H Lin

Improved Model on Minimizing Static Intensity Modulation Delivery Time

In Intensity Modulated Radiation Therapy static intensity modulation delivery, leaf sequencing is basically a process where the intensity map is broken down into subfields or segments that can be implemented by multileaf collimators. The purpose of this study was to investigate a new method to improve the Langer’s integer programming model. The method is to choose the horizontal or orthogonal leaf direction of intensity map according with Xia and Verhy’s alogrithm and invoke a new constraint to optimize multileaf collimator leaf travel distances. A comparative study of three different leaf sequencing methods, Xia and Verhey, Langer et al., and our improved model is presented. The numerical outputs reveal that model modifications can yield better results according with the three criteria: total number of monitor unit, number of segment and leaf travel distances. Keywordsmultileaf collimator; leaf sequencing; leaf travel distances

SU‐E‐T‐21: A Grid Intensity‐Based Dose Algorithm to Realize MLC Irregular and Inhomogeneous Field Modeling for Monte Carlo Clinical Application

Purpose: A grid intensity‐based dose algorithm to realize MLC irregular‐inhomogeneous field modeling is presented for Monte Carlo clinical application in ARTS (Accurate Radiotherapy System). Methods: Linac modeling actually is a multi‐parameter optimization process, and especially depends on the composition and structure detail provided by venders. Maybe a real accelerator use could be substituted by an irregular‐inhomogeneous photon surface source with a compensatory contaminative electron source. MLC intensity map is regarded as many grid gather. The source photon is randomly sampled until its original grid intensity more than zero. The source particle weight is just equal to the grid intensity. The transmission direction of the source particle is decided by SID and MLC position just like a point source irradiation. The measurement data of a set of regular fields are used to commission a XH600D 6MV Linac. The 10cm*10cm field PDD is used to affirm the photon energy spectrum. Their half OARs at iso‐center surface of multiple regular fields are projected to MLC underside and fitted by Boltzmann function. A set of fitting coefficients are deduced. Their OAR differences before 1.5 cm depth are used to deduce the contaminative electron source. The MLC irregular‐inhomogeneous field is divided into several semi‐regular sub‐fields, whose side intensities and sub‐field width are confirmed by the corresponding field coefficients. This algorithm is implemented by adapting the open Monte Carlo code DPM. Results: This algorithm has been benchmarked with experiment data for regular fields. Basically the difference is under 2%/2mm. The rotational irregular‐inhomogeneous multi‐fields also are modeled. Conclusion: A grid intensity‐based dose algorithm is presented, which need not know the composition and structure inside a real Linac. This algorithm can simulate the irregular‐inhomogeneous field formed by MLC, and is promising for Monte Carlo clinical application.

Optimization of Multileaf Collimator Leaf Sequences Based on Multiple Algorithms

To investigate the efficacy of Galvin, Bortfeld and Siochi leaf sequencing algorithm, a leaf sequencing optimization program that compares the results of typical clinical cases is presented. The output of this program is the number of segment and the total number of monitor unit of each clinical case based on the aforementioned three algorithms. Numerical results of each case are analyzed and tested by the movement of multileaf collimator. It shows that this comparison program is effective in clinical.

Electron spectrum reconstruction as nonlinear programming model using micro-adjusting algorithm

In accurate radiotherapy, the electron spectra should be considered to improve the accuracy of dose calculation, because the electron beams that reach the surface traveling through the collimation system of the accelerator are not monoenergetic. For determinining the energy spectra of Clinical electron beams effectively, the nonlinear programming model and a new inversion algorithm “micro-adjusting algorithm” were studied, by using the measued percent depth dose (PDD). The key issue was how to reconconstruct the energy spectra from an ill-pose integral equation linking with the measured PDD, energy spectra and monoenergetic PDD. The monoenergetic PDD was calculated by bipatition model. And the nonlinear programming model with micro-adjusting algorithm was developed to solve this ill-pose integral equation. Testing samples for a Varian Clinac 1800 medical accelerator that considered and ignored the photon contanmination were both used to test this method. The reconstructed results showed even if including 5% photon contanmination the reconstructed energy spectra still agreed well with the original spectra — error of reconstructed PDD was less than 0.05%, while the one that substracted photon contamination was less than 0.006%. It could be concluded that the method of the nonlinear programming model with the micro-adjusting algorithm was feasible to reconstruct electron energy spectra.

