bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐335: Construction of a Surface‐Based 3D Human Model and Monte Carlo Dose Calculations
Authors: Kim C , Jeong J , Yeom Y , Cho S , Cho K
Published: July 2008

Purpose: To construct a surface‐based 3D human model for Monte Carlo dose calculations and compare the model with the conventional voxel‐based model, in terms of computation speed and calculation accuracy. Method and Materials: A voxel‐based 3D human model was converted to a surface‐based model (a polygon model) with 3D‐DOCTOR™. The model was then refined with ‘Remesh’ and ‘Smooth’ functions of Rapidform®2006. The volumes of the organs and tissues were adjusted to the original values with ‘Polygon Offset’ function. Finally, the developed surface model was used, in conjunction with Geant4, to calculate organ radiation doses and the results were compared with those of the voxel model, in terms of computation speed and calculation accuracy. Results: The results show that the surface model is much slower in particle transport or dose calculation than the voxel model. The difference of computation speed was much more than several times, depending on the size of polygons and voxels. The error of the surface model in dose calculation was negligible (i.e., << a few %), as long as the polygon size was not very large (i.e., < 0.5 cm2 on the average). The results also show that the calculation speed of the surface model rapidly increases with the average size of the polygons. Conclusion: The surface model has several advantages over the conventional voxel model. The organs and tissues in the surface model can be easily moved or deformed, which is advantageous for 4D Monte Carlo dose calculations. The surface model is also suitable for accurate modeling of thin structures such as skin, oral mucosa, etc. This work, however, indicates that the computation speed of the surface model is significantly slower than the conventional voxel model. This problem can be addressed by optimizing the particle transport algorithm to polygon surfaces.
SU‐GG‐T‐339: Monte Carlo Calculation to Determine Tandem Ionization Chamber Response to X‐Rays Beam Using PENELOPE
Authors: Pianoschi T , Góes E , Nicolucci P , Costa A , Pelá C
Published: July 2008

Purpose: We used PENEPOLE code to determine the response of a special double‐faced plane parallel ionization chamber to X‐rays considering an energy range of 33 to 115 keV. Method and Materials: We used a plane parallel ionization chamber with a double face, body in Lucite and entrance windows of Mylar (100 μm thick) and aluminum (5 μm thick). One face has an aluminum collecting electrode (A) and the other has a graphite collecting electrode (G), each one with 5 mm thick and 20 cm in diameter. The volumes are approximately 0.6 cm3, filled with dry air (sea level). For this study, the X radiation qualities spectra were calculated using a software that is based on the work of Birch and Marshall. PENELOPE was used to calculate the energy response variation of the modeled tandem chamber. The cutoff energy for photon and electron absorption was assumed as 1 keV. The following parameters were used for all materials: an average angular deflection C1=0.05, a maximum average fractional loss between consecutive hard elastic events C2=0.05, a cutoff energy loss for hard inelastic collisions WCC=5 keV and a cutoff energy loss for hard bremsstrahlung emission WCR=1 keV. Measurements of the energy response variation of the tandem chamber were performed using the radiation qualities established by a standard laboratory. The source‐detector distance was 50 cm and the diameter of circular field was 6.66 cm. Result: The differences between measured and calculated energy response variation are less than 7.0 % for the face A and less than 1.5 % for the face G of the tandem chamber. Conclusion: The results show that PENELOPE code allows a reliable methodology for design and analysis of ionization chambers over the range between 33 to 115 keV of X‐ray beams.
SU‐GG‐T‐340: A Comparison of Geant4 and DPM for 4D Monte Carlo of Lung IMRT
Authors: Hancox C , Seco J , Paganetti H
Published: July 2008

Purpose: Respiratory motion during IMRT for lung tumors presents a particular challenge. Geant4 is an attractive tool for investigating motion effects since it can accommodate time‐dependent geometries. Leaf positions and patient geometry may be updated during the course of the simulation and interplay effects directly studied. Here we investigate the applicability of Geant4 for patient dose calculations in the lung, comparing field‐by‐field results for an IMRT patient with results obtained using the code DPM. Method and Materials: Geant4, being a general purpose code, has many adjustable parameters available to the user and their default values may not be appropriate for patient dose calculations. Additionally, while it can import CT data and accurately model the complex patient geometry, it is not optimized for fast patient dose calculations. In particular, it does not efficiently handle voxel‐to‐voxel variations in material density. We modified the Geant4 (release 9.0) source code to optimize the handling of density variations in the voxelized patient to greatly speed up simulation time. Similar modifications had been made to an earlier release (release 5.0) for proton therapy but this is the first such optimization for IMRT. As part of our project on using Geant4 for 4D dose calculation in the lung we compared the results from Geant4 using different user parameters with the results obtained using DPM for a five‐field IMRT treatment plan for a lung patient. Results: The efficiency of Geant4, version 9.0, was greatly improved by modifying the voxel tracking algorithm. We found that DPM and Geant4 agreed to within a few percent after adjusting just two user parameters, the simulation stepsize limit and the secondary range cutoff (1 mm and 0.1 mm, respectively). Conclusion: Geant4 provides a natural platform for 4D studies and has been shown to be sufficiently accurate for patient dose calculations in lung.
SU‐GG‐T‐424: Volume‐Based Isoeffective Dose Calculations for Hypofractionation and the Analysis of Critical Structure Limits with Respect to Known Toxicity Frequencies
Authors: Price R , Paskalev K , Li J , Ma C , Pollack A
Published: July 2008

