bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐339: Dosimetric Verification of Acuros XB Dose Calculation Algorithm On An Air Cavity for 6‐MV Flattening Filter‐Free Beam
Authors: Kang S , Chung J , Suh T
Published: June 2015

Purpose:

This study was to verify the accuracy of Acuros XB (AXB) dose calculation algorithm on an air cavity for a single radiation field using 6‐MV flattening filter‐free (FFF) beam.

Methods:

A rectangular slab phantom containing an air cavity was made for this study. The CT images of the phantom for dose calculation were scanned with and without film at measurement depths (4.5, 5.5, 6.5 and 7.5 cm). The central axis doses (CADs) and the off‐axis doses (OADs) were measured by film and calculated with Analytical Anisotropic Algorithm (AAA) and AXB for field sizes ranging from 2 Χ 2 to 5 Χ 5 cm2 of 6‐MV FFF beams. Both algorithms were divided into AXB_w and AAA _w when included the film in phantom for dose calculation, and AXB_w/o and AAA_w/o in calculation without film. The calculated OADs for both algorithms were compared with the measured OADs and difference values were determined using root means squares error (RMSE) and gamma evaluation.

Results:

The percentage differences (%Diffs) between the measured and calculated CAD for AXB_w was most agreement than others. Compared to the %Diff with and without film, the %Diffs with film were decreased than without within both algorithms. The %Diffs for both algorithms were reduced with increasing field size and increased relative to the depth increment. RMSEs of CAD for AXB_w were within 10.32% for both inner‐profile and penumbra, while the corresponding values of AAA_w appeared to 96.50%.

Conclusion:

This study demonstrated that the dose calculation with AXB within air cavity shows more accurate than with AAA compared to the measured dose. Furthermore, we found that the AXB_w was superior to AXB_w/o in this region when compared against the measurements.
SU‐E‐T‐349: Effective Dose‐Volume‐Histogram Prediction Method Using Euclidean Distance Volume Histogram for Volumetric Modulated Arc Therapy to Treat Prostate Cancer
Authors: Shin J. , Liang J. , Schild S. , Wong W. , Liu W.
Published: June 2015

Purpose:

Manually checking the plan dose‐volume histograms (DVH) quality is time consuming, thereby limiting the patient throughput and leading to inconsistent plan quality across institutions. We propose to use Euclidean distance volume histogram (EDVH) to automate this process in volumetric‐modulated arch therapy (VMAT) to treat prostate cancer patients by predicting DVH of a new patient based on prior expert patients.

Methods:

52 prostate cancer patients treated by VMAT at our institution were randomly selected. 3D distance transformation was performed on the simulation CTs to get Euclidean distance for each voxel inside every organ‐at‐risk with respect to the tumor. EDVH was then constructed, analogous to computing a DVH. In order to evaluate our proposed method, we utilize the leave‐one‐patient‐out cross validation. For each test patient, we select 5 patients with the closest EDVHs from the remaining 51 patients and determine whether the DVH of this patient is within the band bounded by these 5 patients’ DVHs with a ratio defined as AREA5/AREA6, where AREA5 is the area‐between‐curves (ABC) bounded by the DVHs of the 5 selected patients and AREA6 is the ABC bounded by the DVHs of the 5 selected patients plus the test patient. The higher the ratio is, the better prediction performance is.

Results:

From the process of leave‐one‐patient‐out cross validation, we obtained a prediction ratio (0.94±0.11) for each patient. 48 out of 52 patients (92.3%) for bladder and 46 out of 52 patients (88.5%) for rectum had prediction ratios above 0.80. 41 out of 52 patients (78.9%) for bladder and 38 out of 52 patients (73.1%) for rectum had prediction ratios above 0.90.

Conclusion:

EDVH can effectively capture the overall geometric relationship between the tumor and organ‐at‐risks, which can be used to confidently predict the DVH based on prior expert patients for VMAT to treat prostate cancer.

NIH/NCI K25CA168984, Eagles Cancer Research Career Development, The Lawrence W. and Marilyn W. Matteson Fund for Cancer Research, Mayo ASU Seed Grant, and The Kemper Marley Foundation.
SU‐E‐T‐354: Efficient and Enhanced QA Testing of Linear Accelerators Using a Real‐Time Beam Monitor
Authors: Jung J , Farrokhkish M , Norrlinger B , Wang Y , Heaton R , Jaffray D , Islam M
Published: June 2015

Purpose:

To investigate the feasibility of performing routine QA tests of linear accelerators (Linac) using the Integral Quality Monitoring (IQM) system. The system, consisting of a 1‐D sensitivity gradient large area ion‐chamber mounted at the collimator, allows automatic collection and analysis of beam data.

Methods:

The IQM was investigated to perform several QA constancy tests, similar to those recommended by AAPM TG142, of a Linac including: beam output, MLC calibration, beam symmetry, relative dose factor (RDF), dose linearity, output as a function of gantry angle and dose rate. All measurements by the IQM system accompanied a reference measurement using a conventional dosimetry system and were performed on an Elekta Infinity Linac with Agility MLC. The MLC calibration check is done using a Picket‐Fence type 2×10cm2 field positioned at different off‐axis locations along the chamber gradient. Beam symmetry constancy values are established by signals from an 4×4cm2 aperture located at various off‐axis positions; the sensitivity of the test was determined by the changes in the signals in response to a tilt in the beam. The data for various square field sizes were used to develop a functional relationship with RDF.

