bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐212: Comparison of TG‐43 Dosimetric Parameters of Low and High Energy Brachytherapy Sources Obtained by MCNP Code Versions of 4C, X and 5
Authors: Zehtabian M , Zaker N , Sina S , Meigooni A Soleimani
Published: June 2015

Purpose:

Different versions of MCNP code are widely used for dosimetry purposes. The purpose of this study is to compare different versions of the MCNP codes in dosimetric evaluation of different brachytherapy sources.

Methods:

The TG‐43 parameters such as dose rate constant, radial dose function, and anisotropy function of different brachytherapy sources, i.e. Pd‐103, I‐125, Ir‐192, and Cs‐137 were calculated in water phantom. The results obtained by three versions of Monte Carlo codes (MCNP4C, MCNPX, MCNP5) were compared for low and high energy brachytherapy sources. Then the cross section library of MCNP4C code was changed to ENDF/B‐VI release 8 which is used in MCNP5 and MCNPX codes. Finally, the TG‐43 parameters obtained using the MCNP4C‐revised code, were compared with other codes.

Results:

The results of these investigations indicate that for high energy sources, the differences in TG‐43 parameters between the codes are less than 1% for Ir‐192 and less than 0.5% for Cs‐137. However for low energy sources like I‐125 and Pd‐103, large discrepancies are observed in the g(r) values obtained by MCNP4C and the two other codes. The differences between g(r) values calculated using MCNP4C and MCNP5 at the distance of 6cm were found to be about 17% and 28% for I‐125 and Pd‐103 respectively. The results obtained with MCNP4C‐revised and MCNPX were similar. However, the maximum difference between the results obtained with the MCNP5 and MCNP4C‐revised codes was 2% at 6cm.

Conclusion:

The results indicate that using MCNP4C code for dosimetry of low energy brachytherapy sources can cause large errors in the results. Therefore it is recommended not to use this code for low energy sources, unless its cross section library is changed. Since the results obtained with MCNP4C‐revised and MCNPX were similar, it is concluded that the difference between MCNP4C and MCNPX is their cross section libraries.
SU‐E‐T‐215: Comparison of VMAT‐SABR Treatment Plans with Flattened Filter (FF) Beam and Flattening Filter‐Free (FFF) Beam for Localized Prostate Cancer
Authors: Chung J , Kim J , Kang S , Suh T
Published: June 2015

Purpose:

The purpose of this study is to access VMAT‐SABR plan using flattening filter (FF) and flattening filter‐free (FFF) beam, and compare the verification results for all pretreatment plans.

Methods:

SABR plans for 20 prostate patients were optimized in the Eclipse treatment planning system. A prescription dose was 42.7 Gy/7 fractions. Four SABR plans for each patient were calculated using Acuros XB algorithm with both FF and FFF beams of 6‐ and 10‐MV. The dose‐volume histograms (DVH) and technical parameters were recorded and compared. A pretreatment verification was performed and the gamma analysis was used to quantify the agreement between calculations and measurements.

Results:

For each patient, the DVHs are closely similar for plans of four different beams. There are small differences showed in dose distributions and corresponding DVHs when comparing the each plan related to the same patient. Sparing on bladder and rectum was slightly better on plans with 10‐MV FF and FFF than with 6‐MV FF and FFF, but this difference was negligible. However, there was no significance in the other OARs. The mean agreement of 3%/3mm criteria was higher than 97% in all plans. The mean MUs and deliver time employed was 1701±101 and 3.02±0.17 min for 6‐MV FF, 1870±116 and 1.69±0.08 min for 6‐MV FFF, 1471±86 and 2.68±0.14 min for 10‐MV FF, and 1619±101 and 0.98±0.04 min for 10‐MV FFF, respectively.

Conclusion:

Dose distributions on prostate SABR plans using FFF beams were similar to those generated by FF beams. However, the use of FFF beam offers a clear benefit in delivery time when compared to FF beam. Verification of pretreatment also represented the acceptable and comparable results in all plans using FF beam as well as FFF beam. Therefore, this study suggests that the use of FFF beam is feasible and efficient technique for prostate SABR.
SU‐E‐T‐223: Computed Radiography Dose Measurements of External Radiotherapy Beams
Authors: Aberle C , Kapsch R
Published: June 2015

Purpose:

To obtain quantitative, two‐dimensional dose measurements of external radiotherapy beams with a computed radiography (CR) system and to derive volume correction factors for ionization chambers in small fields.

Methods:

A commercial Kodak ACR2000i CR system with Kodak Flexible Phosphor Screen HR storage foils was used. Suitable measurement conditions and procedures were established. Several corrections were derived, including image fading, length‐scale corrections and long‐term stability corrections. Dose calibration curves were obtained for cobalt, 4 MV, 8 MV and 25 MV photons, and for 10 MeV, 15 MeV and 18 MeV electrons in a water phantom. Inherent measurement inhomogeneities were studied as well as directional dependence of the response. Finally, 2D scans with ionization chambers were directly compared to CR measurements, and volume correction factors were derived.

Results:

Dose calibration curves (0.01 Gy to 7 Gy) were obtained for multiple photon and electron beam qualities. For each beam quality, the calibration curves can be described by a single fit equation over the whole dose range. The energy dependence of the dose response was determined. The length scale on the images was adjusted scan‐by‐scan, typically by 2 percent horizontally and by 3 percent vertically. The remaining inhomogeneities after the system's standard calibration procedure were corrected for. After correction, the homogeneity is on the order of a few percent. The storage foils can be rotated by up to 30 degrees without a significant effect on the measured signal. First results on the determination of volume correction factors were obtained.

