bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐239: Design and Evaluation of a Nozzle Shielding System in Passively Scattered Proton Therapy
Authors: Wroe A , McAuley G , Slater J
Published: June 2015

Purpose:

To design, implement and evaluate a shielding system that will reduce out‐of‐field dose experienced by the patient and associated electronic systems in passively scattered proton therapy treatment.

Methods:

A multi‐stage neutron shielding system was retrofitted to the Gantry 1 treatment nozzle at Loma Linda University Medical Center. The system uses multiple borated polyethylene plates staged after the primary beam modifying devices to attenuate and absorb neutrons produced by such devices. This arrangement locates increasing levels of shielding between the sources of secondary particles in the nozzle and the patient. Additionally, the design of this shielding structure allows it to be easily retrofitted to an existing proton nozzle system without impacting design or treatment beam characteristics. The effectiveness of the shielding was evaluated both through experimental measurements and Geant4 Monte Carlo simulations.

Results:

Measurements were completed with Landauer Luxel+ dosimeters that use optically stimulated luminescence and CR‐39 to detect fast neutrons, thermal neutrons, protons, photons and beta particles. Measurements of a 250 MeV proton beam indicated that the shielding system reduced out‐of‐field dose to the patient by almost half with dose equivalent values at 50 and 40 cm from the field edge decreasing from 0.965 and 1.262 mSv/Gy to 0.596 and 0.777 mSv/Gy respectively. The installation of the multi‐stage shielding system also reduced dose equivalent experienced by electronic systems installed in the treatment room by up to 80%. Geant4 simulations were also used to evaluate the neutron fluence at various positions in the treatment room as well as provide information on microdosimetry spectra within the patient and treatment room.

Conclusion:

The shielding system described above proved to be an effective an inexpensive method of reducing out‐of‐field doses to the patient and electronic systems and can be easily retrofitted to existing passive scattering nozzles.
SU‐E‐T‐245: Detection of the Photon Target Damage in Varian Linac Based On Periodical Quality Assurance Measurements
Authors: Gao S , Balter P , Wang X , Sadagopan R , Pollard J
Published: June 2015

Purpose:

To determine the best dosimetric metric measured by our routine QA devices for diagnosing photon target failure on a Varian C‐series linac.

Methods:

We have retrospectively reviewed and analyzed the dosimetry data from a Varian linac with a target degradation that was undiagnosed for one year. A failure in the daily QA symmetry tests was the first indication of an issue. The beam was steered back to a symmetric shape and water scans indicated the beam energy had changed but stayed within the manufacturer's specifications and agreed reasonably with our treatment planning system data. After the problem was identified and the target was replaced, we retrospectively analyzed our QA data including diagonals normalized flatness (F_DN) from the daily device (DQA3), profiles from an ionization chamber array (IC Profiler), as well as profiles and PDDs from a 3D water Scanner (3DS). These metrics were cross‐compared to determine which was the best early indicator of target degradation.

Results:

A 3% change in FDN measured by the DQA3 was found to be an early indicator of target degradation. It is more sensitive than changes in output, symmetry, flatness or PDD. All beam shape metrics (flatness at dmax and 10 cm depth, and F_DN) indicated an energy increase while the PDD indicated an energy decrease. This disagreement between the beam‐shape based energy metrics (F_DN and flatness) and PDD based energy metric may indicate target failure as opposed to an energy change resulting from changes in the incident electron energy.

Conclusion:

Photon target degradation has been identified as a failure mode for linacs. The manufacturer's test for this condition is highly invasive and requires machine down time. We have demonstrated that the condition could be caught early based upon data acquired during routine QA activities, such as the F_DN.
SU‐E‐T‐249: Determining the Sensitivity of Beam Profile Parameters for Detecting Energy Changes in Flattening Filter‐Free Beams
Authors: Mooney K , Yaddanapudi S , Mutic S , Goddu S
Published: June 2015

Purpose:

To identify the beam profile parameters that can be used to detect energy changes in a flattening filter‐free photon beams.

Methods:

Flattening filter‐free beam profiles (inline, crossline, and diagonals) were measured for multiple field sizes (25×25cm and 10×10cm) at 6MV on a clinical system (Truebeam, Varian Medical Systems Palo Alto CA). Profiles were acquired for baseline energy and detuned beams by changing the bending magnet current (BMC), above and below baseline. The following profile parameters were measured: flatness (off‐axis ratio at 80% of field size), symmetry, uniformity, slope, and the off‐axis ratio (OAR) at several off‐axis distances. Tolerance values were determined from repeated measurements. Each parameter was evaluated for sensitivity to the induced beam changes, and the minimum detectable BMC change was calculated for each parameter by calculating the change in BMC that would Result in a change in the parameter above the measurement tolerance.

Results:

Tolerance values for the parameters were‐Flatness≤0.1%; Symmetry≤0.4%; Uniformity≤0.01%; Slope≤ 0.001%/mm. The measurements made with a field size of 25cm and a depth of d=1.5cm showed the greatest sensitivity to bending magnet current variations. Uniformity had the highest sensitivity, able to detect a change in BMC of BMC=0.02A. The OARs and slope were sensitive to the magnitude and direction of BMC change. The sensitivity in the flatness parameter was BMC=0.04A; slope was sensitive to BMC=0.05A. The sensitivity decreased for OARs measured closer to central axis‐BMC(8cm)=0.23A; BMC(5cm)=0.47A; BMC(2cm)=1.35A. Symmetry was not sensitive to changes in BMC.

Conclusion:

These tests allow for better QA of FFF beams by setting tolerance levels to beam parameter baseline values which reflect variations in machine calibration. Uniformity is most sensitive to BMC changes, while OARs provide information about magnitude and direction of miscalibration.

