bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐496: A Feasibility Study of CT‐Based IMRT Planning for Total Body Irradiation
Authors: Hossain M , Chen Y , Fan J , Lin T , Price R , Li J , Ma C
Published: July 2008

Purpose: In this study we explore patient position options that allow sufficient CT scan acquisition for total body IMRT. We currently perform hand calculations of monitor units and compensator design based on a few direct patient measurements and a limited number of axial CT slices. The SSD available in our treatment room results in a treatment position not suitable for a full body scan. Method and Materials: Option (1) has the patient lying down in a bent knee position and scanned in two parts. In‐house software combines the two image sets into one complete full body scan. Treatment is via two lateral fields at extended SSD. IMRT plans are generated on CMS XiO and compared with Monte Carlo Treatment Plan. Comparisons of plans mimicking our current practice of using two opposed lateral beams are also made. In option (2) the patient is treated AP/PA at 180cm SSD. Full body scans are acquired with the patient lying straight in two parts. The patient is placed on a low cart for treatment and flipped for the PA fields. Three fields are needed to cover the whole body. An IMRT plan is generated on XiO. Results: DVH comparisons between XiO and Monte Carlo indicate that XiO underestimates higher dose and overestimates lower dose. The maximum lung dose and overall dose coverage is lower than hand calculated values in our current protocol. Conclusion: Both methods of CT‐based IMRT are feasible and offer improved distributions and lung sparing. The first approach offers convenient treatment position but requires extended SSD not available in all treatment rooms. The second approach is suitable for treatment in all treatment rooms but adds setup uncertainties and increased treatment time. Comparison of XiO plans with Monte Carlo simulation indicates that an extended SSD beam may have to be commissioned for XiO.
SU‐GG‐T‐497: Comparison of Prostate IMRT Plans From Three Commercial Treatment Planning Systems
Authors: Chi Z , Wang L , Chen L , Fan J , Jin L , Ma C
Published: July 2008

Purpose: Different treatment planning systems (TPS) use different treatment optimization and leaf sequencing algorithms. This work compares prostate IMRT plans optimized with three TPS to investigate the plan quality in terms of target conformity and delivery efficiency. Method and Materials: Eleven prostate cases were planned with the Corvus, Xio and Eclipse TPS using appropriate optimization parameters and dose constraints to meet the acceptance criteria. Plans were normalized for at least 95% of PTV to receive the prescription dose Dp. Dose‐volume histograms and isodose distributions were compared. Other quantities such as Dmin (the minimum dose received by 99% of CTV/PTV), Dmax (the maximum dose received by 1% of CTV/PTV), the volume of CTV/PTV receiving 110%, 105% and 95% of Dp (V110%, V105%, V95%), the volume of rectum and bladder receiving 65 and 40Gy (V65, V40), and the volume of femur receiving 50Gy (V50) were evaluated. Total segments and MUs were also compared. Results: While all plans meet target dose specifications and normal tissue constraints both XiO and Eclipse plans show less target dose heterogeneity (smaller Dmax and V110%) and lower V65 and V40 for the rectum and bladder compared to the Corvus plans. The PTV Dmin is about 2Gy lower for XiO plans than Corvus and Eclipse plans while the XiO and Eclipse plans have slightly higher V50 than the Corvus plans. The Eclipse and XiO plans require significantly less MUs to deliver than the Corvus plans. Conclusions To deliver on a Varian Trilogy accelerator, the Eclipse and XiO plans have better target dose uniformity, slightly less rectal and bladder doses and faster beam delivery. The Corvus plans have less rectal volumes receiving low doses (5–20Gy) while the XiO plans have slightly lower target doses. Overall, the Eclipse TPS is favored for our prostate IMRT planning.
SU‐GG‐T‐500: The Feasibility of Using the Anisotropic Analytical Algorithm (AAA) for IMRT Treatment Planning with the Varian Trilogy High Dose Rate (SRS) Mode
Authors: Hsu A , Mok E
Published: July 2008

Purpose: To determine the feasibility of using the anisotropic analytical algorithm (AAA) for IMRT treatment planning using the Varian Trilogy stereotactic radiotherapy (SRS) high dose rate mode. Method and Materials: Varian Trilogy has a special treatment mode designed for SRS. The SRS flattening filter limits the 6MV field size to 15×15 cm2 and allows a dose rate of 1000 MU/min at isocenter. AAA was implemented at Stanford Cancer Center in 2007 by modeling the intensity profile, extra‐focal photons and electron contamination using diagonal profiles of a 15×15 cm2 and an extrapolated 40×40 cm2 field. 8 IMRT treatment plans and 9 dynamic conformal arc patient plans were delivered to a homogeneous solid water phantom. Verification measurements were made using ionization chamber and film. Results: Percent depth doses calculated using AAA of open fields defined by collimators, agreed with measurement to within 2%. However larger discrepancies, 2.6%, were observed in small fields defined by MLC at depths larger than 20 cm. Of the 8 IMRT patient plans, measurements showed differences from 0.25% to 3.7%. The 9 dynamic conformal arc plans had a consistently higher discrepancy with AAA calculations, 0.78% to 6.5%. It was found that for smaller target volumes for both IMRT and conformal arc plans, differences between AAA and measurements are greater. It was also noted that for the dynamic conformal arc plans, the discrepancy with measurement is related to treatment volume size with larger variations for smaller target volumes. Conclusion: Due to the limited maximum field size and small field sizes used in SRS, implementation of AAA proved to be a challenge. It is shown that acceptable results, (average ∼2%) though not perfect, could be achieved. AAA is still the preferred calculation algorithm for clinical use as the majority of our SRS cases are targeting the thoracic region.
SU‐GG‐T‐509: Impact of Gating On Dose Escalation in Lung Cancer Patients
Authors: Antony J , Loo B , Carlson D , Maxim P , Luxton G , Xing L
Published: July 2008