Purpose: Image guidance and active tracking may lead to PTV reductions allowing for hypofractionation and dose escalation in some disease sites. Variations in total dose and dose/fraction to the normal structures at risk presents difficulties in establishing limits relative to known toxicity rates. In this work we describe a method allowing changes in dose schedules while predicting the appropriate normal tissue limits. Method and Materials: At this institution rectal limits for IMRT delivery to the prostate to 76Gy at 2Gy/fx are set such that no more than 17% and 35% of the rectal volume receives greater than 65Gy and 40Gy and are based on prior randomized studies and arbitrarily, respectively. These values have been utilized in an in‐house, randomized, hypofractionation trial comparing the above fractionation scheme to 2.7Gy/fx to 70.2Gy (1). Two year rectal toxicity data have been analyzed. A follow‐up protocol utilizing IGRT and tracking as well as reduced PTV margins will employ fractionation schemes of 2Gy/fx to 84Gy (2) and 2.7Gy/fx to 78.3Gy (3). Using the equivalent dose at 2Gy/fx (EQD2), alpha/beta ratios of 2.0Gy and 4.0Gy for the prostate and rectum, respectively, and fixing the rectal analysis volume to 17% and 35%, we are able to determine the appropriate rectal limits for varying fractionation schedules. Results: Values for 17% and 35% in scheme (1) above are 59.2Gy and 37.2Gy. Values for scheme (2) are 66.5Gy and 40.7Gy. Values for scheme (3) are 60.9Gy and 38Gy. Conclusion: PTV reductions may decrease the area of overlap into adjacent critical structures. Combined with the inherent conformality of IMRT the resulting critical structure dose distributions may be represented by lower fractional doses leading to uncertainties in correlating with known toxicity rates. By fixing the analysis volume and converting this minimum dose to 2Gy equivalents we may lesson these uncertainties.
SU‐GG‐T‐435: Optimization of Isocenter Location for Intensity Modulated Stereotactic Treatment of Intracranial Targets
Authors: Salter B , Fuss M , Sarkar V , Wang B , Hollingshaus S , Papanikolaou N , Tobler M , Shrieve D
Published: July 2008

Purpose: To quantify the impact of isocenter re‐location/optimization on treatment plan quality for intensity modulated stereotactic treatment of small intracranial lesions. Method and Materials: For 18 patients previously treated by stereotactic intensity modulated radiosurgery (IMRS) or stereotactic intensity modulated radiation therapy (IMRT), a retrospective virtual planning study was conducted wherein the impact of isocenter location on plan quality was measured. Lesions studied included 6 arteriovenous malformations (AVM), 6 acoustic neuromas (ACN) and 6 intracranial metastases, ranging in volume from 0.19–3.21 cm3. Variation of isocenter location causes the geometric grid of pencil beams into which the target is segmented for intensity modulated treatment to be altered. The impact of this pencil beam grid redefinition on achievable conformity index (CI) may be significant for small lesions, where pencil beam size is similar to that of the lesion. Impact on CI was quantified for 3 different collimators (Varian Millennium 120; BrainLab MM3; Nomos binary Mimic) and 3 different treatment planning systems (Varian Eclipse v7.3.1; BrainLab BrainScan v5.31; Nomos Corvus v6.2), resulting in the evaluation of 3,446 different treatment plans. Results: For all patient, collimator and treatment planning system combinations studied, a significant variation in plan quality was observed as a function of isocenter/pencil‐beam‐grid re‐location. Optimization of isocenter location resulted in treatment plan conformity variations as large as 109% (min = 15%, mean = 53%, across all plans) with Millennium 120 mean = 36%, MM3 mean = 70% and Mimic mean = 51%. Conclusion: For intensity modulating treatment plans, variation of isocenter location causes the geometric grid of pencil beams into which the target is segmented to be altered. Optimization of isocenter location for IMRT/IMRS treatment of small intracranial lesions, where pencil beam dimensions are comparable to target dimensions, can result in significant improvements in treatment plan quality.
SU‐GG‐T‐437: Localization and Treatment of Essential Tremors Using Gamma Knife Radiosurgery
Authors: Nair M , Mark R , Smith H , Neumann T , Anderson P
Published: July 2008

Purpose: To present localization technique and results for radiation treatment of functional disorders using Gamma Knife Stereotactic Radiosurgery (GKSRS). Method and Materials: Since one year, we have treated 17 patients with GKSRS. T2 weighted axial MR images were obtained and target coordinates were localized at the planning system to <1mm maximum error and average error of <0.5 mm. The anterior commissure (AC) and poster commissure (PC) planes were identified in the slices. 1/4th of the separation between the AC and PC planes was used for the Y distance from PC and X distance was measured lateral to the affected side 11 mm + half width of the 3rd ventricle. The Z coordinate was located at the slice where AC and PC are visualized or half way between the 2 slices where both planes are identified. The dose delivered to the target was 140Gy using a 4 mm collimator. Results: The center of the radiation lesion was compared with the planning target on the follow up MR images and was accurate to < 1mm. The diameter of the lesion was measured, and varied from 4 to 5 mm. The tremor decreased with the treatment after 1 to 3 months. One patient (N=17) showed partial response, although the lesion was correctly observed on the follow up MR image. Upon DBS, the active neurons were located at 1 mm lateral to the lesion, which supports our proposal for confirmation of the target prior to treatment with microelectrode registration. Conclusion: The GKSRS is a non‐invasive and effective treatment for control of tremor, especially in older patients. The invasive procedures used, may cause the risk of intra or extra cerebral hemorrhage, infection, seizures and bleeding from probe placement. This technique was well tolerated by the patients.
SU‐GG‐T‐446: Dosimetric Characteristics of High Definition Multi‐Leaf Collimator
Authors: Wang Z , Chang Z , Wu Q , Zhou S , Huntzinger C , Yin F
Published: July 2008

