bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐314: Monte Carlo Investigation of the New Cyberknife with a High Dose Rate
Authors: Li JS , Fan J , Ma CM
Published: July 2008

Purpose: This work is aimed at improving beam characteristics by Monte Carlo simulations of the new 6MV Cyberknife (Accuray Inc, Sunnyvalle, CA) linear accelerator (LINAC) with an 800MU/min dose rate. Method and Materials: The BEAMnrc code was used for the linac simulation and the MCSIM code was used for dose calculations in a water phantom. The new linac design included a thinner tungsten target with copper backing and an aluminum plug inserted in the primary collimator opening. The lead slab above the ion chamber was removed from the old model. The effects of the aluminum plug with different length, the impact of the sealed ion‐chamber on the beam and the effects of the dimensions of the primary collimator opening were also investigated for optimal beam characteristics. Results: After many iterations of trial and error with the electron parameters, the final electron energy was chosen to be 6.8MeV and a parallel circular electron beam with a 2.5mm FWHM Gaussian distribution was used as the initial electron source. The agreement on PDDs and dose profiles between Monte Carlo simulations and measurements was excellent for all the cone sizes. Reducing the length of the aluminum plug from 9.58mm to 4.58mm led to a 4.3% increase in the dose rate, a slightly softer beam and little change to the dose at the buildup region. Replacing the open ion‐chamber with the sealed ion‐chamber led to a 4% increase in the dose rate and no significant change in the relative dose distribution. Reducing the straight portion of the primary collimator opening from 3.05cm to 1.05cm led to much flatter dose profiles. Conclusion: Monte Carlo simulations were performed successfully for the new linac geometry of the 800MU/min Cyberknife system. The linac geometry can be modified to further improve the system performance.
SU‐GG‐T‐330: A Feasibility Study of a Monte Carlo Based Dose Verification for a Therapeutic Proton Beam Delivery System
Authors: Shin Jungwook , Kwak Jungwon , Kim Dong Wook , Shin Dongho , Yoon Myongguen , Park Soah , Lim Young Kyung , Kim Jin Sung , Park Sung Yong , Cho Kwan Ho , Lee Se Byeong
Published: July 2008

Purpose: In order to investigate the possibility of the Monte Carlo based dose verification for a therapeutic proton beam delivery system installed at National Cancer Center, Korea (NCC). Method and Materials: The precise modeling of a therapeutic proton beam delivery system and the accurate evaluating of its initial beam characteristics were the primary prerequisite to virtually reproduce a real irradiation. Using the GEANT4.8.2 toolkit, we have implemented the nozzle elements along with the beam delivery path, including the movable components in time and space to produce Spread Out Bragg Peaks (SOBP) and uniform lateral‐dose distributions in the scattering mode. The Magnet fields, which vary in X‐Y direction, also have been realized in our simulation environment for the wobbling mode. Results: We have obtained simulation data that showed a good correlation to the measured dose distributions in several treatment conditions. Conclusion: The results present that when the Monte Carlo commissioning for the overall treatment range are completed we will be able to make practical application of the Monte Carlo simulation as an auxiliary tool for dose verification in addition to standard measurements.
SU‐GG‐T‐341: Energy Distributions of Particles Generated for Proton Interactions in Water: A Simulation with GEANT4 Monte Carlo Code
Authors: Chen Y , Ahmad S
Published: July 2008

Purpose: To simulate using GEANT4 (version 4.8.3), interaction of proton in water with energies of 60, 100, 150, 200, and 250 MeV for accurate determination of energy distributions of particles generated such as protons and neutrons. Method and Materials: A cylindrical water phantom (thickness = 1 cm, diameter = 2 cm, density = 1 g/cm3) was placed in front of a sensitive detector having the same dimension as the phantom. A pencil beam proton directed perpendicularly towards the water slab was used with the detector on the other side recording the energy distributions of particles generated. The simulations were carried out by 4 million incident protons for each of the proton beam energy. Electromagnetic energy loss processes for hadrons, electrons and positrons are categorized in GEANT4 as either “standard” or “low energy”. To extend down particle energies below the standard process, the low energy process was used in the simulation to cover protons with 1 keV; and electrons and photons with 250 eV. Range cut is lowered from the default value of 1mm to 15μm to improve the accuracy of simulation. The Hadronic process includes low energy elastic and inelastic scattering that consists of a pre‐compound nuclear interaction below 170 MeV, and a Bertini cascade model for energies above 150 MeV. The energy distributions of protons and neutrons are then normalized per incident proton. Results: The normalized proton and neutron energy spectrum show that the statistical uncertainties are mostly lower than 10% for most of the points in the proton energy distribution curves and that 20% for neutron curves. Conclusion: We have demonstrated that the GEANT4 toolkit has the ability for the radiation therapy proton beam simulation. This code will primarily be used as a standard code for our proton facility design and planning in our institution.
SU‐GG‐T‐346: Accurate Simulation for Therapeutic Protons Range‐Energy and Range‐Density Tables
Authors: Guo B , Liu Y , Cheng C , Ahmad S , Papanikolaou N
Published: July 2008