Purpose: To investigate the potential impact of gating on treatment planning for lung tumors by evaluating the escalated dose achievable using 3D conformal radiation therapy (3D CRT) on gated tumors. Method and Materials: Twelve lung patients were contoured for gated and non‐gated treatments by keeping using comparable toxicity of normal tissue as dose constraints, and 3D CRT plan was done on targets so that 100% of dose covered 95% of the target volume. The Linear‐quadratic tumor control probability (TCP) model was used to determine the impact of dose escalation on local tumor control. Normal tissue complication probability (NTCP) was calculated using the Lyman‐Kutcher‐Burman model. Results: Three out of twelve patients exhibited significant differences between gated and non‐gated ITV volumes with tumor motion >1cm between the respiratory phases, in both the superior and inferior tumor margins. For three of the 12 patients no dose change was found between gated and non‐gated plans. One patient showed a dose decrease in gated case, due to nonlinearity of the tumor motion with respect to the breathing phase, i.e., tumor in 30% phase may move closer to an OAR than the tumor at 0% or 50% phase. Conclusion: Gating in radiotherapy treatment planning allows reduction in tumor margin, and clinically significant dose escalation up to 12% was found, while staying within clinically acceptable toxicity criteria. Deformation of the tumor becomes highly important in dose escalation, when the target is deformed in a way that alters the proximity of the tumor to an OAR. Tumors in the middle and lower portion of the left lung and mediastinal tumors could benefit, resulting in dose escalation. NTCP increased with mean lung dose for the lung (total lung‐PTV) and TCP increased with modal dose value up to 80 Gy, above which TCP was unity.
SU‐GG‐T‐516: Dose Calculation Verification for Stereotactic Treatment Plan
Authors: Schifter D , Kanner A , Corn B
Published: July 2008

Purpose: Verification of SRS treatment plans was a challenge in the early beginning of stereotactic radio surgery. The large amount of dose deposited in a single fraction through a small collimator or MMLC was a cause of sleepless nights for many physicists. The traditional way was using a “hand calculation”. In some cases a special measurement in phantom was performed (while the patient was in the SRS ring). This work suggests a quick and accurate way using a standard TPS. Method and Materials: The original treatment plans were developed on the Ergo ++ treatment planning system ver. 1.6 for the 6X beam on a Beam Modulator (4mm leaves). All the plans were calculated with heterogeneity corrections turned on. For the purpose of “hand calculation” we used the built‐in second calculation in the Ergo system (the system provides an automatic and independent MU calculation mechanism). For the comparison we used the XIO software version 4.3.3. The beam was carefully modeled and commissioned on both systems for small field sizes. A total number of 20 plans were compared. The transfer was done using DICOM RT protocol. Results: The match between the calculation results on both 3D systems was excellent (in most cases less then 1% discrepancy). The 1D system showed larger errors (3–4% on head cases), which do not exist when we turned the heterogeneity corrections off. The DVH's and isodoses also showed very good agreement. The transfer and recalculation took about 15 minutes per plan. Conclusion: Using a full second 3D calculation on a separate TPS is a fast and accurate way to check SRS plans. It is usually available in most departments and it justifies the effort (if needed) of some extra modeling for small field sizes. “Hand calculation” is much less accurate, especially when inhomogeneities are present.
SU‐GG‐T‐517: Dose Calculation Accuracy of a Commercial Treatment Planning System for Phantom Geometries with Varied Lung Densites
Authors: Babcock K , Cranmer‐Sargison G
Published: July 2008

Purpose: The purpose of this study was to investigate the dose calculation accuracy of a commercial treatment planning system for various water‐lung phantom geometries; specifically, the effects of lung density, chest‐wall thickness and a 3‐field beam configuration. Method and Materials: A comparison was made between collapsed cone convolution (CCC) calculations and DOSXYZnrc Monte Carlo (MC) simulations for: (1) a homogeneous phantom (ρ = 1.00, 0.500, 0.250 and 0.125 g⋅cm−3), (2) a slab phantom with varied chest‐wall thicknesses (dchest = 1.5, 2.25 and 3.0 cm) and lung densities and (3) a 15×15×15 cm3 box of lung surrounded by a 2.25 cm layer of water. We use ρlung = 0.400, 0.150 and 0.250 g⋅cm−3 to simulate full exhalation, inhalation and mean lung density respectively. For the homogeneous and slab phantoms one 6MV 10×10 cm2 field incident on a 50×50×25 cm3 phantom at SSD = 100 cm was simulated. For the box phantom a 3‐field beam configuration was used to simulate a basic lung treatment. Results: For the homogeneous phantom at, , the CCC results were systematically 5% high. The slab phantom results showed that past d = 3.0 cm the accuracy of the CCC calculations were dependent on lung density and independent of chest‐wall thickness. The percent difference was as high as 4% for . The 3‐field box simulations revealed an increased difference with decreasing lung density. Percent differences were as high as 8%, 4%, and 2% for the ρlung = 0.150, 0.250, and 0.400 g⋅cm−3 phantoms. Conclusion: For the homogeneous phantom simulations, the percent difference increased with decreasing density. Dose accuracy was found to be invariant with respect to chest‐wall thickness. From the 3‐field box configuration, we found the total percent difference increased with the number of fields.
SU‐GG‐T‐521: Tissue Heterogeneity Corrections for Thoracic Malignancies: A Dosimetric Evaluation
Authors: Gabrish H , Perkins T , Herman T , Vlachaki M , Ahmad S
Published: July 2008