Research funding provided by Varian Medical Systems. Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.
SU‐E‐T‐316: Dosimetric Effects Resulting From Increased MLC Error and Expanded Dose Dynamic Leaf Tolerances in Sliding Window IMRT
Authors: Netherton T , Ding G
Published: June 2015

Purpose:

To examine the dosimetric effects on the target dose coverage of MLC error (actual vs. planned position) for sliding window IMRT treatment plans.

Methods:

Dynalog files from a treated 7 field H&N sliding window IMRT plan were imported into a Dynalog to .mlc converter. The errors of the MLC's were multiplied by factors of f= 5, 10, 50, and 500 while capping the dose dynamic leaf tolerance at 0.2cm (Eclipse TPS default). Resulting .mlc files were imported into the TPS to study the changes in dose after increasing the magnitude of MLC error. Additionally, the dose dynamic leaf tolerance was expanded to 0.3cm, 0.4cm, and 0.5cm for each case.

Results:

For the linac investigated, A and B MLC error was found in the direction opposite to MLC motion, toward the X1 jaw. This systematic shift in the MLC position results in a shift of the dose distribution per field when the inherent error is multiplied by a factor. The complete treatment plan had maximum and minimum dose differences (modified vs. planned) exhibiting a similar trend as individual fields with dose differences reaching 3% of the prescribed dose with f=10 and an expanded tolerance of 0.3cm.

Conclusion:

A clinically significant difference of 5% of the prescribed dose is seen when the MLC error is multiplied by a factor f∼15 while within the default dose dynamic leaf tolerance of 0.2cm. This suggests that if a large deviation of the MLC's from the planned positions were to occur for just 1/5 of the active leaf positions that differences in the delivered vs. planned dose per fraction could exceed 5%. Using the default MLC tolerance, no clinically significant difference in the mean target dose was found, indicating that the default MLC tolerance of 0.2cm is adequate for accurate IMRT treatment plan delivery.
SU‐E‐T‐324: Dosimetric Evaluation of Soft Docking Sensitivity On An INTRAOP Mobetron IORT Device
Authors: Carlson M , Hessler J , Grzetic S , Gupta N
Published: June 2015

Purpose:

To assess Mobetron soft docking sensitivity thresholds by measuring dosimetric changes resulting from incremental displacements of isolated docking parameters.

Methods:

The Mobetron device uses a sensitive optical system to establish alignment of the collimator with a treatment cone. To assess the limits of this sensitivity, we isolated each docking parameter (transverse, tilt, and vertical) and varied each incrementally within the docking display range. Transverse displacements ranged in 1mm increments from 0.0cm to 0.4cm. Profiles were taken at dmax for each energy (6MeV, 9MeV and 12MeV) along the direction of displacement. Angular displacements ranged in 0.1° increments from 0.0° to 0.4°. Baseline corrected profiles were assessed with a point‐by‐point symmetry analysis and a max‐min flatness metric (90%‐1cm). Vertical displacements ranged from 0.0cm to 0.4cm in 1mm increments. Output factors were compared by % difference from baseline and PDDs were measured at each vertical displacement to assess changes in dmax and R50 values.

Results:

For 1mm lateral displacements, symmetry/flatness deviations for the 10cm cone were +0.48%/+0.38% at 6MeV, +0.95%/+1.04% at 9MeV, and +1.77%/+1.96% at 12MeV. Profiles in the direction orthogonal to gantry motion measured +0.99%/+1.29% symmetry/flatness changes at 4mm displacement. For 0.10 angular displacements, symmetry/flatness deviations for the 10cm cone were +0.07%/+0.05% at 6MeV +0.23%/+0.33% at 9MeV, and +0.34%/+0.59% at 12MeV. The 3cm and 6cm cones were less sensitive to transverse and angular displacements. At 4mm vertical displacement, the 3cm cone differed from baseline by 3.02%, 2.30%, and 2.42% (6MeV, 9MeV, and 12MeV respectively). Maximum combined deviation from baseline in quadrature for one extra docking light (half sensitivity) in each displacement vector was >2%.

Conclusion:

For transverse and vertical displacements, the docking sensitivity range is adequate to ensure flatness and symmetry differences <1.0%. However, for angular displacements, all energies with the 10cm cone deviated at or beyond this threshold.
SU‐E‐T‐331: Dosimetric Impact of Multileaf Collimator Leaf Width On Stereotactic Radiosurgery (SRS) RapidArc Treatment Plans for Single and Multiple Brain Metastases
Authors: Hossain S , Keeling V , Ahmad S , Algan O
Published: June 2015

Purpose:

To determine the effects of multileaf collimator (MLC) leaf width on normal‐brain‐tissue doses and dose conformity of SRS RapidArc treatment plans for brain tumors.

Methods:

Ten patients with 24 intracranial tumors (seven with 1–2 and three with 4–6 lesions) were planned using RapidArc for both Varian Millennium 120 MLC (5 mm leaf width) and high definition (HD) MLC (2.5 mm leaf width). Between 2 and 8 arcs were used with two full coplanar arcs and the rest non‐coplanar half arcs. 6 MV beams were used and plans were optimized with a high priority to the Normal Tissue Objective (to achieve dose conformity and sharp dose fall‐off) and normal brain tissue. Calculation was done using AAA on a 1 mm grid size. The prescription dose ranged from 14–22 Gy. Plans were normalized such that 99% of the target received the prescription dose. Identical beam geometries, optimizations, calculations, and normalizations were used for both plans. Paddick Conformity Index (PCI), V4, V8 and V12 Gy for normal brain tissue and Integral Dose were used for analysis.

