bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐303: Practical Considerations for Maximizing Heat Production in Novel Thermo‐Brachytherapy Seed Prototype
Authors: Gautam B , Shvydka D , Parsai E
Published: June 2012

Purpose: Our recently proposed thermo‐brachytherapy seed offers a convenient approach to radiation sensitization with heat in treatment of solid tumors through concurrent administration of hyperthermia and brachytherapy. The seed consists of a titanium capsule, containing radioactive I–125 and a ferromagnetic core, serving as a source of self‐regulating hyperthermia when placed in an alternating electromagnetic field. We present an experimental study of the magnetic properties of ferromagnetic Ni‐Cu alloy, and develop a protocol for obtaining the material capable of the maximum heat generation. Based on the practically achievable temperature interval we evaluate the effect of thermal expansion on the seed components during the hyperthermia treatment. Methods: Alloy samples of Ni1‐xCux (0.28= × =0.3) were prepared by arc melting method in argon atmosphere. The ingots were annealed in vacuum at 1000°C for 12 hours. These samples were cut into pieces and used for magnetization measurements with SQUID magnetometer. The thermal expansion along greatest dimension of each component of the purposed seed was estimatedfor temperature increase from 37 to 60 ËšC. Results: The annealed samples show sharp Curie transition at temperature TC∼50°C, varying with the alloy concentration. However, the un‐annealed sample does not show the clear transition, thus indicating a strong influence of thermal treatments on the magnetic properties of the Ni‐Cu alloy. The annealing favors atomic diffusion, and leads a sample homogenization, minimizing composition fluctuations and maximizing the heat generation. The effect of the temperature rise on the thermal expansion of each component of the seed was found to be negligible. Conclusions: We have established the thermal annealing protocol resulting in the maximum heat generation from the Ni‐ Cu alloy core. The negligible change in dimensions of the seed components due to heating assures the safety of the implementation of thermo‐brachytherapy seed for hyperthermia treatments. This project is supported through NIH grant # 1 R41 CA153681‐01A1
SU‐E‐T‐306: An Optimized Dosimetry Study Comparing a Multichannel Cylinder versus Single Channel Cylinder in the Treatment of the Vaginal Cuff or Vagina with High Dose Rate Brachytherapy
Authors: Rosen L , Syh J , Patel B , Syh J , Wu H
Published: June 2012

Purpose: Elekta/Nucletron introduced a new CT/MRI multi‐lumen brachytherapy applicator to optimize dosimetry and treatment of vaginal targets. The design includes several peripheral channels isotropically placed 5mm inside cylinder surface with a separate central channel allowing single lumen brachytherapy as an option. We compared optimized dosimetry plans between single channel versus multi‐channel to determine potential dosimetric advantages in vaginal cancer treatment after hysterectomy. Methods: Patients were planned by CT simulation with each fraction of vaginal brachytherapy for postoperative endometrial cancer or vaginal cancer. Twenty CT cases representing five patients were compared. A single channel (standard) treatment approach was planned to the prescribed depth with the goal of providing maximal coverage to the tumor bed (the 5mm rind typically). Doses to the bladder and rectum were recorded by dose volume histogram. A second treatment plan was generated using a multichannel approach. The Oncentra brachytherapy planning system was used to generate both forward and IPSA inverse treatment plans. The target coverage percentage and doses to the bladder and rectum were optimized. Results: The comparison demonstrates an improved V90, V95, V100 of the prescribed dose to the 5mm vaginal cuff rind was achieved in all cases. The average improvement in coverage was 9% (between 6–13%) for the cases evaluated. The multi‐channel plan also reduced the maximal dose (hot spot) to the bladder and rectum within a range of 9–16% and 5–15% respectively. An increased non‐critical vaginal mucosal dose of up to 75% was delivered as a result of the reduced bladder and rectal exposure via multichannel procedure. Conclusions: Optimization of vaginal cuff brachytherapy is possible by using a new multi‐lumen vaginal cylinder. The new device can increase the therapeutic ratio by improving target coverage with a reduction in the potential toxicity of hot spots to the adjacent organs.
SU‐E‐T‐307: Quantitative Assessment of the Source Attenuation for the New CT‐Compatible Titanium Fletcher‐Suit‐Delclos (FSD) Gynecologic Applicator
Authors: Soni N , Gautam B , Shvydka D , Parsai E
Published: June 2012

Purpose: To quantify the effect of 192Ir source attenuation due to titanium material used in manufacturing the new CT‐compatible FSD device during treatment delivery. Methods: A typical cervical cancer treatment includes primary external beam followed by a course of HDR Intra‐Cavitary brachytherapy using FSD applicator. A detailed geometry of the applicator obtained from the manufacturer is implemented using Monte Carlo (MC) simulation package MCNP5. The bending angles of tandem and colpostats are 150 and 1200 respectively. The source geometry is of the VariSource wire model VS2000. We assigned 8 dwell positions in the tandem and 4 dwell positions in each colpostat to calculate dose rate at reference points with and without titanium wall present in the simulation. Results: Based on the MC computation, the titanium tube reduced the overall dose to point A by ∼1.5%, with contributions varying for each dwell position. We also compare MC results with BrachyVision treatment planning system calculations. The clinically used algorithm is based on AAPM report TG‐43, which calculates the dose without inhomogeneity correction. The latest Varian release of the planning software BrachyVision‐Acuros has the updated algorithm capable of inhomogeneity corrected dose calculation. For this study a treatment plan is created based on the exact coordinates of the MC model and used to calculate dose at reference points. The dose to reference point A with BrachyVision traditional algorithm is in excellent agreement with the homogeneous MC model result. Conclusion: The attenuation due to Ti wall is ∼1.5% at point A compared to ∼5% using a non‐CT compatible system. Comparative accuracy of the two treatment planning systems with MC, especially in their abilities to account for the source attenuation due to the applicator titanium walls will also be presented.
SU‐E‐T‐310: Micro‐Dosimetry Study of the Radiation Dose Enhancement at the Gold‐Tissue Interface for Nanoparticle‐Aided Radiation Therapy
Authors: Paudel N , Shvydka D , Parsai E
Published: June 2012

