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All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

TU-C-17A-12: Towards a Passively Optimized Phase-Space Monte Carlo (POPMC) Treatment Planning Method: A Proof of Principle

Purpose: The advent of on-line/off-line adaptive, and biologically-conformal radiation therapy has led to a need for treatment planning solutions that utilize voxel-specific penalties, requiring optimization over a large solution space that is performed quickly, and the dose in each voxel calculated accurately. This work proposes a “passive” optimization framework, which is executed concurrently during Monte Carlo dose calculation, evaluating the cost/benefit of each history during transport, and creates a passively optimized fluence map. Methods: The Monte Carlo code Geant4 v9.6 was used for this study. The standard voxel geometry implementation was modified to support the passive optimization framework, with voxel-specific optimization parameters. Dose-benefit functions, which will increase a particle history’s weight upon dose deposition, were defined in a central collection of voxels to effectively create target structures. Histories that deposit energy to voxels are reweighted based on a voxel’s dose multiplied by its cost/benefit value. Upon full termination of each history, the dose contributions of that history are reweighted to reflect a contribution proportional to the history’s final weight. A parallel-planar 1.25 MeV photon fluence is transported through the geometry, and re-weighted at each dose deposition step. The resulting weight is tallied with the incident spatial and directional coordinates in a phase-space distribution. Results: A uniform incident fluence was reweighted during MC dose calculations to create an optimized fluence map which would generate dose profiles in target volumes that exhibit the same dose characteristics as the prescribed optimization parameters. An optimized dose profile, calculated concurrently with the phase-space, reflects the resulting dose distribution. Conclusion: This study demonstrated the feasibility of passively optimizing an incident fluence map during Monte Carlo dose calculations. The flexibility of the voxel-specific optimization framework allows a variety and combination of optimization parameters to be calculated for each voxel at every transportation step. This work is partially supported by Varian. This work is partially supported by the NIH.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

TU-F-18A-07: To Explore the More Realistic Energy Responses of the In-Depth Photon Counting Detectors

Purpose: We study the effect of the secondary photon events on modeling the energy response functions of the In-depth photon counting X-ray detectors (PCXD) and the potential impact of the spectral distortion on material decompositions. Methods: Square-shape wafers of three potential PCXD materials were constructed (5-by-20-by-30 mm{sup 3} for Si, 4-by-20-by-5 mm{sup 3} for GaAs and 4-by-20-by-3 mm{sup 3} for CdTe), with pixel size of 5-by-4 mm{sup 2} for Si and 5-by-5 mm{sup 2} for GaAs and CdTe. The depth direction (z-direction) was segmented into 5 layers with exponentially increasing thicknesses of each layer. X-rays from 10keV to 120keV with 20000 photons per keV bin was simulated to characterize the energy response function of each PCXD using Geant4. Secondary photons events were recorded and we omitted the photons exiting the detector. The Energy Response Functions (ERFs) from the Monte Carlo (MC) simulations were compared with those from a semi-ideal model developed earlier. Results: For Si, detection of secondary events in the center detector were minimal due to the long aspect ratio of the detector, which results in the agreement between the theoretical prediction and the MC simulation with and without the secondary photons. For CdTe, the secondary photons captured by the center pixel were important, leading to obvious disagreement between the analytical and the simulated ERF. After correction for secondary events, the two curves were more similar except for the escape peaks which are not correctly portrayed by the semi-ideal model. For GaAs, the behavior is in between Si and CdTe. Conclusion: Given the complexity of the In-Depth PCXD's geometry, the uniform semi-ideal model does not fully characterize the ERF at each layer. Therefore, more realistic models need to be explored for better modeling of the spectral distortion.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

WE-D-BRF-02: Acoustic Signal From the Bragg Peak for Range Verification in Proton Therapy