Purpose: The purpose of this work is to generate accurate range‐energy and range‐density tables for therapeutic protons using Monte Carlo simulation in water phantom. Method and Materials The percentage depth dose and the depth peak dose was calculated using Monte Carlo simulation for protons. The simulation was based on Monte Carlo code, MCNPX in water phantom with protons energy ranging from 40 MeV increasing by 10 to 250 MeV. The calculation of the three dimension dose matrix was performed using a 2 × 2 × 1 mm∧3 voxels in the depth peak range in water phantom. The pencil beam was used and the source was uniformly distributed in area with beam diameter as 20 mm. A 3,000,000 history was used in each individual simulation with uncertainty controlled less than 1.0%. Results: The total 22 data points of simulated depth peak were compared to two widely used proton range‐energy tables from International Commission on Radiological Units and Measurements (ICRU) Report 49 and from Data Nuclear Data Tables (DNDT). The simulated range‐energy curve matches very well with ICRU and DNDT with coefficient of determination equal to 1. The difference of the simulation and measurements is 1.17% at maximum and 0.70% at average compared to ICRU, 1.15% and 0.37% compared to DNDT respectively. The relation of range‐density was determined with water density ranging from 0.4 to 2.0 g/cm^3. The exponential relation of range‐density gives very closely matched estimation to the simulated value with the coefficient of determination as 0.9999. Conclusion: The simulated proton range‐energy matches very well to two widely used measurements. The accurate Monte Carlo simulation provides a close benchmark for existed measurements. The simulation offers the chance to replace the infinite experiments for each detailed energy and density. The simulation also expands the feasibility of accurate simulation in heterogeneity.
SU‐GG‐T‐348: Monte Carlo Calculations of Dose Distribution in NSCLC Patients Planned for Stereotactic Body Radiation Therapy
Authors: Moiseenko V , Popescu IA , Liu M , Kristensen S , Gill B , Teke T , Bergman A
Published: July 2008

Purpose: To quantify the accuracy of dose distributions produced according to the stereotactic body radiation therapy (SBRT) protocol BR25 using Monte Carlo (MC). Method and Materials: Eleven NSCLC patients previously treated with conventional radiotherapy, were selected as suitable for SBRT. For each patient three plans were produced in Eclipse TPS. Plan 1 was based on the BR25 protocol, i.e., a conformal plan with no inhomogeneity corrections. Plan 2 had inhomogeneity corrections enabled, but was otherwise identical to plan 1 (as QA requirement for the RTOG 0618 protocol). Plan 3 designed to meet planning objectives with inhomogeneity corrections enabled. Dose distribution from MC simulations for plans 2 and 3 were compared with those generated in Eclipse for target volumes and normal lung. Equivalent uniform dose (EUD) was calculated for PTV, and normal lung minus GTV. V20 was calculated for normal lung minus GTV. Results: Underdosing of PTV was observed in the MC simulations of Eclipse plans. While planning without inhomogeneity corrections gave the desired 100% PTV coverage with 95% of the prescription dose, MC simulation showed that typically less than 90% of the PTV was actually covered with this dose. For a representative patient, the PTV EUD was 60Gy as planned, 61.4Gy if the same plan was calculated with inhomogeneity enabled, and 58.8Gy if MC calculation was used. This underdosing was more pronounced for plans generated with inhomogeneity correction enabled: planned EUD=60.8Gy, and MC EUD=58.6Gy. Conclusion: Current SBRT protocols require that no inhomogeneity corrections are to be applied, which is a better choice than allowing inhomogeneity corrections. However, producing plans with inhomogeneity corrections for QA purposes, as required in these protocols, leads to misrepresentative dose distributions because of improper accounting for lack of backscatter and lateral electronic equilibrium. MC‐based dose calculation should be recommended as a QA tool for lung SBRT.
SU‐GG‐T‐351: Using Dose Mass Histograms (DMH) for the Evaluation of Head and Neck IMRT Plans Calculated by Monte Carlo
Authors: Mora G , Li J , Fan J , Jesus AP , Ma CM
Published: July 2008

Purpose: Dose volume histograms (DVH) represent an important tool for the clinical evaluation of radiotherapy treatment plans. For head and neck regions, however, the presence of air cavities makes the DVH tool less adequate for Monte Carlo calculated IMRT plans. The air cavities may introduce high dose uncertainties in both air cavities and surrounding tissues (interface effects). Since the dose to air is clinically irrelevant DVH becomes less clinically representative in the presence of large air cavities, and hence is no longer a good parameter for plan evaluation. In this work we assess the limitations of DVH for head and neck plan evaluation and investigate the dose mass histogram (DMH) as an alternative to overcome those limitations. Method and Materials: Seven IMRT head and neck cases were included in this study. The Monte Carlo simulation geometry (2mm voxels) was built from patient CT data. Patient dose calculations were performed using the EGS4 based MCSIM code and photon source models for 6 and 10MV beams. Isodose distributions and DVH (including air) were generated for these plans. A new feature was created in order to calculate DMH for both the target (PTV) and critical structures. In addition, DVH excluding air were also generated. Results: Our results for the 7 patients show significant differences (up to 10%) between the DVH calculated including and excluding air for the PTV and critical structures. DMH, on the other hand, eliminates the effect of large Monte Carlo statistical uncertainties in air cavities and is a better parameter than DVH for evaluating head and neck IMRT treatment plans. Conclusion: DMH is a better parameter than DVH for evaluating treatment plans calculated by Monte Carlo simulations that may have large statistical uncertainties in low‐density regions such as air cavities and lung tissues.
SU‐GG‐T‐438: Rind Radiosurgery: Investigation of a Novel Radiation Therapy Technique for the Treatment of Intracranial Lesions
Authors: Hosfeld V , Kirkpatrick J , Oldham M
Published: July 2008