Purpose: Gold nanoparticles (AuNP) have been proposed to be utilized for local dose enhancement in radiation therapy. Due to a very sharp spatial fall‐off of the effect, the dosimetry associated with such an approach is difficult to implement in a direct measurement. This study is aimed at establishing a micro‐dosimetry technique for experimental verification of dose enhancement in the vicinity of gold‐tissue interface. Methods: The spatial distribution of the dose enhancement near the gold‐tissue interface is modeled with Monte Carlo (MC) package MCNP5 in a 1‐dimentional approach of a thin gold slab placed in an ICRU‐4 component tissue phantom. The model is replicating the experiment, where the dose enhancement due to gold foils having thicknesses of 1, 10, and 100μm and areas of 12.5×25mm2 are placed at a short distance from clinical HDR brachytherapy (Ir‐192) source. The measurements are carried out with a thin‐film CdTe‐based photodetector, having thickness <10μm, allowing for high spatial resolution at progressively increasing distances from the foil. Results: Our MC simulation results indicate that for Ir‐192 energy spectrum the dose enhancement region extends over ∼1 mm distance from the foil, changing from several hundred at the interface to just a few percent. The trend in the measured dose enhancement closely follows the results obtained from MC simulations. Conclusions: AuNP's have been established as promising candidates for dose enhancement in nanoparticle‐aided radiation therapy, particularly, in the energy range relevant to brachytherapy applications. Most researchers study the dose enhancement with MC simulations, or experimental approaches involving biological systems, where achievable dose enhancements are difficult to quantify. Successful development of micro‐dosimetry approaches will pave a way for direct assessment of the dose in experiments on biological models, shedding some light on apparent discrepancy between physical dose enhancement and biological effect established in studies of AuNP‐aided radiation therapy. No conflict of interest
SU‐E‐T‐311: Dose Perturbation Due to Thin Layers of High‐Z in HDR Ir‐192 Source Dose Delivery
Authors: Zhang H , Das I
Published: June 2012

Purpose: To evaluate the dose perturbation associated with thin layers of high‐Z materials in HDR 192I source dose delivery through Monte Carlo simulation and experimental measurement. Methods: Dose perturbation of high‐Z thin materials in HDR 192Ir source dose delivery was experimentally and theoretically studied. MCNP5 Monte Carlo simulation code was employed to calculate the dose enhancement effects at upstream and downstream of high‐Z materials. The cutoff energy for photon was 1 keV and for electron 5 keV. A parallel plate ion chamber with window thickness of 0.9 mm, was used to measure the dose rates at the downstream of the high‐Z thin layer. The high‐Z layer was positioned at 0.625 cm from the 192Ir source. The thickness of layer was from 0.1 to 1 mm. The results were normalized by the dose rate in a homogeneous water phantom at the location of the interface. The high‐Z materials used were lead, tin and titanium. In addition, the dose enhancement effects of different high‐Z materials at the shortest distance from the interface were also calculated and measured to examine the Z dependence. Results: The Monte Carlo simulation results agreed very well with the experimental data. At the surface of the 0.1 mm high‐Z interface, Ti, Sn and Pb respectively increase dose rate by 1.2, 1.4 and 2.5 times the dose rate in homogeneous water medium. The dose rate enhancement depends on Z and thickness of the high‐Z layer. The dose enhancement increases with Z. However, an increased thickness in the high‐ Z layer results in less dose enhancement due to attenuation. Conclusions: HDR 192Ir source dose enhancement by a high‐Z material thin layer is similar to that reported from a kilo‐voltage X‐ray beam. Attention should be paid for high‐Z thin layers with HDR source clinically as it could cause significant dose enhancement.
SU‐E‐T‐322: A Dosimetric Comparison of PBI Brachytherapy Techniques: SAVI, Contura, and Tube and Button Applicators
Authors: Park S , Kamrava M , Demanes J
Published: June 2012

Purpose: To evaluate the dosimetry of partial breast irradiation brachytherapy techniques using the Strut Adjusted Volume Implant (SAVI), Contura, and Tube and Button (T&B) applicators. Methods: A total of 51 breast‐cancer patients (23 SAVI, 6 Contura, and 22 T&B) were treated. The target was delineated following NSABP B‐39 guidelines. 3D plans were optimized using the Inverse Planning Simulated Annealing algorithm to deliver 3.4 Gy per fraction to the target and minimize dose to organs at risk (OARs). Graphical optimization was then used to fine tune the final dose distribution. The minimum cavity‐to‐skin distance was measured. Target coverage (V90 and V95) and maximum dose (D0.1cc) to the OARs were evaluated. Dose homogeneity index (DHI = 1‐V150/V100) was calculated. Results: The average cavity‐to‐skin distances were 4.1 mm (0.5–9.6 mm, SAVI) and 11.7 mm (7.1–15.4 mm, Contura). The target‐to‐skin distance for the T&B cases was 8.7 mm (5.0–13.7 mm). The average V90 and V95 to the target were 96.8% and 94.5% (SAVI), 97.0% and 93.0% (Contura), 98.6% and 97.3% (T&B). The mean D0.1cc to the skin, ribs, and lung was 91.5%, 58.8%, 44.5% (SAVI), 93.1%, 51.3%, 40.5% (Contura), 69.1%, 41.5%, and 31.9% (T&B). The average V150 and V200 to the normal breast tissue were 30.4 cc and 14.9 cc (SAVI), 29.5 cc and 7.3 cc (Contura), 18.3 cc and 7.1 cc (T&B). The average DHI for the SAVI, Contura, and T&B cases was 0.55 (0.50–0.60), 0.70 (0.63–0.78), and 0.76 (0.74–0.79). Conclusions: All techniques provided clinically acceptable target coverage and dose to the OARs. The SAVI device provided a lower skin dose at close cavity‐to‐skin distances while providing excellent target coverage. However, the T&B and Contura applicators produced more homogeneous dose distribution (higher DHI) in the target than the SAVI. The correlations between dosimetric properties and follow‐up mammogram results are under investigation.
SU‐E‐T‐323: A CT‐Based Monte Carlo Dose Calculation for Correction of Metal Artifacts Due to the Henschke Applicator
Authors: Yu P , Chao T , Lee C , Tung C , Wu C , Lee H , Nien H , Tu P
Published: June 2012