Purpose: Range verification in ion beam therapy relies to date on nuclear imaging techniques which require complex and costly detector systems. A different approach is the detection of thermoacoustic signals that are generated due to localized energy loss of ion beams. Aim of this work is to study the feasibility of determining the ion range with sub-mm accuracy by use of high frequency ultrasonic (US) transducers and to image the Bragg peak by tomography. Methods: A water phantom was irradiated by a pulsed 20 MeV proton beam with varying pulse intensity, length and repetition rate. The acoustic signal of single proton pulses was measured by different PZT-based US detectors (3.5 MHz and 10 MHz central frequencies). For tomography a 64 channel US detector array was used and moved along the ion track by a remotely controlled motor stage. Results: A clear signal of the Bragg peak was visible for an energy deposition as low as 10{sup 12} eV. The signal amplitude showed a linear increase with particle number per pulse and thus, dose. Range measurements were reproducible within +/− 20 micrometer and agreed well with Geant4 simulations. The tomographic reconstruction does not only allow to measure the ion range but also the beam spot size at the Bragg peak position. Conclusion: Range verification by acoustic means is a promising new technique for treatment modalities where the tumor can be localized by US imaging. Further improvement of sensitivity is required to account for higher attenuation of the US signal in tissue, as well as lower energy density in the Bragg peak in realistic treatment cases due to higher particle energy and larger spot sizes. Nevertheless, the acoustic range verification approach could offer the possibility of combining anatomical US imaging with Bragg Peak imaging in the near future. The work was funded by the DFG cluster of excellence Munich Centre for Advanced Photonics (MAP)

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

WE-E-BRE-11: New Method to Simulate DNA Damage Using Ionization Cross-Sections and a Geometrical Nucleosome Model

Purpose: To obtain probability distributions of various DNA damage types as a function of the incident electron kinetic energy. Methods: Using Geant4-DNA electron ionization cross-sections, we calculated path length distributions for electrons of energies between 10 eV and 1 MeV, defined as the length between two subsequent interactions. These path lengths were then convolved with probability distributions for the creation of same-strand damage, opposite-strand damage, clustered damage, isolated damage, and same DNA strand target damage. These probability distributions of DNA damage were obtained by a Monte Carlo routine calculating probabilities of interaction in DNA targets inside a nucleosome geometrical model. Results represent the probability of a secondary electron, initially created inside a DNA strand target, of undergoing its next interaction: (1) in the opposite strand (DSB), (2) in the same strand (SSB+), (3) in either the opposite or same-strand (clustered), (4) in the same DNA target (multiple-hit) or (5) more than 10 base pairs away (isolated). Results: Electrons with kinetic energy between 50 and 250 eV have a maximal probability of creating DSB, SSB+, clustered damage and multiple-hits in the same target The probabilities for these damage patterns have values of 2.5%, 4.3%, 6.7% and 5.4%, respectively. Isolated damage is most probable between 700 eV to 900 eV with a probability of 0.2%. Conclusion: We obtained DNA damage probability distributions as a function of electron incident energy. We showed that electrons with kinetic energies between 50 and 250 eV have the highest probability of producing complex forms of DNA damage (DSB, SSB+). We also showed that a double ionization within the same DNA target is the most frequent outcome occurring 5% of the time. It is expected that electron slowing down spectra can be convolved with our formalism to calculate source specific DNA damage patterns. Research grants from governments of Canada and Quebec. PP acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290)

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

WE-E-18A-02: Enhancement of Lung Tumor Visibility by Dual-Energy X-Ray Imaging in An Anthropomorphic Chest Phantom Study

Purpose: Intrafractional lung tumor motion during radiotherapy can be compensated for by tracking the tumor position using x-ray imaging and adapting the treatment in real-time. However, locating the tumor with an automated template-matching algorithm is often challenging if the tumor is obscured by ribs. This study investigates the feasibility of creating dual-energy (DE) images of the chest with increased tumor visibility on an Elekta XVI system. Methods: An anthropomorphic chest phantom was imaged at two different energies. Low-energy images were obtained at 80 kVp (0.8 mAs); high-energy images at 129 kVp (0.6 mAs, additional 1.26 mm tin filter). A Geant4 Monte-Carlo framework was developed allowing simulation of the x-ray tube, flat-panel detector and phantom in order to optimize the beam energies, filtration and the weighting factor used to subtract the individual images into a synthetic DE image. The weighting factor was selected to minimize the visibility of bones while maintaining a sufficient tumor visibility. We scored the bone visibility as the contrast of tumor (with bone) to tumor (without bone), and similarly of lung tissue (with bone) to lung tissue (without bone). Tumor visibility was quantified as the contrast between tumor and lung tissue (both without bone). Results: In the experimentally obtained DE image the bone visibility was reduced by 79.2% in tumor and by 96.8% in lung tissue while the overall tumor visibility only decreased by 69.5%. The Monte-Carlo simulation yielded similar results reducing the scores by 90.0%, 85.3% and only 71.9%, respectively. Conclusion: This work demonstrates the feasibility of DE imaging to enhance lung tumor detectability. In the future, we hope to further refine the Monte-Carlo simulation to more accurately predict the weighting factors which would aid real-time implementation. Furthermore, we plan to use the Monte-Carlo framework to simulate DE images of actual lung tumors. The authors would like to thank Paul-Scherrer-Institut and Centre Suisse d'Electronique et Microtechnique for lending us the anthropomorphic phantom. We acknowledge support from Elekta AB under a research agreement. Research at The Institute of Cancer Research is also supported by Cancer Research UK under Programme C46/A10588.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