Conclusion:

With CR, quantitative, two‐dimensional dose measurements with a high spatial resolution (sub‐mm) can be obtained over a large dose range. In order to make use of these advantages, several calibrations, corrections and supporting measurements are needed.

This work was funded by the European Metrology Research Programme (EMRP) project HLT09 MetrExtRT Metrology for Radiotherapy using Complex Radiation Fields.
SU‐E‐T‐224: Considerations for the Proper Treatment of Multiple Cranial Metastases with Single Isocenter Volumetric Modulated Arc Therapy
Authors: Audet C , Poffenbarger B , Hwang A
Published: June 2015

Purpose:

To investigate some limitations of single isocenter VMAT for cranial multiple met cases.

Methods:

A single isocenter VMAT plan (Varian, Eclipse AAA10 commissioned down to 1 cm) was designed for two 7mm diameter spherical targets in a rectangular Solid Water (Gammex) phantom. The targets were separated by a distance of 6cm and the isocenter was centered in one of the targets. The plan was delivered (Varian, Truebeam STx) three separate times with different artificial couch angle errors of 0, 0.5 and 1 degree. The coronal dose distributions were measured with calibrated EBT3 film placed at mid‐phantom. EBT3 film dosimetry was also performed on the delivery of separate multiple arc vmat plans to targets below 6mm in diameter.

Results:

Measurements of the sup/inf dose profiles through the high dose distributions show no movement of the central axis high dose region and shifts of the high dose region intended for the off‐axis target. For the 1 degree rotation error, the high dose region was shifted 1.04mm from the target. This corresponds to the shift expected from triangulation (60mmxTan(1deg)=1.047mm). Furthermore, a streak of 10% interleaf leakage dose was observed and is likely a Result of the off axis target traveling a wide path such that a long length of MLC is exposed for the whole arc. The calculated dose was about 10% to 15% low compared to that measured on film for a 5mm diameter target.

Conclusion:

Judicious use of additional margin for off axis targets or limits on the span of multiple mets treated with one isocenter is recommended. The magnitude of the margin should be based on the rotational errors evaluated for the positioning system and the distance of the target from the isocenter. A lower limit of lesion size that can be accurately treated with VMAT should be determined.
SU‐E‐T‐230: Creating a Large Number of Focused Beams with Variable Patient Head Tilt to Improve Dose Fall‐Off for Brain Radiosurgery
Authors: Chiu J , Ma L
Published: June 2015

Purpose:

To develop a treatment delivery and planning strategy by increasing the number of beams to minimize dose to brain tissue surrounding a target, while maximizing dose coverage to the target.

Methods:

We analyzed 14 different treatment plans via Leksell PFX and 4C. For standardization, single tumor cases were chosen. Original treatment plans were compared with two optimized plans. The number of beams was increased in treatment plans by varying tilt angles of the patient head, while maintaining original isocenter and the beam positions in the x‐, y‐ and z‐axes, collimator size, and beam blocking. PFX optimized plans increased beam numbers with three pre‐set tilt angles, 70, 90, 110, and 4C optimized plans increased beam numbers with tilt angles increasing arbitrarily from range of 30 to 150 degrees. Optimized treatment plans were compared dosimetrically with original treatment plans.

Results:

Comparing total normal tissue isodose volumes between original and optimized plans, the low‐level percentage isodose volumes decreased in all plans. Despite the addition of multiple beams up to a factor of 25, beam‐on times for 1 tilt angle versus 3 or more tilt angles were comparable (<1 min.). In 64% (9/14) of the studied cases, the volume percentage decrease by >5%, with the highest value reaching 19%. The addition of more tilt angles correlates to a greater decrease in normal brain irradiated volume. Selectivity and coverage for original and optimized plans remained comparable.

Conclusion:

Adding large number of additional focused beams with variable patient head tilt shows improvement for dose fall‐off for brain radiosurgery. The study demonstrates technical feasibility of adding beams to decrease target volume
SU‐E‐T‐234: Daily Quality Assurance for a Six Degrees of Freedom Couch Using a Novel Phantom
Authors: Woods K , Woollard J , Ayan A , Sandu A , Sommerfeld J , Laurel A , Gupta N
Published: June 2015

Purpose:

To test the accuracy and reproducibility of both translational and rotational movements for a couch with six degrees of freedom (6DoF) using a novel phantom design

Methods:

An end‐to‐end test was carried out using two different phantoms. A 6 cm3 cube with a central fiducial BB (WL‐QA Sun Nuclear) and a custom fabricated rectangular prism (31 cm × 8 cm × 8 cm), placed on a baseplate with known angular offsets for pitch, roll and yaw with a central fiducial BB and unique surface structures for registration purposes, were used. The end‐to‐end test included an initial CT simulation for a reference study, setup to an offset mark on each phantom, registration of the reference CT to the acquired cone‐beam CT, and final Winston‐Lutz delivery at four cardinal gantry angles. Results for both translational and rotational movements were recorded and compared for both phantoms.