Purpose: Monte Carlo method is the most accurate dose calculation system in the modern world. And we use it to evaluate the influence of computed tomography (CT) metal artifacts resulted from the Henschke applicator in intracavitary brachytherapy dose distribution. Methods: The CT images of the Plastic Water® phantom with Henschke applicator and the applicator associated metal artifacts are applied to the Monte Carlo N‐Particle Transport Code X (MCNPX) for dose calculation. The accuracy of the CT images input is assessed by the phantom dose simulation with and without CT images based on our previous Monte Carlo parameters optimization. To reduce the effect of artifacts, the HU number of metal artifacts and metal materials in CT images are replaced by phantom HU number according to the redefined HU density table. Then the Henschke applicator geometry is superimposed on the corrected CT image. This corrected CT images are applied to MCNPX for dose calculation. The dose calculation with and without metal artifacts can be analyzed for the impact of metal artifacts. Results: This system can be implemented because the dose discrepancy with and without CT images is below 0.05%. We analyze the dose distribution with corrected CT‐based phantom for the impact of metal artifacts. The dose distribution shows the largest dose discrepancy is 8–9% and which is located at the region above 2cm distal to ovoid edge. Conclusions: In this study, the dose at the region above 2cm distal to ovoid edge is significantly under prescription dose. The artifacts resulted from Henschke applicator leads to a certainly inaccurate dose calculation in the planning system. Thus, dose distribution must be evaluated before using new applicator. The CT‐based Monte Carlo method can be used to evaluate the influence of CT metal artifacts in intracavity brachytherapy.
SU‐E‐T‐326: Repeated CT‐Scans in Pulsed Doserate Prostate Brachytherapy: Assessment of Deviations from the Treatment Plan
Authors: Koedooder C , Dinkla A , Pieters B , van Wieringen N , van der Laarse R , van der Grient J , Bel A
Published: June 2012

Purpose: Estimation of deviations between planned and delivered dose in Pulsed Doserate (PDR) brachytherapy for prostate cancer. Methods: A boost of 28.8 Gy is given with PDR brachytherapy in addition to 46 Gy delivered with External Beam RT. Brachytherapy is given in 24 pulses of 120 cGy, with an interpulse period time of 2.0 hours, resulting in a treatment time of over 46 hours. For 31 patients, additional CT‐scans were made apart from the Treatment Planning CT, i.e., one at 24 hours after start of PDR treatment and one shortly before finishing PDR treatment. On the second and third CT, the brachytherapy catheters were newly reconstructed and the treating physician delineated the PTV and organs at risk. Dwell positions and dwell times as used for the original Treatment Plan were imported into the newly reconstructed catheters and the dose distribution was recalculated. Plan comparison parameters were prostate V100 and D90 and rectum and bladder D2cc. Results: Averaged over 3 CT scans and all patients, the prostate V100 decreased 1.2% and D90 decreased 2.7%. For rectum, D2cc was within the tolerance dose (96 cGy/pulse) for all patients on the planning CT, but exceeded the tolerance dose on scan #2 in 7/31 patients with maximally 46% and in scan #3 in 5/25 patients with maximally 29%. Also for bladder D2cc was within the tolerance dose (120 cGy/pulse) for all patients on the planning CT. Here, the tolerance dose was exceeded on scan #2 in 2/31 patients with maximally 14% and in scan #3 in 3/25 patients with maximally 28%. Conclusions: In PDR prostate brachytherapy the relatively long treatment time has no clinically relevant deteriorating effect on the dosimetric quality of the treatment. PTV dose hardly deviates from planned dose, while OAR tolerance doses are rarely exceeded and only in small volumes.
SU‐E‐T‐328: Method for Verifying the Air Kerma Strength of Pre‐Assembled I‐125 Plaques for Ocular Melanoma
Authors: Zimmermann L , Wilkinson D
Published: June 2012