WE-E-18A-08: Towards a Next-Generation Electronic Portal Device for Simultaneous Imaging and Dose Verification in Radiotherapy

Purpose: This work forms part of an ongoing study to develop a next-generation electronic portal imaging device (EPID) for simultaneous imaging and dose verification in radiotherapy. Monte Carlo (MC) simulations were used to characterize the imaging performance of a novel EPID that has previously been demonstrated to exhibit a water-equivalent response. The EPID ' s response was quantified in several configurations and model parameters were empirically validated against experimental measurements. Methods: A MC model of a novel a-Si EPID incorporating an array of plastic scintillating fibers was developed. Square BCF-99-06A scintillator fibers with PMMA cladding (Saint-Gobain Crystals) were modelled in a matrix with total area measuring 150×150 mm{sup 2}. The standard electromagnetic and optical physics Geant4 classes were used to simulate radiation transport from an angled slit source (6 MV energy spectrum) through the EPID and optical photons reaching the photodiodes were scored. The prototype's modulation transfer function (MTF) was simulated and validated against experimental measurements. Several optical transport parameters, fiber lengths and thicknesses of an air gap between the scintillator and photodiodes were investigated to quantify their effects on the prototype's detection efficiency, sensitivity and MTF. Results: Simulated EPID response was more sensitive to variations in geometry than in the optical parameters studied. The MTF was particularly sensitive to the introduction of a 0.5–1.0 mm air gap between the scintillator and photodiodes, which lowered the MTF relative to that simulated without the gap. As expected, increasing the fiber length increased the detector efficiency and sensitivity while decreasing the MTF. Conclusion: A model of a novel water-equivalent EPID has been developed and benchmarked against measurements using a physical prototype. We have demonstrated the feasibility of this new device and are continuing to optimize the design to achieve an imaging response that warrants the development of a next-generation prototype.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

TH-A-19A-01: An Open Source Software for Proton Treatment Planning in Heterogeneous Medium

Purpose: Due to its success in Radiation Oncology during the last decade, interest in proton therapy is on the rise. Unfortunately, despite the global enthusiasm in the field, there is presently no free, multiplatform and customizable Treatment Planning System (TPS) providing proton dose distributions in heterogenous medium. This restricts substantially the progress of clinical research for groups without access to a commercial Proton TPS. The latest implementation of our pencil beam dose calculation algorithm for proton beams within the 3D Slicer open-source environment fulfills all the conditions described above. Methods: The core dose calculation algorithm is based on the Hong algorithm (1), which was upgraded with the Kanematsu theory describing the evolution of the lateral scattering of proton beamlets in heterogeneous medium. This algorithm deals with both mono-energetic beams and Spread Out Bragg Peak (SOBP). In order to be user-friendly, we provide a graphical user interface implemented with the Qt libraries, and visualization with the 3D Slicer medical image analysis software. Two different pencil beam algorithms were developed, and the clinical proton beam line at our facility was modeled. Results: The dose distributions provided by our algorithms were compared to dose distributions coming from both commercialized XiO TPS and literature (dose measurements, GEANT4 and MCNPx) and turned out to be in a good agreement, with maximum dose discrepancies of 5% in homogeneous phantoms and 10% in heterogeneous phantoms. The algorithm of SOBP creation from an optimized weigthing of mono-energetic beams results in flat SOBP. Conclusion: We hope that our efforts in implementing this new, open-source proton TPS will help the research groups to have a free access to a useful, reliable proton dose calculation software.(1) L. Hong et al., A pencil beam algorithm for proton dose calculations, Phys. Med. Biol. 41 (1996) 1305–1330. This project is paid for by NCI/MGH Federal share as a research grant.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2014