Results:

The IQM tracked the beam output well within 1% of the reference ion‐chamber readings. The Picket‐Fence type field test detected a 1mm shift error of one MLC bank. The system was able to detect 2.5% or greater beam asymmetry. The IQM results for all other QA tests were found to agree with the reference values to within 0.5%.

Conclusion:

It was demonstrated that the IQM system can effectively monitor the Linac performance parameters for the purpose of routine QA constancy tests. With minimum user interactions a comprehensive set of tests can be performed efficiently, allowing frequent monitoring of the Linac.

The presenting author's salary is funded by the manufacturer of the QA device. All the other authors have financial interests with the commercialization of this QA device.
SU‐E‐T‐355: Efficient Scatter Correction for Direct Ray‐Tracing Based Dose Calculation
Authors: Chen M , Jiang S , Lu W
Published: June 2015

Purpose:

To propose a scatter correction method with linear computational complexity for direct‐ray‐tracing (DRT) based dose calculation. Due to its speed and simplicity, DRT is widely used as a dose engine in the treatment planning system (TPS) and monitor unit (MU) verification software, where heterogeneity correction is applied by radiological distance scaling. However, such correction only accounts for attenuation but not scatter difference, causing the DRT algorithm less accurate than the model‐based algorithms for small field size in heterogeneous media.

Methods:

Inspired by the convolution formula derived from an exponential kernel as is typically done in the collapsed‐cone‐convolution‐superposition (CCCS) method, we redesigned the ray tracing component as the sum of TERMA scaled by a local deposition factor, which is linear with respect to density, and dose of the previous voxel scaled by a remote deposition factor, D(i)=aρ(i)T(i)+(b+c(ρ(i)‐1))D(i‐1),where T(i)=e^(‐αr(i)+β(r(i))^2) and r(i)=Σ_(j=1,..,i)ρ(j).The two factors together with TERMA can be expressed in terms of 5 parameters, which are subsequently optimized by curve fitting using digital phantoms for each field size and each beam energy.

Results:

The proposed algorithm was implemented for the Fluence‐Convolution‐Broad‐Beam (FCBB) dose engine and evaluated using digital slab phantoms and clinical CT data. Compared with the gold standard calculation, dose deviations were improved from 20% to 2% in the low density regions of the slab phantoms for the 1‐cm field size, and within 2% for over 95% of the volume with the largest discrepancy at the interface for the clinical lung case.

Conclusion:

We developed a simple recursive formula for scatter correction for the DRT‐based dose calculation with much improved accuracy, especially for small field size, while still keeping calculation to linear complexity. The proposed calculator is fast, yet accurate, which is crucial for dose updating in IMRT optimization.
SU‐E‐T‐363: Error Detection Comparison of EPID and MLC Log File Based IMRT QA Systems
Authors: Defoor D , Obeidat M , Linden P , Kirby N , Mavroidis P , Papanikolaou N , Stathakis S
Published: June 2015

Purpose:

In this study we will compare the ability of three QA methods (Delta4, MU‐EPID, Dynalog QA) to detect specific errors.

Methods:

A Varian Novalis Tx with a HD120 MLC and aS1000 Electronic Portal Imaging Device (EPID) was used in our study. Multi‐leaf collimator (MLC) errors, gantry angle and dose errors were introduced into 5 volumetric arc therapy (VMAT) plans. 3D dose distributions calculated with data from the EPID and Dynalog QA methods were compared with the planned dose distribution. The gamma passing percentages as well as percentage error of planning target volume (PTV) dose were used for passing determination. Baselines for gamma passing percentages and PTV dose were established by measuring the original plan 5 times consecutively. Standard passing thresholds as well as thresholds derived from receiver operator characteristic (ROC) analysis and 2 standard deviation (SD) criteria were used.

Results:

When applying the standard 95% pass rate at 3%/3mm gamma analysis 14, 21 and 8 of 30 errors were detected by the Delta4, MU‐EPID and Dynalog QA methods respectively. Thresholds set at 2 SD from our base line measurements resulted in the detection of 18, 9 and 14 of 30 errors for the Delta4, MU‐EPID and Dynalog QA methods respectively. When using D2 of the PTV as a metric the Dynalog QA detected 20 of 30 errors while the EPID method detected 14 of 30 errors. Using D98 of the PTV, Dynalog QA detected 13 of 30 while the EPID detected 3 of 30 errors.

Conclusion:

Although MU‐EPID detected the most errors at the standard 95% cutoff it also produced the most false detections in the baseline data. The Dynalog QA was the most effective when the ROC adjusted passing threshold was used. D2 was more effective as a metric for detecting errors than D98.
SU‐E‐T‐369: Evaluating Intensity Modulated Proton Therapy Relative to Passive Scattering Proton Therapy for Increased Vertebral Column Sparing in CSI of Pediatric Patients
Authors: Seco J , Giantsoudi D , Eaton BR , Adams JA , Paganetti H , MacDonald S
Published: June 2015

Purpose:

To investigate the trade‐off between vertebral column sparing and thecal‐sac target coverage in craniospinal irradiation (CSI) of pediatric patients treated with passive‐scattering (PS) and intensity modulated (IMPT) proton therapy.

Methods:

We selected 2 pediatric patients treated with PS CSI for medulloblastoma. Spinal irradiation was re‐planned with IMPT. For all cases, we assumed prescription dose of 23.4 Gy(RBE), with the spinal canal receiving at least 95% of 23.4 Gy(RBE). PS planning was performed using the commercial system XiO. IMPT planning was done using the Astroid planning system. Beam arrangements consisted of (a) PS posterior‐anterior (PA) field, PS‐PA, (b) IMPT PA field, IMPT‐PA, and (c) two posterior oblique IMPT fields, IMPT2 (‐35°, 35°). Dose distributions were re‐calculated using TOPAS Monte Carlo, along with LET distributions, to investigate LET variations within the target and vertebra anatomy. Variable RBE‐weighed dose distributions were also calculated based on a dose and LET‐dependent biophysical model. Dosimetric data were compared among the plans for the target volume, spinal cord and adjacent critical organs (thecal‐sac and cauda equina).