Purpose: A new type of high‐definition multi‐leaf collimator (HD 120 MLC, Varian Medical Systems) is being introduced into clinical practice to improve the conformality of stereotactic radiation therapy and radiosurgery. Its leaf width at isocenter is 2.5 mm. The interface and casing are the same as Millennium 120 MLC so it can be easily fit inside the gantry for better clearance and efficiency. This study investigated the dosimetric characteristics of this new micro MLC. Method and Materials: The HD 120 MLC was installed in a Novalis TX dedicated image‐guided SRS/SBRT machine. Beam penumbra (defined between 20–80%) was measured with a scanning radiosurgery diode for various field sizes, depth, and beam energies. Dynamic dosimetric gap and leaf transmission were measured with both low (6 MV and 6 MV SRS) and high energy (15 MV) photon beams. The conformity and step effect were studied with a series of circular beams with various radius. Results: The penumbra of HD 120 MLC was measured to be 0.25 cm for 6 MV at 1.5 cm depth with 2 cm × 2 cm field size. It increases to 0.49 cm for 30 cm × 30 cm field size. For 15 MV at 3 cm depth, the penumbra increases from 0.36 cm at 2 cm × 2 cm to 0.60 at 30 cm × 30 cm. For other field sizes and depth, the penumbra values are listed in Table I. The dynamic dosimetric gap was 0.86 cm for 6 MV and 0.98 mm for 15 MV. The leaf transmission factors were 1.17% for 6 MV and 1.33% for 15 MV. Conclusion: HD 120 MLC provides finer beam penumbra, smaller dynamic dose gap, and lower leaf transmission compared to conventional Millennium MLC, and can thus potentially improve the conformality of SRS/SBRT treatments.
SU‐GG‐T‐447: Import of Gamma Knife Model C Treatment Plans Into CERR
Authors: Drzymala R , Karsch K , Alaly J , Khullar D , Wu Y , Deasy J
Published: July 2008

Purpose: To import Gamma Knife treatment plans into an environment that allows for comparison and combination dose distributions with those from other treatment planning systems to include those for linear accelerators. Method and Materials: A set of software tools were developed that allow the binary files of GammaPlan, version 5.4, for Gamma Knife, Model C, to be converted and imported into CERR, version 3.0. The imported data files include CT scan sets, MR scan sets, anatomical contours, the location of the dose matrix relative to patient anatomy, the dose computed by GammaPlan. The functionality of CERR allows us to fuse image sets, contours and doses. Results: We were able to import GammaPlan treatment plan data into CERR for comparison and combination with a Varian Eclipse/Helios treatment plan for recurrent tumor in the same patient. The recurrent tumor extended into anatomical regions that precluded re‐treatment with the Gamma Knife, Model C. This allowed superposition of isodose distributions of the two treatment plans in one viewable format. Areas of dose overlap for the two plans were identified and the progression of disease relative to the initially treatment location was observed. Conclusion: The software tools extend the capabilities of CERR to view GammaPlan treatment plans and combine the doses from other treatment planning systems supported by CERR. Further work will incorporate full plan specifications, including shot locations, gamma angle, and time for each shot so that CERR can export in DICOM RT format. These tools provide the potential for multi‐institutional stereotactic radiosurgery trials that include participation of the Gamma Knife, Model C.
SU‐GG‐T‐448: Online Isocenter Corrections Based On Image Registration and Automatic Target Finding in SRS/SRT Cranial Treatment with the Bite‐Block Frame System
Authors: Song X , Shiu A , Woo S , Kornguth D , Wang H
Published: July 2008

Purpose: Based on the 3D‐3D image registration and mapping the imaging space to the stereotactic frame coordinate space, an online software was developed to automatically update the target isocenter position prior to each non‐invasive SRS/SRT treatment. The prescription dose can then be delivered to the entire target volume according to the treatment plan. Method and Materials: The key concept of SRS/SRT technique is the use of stereotactic coordinates to define the target volume and all other structures within the stereotactic frame space. This software will automatically setup stereotactic coordinate system through localize the 9‐rod positions of the stereotactic frame in computed tomography (CT) images. The CT imaging spaces of both planning and pretreatment are then mapped to the stereotactic coordinate spaces using transformation matrices. The correct target isocenter are determined by fusing pretreatment CT with the planning CT based on the mutual information derived from 3D volume CT images. The final visual verification of the treatment isocenter is coincided with planning isocenter through the registration of a pair of orthogonal digitally reconstructed radiographs (DRRs) generated from pretreatment and planning CT images. Results: A sphere and a rectangle structures within a SRS head phantom were chosen as the targets to evaluate the accuracy of our software. Within the head phantom, the accuracy for repositioned isocenter can be achieved within 0.1 mm for displacements and 0.2 degree for rotations. Also three non‐invasive SRS/SRT patient data sets were used to evaluate the software, and the results were verified with Radionics XKnife RT4 system, the translational discrepancies between reconstructed and optimized isocenters are within 0.5 mm. Conclusion: Using this software for the non‐invasive SRS/SRT treatment, patient setup accuracy within 0.5 mm can be achieved without the discomfort from the pin fixed to the patient's skull.
SU‐GG‐T‐455: Feasibility Study of Treating Multiple Tumors Simultaneously with a 4‐Bank MMLC Stereotactic Radiosurgery System
Authors: Potter L , Lian J , Morris D , Deschesne K , Ewend M , Chang S
Published: July 2008