TH-A-19A-04: Latent Uncertainties and Performance of a GPU-Implemented Pre-Calculated Track Monte Carlo Method

Purpose: Assessing the performance and uncertainty of a pre-calculated Monte Carlo (PMC) algorithm for proton and electron transport running on graphics processing units (GPU). While PMC methods have been described in the past, an explicit quantification of the latent uncertainty arising from recycling a limited number of tracks in the pre-generated track bank is missing from the literature. With a proper uncertainty analysis, an optimal pre-generated track bank size can be selected for a desired dose calculation uncertainty. Methods: Particle tracks were pre-generated for electrons and protons using EGSnrc and GEANT4, respectively. The PMC algorithm for track transport was implemented on the CUDA programming framework. GPU-PMC dose distributions were compared to benchmark dose distributions simulated using general-purpose MC codes in the same conditions. A latent uncertainty analysis was performed by comparing GPUPMC dose values to a “ground truth” benchmark while varying the track bank size and primary particle histories. Results: GPU-PMC dose distributions and benchmark doses were within 1% of each other in voxels with dose greater than 50% of Dmax. In proton calculations, a submillimeter distance-to-agreement error was observed at the Bragg Peak. Latent uncertainty followed a Poisson distribution with the number of tracks per energy (TPE) and a track bank of 20,000 TPE produced a latent uncertainty of approximately 1%. Efficiency analysis showed a 937× and 508× gain over a single processor core running DOSXYZnrc for 16 MeV electrons in water and bone, respectively. Conclusion: The GPU-PMC method can calculate dose distributions for electrons and protons to a statistical uncertainty below 1%. The track bank size necessary to achieve an optimal efficiency can be tuned based on the desired uncertainty. Coupled with a model to calculate dose contributions from uncharged particles, GPU-PMC is a candidate for inverse planning of modulated electron radiotherapy and scanned proton beams. This work was supported in part by FRSQ-MSSS (Grant No. 22090), NSERC RG (Grant No. 432290) and CIHR MOP (Grant No. MOP-211360)

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

TH-A-19A-05: Modeling Physics Properties and Biologic Effects Induced by Proton and Helium Ions

Purpose: Currently, proton and carbon ions are used for cancer treatment. More recently, other light ions including helium ions have shown interesting physical and biological properties. The purpose of this work is to study the biological and physical properties of helium ions (He-3) in comparison to protons. Methods: Monte Carlo simulations with FLUKA, GEANT4 and MCNPX were used to calculate proton and He-3 dose distributions in water phantoms. The energy spectra of proton and He-3 beams were calculated with high resolution for use in biological models. The repair-misrepairfixation (RMF) model was subsequently used to calculate the RBE. Results: The proton Bragg curve calculations show good agreement between the three general purpose Monte Carlo codes. In contrast, the He-3 Bragg curve calculations show disagreement (for the magnitude of the Bragg peak) between FLUKA and the other two Monte Carlo codes. The differences in the magnitude of the Bragg peak are mainly due to the discrepancy in the secondary fragmentation cross sections used by the codes. The RBE for V79 cell lines is about 0.96 and 0.98 at the entrance of proton and He-3 ions depth dose respectively. The RBE increases to 1.06 and 1.59 at the Bragg peak of proton and He-3 ions. The results demonstrated that LET, microdosimetric parameters (such as dose-mean lineal energy) and RBE are nearly constant along the plateau region of Bragg curve, while all parameters increase within the Bragg peak and at the distal edge for both proton and He-3 ions. Conclusion: The Monte Carlo codes should revise the fragmentation cross sections to more accurately simulate the physical properties of He-3 ions. The increase in RBE for He-3 ions is higher than for proton beams at the Bragg peak.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

TH-A-19A-07: The Effect of Particle Tracking Step Size Limit On Monte Carlo- Computed LET Spectrum of Therapeutic Proton Beams