Purpose: Rind radiosurgery is a highly modulated technique that aims to treat the proliferative disease of ring‐enhancing intracranial lesions, and to reduce the dose to normal brain tissue when compared with conventional radiosurgery. This study aims to quantify the potential dose reductions to normal tissue for rind treatments. Method and Materials: A set of IMRT treatment plans were created in the Eclipse planning system to quantify dose sparing for a range of lesions (diameter = 3, 5, 7, 9 cm) and rind thicknesses (0.5, 1.0, 1.5, 2.0 cm). Plans were created for both the conventional (solid tumor) and rind (ring‐enhancing) scenarios. A variety of beam arrangements and IMRT constraint schemes of varying levels of complexity were evaluated. All doses were computed using the pencil beam convolution algorithm with a dose grid size of 1.25 mm. The dose reduction to normal brain was evaluated by comparing the integral dose to a 0.5 cm shell of normal tissue, encapsulating all PTV's. Results: The reduction in normal tissue integral dose ranged from 0 to 5.5% with the greatest savings correlating with smaller lesion sizes. Maximum dose sparing of 5.5% was achieved using a 12 non‐coplanar beam arrangement, and a constraint scheme which held no limits on dose deposited in the center of the tumor. A dramatic increase in the number of monitor units of up to 123% was observed in the delivery of rind distributions. Conclusion: These results indicate that the integral dose to normal brain tissue can be reduced with rind radiosurgery IMRT treatments. The dose sparings were less however than previously reported with radiosurgery cone technique, and this is probably due to leakage associated with the extreme modulation of rind plans.
SU‐GG‐T‐443: Effect of Inhomogeneity On Dose Distribution for Gamma Knife(R) Perfexion(TM) Treatment
Authors: Xiong W , Huang Y , Lee L , Burman C
Published: July 2008

Purpose: To investigate the dose perturbation in Gamma Knife® Perfexion™ Stereotactic Radiosurgery due to inhomogeneity. Method and Materials: We applied Monte Carlo (MC) simulation for Gamma Knife® Perfexion™. Approximation parameters for geometries and materials were used to reconstruct 4mm, 8mm and 16mm collimators in Gamma Knife® Perfexion™ unit. A total of 192 Cobalt‐60 sources with the same activity were simulated. Phase space files stored all particles passing through collimator and reaching a sphere surface of surrounding the isocenter. Patient's geometries were rebuilt from CT data. The doses were calculated using MC simulation with and without inhomogeneity correction. Dose perturbation effect was derived from the comparison of DVH between homogeneous and inhomogeneous geometries. Results: Three patients were investigated. For each patient, at least 2 sites were selected: one in brain and another one close to air cavity. A total of 14 Monte Carlo simulations were performed for 16mm collimator with and without inhomogeneity correction. The statistical error for isocenter dose is around 0.2%. Monte Carlo results show that inhomogeneity effects lowered the dose, on average, by 4.6% in brain site. For sites close to the air cavity, the inhomogeneity effects are mixed. The inhomogeneity effect ranged from 2.7% to −1.3%. Less attenuation from air cavity increases the dose to the site. However, the high density area such as spine results more attenuation and it eventually reduces the dose. Conclusion: Monte Carlo shows that the inhomogeneity effect could cause lower delivered doses from Gamma Knife® Perfexion™ in brain sites and higher or lower doses in those sites close to air cavity. More investigations are necessary to determine accurate dose for treatment sites close to air cavity for Gamma Knife® Perfexion™.
SU‐GG‐T‐444: Electronic Transmission of Gamma Knife Records to a Radiation Oncology Record and Verify System and E‐Mail
Authors: Karsch K , Drzymala R
Published: July 2008

Purpose: Create a mechanism for paperless documentation of Gamma Knife treatment plans (GammaPlan) in PDF format that can be integrated into a record and verify system for radiation oncology. The mechanism must not violate FDA approval of the GammaPlan computer environment. Method and Materials: GammaPlan has a separate binary database that is not directly importable into the Mosaiq record and verify system without the development of in‐house software, but inclusion of “foreign software” into the GammaPlan UNIX‐based computer platform would violate FDA approval. Therefore, a mechanism was devised that uses a combination of scripts in the UNIX environment and a generic Windows XP‐based PC that “listens” on the local network, to convert and combine user selected paper printouts into one PDF file. Various “Freeware” programs were installed on the PC, which include Ghostscript, Fictional Daemon, and ImageMagick. UNIX Korn shell scripts were written for the GammaPlan platform and are launched when each document is printed as a PostScript file by GammaPlan. These files are stored locally before sending them to the PC for conversion to a PDF document. The PDF file is uploaded to the eScan folder of Mosaiq for manual importing into the Documents folder. A Korn shell script uses Elm to e‐mail the PDF document with a comment and HIPAA notice to the physician's e‐mail account. Results: The software has been used for 7 months without significant problems. One potential problem is that the PC must be logged onto in order for the software to work. We check that the computer is up and running from the UNIX platform. Also, we had two instances when the PC was running https which blocks communications between GammaPlan and the PC. Conclusion: The software works well as it was designed under the constraints imposed by the FDA.
SU‐GG‐T‐449: Peripheral Dose Investigation of a Corpus Collosotomy by Gamma Knife, Model C
Authors: Zhao H , Drzymala R , Klein E , Smyth M , Mansur D
Published: July 2008

