D Johnson

Interplay effect of angular dependence and calibration field size of MapCHECK 2 on RapidArc quality assurance.

The purpose of this study is to investigate an effect of angular dependence and calibration field size of MapCHECK 2 on RapidArc QA for 6, 8, 10, and 15 MV The angular dependence was investigated by comparing MapCHECK 2 measurements in MapPHAN‐MC2 to the corresponding Eclipse calculations every 10° using 10 × 10 cm2 and 3 × 3 cm2 fields. Fourteen patients were selected to make RapidArc plans using the four energies, and verification plans were delivered to two phantom setups: MapCHECK 2/MapPHAN phantom (MapPHAN QA) and MapCHECK 2 on an isocentric mounting fixture (IMF QA). Migration of MapCHECK 2 on IMF was simulated by splitting arcs every 10° and displacing an isocenter of each partial arc in the Eclipse system (IMFACTUAL QA). To investigate the effect of calibration field size, MapCHECK 2 was calibrated by two field sizes (10 × 10 cm2 and 3 × 3 cm2) and applied to all QA measurements. The γ test was implemented using criteria of 1%/1 mm, 2%/2 mm, and 3%/3 mm. A mean dose of all compared points for each plan was compared with respect to a mean effective field size of the RapidArc plan. The angular dependence was considerably high at gantry angles of 90° ± 10° and 270° ± 10° (for 10 × 10/3 × 3 cm2 at 90°, 30.6% ± 6.6%/33.4%± 5.8% (6 MV), 17.3% ± 5.3%/15.0% ± 6.8% (8 MV), 8.9% ± 2.9%/7.8% ± 3.2% (10 MV), and 2.2% ± 2.3%/‐1.3% ± 2.6% (15 MV)). For 6 MV, the angular dependence significantly deteriorated the γ passing rate for plans of large field size in MapPHAN QA (< 90% using 3%/3 mm); however, these plans passed the γ test in IMFACTUAL QA (> 95%). The different calibration field sizes did not make any significant dose difference for both MapPHAN QA and IMFACTUAL QA. For 8, 10, and 15 MV, the angular dependence does not make any clinically meaningful impact on MapPHAN QA. Both MapPHAN QA and IMFACTUAL QA presented clinically acceptable γ passing rates using 3%/3 mm. MapPHAN QA showed better passing rates than IMFACTUAL QA for the tighter criteria. The 10 × 10 cm2 calibration showed better agreement for plans of small effective field size (< 5 × 5 cm2) in MapPHAN QA. There was no statistical difference between IMF QA and IMFACTUAL QA. In conclusion, MapPHAN QA is not recommended for plans of large field size, especially for 6 MV, and MapCHECK 2 should be calibrated using a field size similar to a mean effective field size of a RapidArc plan for better agreement for IMF QA. PACS numbers: 87.55.km, 87.55.Qr, 87.56.Fc

Clinical implementation of an empirical method for electron output factor determination.

The objective of this work has been to develop and implement an empirical calculation method for the determination of clinical electron output factors. Electron beams with various energies, field sizes, and source to surface distances using cutouts of varying radii were used to measure dose output at the depth of maximum dose in water. A 30 cm x 30 cm x 17.8 cm water equivalent phantom with a 0.125 cc cylindrical ion-chamber (PTW Model 31010) was used. The calculation model predicted the output factor as a product of the cone factor, radius dependent cutout factor, the effective source to surface distance factor and the area dependent aspect ratio factor. A comparative analysis of clinical cutout output factors, determined through both empirical calculation and direct measurement was performed to evaluate the clinical viability of the calculation method before its implementation in our clinic. A total of 643 output factors for 294 different cutout shapes were determined through both traditional measurement and predictive calculation. Predictive calculation differed from definitive measurement by at most 3.5% for all cases, a majority of cases falling within 1%. The method developed successfully predicts electron output factors on the basis of cutout geometry with accuracy better than 96% for all cases and better then 98% for most cases. This ability holds true for all practical SSD, electron energy, cone, and irregular shape combinations. The method has been clinically implemented and in use at our center since 2007.

Comparison of tumor and normal tissue dose for accelerated partial breast irradiation using an electronic brachytherapy eBx source and an Iridium-192 source.