Purpose: To investigate the effect of charged particle tracking step size limit in the determination of the LET spectrum of therapeutic proton beams using Monte Carlo simulations. Methods: The LET spectra at different depths in a water phantom from a 79.7 MeV spot-scanning proton beam were calculated using Geant4. Five different tracking step limits 0.5 mm, 0.1 mm, 0.05 mm, 0.01 mm and 1 μm were adopted. The field size was set to 10×10 cm{sup 2} on the isocenter plane. A 40×40×6 cm{sup 3} water phantom was modelled as the irradiation target. The voxel size was set to 1×1×0.5 mm{sup 3} to obtain high resolution results. The LET spectra were scored ranging from 0.01 keV/μm to 10{sup 4}keV/μm in the logarithm scale. In addition, the proton energy spectra at different depths were also scored. Results: The LET spectra calculated using different step size limits were compared at four depths along the Bragg curve. At any depths, the spread of the LET spectra increases with the decrease of step size limit. In the dose buildup region (z = 1.9 cm) and in the region proximal to the Bragg peak (z = 3.95 cm), the frequency mean LET does not vary with decreasing step size limit. At Bragg peak (z = 4.75 cm) and in the distal edge (z = 4.85 cm), frequency mean LET decreases with decreasing step size limit. The energy spectrum at any specified depths does not vary with the step size limit. Conclusion: The calculated LET has a spectral distribution rather than a single value at any depths along the Bragg curve and the spread of the computed spectrum depends on the tracking step limit. Incorporating the LET spectrum distribution into the robust IMPT optimization plan may provide more accurate biological dose distribution than using the dose- or fluence-averaged LET. NIH Program Project Grant P01CA021239.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

TH-C-17A-04: Shining Light On the Implementation of Cherenkov Emission in Radiation Therapy

Purpose: We hypothesize that Cherenkov emission (CE) by radiotherapy beams is correlated with radiation dose, CE detection can be maximized by a spectral shift towards the near-infrared (NIR) window of biological tissue, and in certain tissue types (ex. breast/oropharynx), it could prove superior to mega-voltage (MV) imaging. Therefore, we compare CE imaging to onboard MV imaging. Methods: Dose-CE correlation was investigated via simulation and experiment. A Monte Carlo (MC) CE simulator was designed using Geant4. Experimental phantoms include: water; tissuesimulating phantom composed of water, fat emulsion, and beef blood; plastic phantom with solid water insert. The optical spectrometry system consisted of a multi-mode optical fiber and diffraction-grating spectrometer incorporating a front/back-illuminated charge-coupled device (CCD). CdSe/ZnS quantum dots (QDs), emitting at (650±10) nm, were used to achieve NIR shift of the CE signal. CE and MV images were acquired with a complementary metal-oxide-semiconductor (CMOS) camera and an electronic portal imaging device (EPID), respectively. Results: MC and experimental studies indicate a strong linear correlation between radiation dose and CE (Pearson coefficient > 0.99). CE by an 18 MeV beam was effectively shifted towards the NIR in water and in a tissue-simulating phantom, exhibiting a 50% increase at 650 nm for QD depths of ∼3 mm. CE images exhibited relative contrast superior to EPID images by a factor of 30. Conclusion: Our work supports the potential for application of CE in radiotherapy online imaging for patient setup and treatment verification, since CE is intrinsic to the beam and non-ionizing, and QDs can be used to improve CE detectability, yielding image quality superior to MV imaging for the case of low density variability, low optical attenuation materials, such as breast or oropharyngeal cavities. Ongoing work involves microenvironment functionalization of QDs and application of multichannel spectrometry for simultaneous acquisition of dosimetric and tumor oxygenation signals. Funding received from the following organizations: Natural Sciences and Engineering Research Council of Canada, McGill University. YZ acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290)

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

SU-C-16A-07: Sculpting Isodose Lines: Design of An Internally Shielded Tandem for Cervical Cancer Brachytherapy