Purpose: To evaluate and compare dose to tumor and normal tissues in patients with thoracic malignancies calculated using heterogeneity corrections. Method and Materials: Fifteen lung patients were evaluated. Multiple non‐opposing co planner beams of up to seven fields with step and shoot IMRT techniques were used. Treatment planning was performed with the Eclipse system and the prescribed dose was 70.2 Gy with 1.8 Gy per fraction. For clinical use, plans were generated with Modified Batho Power Law correction that uses only the descending part of the TMR/TPR curve for tissue heterogeneity. Additional plans were generated with same beam arrangements and monitor units using a) no correction, b) generalized Batho correction, and c) equivalent tissue air correction. Results: The uncorrected homogeneous plans resulted in lower average PTV mean, minimum and maximum doses by at least 6.4%, 9.4% and 5.0%, respectively (p<0.001). In addition, the percentage of PTV receiving doses below the prescribed dose was higher for the homogeneous plans (p<0.001). There were no significant differences in the average PTV mean, minimum and maximum doses among the three plans with heterogeneity corrections (1.6%, 2.8% and 1.2%, respectively). Moreover, heterogeneity corrections did not significantly impact on the percentage of uninvolved lung volume receiving doses of 30 Gy or higher (p=0.833). The maximum spinal cord doses were less than 25.2 Gy, and comparable for all plans (p=0.952). Conclusion: Heterogeneity corrections impact on tumor but not on normal tissue doses. With the emergence of clinical evidence in support of dose escalation in lung cancer, accounting for heterogeneity corrections is of particular importance to ensure adequate tumor dosing in this setting.
SU‐GG‐T‐525: Improved Proton Yield From a Laser‐Proton Accelerator
Authors: Veltchev I , Tafo A Guemnie , Lin T , Fourkal E , Fan J , Li J , Yogo A , Orimo S , Ma C
Published: July 2008

Purpose: To improve the energy output of the laser proton accelerator. To investigate how target position and thickness influence the final proton energy. To identify which set of laser parameters are most crucial to obtaining higher proton energies. Method and Materials: The experimental set‐up for generation of laser‐accelerated protons has been redesigned. The laser system is a chain of amplifiers capable of delivering up to 25TW in a 40fs pulse. The high‐power laser pulses are delivered to a thin (7μm – 20μm) Al or 7.5μm polyamide target in a shielded vacuum chamber. When the laser beam is focused by an off‐axis parabolic mirror of focal length of 15 cm, the light intensity on the target exceeds 2×1019 W/cm2. All experiments are conducted at oblique (45°) incidence with p‐polarized light. The energy of the generated protons is sampled by a range filter and the transmitted beam is registered on a CR39 track detector. Results: The proton spectra from targets with varying thickness and composition have been recorded. The maximum proton energy in excess of 2MeV was obtained for a 15μm Al target situated 50μm behind the focal plane of the laser beam. Comparison study in 7μm Al and 7.5μm polyamide targets has been conducted, resulting in slightly higher proton flux from the aluminum target and no measurable energy difference. Applying a simplified heat‐transfer model of target destruction we estimated that the laser pre‐pulse is at a level sufficient to destroy Al target of thickness below 14.5μm. Conclusion: The acceleration of protons using 25TW ultra‐short laser pulses to energies in excess of 2MeV has been experimentally demonstrated. The influence of various interaction parameters on the final proton energy has been evaluated. These experimental results are a new step toward the generation of therapeutic proton beams with controlled characteristics using lasers.
SU‐GG‐T‐526: Energy Thresholds in 180 Degree Arc Delivery Proton Therapy
Authors: Sengbusch E , Mackie T , Perez‐Andujar A , DeLuca P
Published: July 2008

Purpose: To quantify the maximum initial proton kinetic energy necessary to treat a given percentage of patients with rotational proton therapy and to examine the impact of this energy threshold on the cost and feasibility of a compact, gantry‐mounted proton accelerator treatment system. Method and Materials: One hundred randomized treatment plans from patients treated with IMRT were analyzed. The maximum radiological pathlength from the surface of the patient to the distal edge of the treatment volume was obtained for 180° continuous arc proton therapy and for 180° split arc proton therapy (two 90° arcs) using CT♯ histograms from the Pinnacle treatment planning system. In each case, the maximum energy necessary to treat a patient with protons was calculated using proton range tables for various media. In addition, Monte Carlo simulations were performed to quantify neutron production in the patient as a function of maximum initial proton energy. Results: The widely accepted value of 250 MeV needed to treat 100 percent of patients with protons was confirmed. However, it was shown that 90 percent of patients could be treated at 198 MeV, and 95 percent of patients could be treated at 207 MeV. Decreasing the maximum proton energy from 250 MeV to 200 MeV decreases the total neutron energy fluence created in the patient by a factor of 2.3. Conclusion: It is possible to significantly lower the requirements on the maximum energy of a compact proton accelerator if the ability to treat a small percentage of patients with rotational therapy is sacrificed. This decrease in maximum energy, along with the corresponding decrease in neutron production, could lower the cost and ease the engineering constraints on a compact proton accelerator treatment facility. Conflict of Interest: An author of this study has financial interest in Tomotherapy, Inc., which has licensed proton accelerator technology.
SU‐GG‐T‐530: MR‐Guided Radiotherapy: Magnetic Field Dose Effects
Authors: Raaijmakers AJE , Raaymakers BW , Lagendijk JJW
Published: July 2008