Results:

IMPT2 resulted in better sparing of the posterior vertebral column (entrance region posterior to thecal‐sac), where planned dose was approximately 6–8Gy(RBE). For IMPT‐PA and PS‐PA the MC‐calculated dose to the posterior vertebral column was, on average, 20Gy and 18Gy respectively. For IMPT2 higher mean‐LET (5keV/µm/(g/cm3)) values were observed in anterior vertebral column (beyond the thecal‐sac) relative to IMPT‐PA and PS‐PA, where mean‐LET was 3.5keV/µm/(g/cm3) and 2.5keV/µm/(g/cm3) respectively. The higher LET region observed for both IMPT plans was in the distal end of treatment fields, where dose delivered was less 5Gy(RBE).

Conclusion:

The two‐oblique proton beams IMPT2 best spared the spinal column, while reducing the dose to the posterior spinal column from 18–20 to 6–8 Gy(RBE). The best LET distribution was obtained with the PS‐PA fields.
SU‐E‐T‐377: Evaluation of a Novel Transmission Detector as a Reference Chamber for Use in Beam Measurements
Authors: Epstein D , Shekel E , Levin D
Published: June 2015

Purpose:

To evaluate a new transmission chamber for use as a reference chamber in measurements of beam data

Methods:

We assessed the performance of a new transmission detector, the Stealth Chamber, manufactured by IBA (IBA Dosimetry). The chamber has an active volume of 249 cm3, with an attenuation equivalent of <0.5mm Al. We mounted the chamber to a TrueBeam linac (Varian Medical Systems, Palo Alto, CA) such that the active area is perpendicular to the beam direction. We performed PDD and profile measurements on field sizes from 1×1 cm2 to 10×10 cm2, as well as for a 4 mm cone. The field detector was either a CC‐13 chamber (IBA Dosimetry) with an active volume of 0.13 cm3 or an Edge Detector (Sun Nuclear, Melbourne, FL) with an active volume of 0.019 cm3. For comparison we repeated all measurements using a CC‐13 or CC‐01 (active volume 0.01 cm3 ) chamber as reference detectors (IBA Dosimetry).All scans were acquired using a Blue Phantom2 (IBA Dosimetry) and IBA OmniPro‐Accept v7.4.24 software.

Results:

All measurements with the Stealth Chamber were identical to those with ion reference chambers. For both regular and filter free 6MV beams there was agreement in PDDs and profiles, including the penumbra region, for all field sizes. This was true for the 4mm cone measurement, as well. The deviation between the Stealth and ion chamber measurements was on average 0.3%.

Conclusion:

The Stealth Chamber gives identical beam data as a conventional ion chamber for all field sizes. The advantage of the Stealth chamber over ion chambers is its efficiency. Once mounted, there is no need to reposition the chamber with varying field sizes. This translates into a huge savings in measurement time, as well as a reduction in potential errors due to reference chamber mispositioning.
SU‐E‐T‐395: Evaluation of Multiple Brain Metastases Stereotactic Treatment Planning in Cyberknife Versus Linac
Authors: Vikraman S , Manigandan D , Rajesh Thiyagarajan , Karrthick Kp , Sambasivaselli R , Senniandavar V , Ramu M , Maragathaveni S , Dhivya N , Muthukumaran M , Tejinder K
Published: June 2015

Purpose:

The purpose of this study was to evaluate multiple brain metastases stereotactic treatment planning of Cyberknife versus linac using dose volume based indices.

Methods:

Fifteen multiple brain metastases patients were taken for this study from Cyberknife Multiplan TPSv4.6.0. All these patients underwent stereotactic treatment in Cyberknife. For each patient VMAT stereotactic treatment plan was generated in MONACO TPSv5.0 using Elekta beam modulator MLC and matched the delivered plan. A median dose of 8.5Gy(range 7–12Gy) per fraction was prescribed. Tumor volume was in the range of 0.06–4.33cc. Treatment plan quality was critically evaluated by comparing DVH indices such as D98, D95, CI, and HI for target volumes. Maximum point doses and volume doses were evaluated for critical organs.

Results:

For each case, target coverage of D98 was achieved with 100% prescription dose with SD of 0.29% and 0.41% in Linac and Cyberknife respectively. The average conformity index(CI) of 1.26±0.0796 SD for Cyberknife and 1.92±0.60SD for linac were observed. Better homogeneity Index (HI) of 1.17±0.09SD was observed in linac as compared to Cyberknife HI of 1.24±0.05SD.All the critical organ doses were well within tolerance limit in both linac and Cyberknife plans. There is no significant difference of maximum point doses for brainstem and optic chiasm. Treatment time and number of monitor units are more in Cyberknife compared to linac. The average volume receiving 12Gy in whole brain was 6% and 12% for Cyberknife and linac respectively. 1000cc of whole brain received 60% lesser dose in Linac compared to Cyberknife in all cases.