Results:

In all cases, HD MLC plans performed better in sparing normal brain tissue, achieving a higher PCI with a lower Integral Dose. The average PCI for all 24 targets was 0.75±0.23 and 0.70±0.23 (p ≤0.0015) for HD MLC and Millennium MLC plans, respectively. The average ratio of normal brain doses for Millennium MLC to HD MLC plans was 1.30±0.16, 1.27±0.15, and 1.31±0.18 for the V4, V8, and V12, respectively. The differences in normal brain dose for all criteria were statistically significant with p‐value < 0.02. On average Millennium MLC plans had a 16% higher integral dose than HD MLC plans.

Conclusion:

Significantly better dose conformity with reduced volume of normal brain tissue and integral dose was achieved with HD MLC plans compared to Millennium MLC plans.
SU‐E‐T‐334: Dosimetric Impacts of Organ at Risk (OAR) Contour Errors in Prostate Volumetric Modulated Arc Therapy (VMAT) Planning: A Sensitivity Study
Authors: Chang J , Gu X , Lu W , Song T , Jia X , Jiang S
Published: June 2015

Purpose:

To investigate the dosimetric impacts of OAR contour errors in prostate VMAT planning with a sensitivity analysis method.

Methods:

Ten randomly selected prostate VMAT patients were used to simulate OAR contour error in rectum and bladder. For each OAR 12 even spaced contour control points were selected in every three slices. For each simulation, only one of the control points was simulated to move inwards or outwards by up to 6 voxels and coordinates of adjacent voxels to that moving control point were moved accordingly with a 3D‐spline smooth function. An in‐house software was used to predict OARs dosimetric endpoints based on geometric relationship between PTV and OAR. In this study, the V75, V70, V65, V60 of rectum and V80, V75, V70, V65 of bladder with and without perturbed contours were calculated and compared.

Results:

The percentage of OAR dose volume difference around the reference OAR contours were plotted as iso‐error lines overlaid on CT images. The significant difference was shown in OAR contours adjacent to the PTV where high dose gradient exists. When one of the 12 points adjacent to the PTV moved up to 6 voxels, maximum errors of 1.99%, 2.46%, 2.89%, and 3.19% were found in V75, V70, V65, and V60 of the rectum, and 1.59%, 1.67%, 1.87%, and 1.90% were found in V80, V75, V70, and V65 of the bladder, respectively.

Conclusion:

We can quantify the dosimetric impact of OAR contouring error by evaluating the percentage of OAR dose volume changes in prostate VMAT planning. For the adaptive radiation therapy, the iso‐error lines can provide the planner a guideline that which portion of contour needed to be checked carefully because dosimetric sensitivity of contour errors are different for different part of contours.
SU‐E‐T‐345: Effect of DLG and MLC Transmission Value Set in the Treatment Planning System (TPS) On Dosimetric Accuracy of True Beam Hypofractionated SRT/SBRT and 2Gy/fx Prostate Rapid Arc Plans
Authors: Wu X , Wang Y
Published: June 2015

Purpose:

Due to limited commissioning time, we previously only released our True beam non‐FFF mode for prostate treatment. Clinical demand now pushes us to release the non‐FFF mode for SRT/SBRT treatment. When re‐planning on True beam previously treated SRT/SBRT cases on iX machine we found the patient specific QA pass rate was worse than iX's, though the 2Gy/fx prostate Result had been as good. We hypothesize that in TPS the True beam DLG and MLC transmission values, of those measured during commissioning could not yet provide accurate SRS/SBRT dosimetry. Hence this work is to investigate how the TPS DLG and transmission value affects Rapid Arc plans’ dosimetric accuracy.

Methods:

We increased DLG and transmission value of True beam in TPS such that their percentage differences against the measured matched those of iX's. We re‐calculated 2 SRT, 1 SBRT and 2 prostate plans, performed patient specific QA on these new plans and compared the results to the previous.

Results:

With DLG and transmission value set respectively 40 and 8% higher than the measured, the patient specific QA pass rate (at 3%/3mm) improved from 95.0 to 97.6% vs previous iX's 97.8% in the case of SRT. In the case of SBRT, the pass rate improved from 75.2 to 93.9% vs previous iX's 92.5%. In the case of prostate, the pass rate improved from 99.3 to 100%. The maximum dose difference in plans before and after adjusting DLG and transmission was approximately 1% of the prescription dose among all plans.

Conclusion:

The impact of adjusting DLG and transmission value on dosimetry might be the same among all Rapid Arc plans regardless hypofractionated or not. The large variation observed in patient specific QA pass rate might be due to the data analysis method in the QA software being more sensitive to hypofractionated plans.
SU‐E‐T‐350: Effective Point of Measurement and Total Perturbation Correction P for Parallel‐Plate Ion Chambers in High‐Energy Photon Beams
Authors: Langner N , Czarnecki D , von Voigts‐Rhetz P , Zink K
Published: June 2015

Purpose:

This paper aims to determine the effective point of measurement and the total perturbation correction p of parallel‐plate chambers for clinical photon dosimetry.

Methods:

The effective point of measurement (EPOM) was calculated using the EGSnrc Monte Carlo code system with the EGSnrc user code egs_ chamber. Depth dose curves of the ionization chambers were calculated in a water phantom for several high energy photon spectra (4, 6, 10, 15, 18 MV‐X). Different normalization criterions (normalization to the maximum of the depth dose curve and normalization to the value in 10 cm depth) have been applied. The EPOM was determined by shifting the normalized depth dose curve of a small water voxel against the depth ionization curve until the disagreement (calculated by the root mean square deviation) reaches a minimum. In addition, the total perturbation correction p was calculated by the ratio of the dose to water and the product of the dose determined in the chamber and the water to air stopping power ratio.