Results:

Translational and rotational measurements were performed with a PerfectPitch (Varian) couch for 10 trials for both phantoms. Distinct translational shifts were [−5.372±0.384mm, −10.183±0.137mm, 14.028±0.155mm] for the cube and [7.520±0.159mm, −9.117±0.101mm, 16.273±0.115mm] for the prototype phantom for lateral, longitudinal, and vertical shifts, respectively. Distinct rotational adjustments were [1.121±0.102o, −1.067±0.235o, −2.662±0.380o] for the cube and [2.534±0.059o, 1.994±0.025o, 2.094±0.076o] for the prototype for pitch, roll, and yaw, respectively. Winston‐Lutz test results performed after 6DoF couch correction from each cardinal gantry angle ranged from 0.26–0.72mm for the cube and 0.55–0.86mm for the prototype.

Conclusion:

The prototype phantom is more precise for both translational and rotational adjustments compared to a commercial phantom. The design of the prototype phantom allows for a more discernible visual confirmation of correct translational and rotational adjustments with the prototype phantom. Winston‐Lutz results are more accurate for the commercial phantom but are still within tolerance for the prototype phantom.
SU‐E‐T‐235: Data Mining for Evaluating Treatment Performances Over a Large Quantity of Data to Monitor and Improve SBRT Workflow
Authors: Feng W , Chu A , Wuu C , Nguyen K
Published: June 2015

Purpose:

To quality assure a large quantity of retrospective treatment cases for treatment performances by randomly sampling is inefficient. Here we provide a method to efficiently monitor and investigate the QA of SBRT workflow over Mosaiq.

Methods:

The code developed with Microsoft SQL Server Management Studio 2008R2 and VBA was used for retrieving and sorting data from Mosaiq (version 2.3–2.6 during 2012–2015). SBRT patients were filtered by fractional dose over 350cGy and total fraction number less than 6, which SBRT prescriptions were defined. The quality assurance on the SBRT workflow was focused on the treatment deliveries such as patient positioning setup, CBCT indicated offsets and couch shifted corrections. The treatment delivery were done by Varian Truebeam systems and the record/verify by Mosaiq.

Results:

Total 82 SBRT patients corresponding to 103 courses and 854 CBCT images were found by the retrieval query. Most centers record daily pre‐treatment (Pre‐Tx: before treatment shift) image‐guided shifts along treatment course for inter‐fraction motion record, and it is useful to also verify it with post‐treatment imaging (Post‐Tx: after treatment CBCT verification) to verify intra‐fraction motion. Analyzing the details of daily recorded shifts can reveals the information of patient‐setup and staff's record/verify behaviors. 3 examples were provided as solid evidences and on‐going rectification for preventing future mistakes.

Conclusions:

The report gave feasible examples for inspector to verify a large amount of data during site investigation. This program can also be extended to a scheduled data mining with software to periodical analyze the timely records in Mosaiq, for example, a various control charts for different QA purposes. As the current trend of automation in radiation therapy field, the data mining would be a necessary tool in the future, just as the automatic plan quality evaluation has been under development in Eclipse.
SU‐E‐T‐241: Design and Implement of An Information Management System for Radiation Workers in a Hospital
Authors: Wu Q , Wu M
Published: June 2015

Purpose:

To meet the special needs of Medical Administration Division, an information management system for radiation workers in a hospital(IMSRWH) has been developed.

Methods:

There are about 200 radiation workers in 20 departments, such as department of radiology, department of radiation oncology, department of nuclear medicine, and so on. An IMSRWH server was used to run a database and web service of Apache+PHP+MySQL. The IMSRWH web service could be accessed with Internet Explorer(IE) to input, search, count, and print the radiation workers’ data. Every worker's data information includes the following tables: registration, personal dose monitoring records, personnel certificate, training certificate, personnel license, medical examination, etc, and each table includes more optional detail information.

Results:

In the past year, the IMSRWH has been successfully developed and implemented in the hospital. According to the needs, the user rights are divided into: the system administrator, office manager, department manager and radiation worker. The different users have the different interfaces, and the appropriate authority permissions mean corresponding operations. The administrator and office manager could manage all personnel information, department manager could manage the staff information of his department, and the workers could view or modify their own information. Because information updates, involving hundreds of each entry and modification are complicated, each table information of all workers can be batch import from or export to a Microsoft Excel CSV format file, to update all together. However, it has only Chinese version now.

Conclusion:

It was demonstrated to be user‐friendly and was proven to significantly improve the efficiency of the hospital management. Since it is an in‐house developed system, more functions can be added or modified to further enhance its potentials in research and clinical practice.
SU‐E‐T‐248: Determining Output Factor of Irregular Electron Cutouts at Extended SSD
Authors: Alkhatib H , Gebreamlak W , Tedeschi D , Mihailidis D
Published: June 2015

Purpose:

To calculate the output factor (OPF) of any irregularly shaped electron beam at extended SSD

Methods:

Using Cerrobend block, circular cutouts with diameter ranging from 2.0cm to the maximum possible size for 15×15 applicator of Varian 2100C and 14×14 applicator of ELEKTA Synergy LINACs were prepared. In addition, two irregular cutouts were made. For each cutout, percentage depth dose (PDD) at standard SSD and point doses at different SSD were measured in water using Scanditronix diodes and PTW 0.125 cm3 ion‐chambers, respectively. In addition, the distance at which electron fluence along the central axis becomes independent of cutout size was determined using EDR2 films. On each LINAC, electron beam energies of 6, 9, 12, and 15/20 MeV were used.

Results:

Using the open applicator as the reference field, the lateral buildup ratio (LBR) for each circular cutout as a function of depth was calculated. From the LBR value and radius of the cutout, the corresponding lateral spread parameter (σR(z)) was determined. Taking the cutout side dependence of σR(z)) into account, PDD of the irregular cutouts at the standard SSD were calculated. From the point dose measurement at different SSDs, the effective SSD of each cutout was calculated. Taking the calculated PDD of the irregular cutout and effective SSD (SSDeff) of the circular cutout into account, OPFs of the irregular cutout at extended SSD values were calculated. Finally, the calculated output factors were compared with the measured values.