Purpose: A novel 4‐bank micro‐multileaf collimator (mMLC) has been commissioned for stereotactic radiosurgery (SRS). Leafs of each bank of mMLC can be individually controlled, which allows to form more complex filed than conventional 2‐bank MLC or circular cone. For cases with multiple brain masses, pre‐treatment QA and treatment are always time consuming. In this study we investigate if multiple tumor sites can share one isocenter and if they can be treated simultaneously without sacrificing the quality of dosimetry. Method and Materials: The microMLC (Alayna Enterprise Corp) consists of 96 tungsten leaves aligned in four banks. Three treated patients, each with two adjacent brain masses, were selected retrospectively in this study. Three plans were made on each patient. Plan 1 used conventional method that was to create two beam sets each with its own isocenter. Treatment was to be delivered in a sequential way. In Plan 2, tumors shared single isocenter but were treated by its respective beam set. In Plan 3, single isocenter was chosen and two masses were to be irradiated simultaneously by one set of beam. For comparisons, the treatment and setup time were measured or estimated. The isodose curves and dose volume histograms (DVH) were analyzed. Results: Plan 2 and Plan 3 save significant time in QA and treatment (∼30mins). Plan 1 and Plan 2 have equally satisfying dosimetric outcome, while the DVHs of Plan 3 degrade significantly. Plan 1 and Plan 2 have individual control of the dose of each target, while Plan 3 doesn't have. Conclusion: We have studied three scenarios of treating multiple isocenter SRS using 4‐bank microMLC. The best method found is to share a common isocenter, but treat the targets individually. This method can reduce the QA and treatment time significantly, and achieve the similar dose coverage as the conventional technique.
SU‐GG‐T‐457: Gamma Knife Quality Assurance Technique for Treatment of Functional Disorders
Authors: Nair M
Published: July 2008

Purpose: Present the technique used for commissioning and validation of Leksell Gamma Knife Radiosurgery machine for treatment of functional disorders. Method and Materials: Leksell Gamma Knife C Radiosurgery machine was commissioned and validated for use for treatment of functional disorders such as Essential Tremor and Parkinson Disease. These patients are treated with 4 mm collimator to a much higher radiation dose of 120 Gy to the nerve. Due to the proximity of the target with vital structure, internal capsule it is very critical that the target localization and delivery accuracy is maintained to less than 1 mm. The isocenter verification of the machine was done using the Elekta target localizer using Gafchromic film, which was calibrated for dose range from 5 Gy to 120 Gy. The isocenter was verified along X,Y and Z orientations. The beam profile for the 4 mm collimator was determined using films exposed along X‐Y and X‐Z plane. The FWHM along both planes were determined from the scans converted to dose, and compared with the data in the Leksell Gamma Plan software. The absolute output for 18 mm collimator was determined using a micro chambers in a spherical phantom. The film from the same batch of calibration was irradiated with 18mm and 4 mm collimator. The ratio of the doses was used for output factor of 4mm relative to 18 mm collimator for dosimetry. The computer generated plan was verified in a phantom with ion chamber, and TLD measurements. Results:The root mean square deviation of target isocenter was 0.6mm. The beam profiles agreed within planning system to 0.8 mm. The absolute dose agreed with computer value to 1.2% Conclusion: Using these tests we have established confidence in the dosimetry and localization for use on patient.
SU‐GG‐T‐466: Quality Assurance and An Assessment of Using Cone‐Beam CT with Extra‐Cranial Frameless‐Stereotactic Radiotherapy
Authors: Nawaz A , Schuller B , Chen G , Kachnic L , Willins J
Published: July 2008

Purpose: Standard quality assurance of Varian's trilogy frameless‐stereotactic radiotherapy(SRT) relies on the a match between the system's optical isocenter and room lasers. It does not match the static kV isocenter of the On‐Board‐Imaging(OBI) or the dynamic kV isocenter of the CBCT, nor does it QA the MV isocenter. Our purpose was to establish a quality assurance(QA) procedure for all isocenters including the MV isocenter for frameless‐SRT. Method and Materials: An acrylic phantom with dimensions of 15×15×15 cm3 was constructed. The phantom included a removable slab that contains two machined holes designed to hold a stainless‐steel ball‐bearing(BB). One machined hole is at the center of the cube, and another is off to one side to established phantom orientation during scanning. The BB is removed during CBCT imaging to minimize artifacts and is replaced for kV and MV images. The Trilogy frameless‐SRT array is mounted atop of the phantom and it is positioned at the LINAC isocenter. Repeated CB‐3D, 2D‐OBI datasets were acquired to determine the system's ability to detect and correct for known table shifts. All shifts observed and recovered by the SRT system were tabulated. Each recovered shift was verified by a Winston‐Lutz(WL) test. The differences between known shift, OBI, CBCT, SRT and the WL test were recorded and tabulated. The couch's ability to recover to a known initial position was also tested and tracked by SRT camera system. Results and Conclusion: Initial results showed the couch had a precision of <1mm. The CBCT, SRT‐array and OBI correctly predicted the movement within 0.13±0.35mm, 0.14±0.24 mm and 0.21±0.38mm, respectively. The mean agreement between the kV, MV and SRT‐optical isocenters was 0.4 mm±0.6 mm via WL. Initial results show that this QA of four isocenters has the potential to allow the use of CBCT and OBI during extra‐cranial frameless‐SRT as an additional modality during treatment.
SU‐GG‐T‐481: 4D‐CT What Is It Good For?
Authors: Meadows J , Wilkinson M , Bichay T
Published: July 2008