Purpose: With the prescription method moving from point A to 3D volume based in cervical cancer HDR brachytherapy, the traditional pear-shaped isodose lines are desired to be sculptured to conform to the irregular shaped target. The standard single channel tandem cannot generate asymmetric isodose lines. Most of the directionally shielded sources proposed in literature are challenging to manufacture and operate. In this study, we proposed a novel internally shielded tandem applicator design which gave users more freedom to manipulate isodose lines while planning. Methods: The proposed tandem design has one centrally located lead cylindrical rod of 8 mm in diameter serving as the internal shield. Multiple source channels with the diameter of 2 mm are evenly spaced and engraved on the central cylindrical rod. The overall diameter of the tandem with polymer encapsulation was kept to be 10 mm. Various number of channels and engraving depths have been tested in the design process. Geant4 Monte Carlo toolkit was used for dose calculation assuming a Varian VS2000 source was placed inside the applicator. A Monte Carlo based planning system has been developed in-house to generate brachytherapy plans. Test plans by using this internally shielded tandem were generated for 3 clinical cases. Results: Water phantom results shown the dose distribution from a VS2000 source in the tandem was strongly distorted towards one direction due to the presence of shielding material. Conformal plans with asymmetric isodose distributions around the tandem can be generated by optimizing dwell times in different channels. Conclusion: An effective and easy-to-use internally shielded tandem was developed. It gave user the freedom to sculpt isodose lines to generate conformal plans for cervical cancer brachytherapy.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

SU-C-BRE-07: Sensitivity Analysis of the Threshold Energy for the Creation of Strand Breaks and of Single and Double Strand Break Clustering Conditions

Purpose: To analyse the sensitivity of the creation of strand breaks (SB) to the threshold energy (Eth) and thresholding method and to quantify the impact of clustering conditions on single strand break (SSB) and double strand break (DSB) yields. Methods: Monte Carlo simulations using Geant4-DNA were conducted for electron tracks of 280 eV to 220 keV in a geometrical DNA model composed of nucleosomes of 396 phospho-diester groups (PDGs) each. A strand break was created inside a PDG when the sum of all energy deposits (method 1) or energy transfers (method 2) was higher than Eth or when at least one interaction deposited (method 3) or transferred (method 4) an energy higher than Eth. SBs were then clustered into SSBs and DSBs using clustering scoring criteria from the literature and compared to our own. Results: The total number of SBs decreases as Eth is increased. In addition, thresholding on the energy transfers (methods 2 and 4) produces a higher SB count than when thresholding on energy deposits (methods 1 and 3). Method 2 produces a step-like function and should be avoided when attempting to optimize Eth. When SBs are grouped into damage patterns, clustering conditions can underestimated SSBs by up to 18 % and DSBs can be overestimated by up to 12 % compared to our own implementation. Conclusion: We show that two often underreported simulation parameters have a non-negligible effect on overall DNA damage yields. First more SBs are counted when using energy transfers to the PDG rather than energy deposits. Also, SBs grouped according to different clustering conditions can influence reported SSB and DSB by as much as 20%. Careful handling of these parameters is required when trying to compare DNA damage yields from different authors. Research funding from the governments of Canada and Quebec. PP acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290)

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

A study of the short- to long-phantom dose ratios for CT scanning without table translation

Purpose: For CT scanning in the stationary-table modes, AAPM Task Group 111 proposed to measure the midpoint dose on the central and peripheral axes of sufficiently long phantoms. Currently, a long cylindrical phantom is usually not available in many clinical facilities. The use of a long phantom is also challenging because of the heavy weight. In order to shed light on assessing the midpoint dose in CT scanning without table movement, the authors present a study of the short- to long-phantom dose ratios, and perform a cross-comparison of CT dose ratios on different scanner models. Methods: The authors performed Geant4-based Monte Carlo simulations with a clinical CT scanner (Somatom Definition dual source CT, Siemens Healthcare), and modeled dosimetry measurements using a 0.6 cm{sup 3} Farmer type chamber and a 10-cm long pencil ion chamber. The short (15 cm) to long (90 cm) phantom dose ratios were computed for two PMMA diameters (16 and 32 cm), two phantom axes (the center and the periphery), and a range of beam apertures (3–25 cm). The results were compared with the published data of previous studies with other multiple detector CT (MDCT) scanners and cone beam CT (CBCT) scanners. Results: The short- to long-phantom dose ratios changed with beam apertures but were insensitive to beam qualities (80–140 kV, the head and body bowtie filters) and MDCT and CBCT scanner models. Conclusions: The short- to long-phantom dose ratios enable medical physicists to make dosimetry measurements using the standard CT dosimetry phantoms and a Farmer chamber or a 10 cm long pencil chamber, and to assess the midpoint dose in long phantoms. This method provides an effective approach for the dosimetry of CBCT scanning in the stationary-table modes, and is useful for perfusion and interventional CT.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