Results:

The EPOM varied slightly depending on the chosen normalization criterion. For all chambers the necessary shift of the EPOM decreased linearly with increasing beam quality specifier TPR20/10. For the Roos and NACP chamber, the results were positive suggesting that the chambers need to be shifted towards the focus. For the Markus chamber, the required shift was negative and for the Advanced Markus chamber partly negative and partly positive. The total perturbation correction p was almost independent of the depth. Only for regions below 1 cm the perturbation correction deviated significantly from unity.

Conclusion:

In the present study, the effective point of measurement and the total perturbation correction p was determined for four parallel‐plate ionization chambers and five clinical relevant photon spectra. Applying the calculated EPOM, the residual perturbation correction p was mostly depth independent.
SU‐E‐T‐358: Empirical Beam Angle Optimization for Lung Cancer Intensity Modulated Radiation Therapy
Authors: Doozan B , Pella S
Published: June 2015

Purpose:

Creating an improved BAO can decrease the amount of time a dosimetrist spends on making a treatment plan, improve the treatment quality and enhance the tools an inexperienced dosimetrist can use to develop planning techniques.

Methods:

Using empirical data created by experienced dosimetrists from 69 patients treated for lung cancer, the most frequently used gantry angles were applied to four different regions in each lung to gather an optimal set of fields that could be used to treat future lung cancer patients. This method, given the moniker FAU BAO, is compared in 7 plans created with the Eclipse BAO choosing 5 fields and 9 fields.

Results:

The results show that the conformality index improved by 30% or 3% when using the 5 and 9 fields. The conformation number was better by 12% from the 5 fields and 9% from the 9 fields. The organs at risk (OAR) were overall more protected to produce fewer nonstochastic effects from the radiation treatment with the FAU BAO.

Conclusion:

Empirical methods for beam angle optimization is a viable method for IMRT treatment planning techniques.
SU‐E‐T‐359: Emulation of Yb‐169 Gamma‐Ray Spectrum Using Metal‐Filtered 250 KVp X‐Rays for Pre‐Clinical Studies of Gold Nanoparticle‐Aided Radiation Therapy
Authors: Reynoso F , Cho S
Published: June 2015

Purpose:

To develop an external beam surrogate of the Yb‐169 brachytherapy source applying a filter‐based spectrum modulation technique to 250 kVp x‐rays. In‐vitro/vivo studies performed with the modulated 250 kVp beam will help gauge the benefits of implementing gold nanoparticle‐aided radiotherapy with the Yb‐169 source.

Methods:

A previously validated MCNP5 model of the Phillips RT‐250 orthovoltage unit was used to obtain the percentage depth dose (PDD) and filtered photon spectra for a variety of filtration and irradiation conditions. Photon spectra were obtained using the average flux F4 tally in air right after all collimation. A 30 × 30 × 30 cm3 water phantom was used to compute the PDD along the central axis (CAX) under the standards conditions of a 10 × 10 cm2 field size at 50 cm SSD. Cylindrical cells of 4 cm in diameter and the energy deposition F6 tally were used along the CAX to score the doses down to 20 cm depth. The number of particle history was set to 2 × 10⁸ in order to keep the relative uncertainty within each cell < 0.3%. The secondary electron spectrum within a gold‐loaded tissue due to each photon spectrum was also calculated using EGSnrc and compared with that due to Yb‐169 gamma rays.

Results:

Under the practical constraints for the spectrum modulation task, 250 kVp x‐rays filtered by a 0.25 mm Erbium (Er) foil produced the best match with Yb‐169 gamma rays, in terms of PDD and, more importantly, secondary electron spectrum.

Conclusion:

Modulation of 250kVp x‐ray spectrum by an Er‐filter was found effective in emulating the gamma ray spectrum of Yb‐169. Possible benefits as predicted from the current MC model such as enhanced radiosensitization with the Er‐filtered beam (as a surrogate of Yb‐169) was confirmed with a separate in‐vitro study.

Supported by DOD/PCRP grant W81XWH‐12‐1‐0198
SU‐E‐T‐366: Estimation of Whole Body Dose From Cranial Irradiation From C and Perfexion Series Gamma Knife Units
Authors: Srivastava S , Andersen A , Lulu B , Cheng C , Das I
Published: June 2015

Purpose:

The Leksell Gamma Knife (GK) B & C series contains 201 Cobalt‐60 sources with a helmet. The new model, Perfexion uses 192 Cobalt‐60 sources without a helmet; using IRIS system for collimation and stereotactic guidance to deliver SRS to brain tumors. Relative dose to extracranial organs at risk (OARs) is measured in phantom in this study for Perfexion and C‐series GK.

Materials & Methods:

Measurements were performed in a Rando anthropomorphic phantom on both systems using a large ion chamber (Keithley‐175) for each collimator. The Keithley‐175 cc ion chamber was sandwiched between phantom slices at various locations in the phantom to correspond to different extracranial OARs (thyroid, heart, kidney, ovary and testis, etc.) The dose measurement was repeated with OSL detectors for each position and collimator.

Results:

A large variation is observed in the normalized dose between these two systems. The dose beyond the housing falls off exponentially for Perfexion. Dose beyond the C‐series GK housing falls off exponentially from 0–20cm then remains relatively constant from 20–40cm and again falls off with distance but less rapidly. The variation of extracranial dose with distance for each collimator is found to be parallel to each other for both systems.