Purpose: In cooperation with Elekta Oncology and Philips Medical Systems, we have designed a 6 MV radiotherapy system with 1.5 Tesla (T) MRI guidance during the actual treatment. In this system, the patient will be irradiated in the presence of a 1.5T magnetic field, which will influence the radiotherapy dose distribution. Method and Materials: The influence of the magnetic field on the dose distribution has been determined using GEANT4 Monte Carlo simulations on geometric water phantoms. The simulations have been validated by comparison to dose measurements in the presence of a magnetic field. Additionally, IMRT treatment plans have been calculated for three patient anatomies (prostate, larynx and oropharynx). Fluence optimization was performed on the basis of pre‐calculated beamlet kernels, resulting in the optimal dose distribution with and without a magnetic field. Results: Results show that when irradiating in the presence of a 1.5T magnetic field, the build‐up distance is reduced, the penumbra dose profile becomes asymmetric and the electron return effect (ERE) causes striking dose increase at all tissue‐air boundaries due to electrons returning into the patient. The build‐up distance and the ERE strongly depend on the surface orientation. Also, the ERE characteristics strongly depend on the magnetic field strength. The validation study shows that the GEANT4 simulations with magnetic field are in agreement to measurements within 3%. IMRT optimized dose distributions with and without a 1.5T magnetic field show that, in spite of all magnetic field dose effects, the same target coverage and sparing of organs at risk can be achieved with and without a 1.5T magnetic field. Conclusion: Irradiating in the presence of a magnetic field results in distinct magnetic field dose effects. However, using IMRT and multiple beam directions, a 1.5T magnetic field does not compromise the ability to achieve desired dose distributions.
SU‐GG‐T‐531: Comparison of Volumetric Modulated Arc Therapy (VMAT) with Serial Tomotherapy and Segmental (step and Shoot) IMRT for Boost Treatment of Prostate Cancer
Authors: Wolff D , Stieler F , Polednik M , Abo‐Madyan Y , Mai S , Steil V , Wenz F , Lohr F
Published: July 2008

Purpose: VMAT is about to be established clinically on a wide basis and may increase the efficiency of Intensity Modulated Radiotherapy (IMRT) treatment. The goal of this investigation was to compare VMAT to established IMRT delivery techniques of serial Tomotherapy and segmental IMRT for prostate cancer. Method and Materials: Using CT data sets of 9 patients treated for prostate cancer, treatment plans for all three techniques were created. Segmental IMRT was planned for a conventional MLC (Elekta MLCi, 2×40leaves, 1cm) and serial Tomotherapy for the MIMiC Multivane‐Collimator (NOMOS, Corp.), using the Corvus 6.3 (Nomos; Pencil beam). VMAT was planned with ERGO++ R1.6 (3D Line Medical Systems/Elekta; Pencil beam). Segmental IMRT plans with MLC used 7 isotropic beams. Serial Tomotherapy plans were calculated in 1cm mode and for a 300° rotation. VMAT plans were created with a 360° and two 100° rotations. Evaluation of plan quality used metrics of conformity index (CI=Vtissue Dmin/Vtarget), homogeneity index (HI=Dmax/Dprescribed), dose to normal tissue and the isodose which encompasses 95% of the target volume. Results: Median indices for conformity and homogeneity of the Corvus plans were CI 2,34/HI 1,21; MLC based segmental IMRT CI 2,89/HI 1,22; and VMAT plans CI 2,02/HI 1,11. 76Gy were prescribed to the median dose of the PTV. The resulting median doses to organ at risk (OAR) for (MIMiC/MLC/VMAT) were as follows: for anterior rectum (52,2Gy/52,4Gy/60,5Gy), posterior rectum (32,3Gy/33,5Gy/34,3Gy) and bladder (44,9Gy/44,1Gy/53,7Gy). Volumes for 50% of prescribed dose (38Gy) in normal tissue were 1091ml/1583ml/1191ml. Dose encompassing 95% of PTV was (MIMiC/MLC/VMAT) 68,8Gy/66,6Gy/70,7Gy. Median treatment time (MIMiC/MLC/VMAT) was 15min/7,5min/3min. Conclusion: Rotational therapy techniques showed higher conformity, coverage and lower normal tissue dose. Homogeneity and dose to posterior rectum were similar for all approaches. VMAT plans are deliverable within a 3minute time slot which is the most efficient treatment option compared to the other approaches.
SU‐GG‐T‐533: Rotational Intensity‐Modulated Therapies: Can Conventional Arcs Match Tomotherapy?
Authors: Alber M
Published: July 2008

Purpose: To investigate the potential of dynamic arc therapy delivered by a conventional linac to match tomotherapy both in terms of plan quality and efficiency. Method and Materials: For a series of different clinical cases, four treatment plans each were generated: a conventional fixed‐beam IMRT plan, an intensity‐modulated arc plan (IMAT), an IMAT plan restricted to one arc (AT) and a tomotherapy plan for the Hi‐Art system. All treatment plans were generated by the same optimization algorithm using EUD‐based cost functions and Monte‐Carlo dose calculation for each modality. The IMRT and IMAT plans were obtained by optimizing MLC segment shapes directly. Results: In general, the intensity‐modulated techniques were all capable of producing comparable plan qualities, sometimes at the price of a large number of beam directions or a reasonably large number of arcs (up to 8). The single arc technique could never match these results fully if normal structures became dose limiting. In general, the trade‐off between target dose homogeneity and organ sparing was worst with AT. In terms of efficiency, IMAT plans were limited by gantry speed, while IMRT plans were limited by MLC constraints. Conclusion: Intensity modulation cannot be replaced fully by rotational delivery, but multiple modulated arcs are more efficient in terms of delivery than fixed beam IMRT when MLC constraints require frequent beam interrupts.
SU‐GG‐T‐536: Characterisation of a Linac Cone‐Beam‐CT Option: What Is the Future Potential for Treatment Planning?
Authors: Cossmann P , Stuessi A , von Briel C
Published: July 2008