Purpose: Corpus collosotomy by Gamma Knife Radiotherapy is a treatment for medically refractory epilepsy. Due to the long treatment duration (3–6 hours), the peripheral dose received by patients becomes a concern. In this study, peripheral doses for a posterior corpus collosotomy by Gamma Knife, Model C were investigated. Method and Materials: A Rando‐Phantom was used to measure the peripheral doses. Three types of dosimetry systems were used, which included ionization chambers, TLDs (TLD‐100 3×3×1mm chips), and OneDose™ MOSFET dosimeters. The dosimeters were placed on the surface of the Rando‐Phantom under 0.5 cm of build up at distances of 5.0, 17.5, 30.0, 42.5, 54.0, 70.0 and 84cm from the center of the treatment volume. Seven cylindrical ionization chambers with buildup caps were placed at the same distances, one chamber at each position. The leakage of the ionization chambers was measured before irradiation and recorded during treatment sessions. The phantom was irradiated using a clinical treatment plan for a posterior corpus collosotomy. The prescription dose was 130Gy (max dose), and the delivery dose was reduced to one third of the prescription dose for measurement purposes. The treatment time was 99.38 minutes. Results: The measured doses by TLD were 17.9, 9.9, 5.8, 2.9, 2.4, 1.5, and 1.1cGy for the seven positions respectively. The measured doses by ionization chambers were 16.9, 12.0, 5.2, 2.9, 2.8, 2.2, and 2.0cGy. The results obtained with the OneDose™ were not consistent due to the decay of the signal over the period of measurement. The doses measured by TLD and ionization chamber for the same position were averaged. The total doses to the measurement positions were 52.2, 33.0, 16.5, 8.7, 7.8, 5.7, and 4.8cGy, respectively, for the full prescription dose of 130Gy. Conclusion: The peripheral doses were significant, especially in the head and neck region.
SU‐GG‐T‐450: Measurement of Relative Output Factors for the Leksell Gamma Knife PERFEXION by Film Dosimetry
Authors: Novotny J , Bhatnagar J , Quader M , Huq M
Published: July 2008

Purpose: To measure relative output factors (ROFs) for 4 and 8 mm collimators for the Leksell Gamma Knife PERFEXION unit. Effective ROFs calculated by Monte Carlo (MC) by ELEKTA and used in the Gamma Plan 8.0 treatment planning system are 0.805, 0.924 for 4 and 8 mm collimators, respectively. Method and Materials: Three types of films, Kodak EDR2, gafchromic EBT and MD‐V2‐55, were used within the spherical polystyrene phantom of diameter of 160 mm. To obtain calibration curve 10 films were exposed to doses of 0–400, 0–800, 0–8000 cGy for EDR2, EBT, MD‐V2‐55 films, respectively. Three samples of each film type were then exposed in identical setup for 4, 8 and 16 mm collimators. Films were scanned by EPSON EXPRESSION 10000 XL scanner with 200 dpi resolution in 16‐bit grayscale for radiographic and 48‐bit color for gafchromic film and imported in red channel. Films were evaluated by FilmQA version 2.0.1215 software. Fifth degree polynomial fit for calibration curve was used and background corrections applied for all films. Doses delivered to the center of each of studied films by 4, 8, 16 mm collimators were obtained and ROFs calculated and corrected for transit dose. Results: Measured ROFs and deviations compared to MC calculated value for 8 mm collimator were 0.899 (−2.7%), 0.917 (−0.8%), and 0.920 (−0.4%) for EDR2, EBT, and MD‐V2‐55 films, respectively. Measured ROFs and deviations compared to MC calculated value for 4 mm collimator were 0.778 (−3.4%), 0.818 (1.6%), and 0.804 (−0.1%) for EDR2, EBT, and MD‐V2‐55 films, respectively. Conclusion: MD‐V2‐55 film seems to be an ideal dosimeter for small stereotactic field ROFs measurements. All results obtained in this study were in close agreement with MC values. Largest deviations noted from the MC were for EDR 2 films. The closest agreement with MC values was observed for MD‐V2‐55 films.
SU‐GG‐T‐451: Clinical Evaluation of New BrainLab Image Guided Frameless SRS System
Authors: Qian Y
Published: July 2008

Purpose: To evaluate the results and accuracy of Image Guided Frameless SRS. Method and Materials: A new image guided frameless system for Novalis has been installed at our institution for Stereotactic Radiosurgery (SRS) treatment. The main features of the system are its utilization of an Infrared camera, a pair of kV x‐ray, and a 6D couch for patient setup. The system works at any couch angle. The tracking computer will generate DRRs for the corresponding couch angle and compares them with OBI kV images to determine the shift corrections in 6 Dimensions (3 Translational and 3 Rotational). The shift corrections are carried out by the 6D robotic couch, The same process is repeated to confirm the correction was successful, and that further correction is no longer needed. The same process is required for every couch angle during the SRS treatment. The SRS fields are shaped by employing either BrainLab micro MLC or BrainLab cones. We have treated various types of brain tumors including trigeminnal nerves. Results: The system is very intutive, and in general only one OBI guided frameless correction procedure is needed for each couch angle. The OBI kV images obtained for confirmation of correction on average show that the mean deviation by automatic imaging fusion computer are 0.5±1, 0.2±1 and 0.3±1 degree for pitch, roll and yaw, 0.3±0.3, 0.3±0.4, 0.2±0.8 mm for longitudinal, lateral and vertical translation movement. Included are the pre and post trigeminal nerve MRI scan that shows clearly that we hit the target of planned treatment. Conclusion: The new type of BrainLab' Image Guided Framless SRS system coupled with an 6D robotic couch, imaging at any couch angles, is accurate, and easy to use. Initial experience shows that clinical results are equivalent to the Frame based SRS system.
SU‐GG‐T‐459: Analysis of Setup and Dosimetric Errors of KV CBCT Guided Stereotactic Intensity‐Modulated Radiosurgery for Spinal Lesions
Authors: Yang Y , Li X , Li T , Fu W , Heron D , Huq M
Published: July 2008