Purpose: Retrospectively analyze a cohort of lung cancer patient's 4D simulation data and calculate the dosimetric impact of using a single respiratory phase CT image to design treatment fields. These fields are then mapped onto the extreme opposite respiratory phase to determine the change in volume (target, normal lung, etc.) and dose to the areas of treatment. Method and Materials: A 40‐slice Siemens SOMATOM® Sensation Open CT unit was utilized for the 4D acquisition of simulation data. The data was separated into ten (20%–100%‐IN, and 0–80%‐EX) respiratory phase bins for visualization of target motion. The 0% IN (inhalation) and 100% EX phases were contoured independently. The treatment fields were optimized on the 0% IN dataset to deliver 66 Gy, and minimized the surrounding critical organs. These fields were then mapped onto the 100%‐EX (exhalation) bin, representing the opposite respiratory extreme, and subsequently re‐calculated for the same volumes of interest. The dose volume histograms were exported into Excel for comparison of these two datasets. Results: Most of the data showed a target dose reduction. Changes in the target volume (size), maximum and mean target doses, as well as the ratio of minimum target dose to prescribed dose were seen. The change in target volume on the two image datasets ranged from +3.6% to −24.6%, with a negative value being a decrease in contoured volume. The change in mean target dose ranged from −0.1% to −15.9%, with a negative value being a decrease in mean dose coverage. Similarly, the ratio of minimum dose to prescribed dose coverage ranged from −3.4% to −21.4%. Conclusion: The results show the importance of using 4D image data to design treatment fields for lung cancer patients. Our knowledge of the target and normal anatomy motion continue to be paramount in designing treatment paradigms in the future.
SU‐GG‐T‐498: Computational Modeling of Beam‐Customization Devices for Heavy‐Charged‐Particle Radiotherapy
Authors: Kanematsu N , Yonai S , Ishizaki A , Torikoshi M
Published: July 2008

Purpose: This work is aimed to improve the computational model of the beam‐customization devices for treatment planning of radiotherapy with heavy charged particles, where only a single collimator and a compensator have been commonly handled with inaccuracy of unphysical collimator dependence in the middle of large fields. Method and Materials: The phase‐space theory is applied to the beam transport through the beam‐customization devices to enable handling of multiple collimators and a compensator in any order. The theoretical model was experimentally tested with a carbon‐ion beam, where two jaw and a multileaf collimators, a 3‐cm PMMA half plate for a range compensator, and an 8‐cm square aperture for a patient collimator were concurrently used. In the model, a matrix of pencil beams was transported through the devices and a two‐dimensional in‐air dose distribution on the isocenter plane was computed within ten seconds. For comparison, the penumbra sizes at the field edges formed by the effective collimators were analytically estimated and the dose profiles along four axes on the isocenter plane were experimentally measured. Results: The model computation agreed with the measurement and analytic estimation at a submillimeter level in penumbra size and reproduced the measured dose fluctuation in the middle of the field due to the range‐compensator scatter. Conclusion: The model computation is fast, accurate, and readily applicable to pencil‐beam algorithms in treatment planning to enable combinational collimation for the best use of beam‐customization devices.
SU‐GG‐T‐499: Deterministic Photon Kerma Distribution Based On the Boltzmann Equation for External Beam Radiation Therapy
Authors: Yuan J , Chen W , Jette D
Published: July 2008

Purpose: There has been rising interest in the medical physics community to calculate the dose with a deterministic transport approach by solving the Boltzmann transport equation (BTE). The purpose of this work is to develop a photon transport algorithm based on the discrete ordinates (SN) solution of the Boltzmann equation for fully three‐dimensional radiotherapy treatment. Method and Materials: The Boltzmann equation was solved in the SN spatial, energy and angular discretization form on an adaptive mesh so that multiresolution can be achieved wherever large gradients occur. An octree‐like data structure has been implemented for generation of the adaptive mesh. Ray‐effects inherent with the discrete ordinate method are mitigated by evaluating the source term in the spherical harmonic expansion and through use of the first collision method. The integral form of the BTE is solved with the method of characteristics along each discrete direction in the Gaussian quadrature for a given energy group. Results: The total group cross sections and Legendre moments of group‐scattering cross sections were calculated using our code MGCS. To validate the algorithm, several examples were employed to calculate the photon flux distribution. The results were compared with Monte Carlo (MC) calculations using EGSnrc. Comparison shows that the agreement for kerma distribution is within the MC uncertainties. Conclusion: The discretization of the space, angle and energy phase space associated with the SN solution of the Boltzmann equation requires large memory storage space. However, with careful implementation such as using adaptive mesh, the deterministic method can provide an alternative approach for computation of accurate radiological quantities.
SU‐GG‐T‐504: Verification of Accuracy of the Monte Carlo Based Electron Treatment Planning System
Authors: Lee J , Chan R
Published: July 2008