Conclusion:

Whole body dose is found to vary significantly between these systems. It is important to measure the extracranial dose, especially for young patients. It is estimated that dose falls off exponentially from the GK housing and is about 1% for large collimators at 75 cm. The dose is two‐orders of magnitude smaller for the 4mm collimator. However, this small dose for patient may be significant radiologically.
SU‐E‐T‐372: Evaluation and Comparison of Second‐Check Monitor Unit Calculation Software with Pinnacle Treatment Planning System
Authors: Tuazon B , Narayanasamy G , Kirby N , Mavroidis P , Papanikolaou N , Stathakis S
Published: June 2015

Purpose:

The purpose of this study was to evaluate and compare the accuracy of dose calculation algorithms in the second check software programs Radcalc, Diamond, IMSure, and MUcheck, against the Pinnacle3 treatment planning system (TPS).

Methods:

Baseline accuracy of the second check software was established by comparison against Pinnacle TPS data using open square fields of 5, 10, 20, 30 and 40cm in a SAD setup. 18 previously treated patients’ files were exported from the Pinnacle3 TPS to each of the four second check softwares, consisting of 146 step and shoot intensity modulated radiotherapy (IMRT) beams and 60 Smart Arcs. Monitor unit (MU) calculated in each of the software were compared with the TPS and the values were represented as a percent difference. Open fields were calculated as a baseline for each software's accuracy using 5×5, 10×10, 20×20, 30×30, and 40×40 fields. Box plots, Pearson correlation, and Bland‐Altman analysis were used for comparison of the results.

Results:

The baseline accuracy was established to within 0.6%, −1.4%, −0.2%, and −1.0% for Diamond, IMSure,MUcheck, and Radcalc, respectively. In the clinical data, the dose difference represented as mean ± 1 standard deviation were 0.7%±0.1%, −0.3%±0.1%, −1.5%±0.1%, and 0.4%±0.0% for Diamond, IMSure, MUcheck, and Radcalc, respectively

Conclusion:

The implementation of Clarkson algorithm for the dose calculation between each of the software in question can vary considerably. The currently used second check software, Radcalc has shown the best agreement on average, variance, and smallest percent range from Pinnacle3 TPS values. The closest in average percent difference from the TPS data was the IMSure software, but has significantly larger variance and percent range. The mean percent differences in Diamond and MUcheck were significantly larger than Radcalc and IMSure.
SU‐E‐T‐379: Evaluation of An EPID‐Based System for Daily Dosimetry Check by Comparison with a Widely‐Used Ionization Chamber‐Based Device
Authors: McDonald D , Koch N , Peng J , Jacqmin D , Ashenafi M , Vanek K
Published: June 2015

Purpose:

To examine the feasibility of using Varian's EPID‐based Machine Performance Check (MPC) system to track daily machine output through comparison with Sun Nuclear's DailyQA3 (DQA) device.

Methods:

Daily machine outputs for two photon energies (6 and 16MV) and five electron energies (6, 9, 12, 16, 20MeV) were measured for one month using both MPC and DQA. Baselines measurements for MPC were taken at the start of the measurement series, while DQA baselines were set at an earlier date. In order to make absolute comparisons with MPC, all DQA readings were referenced to the average of the first three DQA readings in that series, minimizing systematic differences between the measurement techniques due to baseline differences. In addition to daily output measurements, weekly averages were also calculated and compared. Finally, the electron energy dependence of each measurement technique was examined by comparing energy‐specific measurements to the average electron output of all energies each day.

Results:

For 6 and 16MV photons, the largest absolute percent differences between MPC and DQA were 0.60% and 0.73%, respectively. Weekly averages were within 0.17% and 0.23%, respectively. For all five electron energies, the greatest absolute percent differences between MPC and DQA for each energy ranged from 0.49%–0.83%. Weekly averages ranged from 0.07%–0.28%. DQA energy‐specific electron readings matched the average electron output within 0.29% for all days and all energies. MPC energy‐specific readings matched the average within 0.21% for 9–20MeV. However, 6MeV showed a larger distribution about the average with four days showing a difference greater than 0.30% and a maximum difference of 0.51%.

Conclusion:

MPC output measurements correlated well with the widely‐used DQA3 for most beam energies, making it a reliable back up technique for daily output monitoring. However, MPC may display an energy dependence for lower electrons energies, requiring additional investigation.
SU‐E‐T‐384: Evaluation of Dose to Medium Vs. Dose to Water Calcualtion Mode in Acuros XB Algorithm for Lung SBRT
Authors: Pokharel S , Oyewale S , Rana S
Published: June 2015

Purpose:

The purpose of this study it to evaluate the dose to medium vs. dose to water reporting mode of AcurosXB algorithm in Eclipse TPS for lung SBRT

Methods:

Five lung SBRT cases previously calculated with AAA algorithm have been chosen for this study. All high density materials within the body were contoured and assigned HU values. The plans were then recalculated with AcurosXB algorithm using both dose to medium (AXM) and dose to water (AXW) modes. DVHs were then calculated to quantify the differences in dosimetric indices. Also, plan differences (AXW‐AXM) and (AXW‐AXM) were computed to observe the regions where maximum dose differences occur within the body.

Results:

For the same plan normalization, the MU variation between the plans was within ±0.5%. All dosimetric indices of target were within ±0.5% except maximum dose to 2cm (D2‐Max) from the target. D2‐Max was consistently higher with AXW plan with a value as high as 2.71%. All the dosimetric indices for both ipsilateral and contralateral lung appear to be within ±1% on average without any clear trend. Likewise, dosimetric indices for heart and Esophagus appear well within ±1% on average. The maximum dose to spinal cord was recorded consistently higher with dose to water reporting mode with an average of 2.2% (range 1.40% to 4.63%). Also, the plan difference (AXW‐AXM) consistently produced higher maximum dose difference 8.94% (range, 4.2% to 12.4%) as opposed to 2.82%(range, 2.8% to 3.8%) from plan difference (AXM‐AXW).