The objective of this study has been to compare treatment plans for patients treated with electronic brachytherapy (eBx) using the Axxent System as adjuvant therapy for early stage breast cancer with treatment plans prepared from the same CT image sets using an Ir‐192 source. Patients were implanted with an appropriately sized Axxent balloon applicator based on tumor cavity size and shape. A CT image of the implanted balloon was utilized for developing both eBx and Ir‐192 brachytherapy treatment plans. The prescription dose was 3.4 Gy per fraction for 10 fractions to be delivered to 1 cm beyond the balloon surface. Iridium plans were provided by the sites on 35 of the 44 patients enrolled in the study. The planning target volume coverage was very similar when comparing sources for each patient as well as between patients. There were no statistical differences in mean %V100. The percent of the planning target volume in the high dose region was increased with eBx as compared with Iridium (p<0.001). The mean maximum calculated skin and rib doses did not vary greatly between eBx and Iridium. By contrast, the doses to the ipsilateral lung and the heart were significantly lower with eBx as compared with Iridium (p<0.0001). The total nominal dwell times required for treatment can be predicted by using a combination of the balloon fill volume and planned treatment volume (PTV). This dosimetric comparison of eBx and Iridium sources demonstrates that both forms of balloon‐based brachytherapy provide comparable dose to the planning target volume. Electronic brachytherapy is significantly associated with increased dose at the surface of the balloon and decreased dose outside the PTV, resulting in significantly increased tissue sparing in the heart and ipsilateral lung. PACS numbers: 87,53.Jw, 87.55.dk, 87.55.D‐,87.56 b‐,87.56.bg

Dose and linear energy transfer distributions of primary and secondary particles in carbon ion radiation therapy: A Monte Carlo simulation study in water.

The factors influencing carbon ion therapy can be predicted from accurate knowledge about the production of secondary particles from the interaction of carbon ions in water/tissue-like materials, and subsequently the interaction of the secondary particles in the same materials. The secondary particles may have linear energy transfer (LET) values that potentially increase the relative biological effectiveness of the beam. Our primary objective in this study was to classify and quantify the secondary particles produced, their dose averaged LETs, and their dose contributions in the absorbing material. A 1 mm diameter carbon ion pencil beam with energies per nucleon of 155, 262, and 369 MeV was used in a geometry and tracking 4 Monte Carlo simulation to interact in a 27 L water phantom containing 3000 rectangular detector voxels. The dose-averaged LET and the dose contributions of primary and secondary particles were calculated from the simulation. The results of the simulations show that the secondary particles that contributed a major dose component had LETs <100 keV/µm. The secondary particles with LETs >600 keV/µm contributed only <0.3% of the dose.

SU-F-T-58: Dosimetric Evaluation of Breast Tissue Composition for Electronic Brachytherapy (BET) Source In High Dose Rate Accelerated Partial Breast (APBI) Irradiation

PURPOSE To quantitatively evaluate the dosimetric impact of differing breast tissue compositions for electronic brachytherapy source for high dose rate accelerated partial breast irradiation. METHODS A series of Monte Carlo Simulation were created using the GEANT4 toolkit (version 10.0). The breast phantom was modeled as a semi-circle with a radius of 5.0 cm. A water balloon with a radius of 1.5 cm was located in the phantom with the Xoft AxxentTM EBT source placed at center as a point source. A mixed of two tissue types (adipose and glandular tissue) was assigned as the materials for the breast phantom with different weight ratios. The proportionality of glandular and adipose tissue was simulated in four different fashions, 80/20, 70/30, 50/50 and 30/70 respectively. The custom energy spectrum for the 50 kVp XOFT source was provided via the manufacturer and used to generate incident photons. The dose distributions were recorded using a parallel three dimensional mesh with a size of 30 × 30 × 30 cm3 with 1 × 1 × 1 mm3 voxels. The simulated doses absorbed along the transverse axis were normalized at the distance of 1 cm and then compared with the calculations using standard TG-43 formalism. RESULTS All simulations showed underestimation of dose beyond balloon surface compared to standard TG-43 calculations. The maximum percentage differences within 2 cm distance from balloon surface were found to be 18%, 11%, 10% and 8% for the fat breast (30/70), standard breast (50/50), dense breast (70/30 and 80/20), respectively. CONCLUSION The accuracy of dose calculations for low energy EBT source was limited when considering tissue heterogeneous composition. The impact of atomic number on photo-electric effect for lower energy Brachytherapy source is not accounted for and resulting in significant errors in dose calculation.

