bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐859: A New Approach to Improve IMRT Delivery Efficiency by Intensity Map Discretization
Authors: Zhu X , Cullip T , Tracton G , Schreiber E , Xu Z , Chang S
Published: November 2011

Purpose: To minimize IMRT treatment monitor units and delivery time without compromising dosimetric quality in an in‐house treatment planning system. Methods: In‐house treatment planning system PLanUNC employs a two‐step process to generate a MLC‐IMRT treatment plan. First, the index‐dose gradient minimization dose optimization process produces a continuous intensity map. Second, MLC segmentation process converts the intensity map into MLC segments. This two‐step process is sensitive to MLC segmentation technique and can lead to excessive number of segments and dosimetric degradation. We developed a new method that discretizes the intensity map during optimization. The new method considered each beam in turn: its intensity map was optimized using the current dose distribution; the optimized intensity map was discretized according to its statistical distribution; the dose distribution was updated after discretization. In this process, the peaks and valleys in the continuous intensity map were converted into discrete plateaus, and the optimization of remaining beams mitigated possible degradation of dose conformity introduced by the discretization of previous beams. MLC segmentation was applied to the discretized fluence maps. This new approach is being validated using prostate and head and neck cases. Results: For the prostate, with no compromise to dose conformity, the number of segments was reduced by 40%, and the MUs reduced by ∼10%. For the head and neck, the number of segments was reduced by 20%, and a small dose escalation was observed in adjacent normal tissues; this may be due to complex constraints used in the optimization process and could be accommodated by re‐optimizing the beam weight using fixed beam apertures in the future. Conclusions: Discretization based on statistical distribution of beamlet intensity could effectively reduce IMRT beam complexity and result in shorter treatment time.
SU‐E‐T‐860: Developing a Fast‐Monotonic Descent (FMD) Algorithm for RapidArc Radiotherapy
Authors: Yan H , Yin F
Published: November 2011

Purpose: To develop a fast‐monotonic descent (FMD) algorithm for optimization of VMAT treatment planning, and investigate the impacts of arc numbers and beam interval on the plan dose quality. Methods: Fast‐monotonic descent (FMD) algorithm, proved to be one of the fastest convergent methods for optimizing conventional IMRT plans, was used to enhance the aperture optimization in VMAT. The principle consideration in this new algorithm is that the intensities of the to‐be‐delivered apertures will be re‐optimized to take the contribution of the delivered apertures into account. Leaf segmentation algorithm based on power of 2 was implemented to generate deliverable apertures based on power of 2 and a simplified primary beam only pencil beam model was employed. Several typical clinical sites (prostate, spine, lung, and brain) were tested and the effects of various parameter settings on plan dose quality were investigated. For each case, plans using both conventional IMRT and VMAT techniques were created, and their dosimetric metrics were assessed. Results: The mean value of beam intensity per beam varies drastically as beam interval falling into the range [6° ∼ 20°]. It is less obvious as beam interval outside this range. With limited aperture number per beam, the plan dose quality is affected significantly by the beam interval. It is hard to achieve a decent plan with comparable dose to the conventional IMRT one using only one arc and with beam interval less than 3°. Conclusions: To achieve a decent plan dose comparable to the conventional IMRT plan, a VMAT plan needs at least 2 arcs and the minimum beam interval 6°. Highly modulated intensity maps may require more arcs.
SU‐E‐T‐862: Planning and Delivery of Voxel‐Based Dose Painting Plan Using TomoTherapy
Authors: Chen Q , Chen Y , Chen M , Lu W
Published: November 2011

Purpose: Dose painting is useful in prescribing dose based on the biological information and in correcting the hot/cold spot from previously delivered fractions in adaptive therapy. The feasibility of planning and delivering of voxel‐based dose painting plan in TomoTherapy system using a complementary dose approach is studied. Methods: The current intensity modulated radiotherapy (IMRT) treatment planning aiming at uniform target coverage, is not readily adaptable to dose painting that requires voxel‐based prescription. TomoTherapy Optimizer is modified to read a complementary dose from disk. The complementary dose converts the voxel based prescription into the uniform target coverage. Four dose painting plans are created on a phantom for evaluation. The plans are delivered to the phantom with film measurement to verify the deliverability and accuracy. Results: The conformity of plans to prescriptions is found to be exceptional. The difference between 1% volume and 99% volume for quality volume histogram (Q1‐Q99) is around 0.017 for simple plans and 0.1 for complex plans. Since the prescription become a uniform target prescription with complementary dose, the conformity index for the combined dose can be evaluated with conventional metrics. The D1‐D99 is around 1Gy for simple plan and 5.5Gy for complex plan with combined prescribed dose at 80Gy. Although the dose paint plan prescribes maximum dose of 80Gy, the delivery time for most plan is almost the same as the plan with uniform 60Gy prescription. The dose painting plans are delivered to a phantom with TomoTherapy system and the film measured dose showed excellent agreement with planned dose. Conclusions: This study shows the TomoTherapy system is capable of creating and delivery voxel based dose painting plan accurately.