Purpose: For image‐guided radiotherapy (IGRT) the different vendors of linear accelerators offer new kV imaging tools with Cone‐Beam‐CT functionality. The aim of this study was to evaluate the future potential of such a cone beam CT option for therapy planning purposes. Method and Materials: With the Varian On‐Board Imager (OBI) Cone Beam CT (CBCT) option in a single 360° rotation a volumetric CT data set with 14 cm scan length and a 25 or 45 cm field‐of‐view (FOV) can be acquired. In order to calibrate the system with regard to hounsfield units (HU) a special phantom has been designed to include the whole imager area. Removable inserts allow flexible positioning of the probes (in the center/middle/border of the FOV) and the measurement of central axis doses. A planning study was carried out to determine the usability of CBCT data and compare these with diagnostic CT data. Results: Comparisons of data between a diagnostic CT scanner and the 3D‐calibrated CBCT with regard to image quality and HU representation for an humanoid phantom (RSD Alderson) indicate good accordance. Within the field of view the HU variation is up to 5%. Towards the imager edges hounsfield units show small deviations relative to the imager center (max. 8%). Relative dose distributions in CBCT‐based plans show minor differences to plans calculated using a diagnostic CT image dataset (p=0.002) and absolute dosage deviations are within 1% (p=0.001). Central axis doses applied during the acquisition of one volumetric data set are between 0.8 and 1.8cGy depending on the geometry. Results with real patient data show excellent image quality. Conclusion: A properly calibrated CBCT option allows off‐line treatment planning based on CBCT data. Dose distributions and absolute MU's are equivalent to dosimetric calculations based on diagnostic CT data. The image quality is sufficient for contouring of target outlines.
SU‐GG‐T‐542: Arc‐Modulated Radiation Therapy (AMRT): A Novel Method for Rotational Radiation Therapy
Authors: Wang C , Luan S , Tang G , Chen D , Earl M , Yu C
Published: July 2008

Purpose: To develop a prototype treatment planning system for Arc‐Modulated Radiation Therapy (AMRT), which delivers the optimal radiation distribution with only one beam rotation. Method and Materials: The AMRT planning starts with inverse planning, typically using 36 equally‐spaced (fixed) beams and producing 36 continuous intensity maps. After inverse planning, an AMRT leaf sequencing algorithm converts the optimized intensity patterns to a sequence of MLC apertures along a single arc. Finally, a homegrown Monte‐Carlo superposition‐based dose calculation engine is used to quickly and accurately evaluate the plan qualities. A big challenge to AMRT is leaf sequencing. Our algorithm is based on a key observation that if each optimized intensity pattern is delivered through its neighboring beam angles within a ±5° range, there is only negligible error on the resulting dose distribution. Thus, the sequencing problem becomes how to guarantee the interconnectedness of the MLC apertures along the single arc, while reproducing or closely approximating the optimized intensity patterns. We model this problem as computing shortest paths in directed acyclic graphs, and solve it efficiently using various algorithmic techniques. Results: We applied our AMRT approach to 18 clinical cases with a wide range of treatment sites. In terms of dose conformity, we observed that AMRT plans outperform gantry‐fixed IMRT plans and rival intensity‐modulated arc therapy (IMAT) plans. In terms of delivery efficiency, AMRT plans are much more efficient and can be delivered in less than 5 minutes for complex head‐and‐neck cases. The AMRT planning time is currently about 1–2 hours. Conclusion: We developed a prototype treatment planning system for AMRT, which delivers the optimal radiation distribution with only one beam rotation. Experimental studies demonstrated the potentials of our new approach to dynamic IMRT which warrants further investigations.
SU‐GG‐T‐550: Comparison of Electron Beam Characteristics Between Two Elekta™ LINAC Models
Authors: Liu T , Yang C , Perks J , Stern R , Purdy J
Published: July 2008

