Planning System Improvements
Authors: Gordon J , Hashmi S , Hamilton R
Published: November 2011

Purpose: To quantify the impact of modifications to an IMRT planning system using the TG1 19 assessment score as an indicator of improvement. Method and Materials: Evaluation of an IMRT system following the TG1 19 protocol was initially performed to establish baseline results. The TG1 19 evaluation set consisted of five IMRT test cases planned and delivered to a 30 cm × 30 cm × 15 cm solid water phantom. Per‐field measurements were collected using a diode array, and composite measurements were collected using film and an ionization chamber. Measurements were compared to calculation to determine TG1 19 confidence limits. The original plans were then modified to include table attenuation, previously omitted by the planning system. Without modifying the incident radiation fields, the plans were recalculated on the modified phantom, which includes a table model. The resultant calculations were compared to the original measurements, and new TG1 19 confidence limits were determined. Results: The baseline TG1 19 assessment resulted in confidence limits of 4.36%, 23.8% and 9.16% for diode array, composite film, and composite ion chamber measurements, respectively. The TG1 19 assessment after plan modification resulted in confidence limits of 4.36%, 19.5%, and 8.54%. The TG1 19 confidence limits were reduced by 4.30% and 0.62% in film and ion chamber, respectively, which is a relative improvement of 18.1% and 6.8%. Conclusions: Using a single set of measurements, modifications to the planning system via the addition of a table model resulted in quantifiable improvement in TG1 19 assessment results. In this system, the addition of a table model improved the agreement between calculation and measurement, as indicated by the TG1 19 confidence limit results. The improvement was only observed in confidence limits from composite comparisons using film and ion chamber, but not observed for per‐field comparisons using a diode array.
SU‐E‐T‐656: Are We Ready for Biological Planning? A Dosimetric Study
Authors: Turian J , Liao Y , Bernard D , Chu J
Published: November 2011

Purpose: To evaluate the feasibility of using a commercially available IMRT optimization and plan evaluation tool based on radio‐biological models. Methods: A retrospective dose optimization study was performed for 5 lung cases treated using IMRT to evaluate the biological modules available in Pinnacle system. A hybrid objective function consisting of physical dose constraints, for PTVs, and EUD constraints, for organs at risk, was used for optimization. The OARˈs were: spinal cord, esophagus, cardiac ventricle, and both lungs. The “a” value, used in EUD definition, was a variable to allow the optimizer to match the physical dose optimization solution. A biological optimized plan, (BOP) was deemed acceptable if matches or exceeds the quality of a physical dose optimized plan (PDOP). TCP, NTCP, or P+ values were not included in the optimization however; they were used during plan evaluation. The TCP and P+ are based on Poisson model with linear quadratic cell survival for each individual voxel. NTCP module is based on LKB model. For model parameters we choose a combination of published data and our clinical standards. Results: For serial structures the Max EUD constraint was able to bring the dose to levels obtained during physical dose optimization, however it was unable to easily control the magnitude of maximum dose. Values for parameter “a” are 15–20 for serial structures and a=1.0 for parallel structures. Global values for TCP, NTCP, P+ were 56.8% versus 53.4%, 7.4% versus 6.4%, and 49.4% versus 46.9% for BOP and PDOP, respectively. Conclusions: We have performed a retrospective treatment planning study to investigate the feasibility of using biological models for optimization purposes. Although, results between PDOP and BOP plans are comparable it remains to be seen if the clinicians will be willing to accept these concepts in clinical practice without the existence of clinical trials.
SU‐E‐T‐661: Study on Rebuildup after Air Cavity with the Monte Carlo Technique and Ultra‐Thin TLD Dosimeters
Authors: Lee C , Nieh C , Chao T , Yeh C , Tung C
Published: November 2011

Purpose: The purpose of the study is to investigate the re‐buildup after air cavity of various thicknesses for 6MV therapeutic photon beams with different field sizes. Methods: The BEAMnrc code was chosen for Monte Carlo (MC) dose simulation. Complete simulation includes all geometry and components of the treatment head of our Varian 21EX linear accelerator and beam propagation within the water phantom. 6 MV photon beams with field sizes at 3×3 and 10×10 cm2 were studied. Air cavities with thickness 1 and 5 cm were inserted at 4 cm depth. The simulated re‐buildup dose distribution was compared to both those from ultra‐thin TLD measurement (Rexon TLD‐100H, 4.5mm × 0.1mm) and treatment planning calculation (Eclipse, Varian Medical Systems, Palo Alto, CA) using the AAA algorithm.Results: According to the MC simulation results, dose reductions at the re‐buildup surface for 10×10 cm2 fields are 4.6% and 20.5% for 1 and 5 cm air cavities, respectively. Those for the 3×3 cm2 fields are 29.0% and 77.7% for 1 and 5 cm air cavities, respectively. The differences between TLD measurement and MC simulation are within the detector uncertainty. The results indicate that the treatment planning calculation shows minimum ability to model the re‐buildup phenomenon. Conclusions: Dose re‐buildup near an air cavity is a possible cause of target underdose. Results from the current study indicate that it is more profound for large cavity thicknesses and small irradiation fields. Ultra‐thin TLD was also proven an effective detector for measuring dose distribution near re‐buildup region.
SU‐E‐T‐670: Toward Real‐Time Monte Carlo Simulation Using a Commercial Cloud Computing Infrastructure
Authors: Wangb H , Maa Y , Pratxa G , Xinga L
Published: November 2011

Purpose: Monte‐Carlo methods are the gold standard for modeling photon and electron transport in heterogeneous medium; however, their computational cost prohibits their routine use in the clinic. Cloud computing, wherein computing resources are allocated on‐demand from a third party, is a new practical approach for performing ultra‐fast Monte Carlo calculation in radiation therapy. Methods: We deployed the EGS5 Monte‐Carlo package in a commercial cloud environment. Launched from a single local computer with Internet access, a python script allocates a remote virtual cluster. A handshaking protocol designates master and worker nodes. The EGS5 binaries and the simulation data are initially loaded onto the master node. The simulation is distributed among independent worker nodes via the Message Passing Interface (MPI), and the results aggregated on the local computer for display and data analysis. Our novel approach is evaluated for pencil‐beams and broad beams of high‐energy electrons and photons. Results: The output of the cloud‐based Monte‐Carlo simulation is identical to that produced by the single‐threaded implementation. For 1 million electrons, a simulation that takes 2.58 hour on a local computer can be executed in 3.3 minutes on the cloud, a 47X speed‐up. Simulation time scales inversely with the number of parallel nodes. The parallelization overhead is also negligible for large simulations. Conclusion: Cloud computing is a promising platform for Monte Carlo simulation. In addition to the significant speed up, cloud computing builds a layer of abstraction for high performance parallel computing, an abstraction that may change the way dose calculations are performed and radiation treatment plans are completed. Problems of a parallel nature can be solved efficiently using cloud computing.