Purpose: To investigate target localization accuracy and dosimetric influences arising from the setup errors and patient motion for an on‐board kV CBCT‐guided spinal stereotactic intensity‐modulated radiosurgery (IMRS). Method and Materials: Eight spinal patients treated using CBCT‐guided stereotactic IMRS were analyzed for this study. All patients were immobilized using BodyFIX® immobilization system (Medical Intelligence) and set up with the guidance of Varian on‐board CBCT. Four sets of images were acquired for each treatment to evaluate patient's intrafraction motion: CBCT and orthogonal kV images before treatment, kV images in the middle of treatment and another CBCT at the end of treatment. Target localization accuracy and intrafraction motion were evaluated by offline analysis of the registrations between planning CT and CBCTs as well as between DRRs and kV images. CBCT images were imported into Eclipse to perform CBCT‐based dose calculations using patient's real treatment positions and isocenters. The calculated results represent the delivered dose distributions under the real treatment situations and were used to assess the dosimetric changes arising from setup errors and intrafraction motion. Results: The results show that the target localization accuracy is 1.4mm and the overall intrafraction target displacement is less than 2.0mm using our IMRS technique. The averaged target coverage reduction is ∼5% due to the target localization error and 11% due to both the localization error and the intrafraction motion. The largest increase of the maximum cord dose is 4.8Gy due to the target localization error and 7Gy due to the intrafraction motion. Conclusion: CBCT‐guided stereotactic IMRS can provide a target localization accuracy of less than 1.5mm for spinal treatment. With the BodyFIX® system, the intrafraction displacement can be effectively reduced to 1∼2mm. The intrafraction motion at this scale may still have potential influence on the target coverage and the cord dose due to the extreme dose conformity of IMRS.
SU‐GG‐T‐467: On the Sensitivity of Film‐Based, In Vivo Dosimetry to Setup Errors for Stereotactic Body Radiotherapy Treatments
Authors: Giantsoudi D , Gutiérrez A , Shi C , Papanikolaou N
Published: July 2008

Purpose: To determine whether film‐based, in vivo dosimetry can be used to verify precise dose delivery of stereotactic body radiotherapy treatments and to examine its sensitivity in detecting rigid body shifts and introduction of inhomogeneities. Method and Materials: A QUASAR™ respiratory motion phantom was CT scanned, in static mode, with an imaging insert containing two spheres of different sizes and a cube. A SBRT treatment plan was created with TomoTherapy TPS utilizing one of the spheres as the target volume. Dose prescription was 98% of target volume to receive at least 60.0Gy delivered in 5 fractions. The image set was imported into tomotherapy TPS as a patient image set for treatment planning and as a phantom image set for delivery quality assurance verification. Radiographic film was place at the couch table top below the phantom (coronal plane) during irradiation so as to measure the exit dose. Delivery was repeated with the introduction of various lateral shifts and inhomogeneities to the phantom. Calculated exit dose planes were extracted from the DQA analysis software and compared with measured films. Results: Agreement between calculated and measured exit dose planes for the original setup was shown to be within 10%. Comparison profiles of the calculated and measured exit dose planes between the 2cm lateral shift and original setup indicate insufficient sensitivity of the film measurements to distinguish the expected differences and patterns. Similar conclusions were reached for 5cm lateral shifts and introduction of inhomogeneities. Conclusion: According to preliminary results, film‐based, in vivo dosimetry can be suitable for SBRT treatment verification but was shown not to be sensitive to detection of rigid body shifts and inhomogeneities. Further studies of different treatment sites are planned to determine if film‐based, in vivo dosimetry can be used as a secondary technique for patient setup verification.
SU‐GG‐T‐470: Improving Rectal Sparing with a Single Anterior Proton Beam: A Planning Study
Authors: Melancon A , Lee A , Kudchadker R , Dong L
Published: July 2008

Purpose: To demonstrate the potential dosimetric advantage of an anterior proton beam for treatment of prostate cancer patients utilizing the Bragg peak for rectal sparing. Methods to detect the anterior proton beam at depth are being investigated to eliminate the large distal planning margin, enabling abrupt dose falloff at the anterior rectal wall rather than the relatively shallow falloff of the lateral beam penumbra with conventional lateral beam arrangement. Method and Materials: 27 consecutive prostate cancer patients treated with proton therapy were enrolled in a planning study to compare standard IMRT and bi‐lateral proton treatments with a single anterior proton beam treatment. Dose‐population histograms were generated comparing target coverage and normal tissue sparing of all critical structures between the treatment groups. Results: The anterior beam treatment significantly spared the anterior rectal wall more than the IMRT and bilateral beam treatments at the 70 Gy (12.3% vs 19.2% and 19.2%), 60 Gy (15.7% vs 23.8% and 24.3%), 45 Gy (19.4% vs 30.1% and 29.6%), and 30 Gy dose levels (22.7% vs 39.9% and 34.2%). Bladder wall dose was greater in the anterior beam treatment than IMRT and bilateral beam treatments at the 70 Gy (14.1% vs 11.2% and 12.8%), 60 Gy (22.5% vs 13.7% and 16.2%), 45 Gy (29.0% vs 17.9% and 19.8%), and 30 Gy dose levels (32.6% vs 25.1% and 23.1%). No dose was delivered to the femoral heads with the anterior beam treatment while the IMRT and bilateral treatments delivered a mean dose of 17.0 Gy and 25.1 Gy to the femoral heads respectively. Conclusion: With the implementation of in vivo proton beam detection, anterior proton beam treatment of prostate is a possible treatment alternative, sparing dose to the rectum and femoral heads at the cost of a small increase in bladder dose.
SU‐GG‐T‐473: The Effect of 4DCT and 3DCT Image Data Set On Dose Calculation and Optimization: Preliminary Results
Authors: Rassiah‐Szegedi P , Roland T , Szegedi M
Published: July 2008