Conclusion:

In this research, the improved LBR method has been generalized to calculate output factor of irregular cutouts at extended SSD. In a clinically useful region, the percentage difference between the measured and the calculated output factors of the irregular cutouts was within 2%. Similar results were obtained for all available electron energies of both Varian 2100C and ELEKTA Synergy machines.
SU‐E‐T‐250: Determining VMAT Machine Limitations of An Elekta Linear Accelerator with Agility MLC for Accurate Modeling in RayStation and Robust Delivery
Authors: Yang K , Yu Z , Chen H , Mourtada F
Published: June 2015

Purpose:

To implement VMAT in RayStation with the Elekta Synergy linac with the new Agility MLC, and to utilize the same vendor softwares to determine the optimum Elekta VMAT machine parameters in RayStation for accurate modeling and robust delivery.

Methods:

iCOMCat is utilized to create various beam patterns with user defined dose rate, gantry, MLC and jaw speed for each control point. The accuracy and stability of the output and beam profile are qualified for each isolated functional component of VMAT delivery using ion chamber and Profiler2 with isocentric mounting fixture. Service graphing on linac console is used to verify the mechanical motion accuracy. The determined optimum Elekta VMAT machine parameters were configured in RayStation v4.5.1. To evaluate the system overall performance, TG‐119 test cases and nine retrospective VMAT patients were planned on RayStation, and validated using both ArcCHECK (with plug and ion chamber) and MapCHECK2.

Results:

Machine output and profile varies <0.3% when only variable is dose rate (35MU/min‐600MU/min). <0.9% output and <0.3% profile variation are observed with additional gantry motion (0.53deg/s–5.8deg/s both directions). The output and profile variation are still <1% with additional slow leaf motion (<1.5cm/s both direction). However, the profile becomes less symmetric, and >1.5% output and 7% profile deviation is seen with >2.5cm/s leaf motion. All clinical cases achieved comparable plan quality as treated IMRT plans. The gamma passing rate is 99.5±0.5% on ArcCheck (<3% iso center dose deviation) and 99.1±0.8% on MapCheck2 using 3%/3mm gamma (10% lower threshold). Mechanical motion accuracy in all VMAT deliveries is <1°/1mm.

Conclusion:

Accurate RayStation modeling and robust VMAT delivery is achievable on Elekta Agility for <2.5cm/s leaf motion and full range of dose rate and gantry speed determined by the same vendor softwares. Our TG‐119 and patient results have provided us with the confidence to use VMAT clinically.
SU‐E‐T‐251: Developing a Daily Proton Beam Monitoring System
Authors: Yeo I , Ghebremedhin A , Patyal B
Published: June 2015

Purpose:

To develop a daily monitoring system for proton beam output check and beam uniformity check.

Methods:

Designed for continuously irradiated photon and electron beams with a field size of 20 cm × 20 cm, the daily output checker (Sun Nuclear, Inc.) is not suitable for monitoring proton beams with inter‐pulse beam‐off and a field size smaller than 14–16 cm in diameter. To allow such proton beam monitoring, the following tests were performed. 1. Absolute dose and array calibrations which accept continuous irradiation only, were performed using photon beams. 2. Five ion chambers within the central area of 8 cm × 8 cm were utilized to check constancy of output at the center of beam modulation and at distal edge and to check beam symmetry and flatness. 3. To simplify our evaluation, the array calibration was manually modified, such that all five chambers report equal values in spite of their differences in build‐up thicknesses. 4. The chamber at the lower‐right corner is placed under a buildup thickness that can offer dose measurement at the distal edge. This buildup thickness was determined by proton beam range measurements, which established buildup thickness for beam output measurement at the central chamber and range measurement at the corner chamber. 5. The beam‐off delay which allows receipt of pulsed irradiation was activated and optimal delay times were determined for each proton beam at 149.6, 185.6, and 249.5 MeV.

Results:

The above system was tested by miss‐steering proton beams and altering phantom thickness by 1 mm at a time. The system reliably monitored the beam with: 3% tolerance for beam flatness, symmetry and output. The range difference of 0.5 mm could be detected at all energies by setting a tolerance of 20%.

Conclusion:

A quick daily proton beam monitoring system was feasible.
SU‐E‐T‐252: Developing a Pencil Beam Dose Calculation Algorithm for CyberKnife System
Authors: Liang B , Liu B , Zhou F , Xu S , Wu Q
Published: June 2015

Purpose:

Currently there are two dose calculation algorithms available in the Cyberknife planning system: ray‐tracing and Monte Carlo, which is either not accurate or time‐consuming for irregular field shaped by the MLC that was recently introduced. The purpose of this study is to develop a fast and accurate pencil beam dose calculation algorithm which can handle irregular field.

Methods:

A pencil beam dose calculation algorithm widely used in Linac system is modified. The algorithm models both primary (short range) and scatter (long range) components with a single input parameter: TPR20/10. The TPR20/20/10 value was first estimated to derive an initial set of pencil beam model parameters (PBMP). The agreement between predicted and measured TPRs for all cones were evaluated using the root mean square of the difference (RMSTPR), which was then minimized by adjusting PBMPs. PBMPs are further tuned to minimize OCR RMS (RMSocr) by focusing at the outfield region. Finally, an arbitrary intensity profile is optimized by minimizing RMSocr difference at infield region. To test model validity, the PBMPs were obtained by fitting to only a subset of cones (4) and applied to all cones (12) for evaluation.