Purpose: To develop a method of easily verifying the activity or air kerma strength of pre‐assembled COMS and Eye Physics EP917 eye plaques. Methods: A Capintec CRC‐7 Dose Calibrator with its standard vial/syringe sample holder was used to measure the activity of pre‐assembled eye plaques using IsoAid Advantage I‐125 seeds. Plaque activity measurements were made by placing the plaque face up in the center of a 10 cm tall Styrofoam insert in the source holder. The source holder was rotated to four angles (0°, 90°, 180°, and 270°). The average of these four values was compared to the assay air kerma strength, decayed to the plaque measurement date, to establish a plaque calibration factor. The average and standard deviation of several plaque measurements were then calculated to determine the repeatability of the process. Activity measurements were also made to determine the contribution of each seed position to the total measured activity of the EP 917 plaque. Results: Activity measurements were taken on seven vendor‐preloaded Eye Physics EP917 eye plaques. For the EP917, the average plaque calibration factor was 0.344 with a standard deviation of 0.012. For the COMS plaques with a silastic carrier, the plaque calibration factors ranged from 0.237 for a COMS 16N to 0.281 for a COMS 20. The average plaque calibration factor was 0.256 and the standard deviation was 0.015 The individual seed measurements were made using three different EP‐917 plaques. The data from these measurements show that the contribution of a single seed to the total plaque activity is approximately 5.9% or 1/17th of the total activity. Conclusions: Plaque calibration factors based on this methodology may be used to verify the activity of preassembled plaques to within 5%.
SU‐E‐T‐331: Geometric Variations between Pre Planned and or Planned Implant Procedures for Patients Undergoing I‐125 Prostate Brachytherapy Seed Implantation
Authors: Pokhrel D , David J , Sathiaseelan V , Kalapurakal J
Published: June 2012

Purpose: The limitations of pre‐planned prostate brachytherapy seed implantation can be overcome by intraoperative re‐planning in the OR. Main purpose of this report is to compare geometric parameters between pre‐planed and OR‐panned procedures regarding changes in prostate volume/shape, number of seeds/needles and total activity, to address prostate volume/shape changes observed in the OR when the patient is under anesthesia and to present post‐implant dosimetric results.Methods and materials: In 33 clinical 125I cases, ultra‐sound (US) volume studies and plans were performed for both preplanning and in the OR just before seed implantation. Changes in volume/shape and number of seeds/needles used were compared between the two data‐sets and patient received brachytherapy treatment using the OR plan. For each patient, D90 and V100 were obtained from day one post‐implant CT dosimetry. Results: The mean US volume change observed between pre‐plans and OR‐plans was (5.3 ± 4.4)cc, with a maximum of 20cc. Change in the number of seeds between the plans were (8.9 ± 5.7) on an average and showed excellent correlation with prostate volume change. The number of needles remains relatively unchanged. The average total‐activity changed was (2.7 ± 1.6)mCi, with a maximum of 7mCi. Prostate shape changes (in terms of no. of seeds changed) in the OR was seen mainly in the anterior aspect of prostate. The average D90 and V100 were (90.3 ± 4.6)Gy and (148.7 ± 16.6)% of the prescription dose, respectively. Summary and Discussion: Significant variations of prostate volume changes between pre‐planned and the OR‐planned implants were observed in many cases. Largest change in the no. of seeds was due to change in shape of the prostate in its anterior aspect clearly shows the effect of pelvic muscle relaxation and patient re‐positioning in the OR. This supports the argument in favor of performing intraoperative re‐planning and implant‐dosimetry.
SU‐E‐T‐332: A Retrospective Review of Target Motion for Prostate IMRT Patients Using Calypso
Authors: Fredrickson M , Arvan L , Courlas G , Sweeney L , Kaurin D
Published: June 2012

Purpose: This limited study determines the fraction of prostate IMRT patients who do not need setup adjustments, following initial setup, for their first 5 and last 5 fractions. For patients needing setup adjustments during their first or last 5 fractions, the average time to the first adjustment is established for each interrupted fraction. Methods: The Calypso System (Varian Medical Systems, Palo Alto, CA) tracks the position of three implanted fiducials in the prostate. The treatment is manually interrupted if excursions >3mm occur over a time period >5 seconds. Post treatment reports were examined from 18 patients to determine the number of setup changes needed during their first and last 5 fractions and the time of the first setup change for each interrupted fraction. Results: Three of the 18 patients in their first 5 fractions and 5 of the 18 patients in their last 5 fractions did not require setup changes. On average, 7.6 (+/− 7.4) setup changes were needed per patient for the first 5 fractions and 5.9 (+/− 8.5) setup changes for the last 5 fractions. Over the 18 patients, there were a total of 137 setup changes in the first 5 fractions and 106 in the last 5 fractions and only 1 patient with no setup changes. For those patients requiring setup changes, the first setup change occurred, on average, after 6.3 (+/− 4.4) minutes for the first 5 fractions and 5.4 (+/− 2.5) minutes for the last 5 fractions. Conclusions: The majority of prostate patients required adjustments multiple times over the first and last 5 fractions. Using the Calypso System to monitor target motion appears to be beneficial for the majority of prostate patients. Treatment techniques decreasing treatment time, such as VMAT, can be helpful in decreasing the number of interruptions, but still require motion management systems.
SU‐E‐T‐336: ICom ‐ A Communication Interface for Quality Assurance in VMAT Treatment Delivery
Authors: Nicol S , Furstoss C , Wierzbicki W , Munger P
Published: June 2012

Purpose: To present a VMAT QA tool that uses the iCom messaging interface from Elekta Synergy linear accelerators to record and compare, during treatment, real‐time delivery parameters with expected DICOM‐RT plan values. These delivery parameters can also be formatted offline into a new DICOM‐RT plan, then sent back to Pinnacle to compute the 3D delivered dose distribution. Method and Materials: In order to evaluate the sensitivity of our QA tool to mechanical errors and to judge their effects on the delivered dose, both static and dynamic QA plans with and without simulated errors in leaf and gantry position were delivered and analyzed. While static plans were delivered to an EPID, dynamic plans were delivered to an ArcCHECK device embedded with its own cavity insert and a Micro‐Lion ionizing chamber. To clinically assess the dosimetric accuracy of VMAT deliveries as well as treatment degradation due to machine imprecision, several SRS QA plans were finally delivered to a home made gelatine phantom embedded with small plastic inserts with both TLD and nanoDot dosimeters. To validate our approach, comparisons between calculated (planned and reconstructed) and measured absorbed doses were systematically performed. Results: First results demonstrated that real‐time delivery parameters corresponded well to expected plan values. Simulated errors in leaf and gantry position can easily be detected by our QA tool. Although work is still in progress, these deliberately introduced errors seem to have a negligible impact on delivered dose except for small fields or segments. Results obtained with VMAT plans indicate good agreement between reconstructed and planned dose distributions, as well as with dose measurements in the gelatine phantom. Conclusions: In this study, we developed a practical QA software solution based on the Elekta iCom interface to perform a) real‐time VMAT monitoring and b) a volumetric reconstruction of the delivered dose to assess degradation effects produced by machine imprecision. This approach aims to ensure that all critical aspects of VMAT delivery are properly functioning.
SU‐E‐T‐338: Investigations of Fidelity of Dose Delivery at High Dose Rate of Flattening Filter Free (FFF) Beams in VMAT on TrueBeam™
Authors: Kalantzis G , Qian J , Han B , Luxton G
Published: June 2012