Purpose: To compare the electron beam (EB) characteristics of two Elekta™ linear accelerator (LINAC) models having significantly different treatment head designs. Method and Materials: An Elekta™ Synergy™ LINAC and a Synergy‐S™ (Beam Modulator™) are in clinical use at our institution. The Synergy™ treatment head design replaced upper jaws with 40 MLC leaf pairs (1‐cm wide projection at isocenter) followed by a backup diaphragm parallel to MLC leaves; lower conventional jaws are perpendicular to the MLC. Maximum field size at isocenter is 40cm×40cm. The Synergy‐S™ treatment head design replaced both upper and lower jaws with an 80 leaf (40 each side, 0.4cm leaf width at the isocenter) Beam Modulator™ and two pairs of fixed outer diaphragms. Maximum field size projected at isocenter is 21cm×16cm. Electron applicators are attached directly to the treatment head on both machines. A Wellhöfer™ blue water phantom system was used to measure and compare EB PDD, off‐axis profiles and output factors for all EB energies on both machines at several different SSDs. Depths used for profile measurements were (1/2)R90, R90, R70, R50 and Rp+2cm. Field sizes for the output factor measurements are ineer, compromises were required to meet both PDD and flatness specifications. Synergy‐S™ EBs were higher energies (as shown by PDDs co2cm2–10cm2. Results: While beam energy matching was attempted by the eng mparisons) for same nominal energies. Off‐axis profiles for Synergy‐S™ were more rounded than Synergy™. No significant difference between in‐line and cross‐line profiles was noted for either machine. Bremsstrahlung properties of both machines are similar. Output factors for fields defined with 6cm2 applicator of Synergy‐S™ are about 4% higher. Conclusion: Electron beam characteristics of Synergy™ and Synergy‐S™ are significantly different. Special caution should be taken when using the treatment planning beam model for one system to estimate the dose distribution for the other system.
SU‐GG‐T‐551: VMAT Compared to 3D‐RT and Step and Shoot IMRT for Anal Cancer Pelvis Treatment
Authors: Stieler F , Lohr F , Wolff D , Steil V , Abo‐Madyan Y , Wenz F , Mai S
Published: July 2008

Purpose: The shielding of organs at risk (OAR) like testis, small bowel and bladder increases the complexity of a treatment for anal cancer. The new rotation therapy volumetric intensity modulated arc therapy (VMAT) may have the potential to reduce the treatment time while producing a comparable dose distribution as the intensity modulated radiation therapy (IMRT). Method and Materials: CT datasets of 8 patients with anal cancer from our department formed the basis of this study. The routinely applied refined 3D‐Treatment Plan (Masterplan, Theranostic) was compared to a 9 beam step and shoot IMRT (Corvus 6.3, Nomos) and a VMAT plan with 2 rotations generated with ERGO++ 1.6 by Elekta. All three treatment planning systems used the identical CT datasets and the same OAR's. To compare these three techniques, the dose volume histograms (DVH) of PTV and OAR's as well as the total treatment time (TTT) were used. The homogeneity index (HI) and the conformality index (CI) calculated as suggested by the RTOG guidelines were analysed. Results: VMAT provided the best PTV coverage as indicated by the following metrics (isodose as percentage of prescription dose (PD) encompassing 95% of the PTV / percentage of tissue outside the PTV encompassed by 95% of PD). For 3D‐RT the mean values were (94.7±0.6% / 5.4±1.5%), for IMRT (89.8±1% / 0.7±2.6%) and for VMAT (93.7±1% / 2.9±1.3%). The mean CI and HI over all 8 patients was (2.00±0.17, 1.06±0.01) for 3D‐RT, for IMRT (1.56±0.16, 1.15±0.02) and for VMAT (1.71±0.13, 1.09±0.03). The TTT for 3D‐RT (220 seconds) was much shorter than for IMRT (557 seconds). The estimated TTT of the VMAT technique is 180 seconds. Conclusion: The new VMAT technique provides excellent treatment planes with highest conformality and homogeneity. The short treatment delivery time together with low primary monitor units is the most important advantage.
SU‐GG‐T‐552: Statistical Validation of A New Helical Tomotherapy Patient Transfer Station
Authors: Zhuang A , Liu A , Schultheiss T , Wong J
Published: July 2008

Purpose: To evaluate statistically the accuracy of a patient transfer station (PTS, TomoTherapy Inc., Madison, WI) that automatically converts one planning‐station‐generated treatment plan to another one with a different beam model. Method and Materials: In our department we have installed 2 HI⋅ART tomotherapy systems, and patients often need to be transferred from one tomotherapy unit to the other. 30 patients who underwent patient transfer between the two systems were evaluated. For each patient, dose differences between his/her original plan and PTS‐transferred plan were evaluated by comparing doses at 10 randomly selected positions in his/her CT images. The Pearson indexes were calculated to analyze the relationship of the deviations to other parameters, which include absolute dose levels, sites (targets or normal tissues), dose accuracy of original plans and that of transferred plans. The dose accuracy of a treatment plan was determined by comparing delivered doses at the center of a 30cm × 30cm × 12cm solid water phantom to planned doses at the same position. Results: The maximum calculated dose difference between original and transferred plans for target was 1.9%; the average deviation (of non‐directional values) was 0.8% ± 0.005. The errors generated during PTS‐based transferring process were random and did not show correlation with other parameters. More importantly, the PTS took less than 10 minutes to generate a backup plan and reduced the transfer time by ∼90% compared to creating a duplicate plan manually. Conclusion: The results show that a PTS‐transferred plan is an excellent match to the original plan. With a physician's approval, a transferred plan is acceptable for treatment without the necessity of being revalidated in phantom. Thus far, all of our PTS plans have been approved by the treating physician without further optimization.
SU‐GG‐T‐554: A Simple Method to Determine Couch and Collimator Angles in 3‐Field Breast Irradiation
Authors: Lei Sue , Gagnon Gregory , Rashid Abdul , Borzillary Scott
Published: July 2008