Poster — Thur Eve — 18: Cherenkov Emission By High-Energy Radiation Therapy Beams: A Characterization Study

We investigate Cherenkov emission (CE) by radiotherapy beams via radiation dose-versus-CE correlation analyses, CE detection optimization by means of a spectral shift towards the near-infrared (NIR) window of biological tissue, and comparison of CE to on-board MV imaging. Dose-CE correlation was investigated via simulation and experiment. A Monte Carlo (MC) CE simulator was designed using Geant4. Experimental phantoms include: water; tissue-simulating phantom composed of water, Intralipid®, and beef blood; plastic phantom with solid water insert. The detector system comprises an optical fiber and diffraction-grating spectrometer incorporating a front/back-illuminated CCD. The NIR shift was carried out with CdSe/ZnS quantum dots (QDs), emitting at (650±10) nm. CE and MV images were acquired with a CMOS camera and electronic portal imaging device. MC and experimental studies indicate a strong linear dose-CE correlation (Pearson coefficient > 0.99). CE by an 18-MeV beam was effectively NIR-shifted in water and a tissue-simulating phantom, exhibiting a significant increase at 650 nm for QD depths up to 10 mm. CE images exhibited relative contrast superior to MV images by a factor of 30. Our work supports the potential for application of CE in radiotherapy online imaging for patient setup and treatment verification, since CE is intrinsic to the beam and non-ionizing and QDs can be used to improve CE detectability, potentially yielding image quality superior to MV imaging for the case of low-density-variability, low-optical-attenuation materials (ex: breast/oropharynx). Ongoing work involves microenvironment functionalization of QDs and application of multi-channel spectrometry for simultaneous acquisition of dosimetric and tumor oxygenation signals.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

Sci—Fri PM: Topics — 01: A monte carlo model of a miniature low-energy x-ray tube using EGSnrc

The INTRABEAM system (Carl Zeiss, Oberkochen, Germany) is a miniature x-ray generator for use in intraoperative radiotherapy and brachytherapy. The device accelerates electrons to up to 50 keV, which are then steered down an evacuated needle probe to strike a thin gold target. For accurate dosimetry of the INTRABEAM system, it is important that the photon spectrum be well understood. Measurements based on air-kerma are heavily impacted by photon spectra, particularly for low photon energies due to the large photoelectric contribution in air mass energy absorption coefficient. While low energy photons have little clinical significance at treatment depths, they may have a large effect on air-kerma measurements. In this work, we have developed an EGSnrc-based monte carlo (MC) model of the Zeiss INTRABEAM system to study the source photon spectra and half-value layers (HVLs) of the bare probe and with various spherical applicators. HVLs were calculated using the analytical attenuation of air-kerma spectra. The calculated bare probe spectrum was compared with simulated and measured results taken from literature. Differences in the L-line energies of gold were found between the spectra predicted by EGSnrc and Geant4. This is due to M and N shell averaging during atomic transitions in EGSnrc. The calculated HVLs of the bare probe and spherical applicators are consistent with literature reported measured values.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