Purpose: To assess dosimetric fidelity of Varian TrueBeam™ in flattening filter‐free (FFF) modes by dose measurements comparing high versus low dose rate irradiation, particularly for volumetric‐modulated arc therapy (VMAT) plans. Methods: Measurements with a 22cm‐diameter cylindrical PMMA phantom ScandiDos Delta4 multi‐diode detector system and Farmer‐type(0.6 cm3) and PinPoint™(0.015 cm3) cylindrical ionization chambers in rectangular Plastic Water™ phantoms were used to compare photon beam irradiation from 10MV(10XFFF) at 400 and 2400 MU/min and 6MV(6XFFF) at 400 and 1400 MU/min dose rates, respectively, on a Varian TrueBeam™. Two setups were studied: square fields and VMAT plans optimized for stereotactic body radiotherapy treatment of lung tumors. VMAT plans were tested both with and without periodic gating from the Varian RPM™ position‐management gating system. Quantitative comparisons were based on mean and maximum relative dosimetric error (RDE) measured between the two dose rates. Results: Reproducibility was very good for all measurements. For square fields, RDEs of 0.3–0.4% were found with ionization chamber measurements, with negligible contribution from recombination in the detectors. The Delta4 showed consistently higher mean RDE of ∼0.8% for 10XFFF square fields. For non‐gated VMAT plans, mean RDE was ∼1.1% for 10XFFF. Error was independent of delivered dose for square fields. For VMAT plans, RDE increased as dose fraction size decreased. Finally, comparison between gated VMAT plans at high and low dose rates demonstrated a maximum error in low‐dose regions, of 1.5% and 2.3% for 10XFFF and 6XFFF respectively. Conclusions: There is a dosimetric error of the order of ∼0.4 % for 10XFFF for TrueBeam at its highest dose rates. From measurements using the Delta4, VMAT treatment plan deliveries were found to be independent of dose rate to within ∼0.5%. Overall, very good agreement was found between irradiation at high and low dose rates across the spectrum of our measurements.
SU‐E‐T‐339: First Evaluation of Intensity Modulated Radiation Therapy (IMRT) Delivery Accuracy during Real‐Time Tracking Using a Gimbaled X‐Ray Head of Vero4DRT (MHI‐TM2000)
Authors: Utsunomiya S , Miyabe Y , Sawada A , Shiinoki T , Ishihara Y , Mukumoto N , Nakamura M , Yamada M , Monzen H , Mizowaki T , Kokubo M , Hiraoka M
Published: June 2012

Purpose: To evaluate the accuracy of intensity modulated radiation therapy (IMRT) delivery during real‐time tumor tracking using a gimbaled X‐ray head of Vero4DRT (MHI‐Tm2000) (Mitsubishi Heavy Industries, Ltd., Japan). Methods: We have utilized a QUASAR platform phantom (Modus Medical Devices Inc.) where a Kodak EDR2 X‐ray film was placed at 5 cm depth in a solid water phantom. A simple step‐and‐shoot test field with five segmental fields and a clinical field for a prostate IMRT were used. Films were irradiated in following three setups: (1) stationary phantom and gimbal, (2) moving phantom and stationary gimbal and (3) moving phantom and real‐time tracking irradiation. The moving phantom was driven by sine waves at a frequency of 0.25, 0.33 and 0.5 Hz with amplitude of 20 mm along tilt direction. 2D dose distributions and profiles were obtained for each case and analyzed using dose analysis software, DD‐IMRT (R‐TECH.INC, Japan). Results: We observed a large dose blurring effect and as large as 40% dose discrepancies between (1) and (2) for the test field at the high dose gradient. In contrast, the comparison of (1) with (3) shows the differences less than 3% in the most area except the high dose gradient. We also found the good agreement between (1) and (3) for a clinical field. The percentage of values of discrepancy less than 5% and 3% were 97.5% and 82.0%, respectively. Conclusions: We demonstrated the significant dose blurring effect with a moving target in step‐and‐shoot IMRT dose delivery and it was greatly improved by real‐time tumor tracking with Vero4DRT. The system showed an excellent accuracy of IMRT dose delivery during tumor tracking as Result of the first evaluation. This research was supported by the Japan Society for the Promotion of Science (JSPS) through its Funding Program for World‐Leading Innovation R&D on Science and Technology (FIRST Program). Research sponsored in part by Mitsubishi Heavy Industries, Ltd.
SU‐E‐T‐345: A Dosimetric Study of Spatially Fractionated Radiation (GRID) Using Linac Grid Block and Newly Designed Virtual TOMOGRID Template
Authors: Zhang X , Penagaricano J , Sharma S , Clarkson R , Chao M , Chen X , Han E , Corry P , Ratanatharathorn V , Yan Y
Published: June 2012