SU‐F‐T‐527: A Novel Dynamic Multileaf Collimator Leaf‐Sequencing Algorithm in Radiation Therapy

PURPOSE A novel leaf-sequencing algorithm is developed for generating arbitrary beam intensity profiles in discrete levels using dynamic multileaf collimator (MLC). The efficiency of this dynamic MLC leaf-sequencing method was evaluated using external beam treatment plans delivered by intensity modulated radiation therapy technique. METHODS To qualify and validate this algorithm, integral test for the beam segment of MLC generated by the CORVUS treatment planning system was performed with clinical intensity map experiments. The treatment plans were optimized and the fluence maps for all photon beams were determined. This algorithm started with the algebraic expression for the area under the beam profile. The coefficients in the expression can be transformed into the specifications for the leaf-setting sequence. The leaf optimization procedure was then applied and analyzed for clinical relevant intensity profiles in cancer treatment. RESULTS The macrophysical effect of this method can be described by volumetric plan evaluation tools such as dose-volume histograms (DVHs). The DVH results are in good agreement compared to those from the CORVUS treatment planning system. CONCLUSION We developed a dynamic MLC method to examine the stability of leaf speed including effects of acceleration and deceleration of leaf motion in order to make sure the stability of leaf speed did not affect the intensity profile generated. It was found that the mechanical requirements were better satisfied using this method. The Project is sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

MO-F-CAMPUS-J-01: Effect of Iodine Contrast Agent Concentration On Cerebrovascular Dose for Synchrotron Radiation Microangiography Based On a Simple Mouse Head Model and a Voxel Mouse Head Phantom

Purpose: To find effective setting methods to mitigate the irradiation injure in synchrotron radiation microangiography(SRA) by Monte Carlo simulation. Methods: A mouse 1-D head model and a segmented voxel mouse head phantom were simulated by EGSnrc/Dosxyznrc code to investigate the dose enhancement effect of the iodine contrast agent irradiated by a monochromatic synchrotron radiation(SR) source. The influence of, like iodine concentration (IC), vessel width and depth, with and without skull layer protection and the various incident X ray energies, were simulated. The dose enhancement effect and the absolute dose based on the segmented voxel mouse head phantom were evaluated. Results: The dose enhancement ratio depends little on the irradiation depth, but strongly on the IC, which is linearly increases with IC. The skull layer protection cannot be ignored in SRA, the 700µm thick skull could decrease 10% of the dose. The incident X-ray energy can significantly affact the dose. E.g. compared to the dose of 33.2keV for 50mgI/ml, the 32.7keV dose decreases 38%, whereas the dose of 33.7 keV increases 69.2%, and the variation will strengthen more with enhanced IC. The segmented voxel mouse head phantom also showed that the average dose enhancement effect and the maximal voxel dose per photon depends little on the iodine voxel volume ratio, but strongly on IC. Conclusion: To decrease dose damage in SRA, the high-Z contrast agent should be used as little as possible, and try to avoid radiating locally the injected position immediately after the contrast agent injection. The fragile vessel containing iodine should avoid closely irradiating. Avoiding irradiating through the no or thin skull region, or appending thin equivalent material from outside to protect is also a better method. As long as SRA image quality is ensured, using incident X-ray energy as low as possible.

SU-E-T-47: A Monte Carlo Model of a Spot Scanning Proton Beam Based On a Synchrotron Proton Therapy Accelerator

Purpose: To build the model of a spot scanning proton beam for the dose calculation of a synchrotron proton therapy accelerator, which is capable of accelerating protons from 50 up to 221 MeV. Methods: The spot scanning beam nozzle is modeled using TOPAS code, a simulation tool based on Geant4.9.6. The model contained a beam pipe vacuum window, a beam profile monitor, a drift chamber, two plane-parallel ionization chambers, and a spot-position monitor consisted of a multiwire ionization chamber. A water phantom is located with its upstream surface at the isocenter plane. The initial proton beam energy and anglar deflection are modeled using a Gaussian distribution with FWHM (Full Widths at Half Maximum) deponding on its beam energy. The phase space file (PSF) on a virtual surface located at the center between the two magnets is recorded. PSF is used to analyze the pencil beam features and offset the pencil beam position. The source model parameters are verificated by fitting the simulated Result to the measurement. Results: The simulated percentage depth dose (PDD) and lateral profiles of scanning pencil beams of various incident proton energies are verificated to the measurement. Generally the distance to agreement (DTA) of Bragg peaks is less than 0.2cm. The FWHM of Gaussian anglar distribution was adjusted to fit the lateral profile difference between the simulation and the measurement to less than 2∼3cm. Conclusion: A Monte Carlo model of a spot scanning proton beam was bullt based on a synchrotron proton therapy accelerator. This scanning pencil beam model will be as a block to build the broad proton beam as a proton TPS dose verification tool.

SU-E-T-283: Research of the Irradiation Damage to the Skin Cell by the Contaminative Electron in External Radiotherapy.