SU-G-201-12: Investigation of Beta-Emitter 90Sr-90Y Dose Distribution Using Gafchromic EBT3 Film for Application On Conformal Skin Brachytherapy Device

PURPOSE To investigate 90 Sr-90 Y as a high dose rate (HDR) source for application in a conformal skin brachytherapy (CSBT) device. The CSBT device has been previously developed to provide patient specific treatment for small inoperable lesions and irregular surfaces. METHODS A popular beta emitter, 90 Sr-90 Y was tested for feasibility in a CSBT device. A 1 cm diameter plaque was used to deliver dose to a solid water phantom containing EBT3 Gafchromic films arranged at the surface and perpendicular to it. Additionally, a 1 cm diameter 6 MeV electron beam was used to irradiate EBT3 film at 100 cm SSD with a 0.5 cm bolus. Films were digitized with an Epson Expression 10000 XL scanner and calibrated with a 6 MeV electron specific dose curve. Normalized percent depth doses (PDD) and dose profiles for both techniques were analyzed using ImageJ. RESULTS Dose distributions achieved with the 90 Sr-90 Y sources were compared with those of external electron beam radiation therapy (EBRT). Penumbra (20%- 80%) for EBRT and 90Sr-90Y were 4.3 mm and 1.6 mm, respectively. PDD values of 50% (normalized to 2 mm) were 10.1 mm and 2.8 mm for electron and 90 Sr-90 Y, respectively. Flatness (80% of the central beam profile) was 14.1% at a 5 mm depth for EBRT and 4.0% at surface for the 90 Sr-90 Y. CONCLUSION As expected, the PDDs of 90 Sr-90 Y in water are shallower than that of external electron beams for the same field size. 90 Sr-90 Y can be used in CSBT to provide patient specific treatment where shallower depth doses than that provided by electron external beams may be required: e.g. eyelids, nose, lips, ears, etc. The customizability of EBRT could be replicated by using multiple adjacent 90 Sr-90 Y plaque placements.

SU‐E‐T‐424: Feasibility of 3D Printed Radiological Equivalent Customizable Tissue Like Materials

Purpose: To investigate the feasibility of 3D printing CT# specific radiological equivalent tissue like materials. Methods: A desktop 3D printer was utilized to create a series of 3 cm x 3 cm x 2 cm PLA plastic blocks of varying fill densities. The fill pattern was selected to be hexagonal (Figure 1). A series of blocks was filled with paraffin and compared to a series filled with air. The blocks were evaluated with a “GE Lightspeed” 16 slice CT scanner and average CT# of the centers of the materials was determined. The attenuation properties of the subsequent blocks were also evaluated through their isocentric irradiation via “TrueBeam” accelerator under six beam energies. Blocks were placed upon plastic-water slabs of 4 cm in thickness assuring electronic equilibrium and data was collected via Sun Nuclear “Edge” diode detector. Relative changes in dose were compared with those predicted by Varian “Eclipse” TPS. Results: The CT# of 3D printed blocks was found to be a controllable variable. The fill material was able to narrow the range of variability in each sample. The attenuation of the block tracked with the density of the total fill structure. Assigned CT values in the TPS were seen to fall within an expected range predicted by the CT scans of the 3D printed blocks. Conclusion: We have demonstrated that it is possible to 3D print materials of varying tissue equivalencies, and that these materials have radiological properties that are customizable and predictable.

SU-E-T-388: Evaluation of Electronic Brachytherapy Dose Distributions in Tissue Equivalent Materials