Purpose: To perform a dosimetric comparison for spatially fractionated external beam radiation (GRID) using linear accelerator with a dedicated GRID block and Helical Tomotherapy using a virtual TOMOGRID template. Methods: The GRID phantom treatment plan was generated with dedicated GRID block (Radiation Products Design, Inc.) using the Pinnacle v9.0 TPS. The dose profiles were measured using EDR2 films with 100cm SSD at depths of dmax and 10cm for both 6MV X‐rays and 18MV X‐rays beam. TOMOGRID targets were generated using a virtual TOMOGRID template (VTT) provided by an in‐house software (DICOMan). The programmable VTT was configured in various settings to generate TOMOGRID targets (GRID GTV) and a structure of avoidance (SOA). TomoTherapy Plans based on GRID GTVs and SOAs were generated on HiArt TomoTherapy v4.0.4 TPS. Results: For the dose under linac block, the dose decreases with increasing depth under the open area, while the dose under block area is nearly constant. At dmax, the average valley to peak (V/P) ratio is around 0.2 for 6MV X‐rays and 0.3 for 18MV X‐rays. Ten treated linac‐based GRID patients were selected and the same average V/P ratio was obtained. The dose distribution with the properly designed virtual TOMOGRID templates was similar to the linac‐based GRID dose distribution. At isocenter, The V/P ratio is 0.5 and 0.42 for the two TOMOGRID plans used in this study. The V/P ratio is almost constant under linac block across the isocenter plane while the ratio was different under TOMOGRID plans across the isocenter plane. Conclusions: GRID radiation using Tomotherapy is a breakthrough technique. Helical Tomotherapy‐based GRID can provide at least equivalent GTV coverage compared to linac‐based GRID while allowing 3‐D optimization to achieve superior sparing of organs‐at‐risk and no high‐dose spill outside the GRID GTV.
SU‐E‐T‐348: Measurement‐Guided 4D VMAT Dose Reconstruction on an Arbitrary Homogeneous Dataset
Authors: Opp D , Robinson J , Nelms B , Zhang G , Feygelman V
Published: June 2012

Purpose: To develop and validate a VMAT QA tool that takes as input a 2D, low‐density (∼10 mm) empirical dose map from a commercial helical diode array, and outputs a high density, volumetric, time‐resolved (4D) dose matrix on an arbitrary patient dataset. At first, the method validation is limited to a homogeneous ‘patient’. Methods: A VMAT treatment is delivered to a diode array (ArcCHECK, Sun Nuclear Corp., Melbourne, FL). 3DVH software (Sun Nuclear) derives the high‐density volumetric dose using measurement‐guided dose reconstruction (MGDR). MGDR cylindrical phantom results are then used to perturb the 3D TPS dose on the patient dataset, producing a semi‐empirical volumetric dose grid. Four‐dimensional dose reconstruction on the patient is also possible by morphing the sub‐beam instead of composite dose. For validation, TG‐119 structures and objectives were used. 3DVH and TPS cumulative point doses were compared to ion chamber in a cube water‐equivalent phantom (‘patient’). The shape of the phantom is different from the ArcCHECK and the targets were placed asymmetrically. Ion chamber dose sampled at 10Hz was compared to time‐resolved 3DVH point doses. Coronal and sagittal absolute film dose distributions in the cube were compared with 3DVH and TPS. Results: Across four TG‐119 plans, the average PTV point dose difference in the cube between 3DVH and ion chamber is 0.0±0.9%. Average film vs. 3DVH gamma analysis passing rates are 88.6, 96.1, and 99.5% for 1%/2mm, 2%/2mm, and 3%/3mm criteria, respectively. 4D MGDR was also sufficiently accurate. Conclusions: Even for a well‐commissioned TPS, comparison metrics show on average better agreement between MGDR and measurement compared to MGDR and TPS on the arbitrary‐shaped phantom (‘patient’). The method requires no more accelerator time than standard QA, while producing more clinically relevant information. Validation in a heterogeneous thoracic phantom is under way, as is ultimate application to virtual motion studies. Supported in part by a grant from Sun Nuclear Corp.
SU‐E‐T‐355: Volumetric Modulated Arc Therapy Treatment Quality Assurance Using an Ion Chamber Array and a Software‐Modified Phantom
Authors: Han B , Phillips T , Luxton G
Published: June 2012

Purpose: To systematically test an algorithm to correct for the directional response of a 2D ion chamber array (PTW Seven29TM) and implement techniques to use it in quality assurance (QA) of volumetric modulated arc therapy (VMAT). Methods: The 27×27 ion chamber array was placed in sagittal orientation between two slabs of 10‐cm thick Plastic WaterTM to measure the directional response. Photon beams from anterior hemisphere of 10×10cm2 field size and energies of 6, 10 and 15MV with flattening filter, and 6 and 10MV without flattening filter from Varian TrueBeamTM were used with gantry angles ranging from 90° to 270°(IEC). An artificial cylindrical half‐shell of 1‐cm thickness was inserted into the backside 10‐cm slab in the treatment planning system and assigned the CT number of 2000 Hounsfield units to compensate for the Seven29TM directional response. The method was tested using half‐arc plans with 5×5cm2 and 10×10cm2 field size in sagittal orientation and full‐arc VMAT patient treatment plans in both sagittal and coronal orientations. Results: In the absence of the attenuation shell, measurements of the Seven29TM in sagittal orientation indicate a relatively flat response for gantry angles 90° to 15°, decreasing to 97.2% at 0°, and an approximately uniform under response of ∼89% for the beams from the backside quadrant. Using the correction method resulted in agreement within 2.8% for each beam angle and 1.9% for half‐arc deliveries. Results of a typical VMAT QA plan gave gamma test (3 mm, 3%) passing rates of 95.3% and 99.7% in sagittal and coronal orientations, respectively. Conclusions: The combination of Seven29TM and software‐modified Plastic WaterTM phantom proved to be a fast and reliable tool for QA of VMAT. Through this characterization of its directional response, accurate orthogonal 2D dose maps can be generated for independent verification of the treatment delivery from complex VMAT plans.
SU‐E‐T‐359: Comparison of Three Array Calibration Methods of MapCheck for Elekta Beam Modulator LINAC
Authors: Liu T , He R , Huang Z , Vijayakumar S , Yang C
Published: June 2012