Conclusion:

The study shows that dosimetrically comparable plans are achievable Cyberknife and Linac. However, a better conformity, target coverage, lesser OAR dose is achieved with Cyberknife due to greater degrees of freedom with robotic gantry and smaller collimator for multiple targets.
SU‐E‐T‐401: Evaluation of TG‐43 Dose Calculation Accuracy for SAVI‐Based Accelerated Partial Breast Irradiation (APBI) Via Monte Carlo Simulations
Authors: Xu Y , Tian Z , Scanderbeg D , Zhang M , Yashar C , Jiang S , Jia X
Published: June 2015

Purpose:

The current standard TG‐43 dose calculation method for SAVI‐based Accelerated Partial Breast Irradiation (APBI) assumes an ideal geometry of infinite homogeneous water. However, in SAVI treatments, the air cavity inside the device and the short source‐to‐skin distance raise concerns about the dose accuracy of the TG‐43 method. This study is to evaluate TG‐43 dose calculation accuracy in SAVI treatments using Monte Carlo (MC) simulations.

Methods:

We recalculated the dose distributions of 15 APBI patients treated with SAVI devices, including five cases with a size of 6–1, five with 8−1 and five with 10−1, using our in‐house developed fast MC dose package for HDR brachytherapy (gBMC). A phase‐space file was used to model the Ir‐192 HDR source. For each case, the patient CT was converted into a voxelized phantom and the dwell positions and times were extracted from treatment plans for MC dose calculations. Clinically relevant dosimetric parameters of the recalculated dose were compared to those computed via the TG‐43 approach.

Results:

A systematic overestimation of doses was found for the 15 cases in TG‐43 results, with D90, V150, and V200 for PTV_eval 2.8±1.8%, 2.0±2.2%, and 1.8±3.5% higher than MC results. TG‐43 also overestimated the dose to skin with the maximum dose 4.4±8.4% higher on average. The relatively large standard deviation seen in the difference of maximum skin dose is partially ascribed to the statistical uncertainty of MC simulations when computing the maximum dose. It took gBMC ∼1 minute to compute dose for a SAVI plan.

Conclusion:

The high efficiency of our gBMC package facilitated the studies with a relatively large number of cases. An overestimation of TG‐43 doses was found when using this MC package to recompute doses in SAVI cases. Clinical utilization of TG‐43 dose calculation method in this scenario should be aware of this fact.
SU‐E‐T‐410: Evaluation of Treatment Modalities for Stereotactic Lung Radiation Therapy: A Phantom Study
Authors: Mohatt D , Malhotra H
Published: June 2015

Purpose:

To evaluate and verify the accuracy of alternative treatment modalities for stereotactic lung therapy with end‐to‐end testing. We compared three dimensional conformal therapy (3DCRT), dynamic conformal arc therapy (DCAT), intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) treatment using 6 MV, 6 MV flattening filter free (FFF) and 10 MV FFF photons.

Methods:

A QUASAR respiratory motion phantom was utilized with custom ion chamber and gafchromatic EBT2 film inserts. The phantom contained a low density lung medium with a cylindrical polystyrene tumor (35 cc). Pseudo representative structures for various organs at risk (OAR) were created. All treatment plans were created using Eclipse ver. 11 using the same image and structure sets, and delivered via Varian TrueBeam STx linear accelerator equipped with high definition MLC. Evaluation of plan quality followed ROTG 0813 criterion for conformity index (CI100%), high dose spillage, D2cm, and R50%.

Results:

All treatment plans met the OAR dose constraints per protocol and could be delivered without any beam hold offs or other interlocks and hence were deemed clinically safe. For equivalent beam energies, target conformity was improved for all modalities when switching to FFF mode. Treatment efficiency increased for VMAT FFF by a factor of 3–4 over IMRT, and up to factor of 7 when compared to 3DCRT. Pass rates were > 97% for all treatment using gamma criteria of 3%, 3mm. Absolute dose at iso‐center was verified with ion chamber, and found to be within 2% of the treatment planning system.

Conclusion:

The higher dose rate associated with FFF not only reduces delivery times, but in most cases enhances plan quality. The one modality with succeeding best results for all RTOG criterions was VMAT 6 MV FFF. This end‐to‐end testing provides necessary confidence in the entire dose delivery chain for lung SBRT patients.
SU‐E‐T‐421: Failure Mode and Effects Analysis (FMEA) of Xoft Electronic Brachytherapy for the Treatment of Superficial Skin Cancers
Authors: Hoisak J , Manger R , Dragojevic I
Published: June 2015

Purpose:

To perform a failure mode and effects analysis (FMEA) of the process for treating superficial skin cancers with the Xoft Axxent electronic brachytherapy (eBx) system, given the recent introduction of expanded quality control (QC) initiatives at our institution.

Methods:

A process map was developed listing all steps in superficial treatments with Xoft eBx, from the initial patient consult to the completion of the treatment course. The process map guided the FMEA to identify the failure modes for each step in the treatment workflow and assign Risk Priority Numbers (RPN), calculated as the product of the failure mode's probability of occurrence (O), severity (S) and lack of detectability (D). FMEA was done with and without the inclusion of recent QC initiatives such as increased staffing, physics oversight, standardized source calibration, treatment planning and documentation. The failure modes with the highest RPNs were identified and contrasted before and after introduction of the QC initiatives.

Results:

Based on the FMEA, the failure modes with the highest RPN were related to source calibration, treatment planning, and patient setup/treatment delivery (Fig. 1). The introduction of additional physics oversight, standardized planning and safety initiatives such as checklists and time‐outs reduced the RPNs of these failure modes. High‐risk failure modes that could be mitigated with improved hardware and software interlocks were identified.