Purpose: To study the measured and calculated dose distributions for electronic brachytherapy (EBT) in various tissue equivalent homogenous materials. Methods: Calculated dose distributions in water were generated using published TG-43 parameters in Varian BrachyVision software for a 50 kVp, 50 cm Xoft source. Dose distributions were measured within a 3D-scanning tank using dosimeters including: PTW 0.125 cc, pin-point, and parallel-plate ion chambers, Sun Nuclear “Edge” diode and Gafchromic EBT3 film. Multi-channel film dosimetry was used in film analysis. EBT3 film curves were calibrated via radial dose comparison to both independently measured and published data. The resulting film calibration was utilized to measure dose distributions created by titanium filtered source utilized in clinical brachytherapy applications. Data was collected within homogenous PMMA, vinyl, polystyrene, paraffin, and water-equivalent plastic phantoms. Results: Ion-chamber data was corrected to effective points of measurement and normalized prior to comparison between calculated and measured dose distributions. Measurements made in water and water equivalent materials compared well with results from treatment planning software. The maximum percent differences (relative to water) observed between 1 cm and 3.5 cm depth from source for each of the phantom materials are as follows: PMMA 35%, polystyrene 41%, plastic-water 23%, vinyl 115%, and paraffin 46%. Conclusion: The increased probability of photoelectric interactions occurring within the patient during electronic brachytherapy will emphasize the radiological differences between varying human tissues in dose deposition. These differences can Result in clinically significant dose perturbations and it is therefore recommended to move to a model based dose calculation, as outlined in TG-186, to improve the dosimetric accuracy of low energy EBT.

SU‐E‐T‐257: A Monte Carlo Simulation Study of Photon Beam with Energies 6X, 10X, 6FFF, 10FFF From a TrueBeam Linear Accelerator

Purpose: To compare the quality of Monte Carlo simulation results of photon beams with multiple energies with the corresponding measured beam commissioning data from a Varian TrueBeam accelerator. Methods: IAEA phase space files for photon beam energies 6X, 10X, 6FFF, 10FFF from TrueBeam linear accelerator provided by Varian were implemented as source files in the GEANT4 Monte Carlo code. The simulation geometry consisted of upper and lower tungsten alloy jaws and a cubic water phantom (50 × 50 × 50 cm3). Central axis depth dose curves (PDD) for multiple field sizes (1×1, 2×2, 4×4, 6×6, 10×10, 20×20, 30×30 cm2) and radial and transverse dose profiles for identical field sizes at 5 different depths (Dmax, 5, 10, 20, 30 cm) for all four energies were recorded in the phantom. Each simulation was performed with 2.0E9 incident particles. The results of PDD and profiles from simulations were then compared with the corresponding beam commissioning data collected with a Wellhoffer Blue Phantom using a 0.13cc ion‐chamber and a 0.8 × 0.8 mm2 diode. Results: The GEANT4 simulated PDD curve for photon beam compared favorably well, within ∼2%, against the corresponding measured ion‐chamber PDD for all energies and field sizes. The simulated in‐plane and cross‐plane profiles also compared well, within 2 mm, at the 50% level against the measured profiles for all energies, field sizes and depths. Conclusion: For the commissioning of a linear accelerator, the feasibility of utilizing Monte‐Carlo simulated beam data has been demonstrated in this study. The increase of computer speed and capabilities may Result in the adoption of Monte‐Carlo techniques for future machine commissioning and dose calculations.

SU‐E‐T‐726: A Monte Carlo Simulation Study for Production and Subsequent Interaction of Secondary Particles From Carbon‐Ion Radiation Therapy in Water

Purpose: To classify and quantify the secondary particles produced from energetic carbon ion interactions in water, their dose averaged linear energy transfer (LET)s and their dose contributions. Methods: A 1 mm diameter carbon ion pencil beam with various energies per nucleon was used in a GEANT4 simulation to interact with a 27 liter water phantom packed with 3000 rectangular detector sheets. The simulations involved processes arising from electromagnetic interactions (EMI) and hadronic interactions (HI). The EMI govern particles energy loss and straggling with atomic electrons and multiple scattering with atomic nuclei. The HI define ions elastic and inelastic scattering and nuclear interactions with atomic nuclei in the medium. Dose averaged LETs and dose contributions of primary and secondary particles were calculated. Results: The percentages of the total dose deposited in regions A and B of the phantom by primary and secondary particles were 93.54%, 85.61% and 76.60% in A; 28.97%, 17.77% and 13.08% in B, for beam energy/nucleon of 155, 262 and 369 MeV, respectively, where region A covered beam path from phantom surface to 90% distal edge of Bragg peak and region B covered from 90% distal edge of Bragg peak to 5 cm past Bragg peak. The secondary particles proton, 11C, alpha and 11B contributed 4.17%, 9.26% and 15.21% in region A and 49.10%, 55.13% and 58.07% in region B for beam energy per nucleon of 155, 262 and 369 MeV, respectively. Conclusion: The results of the simulations showed that those secondary particles that contributed major dose component had LETs 600 keV/micrometer contributed < 0.3% towards the total dose.