PURPOSE To investigate the micro-damage mechanism of the contaminative electron to the skin cell in external radiotherapy, the cell damage yield was simulated. METHODS The physical interaction and the energy deposited events of contaminative electrons in the cell DNA were modeled based on Geant4-DNA low energy physical model. The densitybased cluster mining algorithm was used to analyze the micro-damage yield and obtain its detailed compositive information. By taking the irradiation sensitive parameter and the clinical feature dose threshold of the skin cell into consideration, the damage features of the low energy contaminative electron to the skin cell were studied. The DSB and SSB yield and ratio, the cluster size and the root mean square radius, the cell SF and the lethal coefficient ε of the complex cDSB were researched. RESULTS For some very low energy electrons such as 20keV and 100keV, the cluster size can be more than 5 SSBs. The irradiation protection will be more crucial for the later response and the high α tissue. The lethal coefficient ε of the complex cDSB will increase with the incident electron number and the accumulative dose increasing for the dose square term in LQ model. However, the ε increment will increase 3%∼15% when the incident electron increases up to 100000. CONCLUSION The simulation of the direct physical damage of the cell can be adjusted by a probability parameter to offset the simulation of the indirect biochemical damage. Thus the micro-damage mechanism of the contaminative electron to the skin cell can be detected at a certain extent by Monte Carlo physical simulation. This damage model of the low energy electron to DNA and these simulated results could be used to evaluate the damage effect of the low energy contaminative electron to the skin cells in the external radiotherapy. Strategic Priority Research Program of the Chinese Academy of Sciences (XDA03040000), Fundamental Research Funds for the Central Universities (2012HGXJ0057,2013HGXJ0193), Seed Foundation of Hefei University of Technology (2012HGZY0007) and Hefei University of Technology undergraduate innovative experiment project(2012CXCY431).

SU-E-T-141: Effect of a Single Gold Nanoparticle with Different Sizes Inside a Small Water Phantom.

PURPOSE This study is to investigate the effects of the gold nanoparticles (GNP), a series of micrometre scale simulations have been constructed with Geant4 to track particles and simulate the effects of those particles as they pass through water phantom. METHODS The simulations were used to calculate the number of secondary electrons which are emitted from the particle tracks and the amount of energy which is deposited in the cell tissue. More electrons means that more water molecules can undergo hydrolysis and create potentially dangerous free radical molecules, therefore breaking up DNA and killing off cells or causing damaging mutations. RESULTS For the 20nm GNP, all three proton energies saw a small increase of electrons above the control, while the X-rays nearly tripled the number of electrons in the phantom. For the 50 nm GNP, the 3 and 2 MeV protons saw a small increase again, however the 1 MeV protons saw a decrease in electrons, the X-rays saw a large increase of nearly 4 times the number of electrons. For the 110nm GNP, all three proton energies saw a decrease in the total number of electrons in the phantom, while the X-rays saw an increase of 8 times as many electrons. CONCLUSION From the range of GNP sizes used, it was found that the X-rays have a larger dose enhancement effect as the GNP size increases, the relation between electron emissions and GNP size was linear. This is because the majority of the dose from the X-rays is delivered to the cell tissue through the initial high energy secondary electrons, any dose lost from the Augerelectrons being trapped inside the GNP volume is small compared to the dose that escapes with the high energy electrons.

RESEARCH ON THE DIGITAL MIXED DENOISING TECHNIQUES FOR ACCELERATING MONTE CARLO SIMULATIONS

Abstract The Monte Carlo method is a stochastic statistic algorithm. It is one of the most accurate dose calculation methods, but its application in clinic is limited because of the long computation time. Generally, to accelerate Monte Carlo simulation and reduce stochastic noise, a digital filtering technique is used to smooth a rough dose distribution (includes evident noise) to a satisfied one. Different types of filters have been applied, such as Gaussian filters, Savitzky-Golay filters, etc., but the ability of a single filtering filter is limited. Therefore, a hybrid filter combining those filtering techniques was used. Two types of mixture methods—parallel and cascade—with three-dimensional Gaussian and Savitzky-Golay filters were researched. In addition, a method that simplifies the mixture filter structure using an equivalent convolution kernel based on convolution theory was introduced. With simulation data from a standard phantom, the rough dose distributions and the dose distribution smoothed by the two types of mixture filters were compared with that of the “benchmark” one. Test results showed that the two types of mixture filters can suppress much of the noise added in Monte Carlo dose distributions and enhance its visualization. As for the research’s test cases, the filtering effect of the cascade mixture filter was slightly better than that of the parallel mixture filter. Filter combinations can provide favorable filtering effects. The filtering effects of different mixture methods are not uniform. The cascade mixture filter has a better filtering effect than the parallel mixture filter.