Purpose: In 3‐field breast irradiation, it is essential to ensure field match between tangential and supraclavicular fields. A perfect field match can be easily accomplished with single‐isocenter technique, no couch or collimator rotations. For tangential field length larger than 20cm, symmetric tangential fields are used and thus appropriate couch and collimator rotations are needed to avoid field overlapping due to beam divergence. There are various methods to determine the couch and collimator angles, from rigorous geometric coordinate transformations to empirical approximations, each having advantages and shortcomings. The purpose of this study is to develop a lookup table method that is both accurate and easy to use. Method and Materials: Assuming symmetric tangential fields Y1=Y2=Y/2, couch and collimator angles needed to achieve a perfect field match are calculated using Siddon's formulae: , , where tan(β)=Y2/SAD. A lookup table of couch and collimator angles as a function of θgantry and Y2 is then created for future easy access. Results: The lookup table helps determine appropriate couch and collimator angles during simulation and planning. For example, for , Y=20cm, it can be easily found from the table that and . The couch and collimator angles increase with increasing field length and decreasing gantry angle. The collimator angle can also be expressed as a fraction of the couch angle. In most cases the ratio of θcollimator to θcouch ranges from 0.3 to 0.6. Conclusion: We have developed a lookup table method to help determine precisely the couch and collimator angles to achieve geometric match in 3‐field breast irradiation. The table is simple and easy to use. It eliminates the need for a calculator or computer program. It becomes a handy tool for dosimetrists, physicists and radiation oncologists. Similar lookup tables can be created for head & neck and craniospinal irradiations.
SU‐GG‐T‐555: Commissioning of a Treatment Planning System with Proton Therapy Capability
Authors: Wang N , Ghebremedhin A , Patyal B
Published: July 2008

Purpose: To describe the commissioning process of a treatment planning system with proton therapy capability. Method and Materials: Simple proton beam arrangements can deliver very precise and highly conformal dose distributions to complex targets and simultaneously conformally avoid critical structures in the vicinity of the target. But proton treatment planning is quite challenging and less forgiving to errors, so commissioning a proton therapy treatment planning system presents its own challenges. Recently, we commissioned a new treatment planning system (Odysey™‐4.01). This treatment planning system has both proton and photon planning capability. We will present the process and results of our experience with commissioning the proton‐planning component of this system. Results: In the non‐dosimetric part of commissioning, various tools and features of the treatment planning system were tested for specifications and were found to be within specs. The fidelity of the data and image transfer from CT to the treatment planning system and from treatment planning system to the treatment rooms was checked and found to be accurate. The treatment devices (compensator boluses and apertures) created by the planning system were checked for accuracy. In the dosimetric commissioning we compared depth dose profiles and beam profiles generated by the treatment planning system with the measured data for regular fields. Dose distributions generated by patch and match field arrangements, and for small stereotactic radio‐surgery (SRS) fields were verified by direct measurements in a phantom. The dose monitor units from treatment planning and hand calculations agreed within 2% for fields larger than 5 cm. Smaller fields needed portal specific calibration. Conclusion: All tests performed as part of commissioning were within specifications. For patch and match fields, dosimetric verification of the plan generated dose distributions is highly recommended.
SU‐GG‐T‐556: Characterization and Evaluation of the Varian 2.5 Mm HD120® High‐Definition MLC
Authors: Poffenbarger B , Audet C
Published: July 2008

Recently, a new MLC has been offered by Varian Medical Systems for complex high resolution static and IMRT field delivery. The MLC has 2.5 mm wide leaves spanning an 8 cm length over isocenter with abutting 7 cm sets of 5 mm leaves. MLC Transmission Factors (TFs) were measured for a 10×10 cm field using a diode array. The transmission values were calculated for each diode within the field then averaged. The average TF values were 1.02% and 1.20% for the 6 MV and 10 MV beams, respectively. The value measured for the Millennium‐120 (5mm leaves, 6 MV) of 1.4% was greater. Film and ion chamber measurements produced similar results. The Dosimetric Leaf Gap (DLG) was measured using a 0.6 mm diameter diode (IBA). Subsequently, the DLG value was adjusted to optimize the agreement between planned and measured dose distributions. Final values of 0.04 cm and 0.05 cm were used for the 6 MV and 10 MV, respectively. Fluence patterns were compared for different MLC leaf widths. The HD120 produced very smooth fluences with superior resolution. Furthermore, fluence delivery was accurate with no more tongue‐and‐groove effect than that observed for wider leaves. A 2D dose distribution for a 9‐field, 6 MV, Nasopharynx IMRT plan was measured using film (EDR2, Kodak) and compared to the planned distribution for both the HD120 and M‐120 MLCs. The planned and measured dose distributions agreed very well and similarly for both MLC models. A complex Ethmoid sinus IMRT case was planned thrice for MLC leaf widths of 2.5, 5 and 10 mm using identical optimization constraints. The HD120 plan produced the sharpest dose fall‐offs, best optical lens and brainstem sparing and the most homogeneous PTV coverage. The HD120 is mechanically stable, provides superior treatment plans and delivers accurate fluences.
SU‐HH‐AUD C‐01: Empirical Investigation of Flat‐Panel Imagers Incorporating High‐Efficiency, Segmented BGO and CsI:Tl Detectors for Radiotherapy Imaging
Authors: Wang Yi , Zhao Qihua , Antonuk Larry E , El‐Mohri Youcef , Koniczek Martin
Published: July 2008