Purpose: To investigate the effect of 4D CT image reconstruction and static helical CT data set (static) on dose calculation and optimization. Method and Materials: A typical breathing cycle was programmed into the CIRS moving tissue equivalent lung phantom which contained a 2 cm diameter target. 4D CT data scans were acquired on the LightSpeed 4‐slice CT (General Electric, WI). The Maximum intensity projection (MIP), average intensity projection (AIP), minimum intensity projection (MinIP) was reconstructed. Two 3D helical scans were also acquired (1) with the phantom moving and (2) with phantom static. Treatment plans were generated on Pinnacle 7.6 to study the effect of different electron densities (pixel value) at the tumor edge predicted by the 5 image sets. The same dose constraints and gantry angles were used to generate plans. The fluence maps for each gantry angle were compared. Results: Both the MIP and the 3D static target produced similar relative intensity maps, where a more homogeneous fluence distribution was observed. The fluence ratio for the center of the target to the edge of the target is greater for the AIP. i.e greater fluence gradient. Conclusion: Planning using MIP or AIP of a 4DCT plays a role in the final dose distribution delivered to the patient. Planning using the MIP will produce a more homogenous delivered dose to the lesion regardless of tumor location, whereas the AI will deliver a higher dose to the lesion at the end of the tumor trajectory. Further investigation is needed to study the impact of the distance of target movement on the final dose distribution obtained via 4D CT.
SU‐GG‐T‐474: A Virtual Linac for External Beam Radiation Therapy Treatment Planning
Authors: Sheu R , Lo Y , Svoboda A
Published: July 2008

Purpose: A virtual linac is developed to graphically simulate external beam radiation therapy treatments in a 3D environment. The patient‐specific simulation is useful for assessing potential collisions and for assisting patient setup. Method and Materials: Interactive Data Language (IDL) is used to create the virtual reality environment. Geometry and dimensions of the simulated treatment devices, including accessories, are obtained from measurements of our own machines as well as limited use of schematics. DICOM filters are implemented to process the planning data exported from the treatment planning system. Results: The patient's body contour, either outlined in the treatment planning system or generated by the IDL program from CT images, can be displayed as part of the 3D virtual linac. All machine components including the gantry, collimator, couch, and accessory selection can be adjusted interactively. Additionally, the patient's position on the couch can be translated in three dimensions. The result is the ability to accurately simulate almost any external beam treatment. Conclusion: Since the 3D graphical models of the treatment units and the patient outline were established, the collision avoidance can be performed by visually simulating the treatment plan. Our virtual linac system includes add‐on accessories such as the electron cones and the SRS head frames; users can easily measure the clearance of device‐to‐patient‐couch and it does not need to use any complicated mathematical model. This virtual reality software provides highly intuitive images for both planners and therapists for designing treatment plans and positioning patients. It can be applied as a pre‐treatment QA tool.
SU‐GG‐T‐479: Dosimetric Comparison of Whole Breast Irradiation Using Helical Tomographic Technique and Static Multileaf Collimated Field‐In‐Field Technique
Authors: Hsieh H , Chang S , Yuan J , Pino‐y‐Torres J , Chen W
Published: July 2008

Purpose: To review the dosimetry results of whole breast irradiation using a helical Tomotherapy Hi‐Art device and discuss the feasibility of such treatment technique by comparing the data with linear accelerator based Eclipse field‐in‐field (FIF) technique. Method and Materials: Twenty breast plans were generated using a Tomotherapy Hi‐Art planning system. Among them, 11 were left breast cases and 9 were right breast cases. Treatment planning techniques such as contour delineation and optimization strategy/parameters were described. Key dosimetric results including target dose coverage and organs‐at‐risk (OAR) dose statistics were compiled and presented. Radio‐biological consequences to the OARs were studied using the normal tissue complication probability (NTCP) models. A retrospective study using the forward FIF planning technique was also performed for those 20 patients as comparison benchmark. Results: On the ipsilateral side, the Tomotherapy plans performed slightly better than the forward FIF plans in target coverage and dose uniformity (p=0.0018), and significantly better in OAR high dose statistics such as V20 in lung (p=0.0001) and V30 in heart (p=0.0003). Meanwhile, the Tomotherapy plans delivered slightly higher average dose to both ipsilateral and contralateral OAR, including the heart, the lung, the breast, the esophagus, and the spinal cord. However, the differences in NTCPs between the two techniques for the total lung (p=0.298) and the heart (p=0.268) were statistically insignificant, whereas the NTCPs for the esophagus were too low to compare. Conclusion: The Tomotherapy technique can provide better target coverage for breast cancer treatment. The differences in NTCPs associated with slightly higher mean doses to the OAR were insignificant comparing with the forward FIF technique.
SU‐GG‐T‐483: Practical Guidelines for Radiation Treatment Planning and Dose Constraints for Gastro‐Intestinal Cancers
Authors: Bouchard M , Briere T , Beddar S , Das P , Delclos M , Krishnan S , Crane C
Published: July 2008