Results:

With RMS values normalized to the dmax and all cones combined, the average RMSTPR at build‐up and descending region is 2.3% and 0.4%, respectively. The RMSocr at infield, penumbra and outfield region is 1.5%, 7.8% and 0.6%, respectively. Average DTA in penumbra region is 0.5mm. There is no trend found in TPR or OCR agreement among cones or depths.

Conclusion:

We have developed a pencil beam algorithm for Cyberknife system. The prediction agrees well with commissioning data. Only a subset of measurements is needed to derive the model. Further improvements are needed for TPR buildup region and OCR penumbra. Experimental validations on MLC shaped irregular field needs to be performed.

This work was partially supported by the National Natural Science Foundation of China (61171005) and the China Scholarship Council (CSC).
SU‐E‐T‐253: Development of a GDPM Monte Carlo Based Quality Assurance Tool for Cyberknife
Authors: Webster M , Ouyang L , Folkerts M , Tian Z , Jia X , Jiang S , Gu X
Published: June 2015

Purpose:

To develop a 3D quality assurance (QA) tool for Cyberknife plans using a GPU‐based Monte Carlo dose calculation package, gDPM. The developed QA tool will be used as a secondary check for Cyberknife treatment planning software (TPS) reported dose distributions.

Methods:

Phase space files of the 5, 7.5, 10, 15, and 60 mm iris cones were downloaded from IAEA phase‐space database. From the 60mm iris cone phase space file, we were able to generate raw phase space files for all other cones using a particle rejection method. The particles in the raw phase space files were binned with respect to the radial position and energy for commissioning. During the commissioning process, gDPM calculated percent depth dose and off‐center ratios which were compared to those measured in the Blue Phantom. The off‐axis data was measured with an SAD setup at depths of 15, 50, 100, 200, and 300 mm. At the commissioning stage, an optimization problem was solved to adjust the binned particles weights to minimize the difference between the calculated and measured ones. Commissioning validations will be performed by measuring point dose data of patient specific plans delivered to water phantoms. With commissioned phase space files, 3D patient‐specific dose distributions will be calculated and compared against TPS reported dose.

Results:

Beyond the initial buildup region, the root mean square difference between the calculated and measured percent depth dose differences was less than 0.5%. The full‐width half‐max data from the off‐axis ratio calculations was found to be within 0.1 mm of the measured data.

Conclusion:

Our current work showed excellent agreement between the gDPM calculated dose and the measured data for all cone sizes and types for the Cyberknife system. Implementation of a fast and accurate QA tool for patient specific plans will be feasible with this tool.
SU‐E‐T‐318: Dosimetric Evaluation of ArcCHECK and 3DVH System Using Customized Polymer Gel Dosimeter
Authors: Ono K , Fujimoto S , Hayashi S , Miyazawa M , Akagi Y , Hirokawa Y
Published: June 2015

Purpose:

ArcCHECK and 3DVH system (Sun Nuclear) can reconstruct the three‐dimensional (3D) dose distribution and provide the DVH analysis in a patient. The aim of this study was to evaluate dosimetric accuracy of this system using customized polymer gel dosimeter, and also Gafchromic EBT3 films.

Methods:

Polyacrylamide‐based gel contained magnesium chloride as a sensitizer (iPAGAT) was used in this study. Volumetric‐modulated arc therapy (VMAT) plan was performed for the C‐shape structure by the Eclipse treatment planning system (Varian) and used to irradiate the ArcCHECK by the Novalis Tx linear accelerator (Varian/BrainLAB). The cubic phantom filled with iPAGAT and EBT3 films placed in three orthogonal planes (axial, sagittal, and coronal) inserted into the I'mRT Phantom (IBA Dosimetry) simulated a patient were irradiated with the same VMAT plan. The measurement‐guided 3D dose distribution was reconstructed using 3DVH software from the measured data of the ArcCHECK. The 3D dose distribution in iPAGAT was read out by Signa 1.5 T MRI system (GE), and 2D dose distribution on EBT3 was read out by color scanner (Epson). The comparison of all the dose distributions was performed with dose profiles and gamma index analysis in orthogonal planes using in‐house developed software.

Results:

A good agreement was observed by overlaying the dose profiles of 3DVH, EBT3, and iPAGAT. The mean pass rates by gamma index analysis with 3%/3 mm criteria in orthogonal planes were 94.3% (3DVH vs EBT3), 91.1% (3DVH vs iPAGAT), and 96.4% (iPAGAT vs EBT3), respectively.

Conclusion:

3D dose distribution reconstructed by ArcCHECK and 3DVH system was estimated accurately in a patient. However, slightly differences were observed between 3DVH and iPAGAT because of MRI noise, therefore further study is required to improve the accuracy of MRI based polymer gel dosimetry for the DVH analysis.
SU‐E‐T‐326: Dosimetric Impact of Beam Energies and Jaw Tracking On Intracranial Tumors Using RapidArc
Authors: Hossain S , Keeling V , Ali I , Ahmad S , Algan O
Published: June 2015

Purpose:

To determine the dosimetric impact of jaw tracking and beam energies on dose conformity and normal‐brain‐tissue doses for intracranial tumors using VMAT (RapidArc).