Conclusion:

It appears that dose reporting mode in AcurosXB algorithm has insignificant effects in dosimetric indices for SBRT lung. The only significant difference observed was the maximum dose difference within the body which may not be clinically important. This may be due to the fact that AcurosXB algorithm transport the particles in medium regardless of dose reporting mode chosen.
SU‐E‐T‐385: Evaluation of DVH Change for PTV Due to Patient Weight Loss in Prostate VMAT Using Gaussian Error Function
Authors: Viraganathan H , Jiang R , Chow J
Published: June 2015

Purpose:

We proposed a method to predict the change of dose‐volume histogram (DVH) for PTV due to patient weight loss in prostate volumetric modulated arc therapy (VMAT). This method is based on a pre‐calculated patient dataset and DVH curve fitting using the Gaussian error function (GEF).

Methods:

Pre‐calculated dose‐volume data from patients having weight loss in prostate VMAT was employed to predict the change of PTV coverage due to reduced depth in external contour. The effect of patient weight loss in treatment was described by a prostate dose‐volume factor (PDVF), which was evaluated by the prostate PTV. Along with the PDVF, the GEF was used to fit into the DVH curve for the PTV. To predict a new DVH due to weight loss, parameters from the GEF describing the shape of DVH curve were determined. Since the parameters were related to the PDVF as per the specific reduced depth, we could first predict the PDVF at a reduced depth based on the prostate size from the pre‐calculated dataset. Then parameters of the GEF could be determined from the PDVF to plot the new DVH for the PTV corresponding to the reduced depth.

Results:

A MATLAB program was built basing on the patient dataset with different prostate sizes. We input data of the prostate size and reduced depth of the patient into the program. The program then calculated the PDVF and DVH for the PTV considering the patient weight loss. The program was verified by different patient cases with various reduced depths.

Conclusion:

Our method can estimate the change of DVH for the PTV due to patient weight loss quickly without CT rescan and replan. This would help the radiation staff to predict the change of PTV coverage, when patient's external contour reduced in prostate VMAT.
SU‐E‐T‐386: Evaluation of EBT3 Film Response in Different Batches
Authors: Escarcia F , Herrera J , Garcia O
Published: June 2015

Purpose:

To investigate the uniformity of film response of EBT3 film of two different film batches. It has been reported that the response of the EBT film family is not homogenous between film batches. The later may have an impact in the dosimetry of small radiotherapy beams.

Methods:

A solid water phantom was used for dosimetric measurements. EBT3 film irradiation was performed with a 6 MV photon beam at 5 cm depth with a SAD of 100 cm. All irradiations were performed perpendicularly to the film plane covering the dose range 1 to 10 Gy. Three square field sizes were used to analyze the film response energy dependence: 10, 5 and 1 cm2. Two batches of film EBT3 were used #A03181302 (B1) and #03031403 (B2). Film read out was carrying out with an Epson Perfection V750‐Pro flatbed scanner in transmission mode with a spatial resolution of 72 dpi, with all post‐processing and colour management options turned off, using 48 bits RGB colour depth. The scans were analyzed with the red channel.

Results:

The results shown that there were differences between the film response for each batch. The differences between batches for 1 Gy were 2%, 6% and 12% for 10, 5 and 1 cm2 square field sizes, respectively. The differences found for 10 Gy were 13%, 14% and 13% for 10, 5 and 1 cm2 square field sizes, respectively. It can be observed that the dependence with field size dismissed for higher doses. The later may be due to film response saturation at 10 Gy.

Conclusion:

The EBT3 film ‐as its predecessors‐, it suffer for inter‐batch variability in the film response. Further research is required to assess the possible impact in small beam dosimetry.
SU‐E‐T‐389: Evaluation of Flattening‐Filter‐Free Arcs for Lung SBRT
Authors: Ouyang L , Lee H , Pompos A , Yan Y , Jiang S , Foster R
Published: June 2015

Purpose:

To evaluate dynamic conformal arc therapy (DCAT) and volumetric‐modulated arc therapy (VMAT) using flattening‐filter‐free (FFF) beams for treating non‐small cell lung cancer (NSCLC) with stereotactic body radiation therapy (SBRT).

Methods:

Five clinical patients, previously treated with SBRT using non‐coplanar 3D conformal radiation therapy (3DCRT), were selected and re‐planned with DCAT and VMAT with both FFF beam (6xFFF with a dose rate of 1200MU/min) and flattened beams (6x with 600 MU/min). All the arc plans were planned with one 360° arc and normalized to the same PTV coverage (100% prescription cover 95% PTV volume) for comparison. Treatment planning metrics such as R100%, R50%, D2cm and lung V20 were compared to the original plans. To evaluate the treatment efficiency differences, all the arc plans were delivered and plan delivery time was compared to that of the clinical treatment as recorded in Mosaiq.

Results:

All plans meet RTOG conformality constraints and normal tissue tolerances. Average R100% was similar for FFFDCAT (1.07±0.05), FFFVMAT (1.03±0.08) and flattened VMAT (fVMAT) (1.03±0.09) while flattened DCAT (fDCAT) (1.09±0.07) and 3DCRT (1.11±0.06) were significantly inferior (p<0.05, t test). FFFDCAT produced the best average intermediate dose conformality as indicated by R50% (3.86±0.44) and D2cm (43.7±5.3%) when compared to all the other techniques. Significant improvement (p<0.05) in lung V20 was also found with FFFDCAT (2.33±2.06%) when compared to FFFVMAT (2.48±2.03%), fVMAT (2.52±2.07%) and fDCAT (2.64±2.11%) and was slightly better than 3DCRT (2.43±2.04%), though not significant. The FFFDCAT delivery significantly improves the treatment efficiency with an average plan delivery time of 2.70±1.57 min (p<0.05), as compared to fDCAT (5.98±3.45min), fVMAT (6.51±2.94) and 3DCRT (25.14±5.67), but is not significantly better than the FFFVMAT (3.18±1.04).