Conclusion:

The FMEA analysis identified the steps in the treatment process presenting the highest risk. The introduction of enhanced QC initiatives mitigated the risk of some of these failure modes by decreasing their probability of occurrence and increasing their detectability. This analysis demonstrates the importance of well‐designed QC policies, procedures and oversight in a Xoft eBx programme for treatment of superficial skin cancers. Unresolved high risk failure modes highlight the need for non‐procedural quality initiatives such as improved planning software and more robust hardware interlock systems.
SU‐E‐T‐425: Feasibility of Establishing a National Postal Dosimetry System Using EBT3 Gafchromic Film
Authors: Abdel‐Rahman W , Mohammed I , Bamajboor S
Published: June 2015

Purpose:

To evaluate the performance of a postal dosimeter consisting of EBT3 films and a miniature Lucite phantom to be used for photon beams output audits for mega‐voltage photon beams across the Saudi Arabia.

Methods:

Two batches of EBT3 films were calibrated in 5 mega‐voltage photon beams with energies ranging between 6 MV and 18 MV in the dose range between 0 to 4 Gy. A 4×4×20 cm3 lucite phantom was constructed to be used as an irradiation phantom. The lucite irradiation phantom consists of two parts and allows for embedding a 2.5×2.0 cm2 EBT3 film at 8.9 cm depth. Factors that convert the dose measured in the lucite irradiation phantom at 8.9 cm depth to the dose at 10 cm depth in water for 10×10 cm2 field for different photon energies were calculated using the dosxyznrc/EGSnrcMP user code. The performance of the proposed postal dosimeter was tested in 17 different photon beams across 4 radiotherapy centres in Saudi Arabia. The outputs of the 17 beams are monitored by either the International Atomic Energy Agency or by the Radiological Physics Centre.

Results:

For the 17 photon beams, the average of the ratios of measured to stated outputs was 1.01 ± 0.02 and with a maximum ratio of 1.05.

Conclusion:

The results of our work indicate that the proposed postal dosimetry system can be used for national auditing of outputs for mega‐voltage photon beams. The service can be offered to other national radiotherapy centres or to a be used for credentialing of centres participating in national trials.
SU‐E‐T‐426: Feasibility of Stereotactic Body Radiation Therapy (SBRT) Treatment of Pancreatic Cancer Using Volumetric Modulated Arc Therapy (VMAT) with Active Breathing Control (ABC)
Authors: Zhang Y , Jackson J , Davies G , Herman J , Forbang R Teboh
Published: June 2015

Purpose:

SBRT shows excellent tumor control and toxicity rates for patients with locally advanced pancreatic cancer (PCA). Herein, we evaluate the feasibility of using VMAT with ABC for PCA SBRT.

Methods:

Nine PCA patients previously treated via SBRT utilizing 11‐beam step‐and‐shoot IMRT technique in our center were retrospectively identified, among whom eight patients received 3300cGy in 5 fractions while one received 3000cGy in 5 fractions. A VMAT plan was generated on each patient's planning CT in Pinnacle v9.8 on Elekta Synergy following the same PCA SBRT clinical protocol. Three partial arcs (182°–300°, 300°‐60°, and 60°‐180°) with 2°/4° control‐point spacing were used. The dosimetric difference between the VMAT and the original IMRT plans was analyzed. IMRT QA was performed for the VMAT plans using MapCheck2 in MapPHAN and the total delivery time was recorded. To mimic the treatment situation with ABC, where patients hold their breath for 20–30 seconds, the delivery was intentionally interrupted every 20–30 seconds. For each plan, the QA was performed with and without beam interruption. Gamma analysis (2%/2mm) was used to compare the planned and measured doses.

Results:

All VMAT plans with 2mm dose grid passed the clinic protocol with similar PTV coverage and OARs sparing, where PTV V_RxDose was 92.7±2.1% (VMAT) vs. 92.1±2.6% (IMRT), and proximal stomach V15Gy was 3.60±2.69 cc (VMAT) vs. 4.80±3.13 cc (IMRT). The mean total MU and delivery time of the VMAT plans were 2453.8±531.1 MU and 282.1±56.0 seconds. The gamma passing rates of absolute dose were 94.9±3.4% and 94.5±4.0% for delivery without and with interruption respectively, suggesting the dosimetry of VMAT delivery with ABC for SBRT won't be compromised.

Conclusion:

This study suggests that PCA SBRT using VMAT with ABC is a feasible technique without compromising plan dosimetry. The combination of VMAT with ABC will potentially reduce the SBRT treatment time.
SU‐E‐T‐431: Feasiblity of Using CT Scout Images for 2D LDR Brachytherpay Planning
Authors: Ha J , Weaver R
Published: June 2015

Purpose:

i) To show the feasibility of using CT scout images for 2D low‐dose rate brachytherapy planning with BrachyVision (version 10.4); ii) to show their advantages and disadvantages over DRRs.

Methods:

A phantom was constructed to house a Fletcher‐Suite applicator. The phantom is made of Styrofoam with metal BBs positioned at well‐defined separations. These markers are used to assess the image distortion in the scout images. Unlike DRRs, scout images are distorted only in the direction normal to the couch direction; therefore, they needed to be scaled unidirectionally prior to importing into BrachyVision. In addition to confirming the scaling is performed correctly by measuring distances between well‐positioned BB, we also compare a LDR plan using scout images to a 3D CT‐based plan.