Methods:

Seven patients with 1–2 and three patients with 4–6 intracranial tumors were planned using RapidArc for Varian TrueBeam STx machine with beam energies 6MV‐FFF (Flattening‐Filter‐Free), 8MV, 10MV, and 10MV‐FFF. The prescription dose ranged from 14–23Gy. Between 2 and 8 arcs were used with the following geometries: 2 full coplanar arcs and the non‐coplanar half arcs. Plans were optimized (jaw tracking ON) with a high priority to Normal‐Tissue‐Objective and normal‐brain‐tissue. Plans were calculated on 1mm grid size using AAA algorithm and then normalized so that 99% of each target volume received the prescription dose. Plans for the 6MV‐FFF were also optimized without jaw tracking (No‐JT) for comparison. Plan quality was assessed by target coverage using Paddick Conformity Index (PCI), sparing of normal‐brain‐tissue through analysis of V4Gy, V8Gy and V12Gy, and integral dose.

Results:

The average PCI ± standard deviation was 0.76±0.21 and 0.76±0.22 for 6MV‐FFF and 10 MV‐FFF, respectively. The average ratio in normal brain tissue volume (reported as follows V4,V8,V12) were (1.12±0.07,1.12±0.07,1.14±0.04), (1.12±0.08,1.12±0.09,1.13±0.06), (1.19±0.10,1.18±0.10,1.20±0.04), and (1.04±0.03,1.03±0.03,1.03±0.04) for 8MV/6MV‐FFF, 10MV‐FFF/6MV‐FFF, 10MV/6MV‐FFF, 6MV‐FFF No‐JT/6MV‐FFF, respectively. Statistically significant differences in normal‐brain‐tissue for V4, V8, and V12 were observed in all cases for the different energies (p‐values <0.05). V4 data shows significant differences in JT vs. No‐JT (p=0.04), however no difference was found for V8 and V12. Brain tissue sparing from best to worst occurred in this order 6MV‐FFF, 6MV‐FFF no‐JT, 10MV‐FFF, 8MV, and 10MV. The average ratio of integral brain dose was 1.05±0.04 (p=0.21), 1.04±0.05 (p=0.33), 1.09±0.06 (p=0.04), and 1.02±0.06 (p=0.61) for 8MV/6MV‐FFF, 10MV‐FFF/6MV‐FFF, 10MV/6MV‐FFF, and 6MV‐FFF No‐JT/6MV‐FFF, respectively.

Conclusion:

Normal brain tissue and integral dose improved using the lower energy and FFF beams, though plan conformity showed energy independence.
SU‐E‐T‐327: Dosimetric Impact of Beam Energy for Intrabeam Breast IORT with Different Residual Cancer Cell Distributions After Surgery
Authors: Schwid M , Zhang H
Published: June 2015

Purpose:

The purpose of this study was to evaluate the dosimetric impact of beam energy to the IORT treatment of residual cancer cells with different cancer cell distributions after breast‐conserving surgery.

Methods:

The three dimensional (3D) radiation doses of IORT using a 4‐cm spherical applicator at the energy of 40 keV and 50 keV were separately calculated at different depths of the postsurgical tumor bed. The modified linear quadratic model (MLQ) was used to estimate the radiobiological response of the tumor cells assuming different radio‐sensitivities and density distributions. The impact of radiation was evaluated for two types of breast cancer cell lines (α /β=10, and α /β =3.8) at 20 Gy dose prescribed at the applicator surface. Cancer cell distributions in the postsurgical tissue field were assumed to be a Gaussian with the standard deviations of 0.5, 1 and 2 mm respectively, namely the cancer cell infiltrations of 1.5, 3, and 6 mm respectively. The surface cancer cell percentage was assumed to be 0.01%, 0.1%, 1% and 10% separately. The equivalent uniform doses (EUD) for all the scenarios were calculated.

Results:

The EUDs were found to be dependent on the distributions of cancer cells, but independent of the cancer cell radio‐sensitivities and the density at the surface. EUDs of 50 keV are 1% larger than that of 40 keV. For a prescription dose of 20 Gy, EUDs of 50 keV beam are 17.52, 16.21 and 13.14 Gy respectively for 0.5, 1.0 and 2.0 mm of the standard deviation of cancer cell Gaussian distributions.

Conclusion:

The impact by selected energies of IORT beams is very minimal. When energy is changed from 50 keV to 40 keV, the EUDs are almost the same for the same cancer cell distribution. 40 keV can be safely used as an alternative of 50 keV beam in IORT.
SU‐E‐T‐328: Dosimetric Impact of Cobalt‐Chrome Stabilization Hardware in Paraspinal Radiation Therapy
Authors: Tang G , LoSasso T , Saleh Z , Mechalakos J , Lim S , Laufer I , Bilsky M , Yamada J , Lovelock D
Published: June 2015

Purpose:

Due to saturation, high density materials Result in an apparent density of 3.2 g/cm3 in CT images. The true density of traditional titanium stabilization rods (∼4.4 g/cm3) is typically ignored in treatment planning. This may not be acceptable for new cobalt‐chrome rods with a density of 8.5 g/cm3. This study reports the dosimetric impact of cobalt‐chrome rods in paraspinal radiotherapy.

Methods:

For titanium and cobalt‐chrome rods, two planning studies were done for both IMRT and VMAT in Varian Eclipse using AAA. 1) The effect of planning without assigning the true rod density was assessed by comparing plans generated with the apparent density and recalculated with the true density for titanium and cobalt‐chrome. 2) To test if TPS can compensate for high density rods during optimization. Furthermore, TPS calculation accuracy was verified using MapCheck for a single 20 × 10 cm2 field. The MapCheck was incrementally shifted to achieve measurement resolution of 1 mm.