On the consistency of Monte Carlo track structure DNA damage simulations

Purpose: Monte Carlo track structures (MCTS) simulations have been recognized as useful tools for radiobiological modeling. However, the authors noticed several issues regarding the consistency of reported data. Therefore, in this work, they analyze the impact of various user defined parameters on simulated direct DNA damage yields. In addition, they draw attention to discrepancies in published literature in DNA strand break (SB) yields and selected methodologies. Methods: The MCTS code Geant4-DNA was used to compare radial dose profiles in a nanometer-scale region of interest (ROI) for photon sources of varying sizes and energies. Then, electron tracks of 0.28 keV–220 keV were superimposed on a geometric DNA model composed of 2.7 × 10{sup 6} nucleosomes, and SBs were simulated according to four definitions based on energy deposits or energy transfers in DNA strand targets compared to a threshold energy E{sub TH}. The SB frequencies and complexities in nucleosomes as a function of incident electron energies were obtained. SBs were classified into higher order clusters such as single and double strand breaks (SSBs and DSBs) based on inter-SB distances and on the number of affected strands. Results: Comparisons of different nonuniform dose distributions lacking charged particle equilibrium may lead to erroneous conclusions regarding the effect of energy on relative biological effectiveness. The energy transfer-based SB definitions give similar SB yields as the one based on energy deposit when E{sub TH} ≈ 10.79 eV, but deviate significantly for higher E{sub TH} values. Between 30 and 40 nucleosomes/Gy show at least one SB in the ROI. The number of nucleosomes that present a complex damage pattern of more than 2 SBs and the degree of complexity of the damage in these nucleosomes diminish as the incident electron energy increases. DNA damage classification into SSB and DSB is highly dependent on the definitions of these higher order structures and their implementations. The authors’ show that, for the four studied models, different yields are expected by up to 54% for SSBs and by up to 32% for DSBs, as a function of the incident electrons energy and of the models being compared. Conclusions: MCTS simulations allow to compare direct DNA damage types and complexities induced by ionizing radiation. However, simulation results depend to a large degree on user-defined parameters, definitions, and algorithms such as: DNA model, dose distribution, SB definition, and the DNA damage clustering algorithm. These interdependencies should be well controlled during the simulations and explicitly reported when comparing results to experiments or calculations.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Latent uncertainties of the precalculated track Monte Carlo method

Purpose: While significant progress has been made in speeding up Monte Carlo (MC) dose calculation methods, they remain too time-consuming for the purpose of inverse planning. To achieve clinically usable calculation speeds, a precalculated Monte Carlo (PMC) algorithm for proton and electron transport was developed to run on graphics processing units (GPUs). The algorithm utilizes pregenerated particle track data from conventional MC codes for different materials such as water, bone, and lung to produce dose distributions in voxelized phantoms. While PMC methods have been described in the past, an explicit quantification of the latent uncertainty arising from the limited number of unique tracks in the pregenerated track bank is missing from the paper. With a proper uncertainty analysis, an optimal number of tracks in the pregenerated track bank can be selected for a desired dose calculation uncertainty. Methods: Particle tracks were pregenerated for electrons and protons using EGSnrc and GEANT4 and saved in a database. The PMC algorithm for track selection, rotation, and transport was implemented on the Compute Unified Device Architecture (CUDA) 4.0 programming framework. PMC dose distributions were calculated in a variety of media and compared to benchmark dose distributions simulated from the corresponding general-purpose MC codes in the same conditions. A latent uncertainty metric was defined and analysis was performed by varying the pregenerated track bank size and the number of simulated primary particle histories and comparing dose values to a “ground truth” benchmark dose distribution calculated to 0.04% average uncertainty in voxels with dose greater than 20% of D{sub max}. Efficiency metrics were calculated against benchmark MC codes on a single CPU core with no variance reduction. Results: Dose distributions generated using PMC and benchmark MC codes were compared and found to be within 2% of each other in voxels with dose values greater than 20% of the maximum dose. In proton calculations, a small (≤1 mm) distance-to-agreement error was observed at the Bragg peak. Latent uncertainty was characterized for electrons and found to follow a Poisson distribution with the number of unique tracks per energy. A track bank of 12 energies and 60000 unique tracks per pregenerated energy in water had a size of 2.4 GB and achieved a latent uncertainty of approximately 1% at an optimal efficiency gain over DOSXYZnrc. Larger track banks produced a lower latent uncertainty at the cost of increased memory consumption. Using an NVIDIA GTX 590, efficiency analysis showed a 807 × efficiency increase over DOSXYZnrc for 16 MeV electrons in water and 508 × for 16 MeV electrons in bone. Conclusions: The PMC method can calculate dose distributions for electrons and protons to a statistical uncertainty of 1% with a large efficiency gain over conventional MC codes. Before performing clinical dose calculations, models to calculate dose contributions from uncharged particles must be implemented. Following the successful implementation of these models, the PMC method will be evaluated as a candidate for inverse planning of modulated electron radiation therapy and scanned proton beams.

07 ISOTOPE AND RADIATION SOURCES↗