Purpose: To compare three array calibration methods of MapCheck for an Elekta Beam Modulator LINAC. Methods: Both Elekta Synergy and Elekta Beam Modulator LINACs are used in our clinic. While an Elekta Synergy LINAC is a conventional Linac with the maximu field size 40 × 40 cm2, an Elekta Beam Modulator LINAC only has a maximum field size of 21 × 16 cm2. Because the MapCheck requires a field size of 26 × 26 cm2 for array calibration according to its user manual, it is a challenge to calibrate MapCheck array for Beam Modulator. We have tested three methods for this Purpose: 1) use the array calibration file from Elekta Synergy; 2) calibrate the MapCheck using Beam Modulator at 135cm SSD; 3) calibrate the MapCheck with both Beam Modulator and Elekta Synergy, then combine the calibration files. For the third method, the MapCheck was first calibrated with Beam Modulator at both collimator angles 0 and 90 degrees at 110cm SSD. Then the MapCheck was calibrated with Elekta Synergy using the standard method at 100cm SSD. The three array calibration files were combined such that it best describes the response of MapCheck detectors to Beam Modulator and all the detectors are covered. After calibrating the MapCheck detector array with the three methods, four prostate and four H&N IMRT treatment plans were delivered. The measurements were compared with the calculations. The Gamma Index (Gamma Index < 1) pass rates with a criterion of 3% and 3mm from all the three calibration methods were compared. Results: The pass rate of Gamma Index ranges from 90.4% to 100%. The null hypothesis that all three methods come with the same QA pass rates can not be rejected (p>0.1). Conclusions: For the testing cases, no significant difference was found among the three array calibration methods in term of Gamma Index pass rate.
SU‐E‐T‐367: A Method of Improving the Effective Spatial Resolution of IMRT QA for 2D Array Devices
Authors: Zhang J , Laub W
Published: June 2012

Purpose: To improve the effective spatial resolution of small field IMRT QA with 2D‐array device. Methods: When small field IMRT QA is measured with 2D‐array device, the spatial resolution (5–10mm) may beconsidered too sparse to accurately verify dose over high gradient area. Reducing the incident angle can improve spatial resolution along the transverse direction of the beam eye view (BEV). Rotating collimator can improve resolution along the other direction. However, detector angular dependency and electron scattering may potentially degrade measurement accuracy when the incident angle becomes too small. In this study, a Scandidos Delta4 device was used for IMRT QA measurements. It has two orthogonal dose planes with 5mm spatial resolution in central 6×6 cm. We measured IMRT plans with two settings: one with 40 and 50 incident angles and the other with 15 and 75 incident angles. The measured 2D dose planes were compared with the ones calculated by TPS. 3D doses were estimated by interpolating two measurement points along beamlets based on the PDD curve and compared with the one from TPS. Results: Calculation showed that for single detector plane, the spatial resolution along the transverse irection of the BEV is improved from 3.21 mm (for the 40 degree) to 1.29 mm (for the 15 degree). When both orthogonal planes were considered, the spatial resolution was improve from 1.75 mm to 1.02 mm. The QAmeasurements showed that the average passing rate with 2% dose deviation criterion was 74 % for 15‐degree incident angle, only slightly lower than the passing rate of 80% for the 40‐degree incident angle. On the other side, the 15/75 setting reconstructed 3D dose in high gradient region more accurately than the 40/50 setting due to higher spatial resolution. Conclusions: A moderately small incident angle can improve effective spatial resolution without significant accuracy loss.
SU‐E‐T‐368: A Comparison of Small MU Sub‐Field Dosimetry for Step‐And‐Shoot IMRT Fields on Varian IX and Truebeam Machines
Authors: Dolan J , Ambrose R , Furhang E , Harrison L
Published: June 2012

Purpose: To compare measured and calculated dose distributions for step‐and‐shoot IMRT fields comprised of sub‐fields with small MUs on Varian IX and Truebeam machines. Methods: The Eclipse treatment planning system (v 8.9.9) was used to create step‐and‐shoot IMRT fields comprised of a sequence of rectangular sub‐fields of decreasing size. MU settings of 1, 2 and 50 MU per sub‐field and energies of 15x and 6x were investigated on both an IX (console v7.8.05) and a Truebeam (console v1.5.10.2). Dose rates of 300 MU/min and 600 MU/min were investigated. All fields were delivered with the sub‐fields merged into a single field and with them unmerged as separate fields. The delivered dose distributions were measured and compared to the calculated distributions using the Mapcheck QA system(v 4.01.01). Results: For unmerged fields, the measured and calculated dose distributions showed similar agreement for both machines. For the merged 1 and 2 MU per sub‐field fields on IX, a previously described ‘overshoot’ phenomenon was readily apparent. In this phenomenon, a slight discrepancy in the beam off timing between sub‐fields seems to introduce a systematic error in dose delivery. This phenomenon was not evident for the merged 50 MU per sub‐field field on the IX or for any merged fields on the Truebeam. The magnitude of the overshoot phenomenon on the IX was found to increase with increasing dose rate. Conclusions: The Truebeam appears to be able to deliver low MU step‐and‐ shoot IMRT fields more accurately than the IX. On the Truebeam, neither field merging nor dose rate appear to affect the accuracy of step‐and‐shoot delivery. On the IX, merging fields introduces a small systematic error that may suggest the need for limiting dose rate and minimum sub‐field MU in certain applications.
SU‐E‐T‐372: Optimal Selection of Plane and Alignment Based on Quantitative Evaluation Using Three‐Dimensional (3D) Dose Data for IMRT and VMAT Dosimetry
Authors: Tachibana H , Takahash R , Umeda T
Published: June 2012