Purpose: With the advent of conformal radiation therapy (3D‐CRT) and intensity‐modulated radiation therapy (IMRT), dose constraints to organs at risk (OARs) based on 2D planning toxicity data have become obsolete. The purpose of this report is to provide practical guidelines to the treatment planner for gastro‐intestinal cancers, based on the standards of practice at our institution and our experience from a large patient population. Method and materials: We describe our standard clinical practice for constraints on OAR from inhomogeneous irradiation data (in equivalent 2‐Gy/fractions) for gastro‐intestinal treatment planning, delineation of OARs and margins added to the target volumes. We use the same constraints for 3D‐CRT and IMRT. Results: For upper abdominal sites, we use the following dose‐volume histogram (DVH) constraints: esophagus (V50Gy < 50%, maximum dose < 70Gy), stomach (maximum 54Gy), heart (V40Gy < 40%, V60Gy < 25–30%), lungs (V20Gy < 35%), kidneys (V20Gy < 33% each), and spinal cord (maximum 45Gy). For the whole normal‐functioning liver, we limit V20Gy < 66%, V30Gy < 50% and NTCP < 10%. For lower abdomen sites, we keep the DVH of the small bowel limited to V20Gy < 50%, V45Gy < 10% and maximum 50Gy; bladder, V45Gy < 20%; genitalia, V20Gy < 67% and V30Gy < 20%. A predetermined Internal Target Volume (ITV) margin of 1cm radially and 2cm superior‐inferiorly is added for neoadjuvant gastric radiation treatment. The ITVs for the esophagus and liver are determined using 4D‐CT. A 10‐mm planning target volume margin is used when imaging is performed weekly, and 5 mm when imaging daily. Conclusion: Guidelines on constraints and margins for 3D‐CRT planning provide practical tools to the planner. Uniformity of the planning process might lead to more accurate indications about treatment risks, with planning efficiency and treatment safety being the ultimate goals.
SU‐GG‐T‐490: Analysis of Clinical Data Sets by Comparative DVH Analysis
Authors: Bice W
Published: July 2008

Analysis of large clinical dosimetry data sets has been limited to evaluation of single number dosimetric quantifiers (like V20 or D90). The quantifiers are linked with clinical outcomes and conclusions drawn based on comparative statistical tests. These quantifiers are often a matter of guesswork, at best chosen by recommendation of investigators who have performed similar analyses. A tool for comparative analysis of the entire dose volume histogram for large data sets has been developed and is described. This tool has been used as an interface between several treatment planning systems, external beam and brachytherapy. The downloaded structure dose volume histograms and data set analysis and comparison are performed within the tool. As an example of this process, a series of implants, performed at multiple institutions, has been analyzed following import from Variseed (Varian Medical Systems). Ten patients who had RTOG Grade 2 rectal complications following the implant were compared to twenty‐three patients who did not experience any rectal complications. While a similar analysis has been performed previously, this work was limited to analyzing rectal dosimetry in terms of 2 or 3 rectal dose quantifiers. While the previous effort was able to show statistical significance with R100, it is apparent from the present work that the critical doses for rectal complications are much more likely associated with doses to the rectum which are much lower than the prescription dose, in the range of 40–50 Gy. This is verified by improved significance of the correlation between the two arms of the cohort at these dose levels. The monotherapy dataset demonstrates the utility of the tool and the associated process. The tool has also been used to analyze external beam data sets to demonstrate the dosimetric improvement of inverse planning over conventional planning for external beam radiotherapy.
SU‐GG‐T‐494: Dosimetric Uncertainties in Matched IMRT and Conventional Fields for Head and Neck Cancer Radiotherapy
Authors: Hu A , Rembert J , Akazawa C , Yom S , Quivey J , Xia P
Published: July 2008

Purpose: Two techniques using IMRT for treatment of head and neck tumors are commonly employed: (A) extended‐field IMRT (EF‐IMRT) with a single isocenter; (B) Beam‐split method with an AP field for the low neck/supraclavicular fossa. Beam‐split technique used in the latter approach introduced dosimetric uncertainties at the matchline due to imprecise calibration of the independent jaw or MLC positions. The objective of this study is to evaluate the potential matchline dose variation if such a systematic error occurs. Method and Materials: Five patients with nasopharyngeal cancer treated with EF‐IMRT technique were replanned with the beam‐split approach. A midline cord block was used in the AP fields. To simulate the effect of a systematic error in jaw/MLC positions, analyses were performed with the conventional field(s) matched exactly and shifted ± 1,2,&3mm (superior‐inferior) relative to the IMRT fields. Dose covering 95% of the tumor volume (D95) was measured for a 1.2 cm thick volume of the CTV centered at the matchline (mCTV) to assess tumor volume dose coverage. Dose encompassing 1 cc (D1cc) of the spinal cord volume within the IMRT fields were analyzed. Results: For the beam‐pslit technique, overlap of the AP beam with the IMRT fields led to dose increases in D1cc of the spinal cord by 3.14Gy ±0.57Gy, 5.45Gy ± 1.97Gy, and 9.9Gy ± 5.33Gy for 1, 2, and 3 mm overlaps, respectively. A gap between the AP field and the IMRT fields led to dose reductions in D95 of the mCTV by 4.55 ±1.86Gy, 8.38 ± 2.52 Gy, 13.92 ± 0.36Gy for 1, 2 and 3 mm gaps, respectively. Conclusion: Systematic errors due to independent jaw/MLC imprecision can potentially lead to concerning dosimetric variations when matching conventional and IMRT fields in head and neck radiotherapy. Stringent quality assurance on MLC/jaw positions is required when the beam‐split method is applied.
SU‐GG‐T‐501: Dosimetric Effects Caused by Interplay Between MLC‐Delivery Dynamics and Organ Breathing‐Motion in IMRT for Lung
Authors: Seco J , Sharp G , Paganetti H
Published: July 2008