Results:

There is no distortion of the image along the couch direction due to the collimation of the CT scanner. The distortion in the transverse plane can be corrected by multiplying by the ratio of distances between source‐to‐isocenter and source‐to‐detector. The results show the distance separations between BBs as measured in scout images and by a caliber are within a few millimeters. Dosimetrically, the difference between the dose rates to points A and B based on scout images and on 3D CT are less than a few percents. The accuracy can be improved by correcting for the distortion on the transverse plane.

Conclusion:

It is possible to use CT scout images for 2D planning in BrachyVision. This is an advantage because scout images have no metal artifacts often present in CT images or DRRs. Another advantage is the lack of distortion in the couch direction. One major disadvantage is that the image distortion due to beam divergence can be large. This is due to the inherent short distance between source‐to‐isocenter and source‐to‐detector on a CT scanner.
SU‐E‐T‐432: Field Size Influence On the Electron and Photon Spectra Within Small MV Field Detectors
Authors: Benmakhlouf H , Andreo P
Published: June 2015

Purpose:

To investigate the influence of photon field size on the electron and photon fluence spectra in the active volume of small field detectors.

Methods:

The PENELOPE MC system based usercode PenEasy was used to calculate the material influence on the spectra by scoring the differential fluence in inserts of silicon, carbon, phosphorus and aluminium having 3 mm diameter and height. The spectra were then calculated inside the active volume of eleven detectors (ion chambers and solid‐state detectors) whose geometry was simulated with great detail. The inserts/detectors were placed at 10 cm depth in a 30 cm × 30 cm × 30 cm water phantom and irradiated with 2.5 MeV photons and Varian Clinac 6 MV beams of small, medium and large size.

Results:

For all configurations, photon spectra in the scoring volume were similar to that in a small water volume except for additional characteristic x‐ray peaks resulting from the material itself and from the materials surrounding the detectors (i.e. high‐Z shielding the silicon). Electron fluence calculated in the inserts were up to 60% larger than in water; the difference increased with material density and decreasing field size. MC‐calculated doses were compared to analytically determined collision kerma and restricted cema (cut‐off=15keV). For the inserts, with large and medium fields K_col agreed with MC‐dose, but K_col overestimated the dose for small fields due to lack of lateral CPE. For the detectors, up to 15% differences between K_col and the MC‐dose were found. For all configurations the C‐delta and MC‐dose agreed within ±2%.

Conclusion:

The most relevant findings were that shielding affects substantially the photon spectra and material conditions the electron spectra, their field size dependence varying with the geometry configuration. These affect the values of factors entering into relative dosimetry.
SU‐E‐T‐441: Gamma Passing Rates for IMRT QA and VMAT QA
Authors: Kremmel E , Giaddui T , Keller J , Fu M , Peng C , Doyle L , Yu Y , Harrison A
Published: June 2015

Purpose:

This study compares gamma passing rates for a cohort of similar intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) quality assurance (QA) plans to determine the equivalence of the patient specific QA plan delivery. The passing criterion is 90% gamma index with 3% dose difference (DD), 3mm distance‐to‐agreement (DTA) and a dose threshold of 10%.

Methods:

Gamma passing results of previously treated IMRT QA plans, delivered to Sun Nuclear MapCheck phantom, and VMAT QA plans, delivered to ScandiDos Delta4 phantom, are organized by anatomical site and treatment machine. Both Varian and Elekta machines are included. Pairs of IMRT and VMAT plans are matched based on site, machine, and PTV volume to ensure similar plan cohorts. A two‐tailed t‐test analysis of the data with an alpha of 0.05 determines if there exists a statistically significant difference. Power was calculated to detect a difference of 4%; all data sets were strong with above a 0.8 power.

Results:

The first data set consisting of 20 matched pairs of prostate plans was statistically insignificant (p‐value=0.90, Power=0.99). The 14 matched pairs set of head and neck plans has a statistically significant Result (p‐value=0.028, Power=0.88). The head and neck IMRT gamma indexes have a mean of 93.1% and range of 82%‐100% while the VMAT gamma indexes have a mean of 96.7% and range of 92%‐100%. The two combined data sets of matched plans had a statistically insignificant Result (p‐value=0.073, Power=0.99).

Conclusion:

Overall, IMRT and VMAT have equivalent passing rates when comparing the gamma analysis using a passing criterion of 3% DD and 3mm DTA. When separated by site, prostate IMRT and VMAT plans have equivalent passing rates while head and neck plans have a statistically significant variation of passing rates. The passing rates for the two modalities are independent of delivery machine for matched PTV target volumes.
SU‐E‐T‐442: Geometric Calibration and Verification of a GammaPod Breast SBRT System
Authors: Yu C , Niu Y , Mutaf Y , Maton P , Hoban P
Published: June 2015

Purpose:

The first GammaPod™ unit for prone stereotactic treatment of early stage breast cancer has recently been installed and calibrated. Thirty‐six rotating circular Co‐60 beams focus dose at an isocenter that traverses throughout a breast target via continuous motion of the treatment table. The breast is immobilized and localized using a vacuum‐assisted stereotactic cup system that is fixed to the table during treatment. Here we report on system calibration and on verification of geometric and dosimetric accuracy.

Methods:

Spatial calibration involves setting the origin of each table translational axis within the treatment control system such that the relationship between beam isocenter and table geometry is consistent with that assumed by the treatment planning system. A polyethylene QA breast phantom inserted into an aperture in the patient couch is used for calibration and verification. The comparison is performed via fiducial‐based registration of measured single‐isocenter dose profiles (radiochromic film) with kernel dose profiles. With the table calibrations applied, measured relative dose distributions were compared with TPS calculations for single‐isocenter and dynamic (many‐isocenter) treatment plans. Further, table motion accuracy and linearity was tested via comparison of planned control points with independent encoder readouts.