Purpose: Gamma, DTA and dose difference (DD) evaluations could take into account only the two axes on the evaluation plane for IMRT and VMAT plan verification dosimetry. However, some steep dose gradient region on another axis with the errors of measurement setup and the laser alignment can decrease the agreement between the calculation and measurement on the plane. In this study, we developed an optimal selection method of finding optimal plane of the evaluation and adequate alignment of the measurement data using three‐dimensional dose data. Methods: After the isocenter alignment of the measurement data manually, the optimal selection engine reconstructs dose planes at certain locations on the axis perpendicular to the evaluation plane from the 3D data of dose calculation. then, the calculation planes are compared with the measurement plane using gamma evaluation in order to choose the candidates of the best plane of the evaluation. Using the candidates with the combinations with rotations and shifts, DD evaluation is conducted to find out the best plane and the best combination of shift and rotation. To assess the effectiveness of the method, a head and neck IMRT plan measurement was performed and the path ratios of gamma evaluation (3%/3mm criteria) were measured with optimal selection and non‐optimal selection method, respectively. Results: Average pass ratio of gamma evaluation with the optimal selection method was14.2%‐better than with the non‐optimal selection method. Additionally, the method found 0.5mm‐ and 1.5mm‐deviations in the two‐axis on the evaluation plane, respectively. 1.0mm‐deviation was also measured on the axis perpendicular to the plane. Conclusions: IMRT and VMAT verification measurement with the method could provide us not only the dosimetric check of the plan with better result but also the mechanical test of isocenter laser alignment.
SU‐E‐T‐373: The Comparative Research of Monte Carlo Simulation Based Inhomogeneous Tissue Correction Algorithm
Authors: Li G , Yan B , Wu A , Jing J , Wang Q
Published: June 2012

Purpose: Two kinds of improved Batho method are proposed in this paper, the dose correction results for test example are compared also.Method and Materials:[Method1].The MC simulation model is : a 1cm3 cavity is placed in the central axis of 30cm3 water phantom. The center of the cavity is (0,0,0.5),'¦(0,0,29.5). The dose distribution of the cavity is simulated, the relative error with homogeneous water phantom is obtained and pre‐treated by the relative error databases. The corrected dose distribution is got in two steps, the cavity region and boundary point are corrected corresponding to the relative error databases firstly, then, the inhomogeneous tissue outside the cavity is corrected according to the Batho method.[Method2].The density of human tissue is divided into four corresponding intervals in terms of the value of CT. The simulation model is: a 1 cm3 typical inhomogeneous tissue is placed at the central axis of 30 cm3 homogeneous tissue phantom. Compared with the PDD of pure homogeneous tissue phantom, the relative error database can be obtained. The program judges the destiny change along the primary ray path. The Batho algorithm is used for dose correction when the destiny of tissue is relatively uniform. In the inhomogeneous interface between different tissues and inside cavity, the program will quickly find the relative error by index according to tissue density changes and depth, read the corresponding data to gain accurate correction results. Results: The method 1 obtain better correction result in the boundary point compared to MC simulation, but, in the cavity, the maximum error correction is 9.661%. Accordingly, the maximum correction error in the cavity region by method2 is 4.73%, in the boundary point is 2.47%, can satisfy the clinical precision requirement of 5%. Conclusions: The method 2 can obtain satisfactory correction Result in inhomogeneous tissue interface and the cavity area. The work is supported by the National Natural Science Foundation of China (60872112 and 10805012), the Natural Science Foundation of Anhui Province (11040606M132) and the Fundamental Research Funds for the Central Universities and the Science Research and Development Fund of Hefei University of Technology (2012HGXJ0062).
SU‐E‐T‐375: Ion Recombination Correction Factors (Pion) for Varian TrueBeam High Dose Rate Therapy Beams
Authors: Kry S , Popple R , Molineu A , Followill D
Published: June 2012

Purpose: Ion recombination is approximated in TG‐51 by Pion, which is calculated by a 2‐voltage measurement. This approximation is sufficiently accurate for the calibration of beams only when Pion is less than 1.05. High‐dose per pulse beams, such as flattening filter free beams, may increase Pion to an unacceptable level for the 2‐voltage measurement approach to be adequate. Methods: Pion was measured for flattened beams of 6, 10, 15, and 18 MV, as well as flattening filter free beams of 6 and 10 MV. Pion was also measured for electron beams of 6–18 MeV at standard as well as high dose rate (1000 MU/min). The measurements were made at a range of depths and with a variety of clinical ion chambers, including PTW, NEL, and Exradin chambers. 1/V versus 1/I curves were generated for the scenarios generating the maximum measured Pion values in order to confirm the accuracy of the 2‐voltage technique. Results: Consistent with the increased dose per pulse, Pion was higher for FFF beams than for FF beams. However, for all beams, measurement locations, and chambers examined, Pion never exceeded 1.018. Compared to the 1/V versus 1/I results, the 2 voltage technique was always accurate within 0.3%. Conclusions: Recombination can be adequately accounted for in high dose rate flattening‐filter free beams using the standard 2‐voltage technique to determine Pion.