Purpose: Megavoltage active matrix flat‐panel imagers (AMFPIs) are routinely used for portal imaging in external beam radiotherapy. However, conventional MV AMFPIs are inefficient, utilizing only ∼2% of the incident radiation, which leads to a detective quantum efficiency (DQE) of only ∼1%. Recent theoretical studies have shown that incorporation of thick, segmented scintillating detectors in MV AMFPIs can significantly increase DQE performance, leading to improved image quality at low dose and the possibility for dose‐efficient, cone‐beam computed tomography (CBCT). Method and Materials: Building upon the findings of our earlier studies, four prototype AMFPIs have recently been constructed, each consisting of a segmented BGO (11.3 mm thick) or CsI:Tl (11.4, 25.6 and 40.0 mm thick) detector coupled to an indirect detection flat‐panel array. Each detector consists of a matrix of 120 × 60 scintillator elements at 1.016 mm pitch, covered by a black or mirror top reflector. X‐ray sensitivity, MTF, NPS, DQE and phantom images were obtained for each prototype using a 6 MV photon beam at extremely low doses (e.g., 1 beam pulse, equivalent to 0.028 MU). Results: The BGO prototype shows better MTF (∼20% at the Nyquist frequency) than the CsI:Tl prototype at similar thickness. The measured DQE(0) at 1 beam pulse for the BGO and the three CsI:Tl prototypes were ∼19%, 12%, 19% and 23%, respectively. Moreover, the BGO prototype offers higher DQE values compared to the CsI:Tl prototypes over most of the spatial frequency range. Images of a contrast detail phantom using the BGO prototype at 2 beam pulses are comparable to that obtained from a conventional AMFPI with 18 times more dose. Conclusion: Prototype AMFPIs employing thick, segmented BGO and CsI:Tl detectors offer significant improvement in image quality at extremely low doses, leading to the possibility of soft tissue visualization using MV CBCT at clinically practical doses.
SU‐HH‐AUD C‐05: Low Dose Rate Prostate Brachytherapy: A Tomosynthesis‐Based Intra‐Operative Post‐Implant Dose Evaluation
Authors: Brunet‐Benkhoucha M , Verhaegen F , Lassalle S , Béliveau‐Nadeau D , Reniers B , Donath D , Taussky D , Carrier J
Published: July 2008

Purpose: To develop an intra‐operative dose assessment procedure that can be performed after an I‐125 prostate seed implantation, while the patient is still under anaesthesia. To accomplish this, we reconstruct the 3D position of each seed and co‐register it with the prostate contour. Method and materials: Our seed detection method involves a tomosynthesis‐based filtered reconstruction of the volume of interest. For 24 patients, the required cone‐beam images were obtained from 7 projections acquired over an angle of 60° with an isocentric imaging system adjacent to the treatment table. A graphical user interface (GUI) has been developed to allow visualization of the final seed positions and to interactively introduce corrections in the seeds positioning, if needed. The co‐registration between the tomosynthesis‐based seed positions and the TRUS‐based prostate contour is performed by applying the same rigid transformation as the one derived from the best match between the planned and the reconstructed seed positions. Dose analysis is then performed based on the co‐registered images. Results: In a patient study with an average of 56 seeds per implant, the automatic tomosynthesis‐based reconstruction yields a detection rate of 96% of the seeds and less than 1 false‐positive seed per implant. The GUI allows the user to achieve a 100% detection rate in less than 5 minutes. The seed localization error obtained with a phantom study is (0.4±0.4) mm. This leads to small dosimetric relative errors on D90 and V100 of respectively 1.5% and 0.3%. Patient dose analyses have shown a significant reduction in the dosimetric parameters between the planned and the post‐operative dosimetry. The relative difference between planned and intra‐operative D90 and V100 are respectively (11±8)% and (4±3)%. Conclusion: Our reconstruction method has the potential to provide accurate intra‐operative prostate dosimetry, all in less than 10 minutes extra time added to the whole implantation procedure.
SU‐HH‐AUD C‐06: A Novel Approach to Multi‐Parametric Dynamic Chemical Shift Imaging
Authors: Taylor BA , Hwang KP , Hazle JD , Stafford RJ
Published: July 2008

Purpose: To integrate modern super‐resolution spectral quantification techniques with multi‐echo magnetic resonance imaging in order to provide multi‐parametric chemical shift imaging in real‐time. Method and Materials: A technique integrating the Steiglitz‐McBride algorithm and Cauchy's Calculus of Residues was developed to determine the proton resonance frequency(PRF), T2*, and T1‐wieghted amplitude for multiple species in a chemical shift MR signal encoded using multiple gradient echoes. Monte Carlo simulation was implemented evaluating the performance of the algorithm under varying acquisition parameters and results compared to the Cramer‐Rao lower bound (CRLB). The PRF values in a mayonnaise‐lemon juice phantom were measured during heating demonstrating the ability of performing PRF thermometry in the presence of lipid. Phantom scans were performed comparing the T2* values from this technique and from spoiled gradient echo(SPGR) acquisitions. To demonstrate the use of this technique for temperature imaging in vivo, an image‐guided laser treatment in a canine brain was performed. Temperature maps were created at spatial resolutions of 1.5×1.5×4mm3 every five seconds in addition to T1‐weighted and T2* images providing multi‐parametric monitoring during treatment. Results: The technique achieves the CRLB demonstrating robust estimation of the PRF and T2* in the presence of noise. The uncertainty of the technique was theoretically and experimentally shown to be inversely proportional to the image SNR. The mayonnaise‐lemon juice phantom showed a PRF shift with temperature consistent to published literature. T2* values provided by the technique did not statistically differ from SPGR‐based T2* mapping. Monitoring of the canine brain treatment was successful providing highly sensitive temperature measurements. T2*‐corrected T1‐W images detected a treatment‐induced hemorrhage demonstrating the importance of multi‐parametric monitoring. Conclusion: A technique is presented that provides chemical shift imaging at higher spatiotemporal resolutions than what was previously available. The ability of this technique to provide multiple parameters demonstrates great promise for image‐guided thermal therapies.