Purpose: To verify accuracy of the Monte Carlo based electron treatment planning system (Varian Eclipse, eMC 8.1) in modeling blocked electron fields. Method and Materials: Dose distributions, for open field with cones sizes 10×10cm2 and 15×15cm2 at standard source‐to‐surface distance (SSD) and extended SSD (113cm); plus blocked fields of 3×3cm2, 4×4cm2, 5‐cm diameter circle, and an irregular field at 100SSD, using various electron energies, measured using MapCheck device, are compared to those generated by eMC algorithm. γ index (3%/3mm) is used for analysis. Inhomogeneous (air and 3mm Al) and angular effects are also evaluated. Output measurements are confirmed with ion chamber. Results: There is a good agreement between measured and planned dose distributions for 10×10cm2 and 15×15cm2 cones at 100SSD and 113SSD at the depth of 2.5cm for 9 MeV and 3cm for 12–22 MeV, except eMC algorithm overestimates low dose regions in all energies. For 12 MeV, the algorithm overestimates dose less than 20cGy by 40%. The eMC algorithm can predict dose distributions well when different cutouts are inserted. However, 34% of measured points had γ >1 (TH= 10%) for irregular field with 18 MeV. Obliquity effect has test for 9 and 15 MeV. γ values for 9 and 15 MeV, measured at 105SSD with 10×10 cm2 cone, are 97.7% and 89.2% for gantry angle of 10° and 94.7% and 91.5% for 20°. The algorithm also shows a good agreement when inhomogenity present. But it underestimates dose under an air cavity for 9 MeV by 13.7%. Dose measured when computed MU delivered is within tolerance. Conclusion: Study shows that eMC can model dose distributions for blocked fields and different setup geometries with more than 90% of accuracy. This provides better dose estimation for treatment over critical organs. Further study is need for air cavity interface.
SU‐GG‐T‐505: Second Cancer Risks Following Proton Therapy and Intensity Modulated X‐Ray Therapy for Prostate Cancer
Authors: Fontenot J , Newhauser W
Published: July 2008

Purpose: To assess the risk of second cancers from proton therapy for a typical prostate patient relative to 6‐MV IMXT, taking into account contributions from both primary and secondary (i.e., scatter and leakage) sources of radiation. Method and Materials: A 250‐MeV proton plan and a 6‐MV IMXT plan were constructed for a typical prostate patient using a commercial treatment planning system. Doses from the primary fields delivered to organs at risk of a developing a second cancer were taken from dose‐volume histograms provided by the planning system. Secondary doses from the proton plan were determined in a computerized anthropomorphic phantom using a detailed Monte Carlo model of the double‐scattering beamline at the M. D. Anderson Proton Therapy Center. Secondary doses from IMXT of the prostate were determined from published thermoluminescent detector measurements in an anthropomorphic phantom. Second cancer risks were estimated from primary and secondary doses on an organ‐by‐organ basis using modified risk coefficients from Report 116 of National Council on Radiation Protection and Measurements. Results: Proton therapy reduced risk of a second cancer by 17% compared to IMXT. This reduction was attributed the substantial reduction of dose to the rectum and bladder provided by the proton plan. Secondary doses from proton therapy were smaller than IMXT near the field, but larger far from the field. Much of the risk for both modalities emanated from the in‐field organs, i.e., the rectum and bladder. However, the risks from the in‐field organs were considerably lower from the proton plan (0.98%) compared to the IMXT plan (1.40%). The risks from secondary doses to out‐of‐field organs were similar for both plans (1.05% and 1.04% for the proton and IMXT plans, respectively). Conclusion: When considering primary and secondary doses, proton therapy can reduce the incidence of radiation‐induced cancers in prostate patients compared to contemporary IMXT.
SU‐GG‐T‐511: Towards Real‐Time Radiation Therapy: Superposition/Convolution at 4fps
Authors: Jacques R , Taylor R , Wong J , McNutt T
Published: July 2008

Purpose: To enable novel radiation therapy workflows by providing superposition/convolution dose calculation at interactive rates. Method and Materials: The Superposition/Convolution (S/C) dose computation algorithm utilizes ray‐tracing techniques extensively and is well suited to acceleration using consumer graphics processing units (GPU). The standard S/C algorithm was parallelized and implemented using NVIDIA's CUDA GPU programming environment. The basic S/C method generates incident fluence, using a source model, and transports it from the radiation source, using ray‐tracing, to deposit a TERMA grid. Then superposition spreads the dose deposition kernel out by ray‐tracing from each point in the TERMA grid. There are several issues with parallelizing this algorithm. Calculation of the TERMA grid suffers from read‐write conflicts. We solve this by running groups of spatially separated divergent rays. The forward S/C kernel also suffers from read‐write conflicts, but the inverse kernel does not. We reduced memory bandwidth by using a variable step ray‐tracer, independently attenuating each spectral energy bin and caching lookup‐tables. We explored the concept of using volumetric mip‐maps to approximate the ray as a true solid angle. Results: We compared our implementation to Pinnacle3 for low (64×64×64 cube with 4 mm voxels) and high resolution (128×128×128 cube with 2mm voxels) water phantoms using as similar settings as possible. Pinnacle3 (Philips ‐ Madison, WI) was run on a SunFire v250 machine with times of 30.961s and 365.944s respectively. We performed our calculations on a single NVIDIA GeForce 8800 GTX with times of 0.217s and 2.197s respectively. Preliminary experiments using volumetric mip‐maps showed additional performance improvements with minor accuracy loss. Conclusion: We have completed a GPU accelerated superposition/convolution dose engine, providing a substantial performance gain over CPU based implementations — indicating that real time dose computation is feasible with the accuracy levels of the S/C algorithm.
SU‐GG‐T‐528: Configuration of a Treatment Planning System with Monte Carlo Simulations for a Compact Proton Therapy Unit
Authors: Zheng Y , Klein E , Rosenthal S
Published: July 2008