Results:

After table calibration, comparison of measured and calculated single‐isocenter dose profiles show agreement to within 0.5 mm for each axis. Gamma analysis of measured vs calculated profiles with 3%/2mm criteria yields a passing rate of >99% and >98% for single‐isocenter and dynamic plans respectively. This also validates the relative dose distributions produced by the TPS. Measured table motion accuracy was within 0.05 mm for all translational axes.

Conclusion:

GammaPod table coordinate calibration is a straightforward process that yields very good agreement between planned and measured relative dose distributions. The dynamic table motions used for dose painting are consistent with planned control points.

CY, YN, PM, PH are employees of Xcision Medical Systems
SU‐E‐T‐443: Geometric Uncertainties in Eye Plaque Dosimetry for a Fully Loaded 16 Mm COMS Plaque
Authors: Morrison H , Menon G , Jans H , Larocque M , Sloboda R
Published: June 2015

Purpose:

To determine the effect of geometric uncertainties in the seed positions in a COMS eye plaque on the central axis (CAX) dose.

Methods:

A Silastic insert was placed into a photopolymer 3D printed 16 mm COMS plaque, which was then positioned onto a custom‐designed PMMA eye phantom. High resolution 3D images were acquired of the setup using a Siemens Inveon microPET/CT scanner. Images were acquired with the plaque unloaded and loaded with IsoAid I‐125 seed shells (lack of silver core to minimize metal artifacts). Seed positions and Silastic thickness beneath each slot were measured. The measured seed coordinates were used to alter the seed positions within a standard 16 mm COMS plaque in Plaque Simulator v5.7.3 software. Doses along the plaque CAX were compared for the original and modified plaque coordinates using 3.5 mCi seeds with treatment times set to deliver 70 Gy to tumour apexes of 3.5, 5, and 10 mm height.

Results:

The majority of seeds showed length‐wise displacement, and all seeds showed displacement radially outward from the eye center. The average radial displacement was 0.15 mm larger than the expected 1.4 mm offset, approximately half of which was due to increased Silastic thickness beneath each slot. The CAX doses for the modified seed positions were consistently lower for all tumour heights due to geometric displacement of the seeds; dose differences were found to increase to a maximum of 2.6% at a depth of ∼10 mm, after which they decreased due to the inverse square dose fall‐off minimizing this effect.

Conclusion:

This work presents initial results of a broader dosimetric uncertainty evaluation for fully loaded COMS eye plaques and demonstrates the effects of seed positioning uncertainties. The small shifts in seed depths had noticeable effects on the CAX doses indicating the importance of careful Silastic loading.

Funding provided by Alberta Cancer Foundation Grant #26655, Vanier Canada Graduate Scholarship, and Alberta Innovates Health Sciences Graduate Studentship
SU‐E‐T‐444: Gravity Effect On Maximum Leaf Speed in Dynamic IMRT Treatments
Authors: Olasolo J , Pellejero S , Artacho JM , Gracia M , Gallardo N , Martin ML , Lozares S , Maneru F , Bragado L , Miquelez S
Published: June 2015

Purpose:

A leaf sequencing algorithm has been recently developed in our department. Our purpose is to utilize this algorithm to reduce treatment time by studying the feasibility of using several maximum leaf speeds depending on gantry angle and leaf thickness (0.5 or 1 cm at isocenter). To do so, the gravity effect on MLC performance has been examined by means of analysing the dynalog files.

Methods:

Leaf position errors has been ascertained according to gantry angle and leaf speed in MLC Millenium120 (Varian). In order to do this, the following test has been designed: all leaves move in synchrony, with same speed and 1 cm gap between opposite leaves. This test is implemented for 18 different speeds: 0.25‐0.5‐0.75‐1‐1.25‐1.5‐1.75‐2‐2.1‐2.2‐2.3‐2.4‐2.5‐2.6‐2.7‐2.8‐2.9‐3.0 cm/s and 8 gantry angles: 0‐45‐90‐135‐180‐225‐270‐315. Collimator angle is 2 degrees in all cases since it is the most usual one in IMRT treatments in our department. Dynamic tolerance is 2 mm. Dynalogs files of 10 repetitions of the test are analysed with a Mathlab in‐house developed software and RMS error and 95th percentiles are calculated. Varian recommends 2.5 cm/s as the maximum leaf speed for its segmentation algorithm. In our case, we accept this speed in the most restrictive situation: gantry angle 270 and 1 cm leaf thickness. Maximum speeds for the rest of the cases are calculated by keeping the difference between 95th percentile and dynamic tolerance. In this way, beam hold‐off probability does not increase.

Results:

Maximum speeds every 45 degrees of gantry rotation have been calculated for both leaf thickness. These results are 2.9‐2.9‐2.9‐2.9‐2.7‐2.6‐2.6‐2.7 cm/s for 0.5 cm leaf thickness and 2.7‐2.7‐2.7‐2.7‐2.6‐2.5‐2.5‐2.6 cm/s for 1 cm leaf thickness.

Conclusion:

Gravity effect on MLC positioning has been studied. Maximum leaf speed according to leaf thickness and gantry angle have been calculated which reduces treatment time.
SU‐E‐T‐446: Group‐Sparsity Based Angle Generation Method for Beam Angle Optimization
Authors: Gao H
Published: June 2015

Purpose:

This work is to develop the effective algorithm for beam angle optimization (BAO), with the emphasis on enabling further improvement from existing treatment‐dependent templates based on clinical knowledge and experience.