Purpose: Interplay between IMRT multi‐leaf collimator (MLC) delivery and patient's breathing motion can affect the delivered dose distribution. Interplay is the correlated effect between MLC‐delivery and organ‐motion, due to either MLC motion or initial breathing phase dependence of MLC‐delivery. This study investigated the clinical relevance of interplay for lung IMRT plans assuming free‐breathing delivery. Method and Materials: An in‐house developed 4D Monte Carlo system was used for patient dose calculations. Three patients with lung carcinoma were studied, where motion‐amplitude was 1–2cm. A voxel‐displacement‐map was used to quantify the non‐rigid motion of organs in the CT‐geometry. In order to quantify the MLC position relative to the breathing phase, each photon going through the linac head is tagged to indicate the associated breathing phase during which it will enter the patient. 4DCT data was sorted into 6‐phases, inhale, exhale, 2 phases from inhale to exhale, and 2 phases from exhale to inhale. The exhale phase was used as reference phase. Interplay effects were studied on a beam‐by‐beam basis for a treatment in 30 fractions. Different starting initial MLC phases relative to the breathing motion were selected per daily fraction. Results: The volume of ITV covered by 5–15Gy isodose lines can vary up to 10% relative to the average volume covered per beam over 30‐fractions. Organ motion causes in some instances the dose to be primarily delivered to phases close to inhale and in other cases delivered to phases close to exhale, i.e. the reference phase. For hypo‐fractionation (10–15 fractions), the volume of ITV covered by 5–15Gy isodose lines can vary up to 12–15% relative to mean volume covered over 30‐fractions. Conclusion: Interplay between MLC‐delivery and organ motion can lead to under‐dosed regions in the ITV volume.
SU‐GG‐T‐503: Dose Calculation On Cone‐Beam Scans: A Correction Algorithm for Improved Dosimetry
Authors: Rowbottom CG , Marchant TE , Moore CJ
Published: July 2008

Purpose: The introduction of cone‐beam CT (CBCT) based IGRT has greatly improved our knowledge of patient anatomy during treatment. However, the calculation of dose distributions based on CBCT images is problematic, due to poor consistency of density calibration and image artefacts. A method for correcting CBCT images for use in dose calculations has been developed and implemented at our institution. Method and Materials: Post processing of the Elekta Synergy CBCT scans was performed to remove artefacts and spurious density variations. In the algorithm the pre‐treatment CT scan was used as a reference for the density of regions in the CBCT image, providing a ‘shading map’ of the differences between the two. Smoothing of the shading map was performed to remove high frequency content. Testing of the correction algorithm compared density values for corresponding tissue types, certified that non‐artefact density changes are maintained, and compared dose calculations based on corrected CBCT scans to pre‐treatment CT calculations. Results: CBCT based dose calculations have been developed for prostate & lung treatment sites. For both sites the densities within the corrected CBCT scan were closer to those in the pre‐treatment CT scan. In the example of prostate patients CT numbers in regions of fat and muscle tissue in the corrected CBCT were within 1% of the values in the planning CT, as opposed to 10–20% different for the un‐corrected CBCT. The improvement in the density within the CBCT scan improves the dosimetric quality of the scan, providing a means of more automated treatment planning based on CBCT scans. Conclusion: A method for correcting CBCT scans for re‐calculation of delivered dose has been developed and validated. The enhanced CBCT scans greatly improve the agreement between CBCT calculated doses and the expectation based on the planning CT.
SU‐GG‐T‐506: Calculation of Positron Emitters' Matrix for Absolute Dose Reconstruction in Proton Therapy
Authors: Fourkal E , Fan J , Ma C
Published: July 2008

Purpose: The short‐lived radioisotopes, mainly , and produced during proton therapy allow imaging of three dimensional in‐vivo activity distribution using positron emission tomography (PET). In our previous study we explored the possibility of reconstructing in‐vivo absolute dose distribution using PET imaging information. Developed mathematical model requires the knowledge of the positron emitters' matrix ΔÑk (r, Ei), which can either be pre‐calculated using Monte Carlo method (time‐consuming but easily done) or calculated analytically (time‐efficient, but requires some abstract representations). The purpose of this study is to develop an analytical model for the calculation of the positron emitters' kernels which would significantly speed up the dose reconstruction procedure. Method and Materials: The positron emitters' matrix/kernel is calculated via the energy integration of the given nuclear activation cross section with the proton energy fluence spectrum at position r. The proton energy fluence spectrum in turn is found from the solution to the Boltzmann kinetic equation. Results: As a first step, the calculated proton energy fluence spectra for different initial proton energies have been compared to those obtained from Fluka simulations at different spatial points r. An excellent agreement between both approaches is found. In the second step, the nuclear activation cross sections for production of different species have been integrated with the proton energy fluence to obtain the positron emitters' kernel. In the final third step, the positron emitters' kernel is used to calculate the activity distribution and compare it to that simulated using Fluka MC code. An excellent agreement between the results of analytical calculations and computer simulations is also found. Conclusion: The developed analytical approach offers significant improvement in time efficiency for the problem of dose reconstruction from the measured PET distribution. It constitutes a necessary step forward needed for practical application of this imaging modality in in‐vivo proton beam dosimetry.