Conclusion:

Combining FFF beams and DCAT provide promising improvements in NSCLC SBRT treatment in both plan quality and treatment efficiency and unlike VMAT, tumor coverage is not affected by MLC interplay effects.
SU‐E‐T‐393: Evaluation of Large Field IMRT Versus RapidArc Planning for Carcinoma Cervix with Para‐Aotic Node Irradiation
Authors: Raman S Kothanda , Girigesh Y , MISHRA M , Lalit K
Published: June 2015

Purpose:

The objective of this work is to evaluate and compare Large field IMRT and RapidArc planning for Carcinoma Cervix and Para‐aotic node irradiation.

Methods:

In this study, ten patients of Cervix with para‐aotic node have been selected with PTV length 35+2cm. All plans were generated in Eclipse TPS V10.0 with Dynamic IMRT and RapidArc technique using 6MV photon energy. In IMRT planning, 7 fields were chosen to get optimal plan and in RapidArc, double Full arc clockwise and counter clockwise were used for planning. All the plans were generated with single isocenter and calculated using AAA dose algorithm. For all the cases the prescribed dose to PTV was same and the plan acceptance criteria is; 95% of the PTV volume should receive 100% prescribed dose. The tolerance doses for the OAR's is also taken in to account. The evaluation criteria used for analysis are; 1) Homogeneity Index, 2) Conformity Index, 3) Mean Dose to OAR's, 4)Total monitor units delivered.

Results:

DVH analysis were performed for both IMRT and RapidArc planning. In both the plans, 95% of PTV volume receives prescribed dose and maximum dose are less than 107%. The conformity index are same in both the techniques. The mean Homogeneity index are 1.036 and 1.053 for IMRT and RapidArc plan. The mean (mean + SD) dose of bladder and rectum in IMRT is 44.2+1.55, 42.05+2.52 and RapidArc is 46.66+1.6, 44.2+2.75 respectively. There is no significant difference found in Right Femoral head, Left Femoral head and Kidney doses. It is found that total MU's are more in IMRT compared with RapidArc planning.

Conclusion:

In the case of cervix with Para‐arotic node single isocenter irradiation, IMRT planning in large‐field is better compared to RapidArc planning in terms of Homogeneity Index and mean dose of Bladder and Rectum.
SU‐E‐T‐394: Evaluation of Microwave Ablation Zone Variance Based On Reported Changes in Thermal and Electrical Properties of Tissues
Authors: Deshazer G , Prakash P , Hagmann M , Dupuy D , Merck D
Published: June 2015

Purpose:

Image‐guided percutaneous ablation is an, effective, inexpensive, and accessible treatment for liver, lung, and kidney cancers. However, its relatively high recurrence rate makes it more palliative. We hypothesize that higher rates are due in part to inadequate margin estimation, resulting from over‐simplified geometric planning. Better predictive planning is needed, however, this requires numerical model accounting for the physical characteristics of heterogeneous tissues around the target. Literature suggests that pathologies such as cirrhosis and malignant tissue have varying thermal and electrical properties when compared to normal tissue. The objective of this study is to employ a simulation based approach to determine the impact of heterogeneous tissue physical properties on treatment outcome.

Methods:

An electromagnetic‐thermal finite‐element model of the BSD Medical ST antenna was created. The antenna placed inside the center of a tumor that was immersed within liver tissue. The variance of ablation zone sizes with tissue physical properties was evaluated by assigning varying tissue dielectric and thermal properties to the tumor, consistent with data reported in the literature. 10–30 % variations in tissue effective electrical conductivity, and varying perfusion levels for tumor, normal, and cirrhotic liver (3–18 kg/m3/s) were considered. The CEM43 > 240 min threshold was used to estimate ablation zone extents.

Results:

Less than 4 mm variation in ablation zone extents was observed when considering 10–30 % differences in effective electrical conductivity between malignant and normal tissue. Perfusion variations showed the largest difference when compared to homogeneous simulation (∼ 4.5 mm − 1.0 cm).

Conclusion:

The results suggest that dielectric inhomogeneities have limited impact on predicted ablation zone extents. However, variances in tumor and normal liver tissue blood perfusion, within the range of clinically measured values reported in the literature, significantly affects predicted ablation zone volumes. Further research investigating the impact of other tissue properties is warranted.
SU‐E‐T‐405: Evaluation of the Raystation Electron Monte Carlo Algorithm for Varian Linear Accelerators
Authors: Sansourekidou P , Allen C
Published: June 2015

Purpose:

To evaluate the Raystation v4.51 Electron Monte Carlo algorithm for Varian Trilogy, IX and 2100 series linear accelerators and commission for clinical use.

Methods:

Seventy two water and forty air scans were acquired with a water tank in the form of profiles and depth doses, as requested by vendor. Data was imported into Rayphysics beam modeling module. Energy spectrum was modeled using seven parameters. Contamination photons were modeled using five parameters. Source phase space was modeled using six parameters. Calculations were performed in clinical version 4.51 and percent depth dose curves and profiles were extracted to be compared to water tank measurements. Sensitivity tests were performed for all parameters. Grid size and particle histories were evaluated per energy for statistical uncertainty performance.