Methods:

The proposed BAO algorithm utilizes a priori beam angle templates as the initial guess, and iteratively generates angular updates for this initial set, namely angle generation method, with improved dose conformality that is quantitatively measured by the objective function. That is, during each iteration, we select “the test angle” in the initial set, and use group‐sparsity based fluence map optimization to identify “the candidate angle” for updating “the test angle”, for which all the angles in the initial set except “the test angle”, namely “the fixed set”, are set free, i.e., with no group‐sparsity penalty, and the rest of angles including “the test angle” during this iteration are in “the working set”. And then “the candidate angle” is selected with the smallest objective function value from the angles in “the working set” with locally maximal group sparsity, and replaces “the test angle” if “the fixed set” with “the candidate angle” has a smaller objective function value by solving the standard fluence map optimization (with no group‐sparsity regularization). Similarly other angles in the initial set are in turn selected as “the test angle” for angular updates and this chain of updates is iterated until no further new angular update is identified for a full loop.

Results:

The tests using the MGH public prostate dataset demonstrated the effectiveness of the proposed BAO algorithm. For example, the optimized angular set from the proposed BAO algorithm was better the MGH template.

Conclusion:

A new BAO algorithm is proposed based on the angle generation method via group sparsity, with improved dose conformality from the given template.

Hao Gao was partially supported by the NSFC (#11405105), the 973 Program (#2015CB856000) and the Shanghai Pujiang Talent Program (#14PJ1404500).
SU‐E‐T‐448: Heightened Apical Positivity of 2‐Year Post‐Radiotherapy Biopsies Is Not Related to Suboptimal Dosimetry
Authors: Studenski M , Stoyanova R , Abramowitz M , Dogan N , Pollack A
Published: June 2015

Purpose:

Previous research has demonstrated that following radiation therapy for prostate cancer, there is a relative increase in positive biopsies in the apex versus the rest of the prostate. The increase could be due to: 1) Inter‐fraction apex motion or deformation, 2) Intra‐fraction apex motion or deformation, 3) Suboptimal dose coverage in the apex, 4) Tissue composition in the apex and/or 5) Prostate size. In this initial study, the potential for suboptimal dose coverage in the apex was assessed by splitting the prostate planning target volume into the apex (inferior third) and remainder.

Methods:

69 patients were selected from 303 patients treated on a clinical radiotherapy trial for prostate cancer. These patients were selected as they had both a localized (sextant template) 2‐year post‐treatment biopsy and 3D dose information. Of these patients, 10 had positive biopsies in the apex, 8 in the remainder and 11 in both locations. For all patients, the following dosimetric data was acquired from the apex dose volume histogram: Dmean, Dmax, Dmin, D95% and V100%. Unpaired, one‐tailed t‐tests were used to test for statistical significance (p < 0.05) between all dosimetric parameters for patients with positive versus negative apical biopsies. Additionally, D95% for the apex was plotted against D95% of the remainder.

Results:

There was no statistical difference for the selected apical dosimetric parameters for patients with positive versus negative biopsies (p‐values > 0.05). No correlation was found between D95% (normalized to the prescription dose) for the apex and remainder (R2 = 0.0116).

Conclusion:

No correlation was found between positive apical biopsy and suboptimal dosimetric coverage. Current research is looking into inter‐fraction apex motion and deformation as a potential source of the increased apical failure using daily CBCT images.
SU‐E‐T‐451: Hybrid‐VMAT: A Novel Technique Combining VMAT and 3D in Planning Whole Breast Radiotherapy with a Simultaneously‐Integrated Boost (WBRT+SIB)
Authors: Guida K , Qamar K , Thompson M
Published: June 2015

Purpose:

The RTOG 1005 trial offered a hypofractionated arm in delivering WBRT+SIB. Traditionally, treatments were planned at our institution using field‐in‐field (FiF) tangents with a concurrent 3D conformal boost. With the availability of VMAT, it is possible that a hybrid VMAT‐3D planning technique could provide another avenue in treating WBRT+SIB.

Methods:

A retrospective study of nine patients previously treated using RTOG 1005 guidelines was performed to compare FiF+3D plans with the hybrid technique. A combination of static tangents and partial VMAT arcs were used in base‐dose optimization. The hybrid plans were optimized to deliver 4005cGy to the breast PTVeval and 4800cGy to the lumpectomy PTVeval over 15 fractions. Plans were optimized to meet the planning goals dictated by RTOG 1005.

Results:

Hybrid plans yielded similar coverage of breast and lumpectomy PTVs (average D95 of 4013cGy compared to 3990cGy for conventional), while reducing the volume of high dose within the breast; the average D30 and D50 for the hybrid technique were 4517cGy and 4288cGy, compared to 4704cGy and 4377cGy for conventional planning. Hybrid plans increased conformity as well, yielding CI95% values of 1.22 and 1.54 for breast and lumpectomy PTVeval volumes; in contrast, conventional plans averaged 1.49 and 2.27, respectively. The nearby organs at risk (OARs) received more low dose with the hybrid plans due to low dose spray from the partial arcs, but all hybrid plans did meet the acceptable constraints, at a minimum, from the protocol. Treatment planning time was also reduced, as plans were inversely optimized (VMAT) rather than forward optimized.

Conclusion:

Hybrid‐VMAT could be a solution in delivering WB+SIB, as plans yield very conformal treatment plans and maintain clinical standards in OAR sparing. For treating breast cancer patients with a simultaneously‐integrated boost, Hybrid‐VMAT offers superiority in dosimetric conformity and planning time as compared to FIF techniques.