Results:

PTV coverage was ∼0.3% and ∼4.7% lower in plans that were recalculated with the true rod density of titanium and cobalt‐chrome, respectively. PTV coverage can be maintained if the correct density is used in optimization. Measurements showed that TPS overestimated the dose locally by up to 11% for cobalt‐chrome rods and up to 4% for titanium rods if the density is incorrect. With density corrected, maximum local differences of 6% and 3% were seen for cobalt‐chrome and titanium rods, respectively. At 2 cm beneath a rod, electrons scattered from the side of the rod increased the lateral dose and diminished as depth increases. TPS was not able to account for this effect properly even with the true rod density assigned.

Conclusion:

Neglecting the true density of cobalt‐chrome rods can cause under coverage to the PTV. Assigning the correct density during treatment planning can minimize unexpected decrease in PTV dose.
SU‐E‐T‐329: Dosimetric Impact of Implementing Metal Artifact Reduction Methods and Metal Energy Deposition Kernels for Photon Dose Calculations
Authors: Huang J , Followill D , Howell R , Liu X , Mirkovic D , Stingo F , Kry S
Published: June 2015

Purpose:

To investigate two strategies for reducing dose calculation errors near metal implants: use of CT metal artifact reduction methods and implementation of metal‐based energy deposition kernels in the convolution/superposition (C/S) method.

Methods:

Radiochromic film was used to measure the dose upstream and downstream of titanium and Cerrobend implants. To assess the dosimetric impact of metal artifact reduction methods, dose calculations were performed using baseline, uncorrected images and metal artifact reduction

Methods:

Philips O‐MAR, GE's monochromatic gemstone spectral imaging (GSI) using dual‐energy CT, and GSI imaging with metal artifact reduction software applied (MARs).To assess the impact of metal kernels, titanium and silver kernels were implemented into a commercial collapsed cone C/S algorithm.

Results:

The CT artifact reduction methods were more successful for titanium than Cerrobend. Interestingly, for beams traversing the metal implant, we found that errors in the dimensions of the metal in the CT images were more important for dose calculation accuracy than reduction of imaging artifacts. The MARs algorithm caused a distortion in the shape of the titanium implant that substantially worsened the calculation accuracy. In comparison to water kernel dose calculations, metal kernels resulted in better modeling of the increased backscatter dose at the upstream interface but decreased accuracy directly downstream of the metal. We also found that the success of metal kernels was dependent on dose grid size, with smaller calculation voxels giving better accuracy.

Conclusion:

Our study yielded mixed results, with neither the metal artifact reduction methods nor the metal kernels being globally effective at improving dose calculation accuracy. However, some successes were observed. The MARs algorithm decreased errors downstream of Cerrobend by a factor of two, and metal kernels resulted in more accurate backscatter dose upstream of metals. Thus, these two strategies do have the potential to improve accuracy for patients with metal implants in certain scenarios.

This work was supported by Public Health Service grants CA 180803 and CA 10953 awarded by the National Cancer Institute, United States of Health and Human Services, and in part by Mobius Medical Systems.
SU‐E‐T‐330: Dosimetric Impact of Intrafraction Respiratory Motion On Lung SBRT Treatment Using Cyberknife 0‐View Tracking Mode
Authors: Rao M , Chen F , Cotrutz C , Ye J
Published: June 2015

Purpose:

To investigate the influence of respiratory motion on the delivered dose in lung stereotactic body radiotherapy (SBRT) using Cyberknife (CK) 0‐View tracking mode.

Methods:

CT scans at inspiration and expiration of an anthropomorphic motion phantom were fused base on the spine and an internal target volume (ITV) was created. A 5mm expansion around the ITV resulted in the planning target volume. Three CK plans were generated in Accuray MultiPlan using Lung Optimization Tracking 0‐View technique with the minimum MU per beam set to (a) 5MU, (b) 15MU and (c) 30MU, respectively. Doses were calculated on the expiration CT using Monte‐Carlo algorithm. Each plan was delivered 5 times with a range of different starting phases in the respiratory cycle to assess the dose variation due to interplay effect. The delivered dose was measured with EBT3 Gafchromic film which was inserted in the moving target of the phantom. The target motion range is 3 cm in superior‐inferior (SI) direction with the breathing period of 5 seconds.

Results:

The gamma analysis (5%/2mm) of the dose with the films in the transverse plane resulted in average passing rate of 95.5±4.1%, 96.7±2.6%, and 96.2±2.5% for plan (a), (b), and (c), respectively. For the sagittal films, the average passing rate was 91.1±4.9%, 92.1±3.6%, and 92.3±2.9% for the three plans, respectively. The disagreement between measurement and dose calculations were mostly on the target edges in SI direction. The mean measured versus calculated dose differences at the edge of target in SI direction were (a) 3.9±4.8%, (b) 2.4±3.3%, and (c) 2.2±3.2% for the three plans, respectively.

Conclusions:

The plans with low‐MU beams (below 10MU) tend to cause slightly larger dose variation. However in terms of target coverage, the overall clinical dosimetric impact of the intrafraction respiratory motion in lung SBRT is insignificant when averaged over 3∼5 fractions.
SU‐E‐T‐342: Dynamic Accuracy and Stability of Novel Multi‐Leaf Collimator (MLC) for Volumetric Modulated Arc Therapy (VMAT)
Authors: Nakaguchi Y
Published: June 2015

Purpose:

We have investigated the dynamic accuracy and long‐term stability of an Agility (Elekta, Stockholm, Sweden) multi‐leaf collimator (MLC) for volumetric modulated arc therapy (VMAT).