Purpose: A compact proton therapy unit Monarch250 (Still River Systems, Littleton, MA) is under development and the first unit will be installed at our institution. Our purpose is to generate dose profiles and test beam characteristics for the proton therapy unit with Monte Carlo simulation and configure a commercial treatment planning system (Varian, Palo Alto) using the simulated beam data. Method and Materials: Since the beam delivery apparatus is not installed yet, we used the design data provided by the manufacturer to model the beam line and calculate machine specific parameters such as water equivalent thickness of each component. We modeled the range modulator, second scatterer, range shifter, applicator, aperture and other components in detail. A Monte Carlo code (MCNPX, Los Alamos National Laboratory) was used to simulate the beam profiles. Up to 108 proton histories were tracked per simulation to achieve < 2% statistical uncertainties. The dose profiles and machine‐specific parameters were then entered into the treatment planning system for configuration. Results: A complete set of configuration parameters and beam profiles will be generated. The beam characteristics such as flatness, symmetry and penumbra and spread‐out Bragg peak will be evaluated against the machine specifications. The treatment planning system will be configured with the dose profiles from Monte Carlo simulation and machine specific parameters provided by the manufacturer. Conclusion: The Monte Carlo tool allows us to test the beam characteristics and configure a treatment planning system for a compact proton therapy unit before it is constructed. Validation of the Monte Carlo modeling will commence. Using a validated MC model to (partially or fully) commission the treatment plan and proton therapy machine is under investigation, which could significantly reduce the overall time and cost to model and commission proton beams for clinic use.
SU‐GG‐T‐535: Laser‐Proton Inter‐Track Effect and the DNA Double‐Strand Break
Authors: Fan J , Fourkal E , Tafo A Guemnie , Veltchev I , Price R , Luo W , Li J , Eldib A , Chen L , Ma C
Published: July 2008

Purpose: Ionizing radiation deposits energy and generates ionizing events non‐uniformly in the medium through which it passes. Particle track and event spatial distribution may have a strong impact on the DNA damage. The aim of this work was to determine the increase of the Most Effective Event pairs (MEE, adjacent event pairs with distance between 1 and 3 nm) by inter‐track effect of a laser‐proton beam. Method and Materials: The proton beam from ultra‐short pulse laser acceleration is pulsed radiation with extremely short durations. Events (spurs and blobs) from proton tracks of one single proton pulse are almost generated at the same time, at which inter‐track effects may become significant. A model was built for the tracks of laser protons. In this model radicals are supposed to be formed in spherical events, each having a fixed amount of energy deposit (∼200 eV). The spacing of events is random although the number of events is made to agree with the LET of the proton track. Proton tracks of one pulse were assumed to be parallel to each other while the spacing is random. The variation of MEE was considered, which is supposed to be the major cause of DNA lethal lesions. Results and Conclusion: Calculations were carried out assuming different combinations of parameters characterizing the production of MEE. No large variations in the yields of MEE were found at low doses. However, as the given dose becomes higher, the ratio of MEE numbers between the laser‐proton beam and conventional proton beam increases quickly because of the intra‐track effect. The ratio reaches about 1.09 in the radiosurgery dose range, which may affect the final RBE significantly for laser‐proton radiation therapy.
SU‐GG‐T‐541: A Method of Delivering a Low Dose Fraction Using a TomoTherapy Unit
Authors: Rasmussen K , Schubert L , Westerly D , Tome W
Published: July 2008

Purpose: To create a method of delivering PRDR for re‐irradiation of large brain tumors on the TomoTherapy Hi‐ART® system Method and Materials: Pulsed Reduced Dose Rate (PRDR) delivers a standard 2Gy fraction delivered over ten 0.2Gy sub‐fractions with a 3 minute break between sub‐fractions to give a time averaged dose rate of 4Gy/hr. The accelerator on a tomotherapy unit has a fixed output rate of 8Gy/min. If the dose per fraction is conventionally planned at <0.6Gy/fraction, the result is a clinically unacceptable DVH. Our method involves a virtual grid style blocking scheme, where half of the beam angles are blocked using 15 equally spaced directional segments surrounding the center of the image set. Ten patients treated using conventional PRDR with an average PTV volume of 344cc were retrospectively re‐planned using five techniques (standard 2Gy fraction, 2Gy in 0.2Gy fractions without grid blocking, two grid patterns, and a combination dataset incorporating both grids) and analyzed with conformation numbers (CN), homogeneity indexes (HI), and dose volumes to normal tissues. Plans were optimized using equal constraints and machine parameters. Results: Our method allowed for clinically acceptable treatment plans at 0.2Gy with a treatment time ⩽3min per subfraction. The average HI was slightly poorer for the combination plan versus the standard 2Gy fraction (0.064 versus 0.027) and the CN was similar over all techniques (0.72–0.73). Normal tissue dose volumes for each patient were also similar for each technique. Initial ion chamber measurements agree with predicted values for a 0.2Gy subfraction. Conclusion: PRDR is deliverable on a TomoTherapy system using a virtual blocking method. Results can be slightly improved through the use of two grids alternated on a daily basis. The dose to normal structures for individual patients was similar for all methods. Conflict of Interest: The author has a financial interest in TomoTherapy Inc.
SU‐GG‐T‐543: 4D Imaging, Planning and Treatment with Cyberknife System
Authors: Lo A , Hancock S
Published: July 2008