Results:

Model accuracy for air profiles is poor in the shoulder and penumbra region. However, model accuracy for water scans is acceptable. All energies and cones are within 2%/2mm for 90% of the points evaluated. Source phase space parameters have a cumulative effect. To achieve distributions with satisfactory smoothness level a 0.1cm grid and 3,000,000 particle histories were used for commissioning calculations. Calculation time was approximately 3 hours per energy.

Conclusion:

Raystation electron Monte Carlo is acceptable for clinical use for the Varian accelerators listed. Results are inferior to Elekta Electron Monte Carlo modeling. Known issues were reported to Raysearch and will be resolved in upcoming releases. Auto‐modeling is limited to open cone depth dose curves and needs expansion.
SU‐E‐T‐406: Evaluation of the Stealth Chamber by IBA Under Large Field Conditions
Authors: Quino L Vazquez , Hernandez C Huerta , DeWeese M
Published: June 2015

Purpose:

To evaluate the performance of the stealth chamber by IBA under large field conditions.

Methods:

Measurement of PDDs and profiles for fields of 10×10, 15×15, 20×20 and 25×25 cm2 were acquired in a blue phantom2 scanning system for 6 and 15 MV beams in a Varian TrueBeam linear accelerator. The stealth chamber has a field size limit of 25×25 cm2. Measurements with two different detector set were performed. First a cc13 chamber as field and reference chamber was acquired. And with a cc13 chamber as field detector and the stealth chamber as the reference chamber. PDDs for the fields mentioned above were conducted and profiles with the same fields at depth of dmax, 5, 10 and 20 cm were acquired for comparison.

Results:

Examination of the PDDs and profile comparison with a criterion of 1% and 1 mm agreement was performed. Very good agreement between the cc13 and stealth chamber set was observed within the chosen criterion.

Conclusion:

This work indicates the comparable results obtained with the use of the stealth chamber and cc13 as the reference chamber in relative dosimetry measurements.
SU‐E‐T‐412: Evaluation of Tungsten‐Based Functional Paper for Attenuation Device in Intraoperative Radiotherapy for Breast Cancer
Authors: Kamomae T , Monzen H , Okudaira K , Miyake Y , Oguchi H , Komori M , Kawamura M , Itoh Y , Kikumori T , Naganawa S
Published: June 2015

Purpose:

Intraoperative radiotherapy (IORT) with an electron beam is one of the accelerated partial breast irradiation methods that have recently been used in early‐stage breast cancer. A protective acrylic resin‐copper disk is inserted between the breast tissue and the pectoralis muscle to minimize the dose received by the posterior structures. However, a problem with this protective disk is that the surgical incision must be larger than the field size because the disk is manufactured from stiff and unyielding materials. The purpose of this study was to assess the applicability of a new tungsten‐based functional paper (TFP) as an alternative to the existing protective disk in IORT.

Methods:

The newly introduced TFP (Toppan Printing Co., Ltd., Tokyo, JP) is anticipated to become a useful device that is lead‐free, light, flexible, and easily processed. The radiation shielding performance of TFP was verified by experimental measurements and Monte Carlo (MC) simulations using PHITS code. The doses transmitted through the protective disk or TFP were measured on a Mobetron mobile accelerator. The same geometries were then reproduced, and the dose distributions were simulated by the MC method.

Results:

The percentages of transmitted dose relative to the absence of the existing protective disk were lower than 2% in both the measurements and MC simulations. In the experimental measurements, the percentages of transmitted dose for a 9 MeV electron beam were 48.1, 2.3, and 0.6% with TFP thicknesses of 1.9, 3.7, and 7.4 mm, respectively. The percentages for a 12 MeV were 76.0, 49.3, 20.0, and 5.5% with TFP thicknesses of 1.9, 3.7, 7.4, and 14.8 mm, respectively. The results of the MC simulation showed a slight dose increase at the incident surface of the TFP caused by backscattered radiation.

Conclusion:

The results indicate that a small‐incision procedure may be possible by the use of TFP.
SU‐E‐T‐429: Feasibility Study On Three‐Dimensional GRID Therapy in Conventional Linacs
Authors: Yan Y , Meyer J , Timmerman R , Hrycushko B , Chen B , Saha D , Jiang S
Published: June 2015

Purpose:

Two‐dimensional GRID therapy, traditionally planned and delivered using a dedicated GRID block or MLC modulation, has shown clinical efficacy in treating bulky tumors. However, the large dose to normal tissues outside target can be limiting. We hypothesize that modulation in the third dimension will improve dose sparing of normal tissues, maximize the bystander effect within the target, and ultimately improve the therapy effectiveness. This study aims to investigate the feasibility of a three‐dimensional GRID technique using conventional LINACs to achieve a 3D lattice of high dose volumes within a target.

Methods:

Datasets of patient's having large tumor sizes were used to investigate the planning and delivering of 3D GRID using a Varian TrueBeam linac. Original patient contours of PTV are exported from a TPS to DICOManTX where 3D GRID targets are generated in programmable configurations. A structure of avoidance (SOA), i.e., PTV minus GRID targets, is also generated to facilitate inverse planning to achieve the desired pattern. The artificial structures were sent back to the TPS where an IMRT or VMAT plan is designed to deliver a desired high dose to GRID targets while minimizing the dose to the SOA as much as possible.

Results:

The programmable GRID target generator enables us to modify the target geometry to maximize the peak‐to‐valley ratio. Preliminary results show that plans based on spherical GRID targets achieve a higher peak‐to‐valley dose ratio compared with cylindrical targets. High dose spillage outside the target was eliminated. IMRT planning requires the number of beams to be larger than 16, while for VMAT the number of arcs should be at least 4 in order to achieve dosimetric goals.

Conclusion:

Planning and delivering 3D GRID therapy using conventional LINACs was shown to be feasible. More research and development are required before this new modality can be implemented clinically.