Methods:

The MLC leaf position accuracy during VMAT delivery was evaluated with the use of three different quality assurance tests for several months: (1) a leaf gap‐width test between opposing leaves by measurement of the isocenter dose during constant‐gap sliding‐window delivery with varied dose rates, MLC leaf speeds, and gantry angles; (2) a leaf position test by picket‐fence delivery with and without gantry rotation; and (3) the complicated VMAT test plans were evaluated by measurement using gamma analysis with criteria of 3 mm distance‐to‐agreement (DTA) and 3% percent dose difference (DD).

Results:

The leaf gap‐width deviation once a week for five months was within 0.15 mm under all the test fractions. There were not clear differences of the accuracy depending on the dose rates, the leaf speeds, and gantry angles. Agreement in the static vs. rotation picket fence test was better than 0.5 mm. This difference is near to the limit of the measurement and is not the difference between static and rotation. The pass rates of each complicated VAMT test plans once a day for a month were more than 90% in the gamma analysis. These pass rates are accuracy enough for VMAT because the test plans included strict test plans.

Conclusion:

MLC leaf position accuracy and long‐term stability were confirmed by the proposed procedures. The Agility MLC showed enough MLC leaf position accuracy and long‐term stability for VMAT.
SU‐E‐T‐346: Effect of Jaw Position On Dose to Critical Structures in 3‐D Conformal Radiotherapy Treatment of Pancreatic Cancer
Authors: Paudel N , Han E , Liang X , Morrill S , Zhang X , Hardee M , Penagaricano J , Ratanatharathorn V
Published: June 2015

Purpose:

Three‐dimensional conformal therapy remains a valid and widely used modality for pancreatic radiotherapy treatment. It usually meets dose constraints on critical structures. However, careful positioning of collimation jaws can reduce dose to the critical structures. Here we investigate the dosimetric effect of jaw position in MLC‐based 3‐D conformal treatment planning on critical structures.

Methods:

We retrospectively selected seven pancreatic cancer patients treated with 3‐D conformal radiotherapy. We started with treatment plans (Varian Truebeam LINAC, Eclipse TPS, AAA, 18MV) having both × and y jaws aligned with the farthest extent of the block outline (8mm around PTV). Then we subsequently moved either both x‐jaws or all × and y jaws outwards upto 3 cm in 1 cm increments and investigated their effect on average and maximum dose to neighboring critical structures keeping the same coverage to treatment volume.

Results:

Lateral displacement of both x‐jaws by 1cm each increased kidney and spleen mean dose by as much as 1.7% and 1.3% respectively and superior inferior displacement increased liver, right kidney, stomach and spleen dose by as much as 2.1%, 2%, 5.2% and 1.6% respectively. Displacement of all × and y‐jaws away by 1cm increased the mean dose to liver, right kidney, left kidney, bowels, cord, stomach and spleen by as much as 4.9%, 5.9%, 2.1%, 2.8%, 7.4%, 10.4% and 4.2% respectively. Percentage increase in mean dose due to 2 and 3cm jaw displacement increased almost linearly with the displaced distance. Changes in maximum dose were much smaller (mostly negligible) than the changes in mean dose.

Conclusion:

Collimation jaw position affects dose mostly to critical structures adjacent to it. Though treatment plans with MLCs conforming the block margin usually meet dose constraints to critical structures, keeping jaws all the way in, to the edge of the block reduces dose to the critical structures during radiation treatment.
SU‐E‐T‐357: Electronic Compensation Technique to Deliver Total Body Dose
Authors: Lakeman T , Wang I , Podgorsak M
Published: June 2015

Purpose:

Total body irradiation (TBI) uses large parallel‐opposed radiation fields to suppress the patient's immune system and eradicate the residual cancer cells in preparation of recipient for bone marrow transplant. The manual placement of lead compensators has conventionally been used to compensate for the varying thickness through the entire body in large‐field TBI. The goal of this study is to pursue utilizing the modern electronic compensation technique to more accurately and efficiently deliver dose to patients in need of TBI.

Methods:

Treatment plans utilizing electronic compensation to deliver a total body dose were created retrospectively for patients for whom CT data had been previously acquired. Each treatment plan includes two, specifically weighted, pair of opposed fields. One pair of open, large fields (collimator=45°), to encompass the patient's entire anatomy, and one pair of smaller fields (collimator=0°) focused only on the thicker midsection of the patient. The optimal fluence for each one of the smaller fields was calculated at a patient specific penetration depth. Irregular surface compensators provide a more uniform dose distribution within the smaller opposed fields.

Results:

Dose‐volume histograms (DVH) were calculated for the evaluating the electronic compensation technique. In one case, the maximum body doses calculated from the DVH were reduced from the non‐compensated 195.8% to 165.3% in the electronically compensated plans, indicating a more uniform dose with the region of electronic compensation. The mean body doses calculated from the DVH were also reduced from the non‐compensated 120.6% to 112.7% in the electronically compensated plans, indicating a more accurate delivery of the prescription dose. All calculated monitor units were well within clinically acceptable limits.

Conclusion:

Electronic compensation technique for TBI will not substantially increase the beam on time while it can significantly reduce the compensator setup time and the potential risk of errors in manually placing lead compensators.