Purpose: Complete 4D imaging, planning and treatment delivery for pancreatic cancer. Method and Materials: The patient was scanned using a GE CT scanner while recording respiratory excursion of the chest by monitoring an infrared reflector. Images and respiratory recordings were binned into 10 phases on a GE Advantage workstation. 4D animation was used to observe organ movement. Phase 5 was the most stable and selected as the primary planning CT for contouring on the Cyberknife system. Phases 0, 2, 4, 5, 6, 8 were sent to Cyberknife as a 4D CT with phase tags. Multiplan loaded the primary CT, RT structures and 5 phase tagged CTs. 2 gold seeds inside the pancreas PTV were identified in the primary CT and automatically propagated and registered in the 5 phases. Automatic deformable registration based on intensity was performed. Inverse simplex planning was performed and optimized to give 30Gy in 5 fractions to PTV (GTV+3 mm margin in all directions except toward the duodenum). Results: In the static plan, the 79% isodose‐line (30Gy, maximum 38Gy) covered 96% of the PTV. With organ movement assigned an equal weight for each phase the 79% isodose‐line (29.3Gy, maximum 37Gy) covered 91% of the PTV in the 4D plan. During treatment delivery, detectors monitored respiratory excursion of infrared reflectors that were placed on the chest of the patient and orthogonal x‐ray images were taken every 3 beams. The fiducial seeds were identified in the x‐ray images and correlated with the movement of the infrared reflectors. A movement prediction model of the seeds in Synchrony tracking guided the robot during radiation delivery. Conclusion: True 4D dose delivered by the Cyberknife tracking and delivery system by taking into account of organ motion can be estimated with 4D images and dose calculation during all phases of the respiratory cycle.
SU‐GG‐T‐546: How Many Plans Are Required to Represent Pareto Surfaces in Multi‐Objective IMRT Planning?
Authors: Craft D , Bortfeld T
Published: July 2008

Purpose: To demonstrate that the number of IMRT plans needed to fully represent the trade‐off space in multi‐objective optimization is small (<50). Method and Materials: For clinical IMRT cases, we compute a database of Pareto optimal plans using established methods. A treatment plan is Pareto optimal if there does not exist another plan which is better in at least one objective and at least as good in every objective. We analyze Pareto optimal plan databases by three methods. The first method examines the points as they are added sequentially, and for each newly added point we determine how “new” that point is (i.e. how far that point is from the convex hull of the previously generated points). Principal component analysis (PCA) on the set of beamlet solutions is used to understand the shape of the Pareto surface in beamlet space. Correlations between objectives are found by computing Spearman correlation coefficients on each pair of objectives. Results: We compute Pareto surfaces from 3 to 10 dimensions for pancreas, prostate, and brain cases. For all cases, we find that after about 40 plans, subsequently generated plans are very close to what can be achieved by averaging the previous plans. Therefore, allowing for averaging of plans, which is possible if the plans have not been sequenced yet, the entire Pareto surface can be well approximated by a small number of plans. Conclusion: Since the shape of the feasible set is relatively simple, as demonstrated by PCA, the Pareto surface is simple to describe, regardless of the number or types of objectives used. Once plans that span this space are computed, further plans do not add to the diversity of solutions available. Based on the analyses, we offer a simplified strategy for finding a small set of spanning Pareto optimal plans.
SU‐GG‐T‐549: Dosimetry of An X‐Ray Endocavitary Proctoscope Adapted for Use with the Axxent® Electronic Brachytherapy System
Authors: Rusch T , Klein E , Kelley L , Myerson R , Axelrod S
Published: July 2008

Purpose: To analyze the dose rate and uniformity at the aperture of an Electro‐Surgical Instrument (ESI). Proctoscope when used with an Axxent® Model S700 X‐ray Source. Method and Materials: The ESI proctoscope was originally designed for use with a Philips RT‐50 x‐ray contact therapy unit. Because these units are no longer supported, an alternative x‐ray source is being sought which allows for duplication of the Papillon technique for treating anal‐rectal lesions. A source holder was fabricated to position the Axxent model S700 X‐ray Source collinearly on axis within a lead‐lined proctoscope possessing a 24 mm inner diameter aperture. This mechanism allows the distance from the proctoscope aperture to the source to be adjusted from 1 to 10 cm and allows a filter to be placed at the distal end of the source to harden the beam and/or flatten the dose distribution. The dose rate was measured at the proctoscope aperture using a PTW 34013 Soft X‐ray Chamber set into the surface of a Gammex RMI 457 solid water phantom. GAFChromic EBT radiochromic film was used for profile measurements. Results: For operation at 50 kVp and 0.30 mA beam current with a 0.5 mm thick Al filter and source‐to‐aperture distance of 3.4 cm, the dose rate was 1.1 Gy per minute. Profile evaluation indicated a dose uniformity of 5%, 8% and 15% across 80%, 93% and 98% of the aperture diameter, respectively, without use of a flattening filter. Conclusion: Measurements of dose rate and uniformity at the aperture of a 24 mm diameter ESI proctoscope indicate that the Axxent S700 x‐ray source may be a suitable alternative to the Philips RT‐50 Endocavitary Unit for treatment of anal‐rectal lesions. Conflict of interest: Research sponsored by Xoft, Inc.