The authors are current and former employees of TomoTherapy Inc.
SU‐E‐T‐863: Feasibility Study of Using the HybridArc Treatment Technique for Preoperative Irradiation of Rectal Cancer: Comparison of Dose Patterns in Lesion and Organ at Risk with the Tomotherapy
Authors: Reynders T , Gevaert T , Engels B , Verellen D , De Coninck P , Duchateau M , Tournel K , Depuydt T , Ridder M De
Published: November 2011

Purpose: HybridArc is a novel treatment technique blending aperture‐enhanced dynamic conformal arcs with discrete IMRT beams, allowing selection of dynamic arcs with a set of static IMRT beams at specified intervals along each arc. The aim was to evaluate the new technique with regard to achievable plan quality and treatment efficiency and to compare HybridArc against the well establish IMRT treatment technique, Tomotherapy, for the preoperative treatment of rectal cancer. Methods: Twelve patients treated with Tomotherapy were considered and simulated with HybridArc. Treatment plans were designed to deliver in one process a homogeneous dose of 46.0Gy with an integrated boost of 55.2Gy. Planning objectives were to give at least 95% of the prescribed dose to 95% of the PTVs while keeping irradiated volumes of the organs‐at‐risk (small bowel and bladder) as low as possible. The gradient toward the boost region (GI) (ratio volume receiving 52.5Gy on PTV55.2Gy volume), the volumes receiving: 95% of the dose for PTV, 15Gy (V15) for small bowel and 21Gy (V21) for bladder were analyzed. Results: All plans achieved the objectives. Comparable GI was found 1.31 SD0.20 (HybridArc) and 1.32 SD0.16 (Tomotherapy). The 95% PTV coverage was 95.70% (SD1.16) and 97.80% (SD1.70) for PTV46Gy; and 99.19% (SD1.60) and 99.29% (SD0.72) for PTV55.2Gy for HybridArc and Tomotherapy, respectively. In terms of organ at risk sparing, a small advantage for Tomotherapy was found. The V15 of the small bowel was 145,90cc (SD75,94cc) and 112,71cc (SD73,43cc), for HybridArc and Tomotherapy, respectively. The V21 for the bladder was 45.48% (SD10.19) and 39.68% (SD10.67), for HybridArc and Tomotherapy, respectively. Conclusions: HybridArc is clinically possible for the irradiation of preoperative rectal cancer. Furthermore, HybridArc, using one single enhanced dynamic conformal arc and 6 discrete IMRT beams, can achieve equivalent treatment plans (PTV coverage and organ at risk sparing) than the Tomotherapy.
SU‐E‐T‐865: Biologically Optimized 4D Dose Distributions for the Treatment of Incurable Glioblastoma
Authors: Holdsworth C , Corwin D , Stewart R , Rockne R , Swanson K , Phillips M
Published: November 2011

Purpose: A patient‐specific method to optimize IMRT treatment of incurable disease by maximizing tumor cell death and minimizing dose to normal tissue is proposed. Methods: A multiobjective evolutionary algorithm was used to optimize IMRT dose distributions on an example patient throughout treatment using a published patient‐specific 4D mathematical model of glioblastoma proliferation and invasion [Rockne 2010] to calculate the distribution of diffusely invaded tumor cells, hypoxia, and radiosensitivity and recalculate distributions after each week of treatment. Each optimized IMRT plan was designed to maximize EUD for tumor cells while holding constant the EUD for normal tissue. Dose distributions were scaled such that the EUD delivered to the normal tissue for each fraction was equal to the EUD delivered from one fraction using the current clinical protocol of 1.8 Gy to the frank tumor with a 2.5 cm margin. Results: The mathematical model predicted that the total volume of tumor was reduced by 41.0% after one week and 72.5% after two weeks of treatment using the proposed protocol. The current clinical protocol was shown to reduce volume by only 5.5% after one week and 11.2% after two weeks for the same EUD delivered to normal tissue. After 7 weeks of treatment using the clinical protocol and 250% higher normal tissue EUD, the tumor burden was only reduced by 40.4%. Conclusions: A patient‐specific mathematical model showed that a 4D treatment plan that maximizes tumor cell killing while sparing normal tissue reduces the tumor volume more in one week than in seven weeks using current clinical protocol for the same EUD delivered to normal tissue. This approach has the potential to optimize and tailor IMRT treatment planning to individual patientˈs disease through an iterative dialog between a mathematical model for disease and response to therapy with objective‐based treatment optimization.
SU‐E‐T‐866: IMRT Dose Optimization in the Limit of Large Number of Incident Beams
Authors: Kim H , Zhu L , Kim T , Xing L
Published: November 2011

Purpose: Recently proposed algorithm with total‐variation regularization greatly contributes to simplifying fluence‐map in IMRT inverse planning. In practice, however, there exist several cases where fluence‐map cannot be simplified as PTV may be complicated and critical structures are close to PTV, requiring high dose conformality. To overcome these difficulties, this study proposes increasing the number of beams so that it could achieve improvement in the quality of planning with similar or smaller number of beam segments. Methods: High number of beams is arranged at the uniform angular distance. For optimizing a fluence‐map, a new L1‐solver, TFOCS, was used due to many benefits for IMRT optimization. As a criterion to assess the dose conformality of PTV, conformation number (CN) was adopted for all cases with different number of beams. Other factors any other than beam number, such as the way of determining weighting factors and the prescribed residues on PTV and OAR, should be determined in the same way for fair comparisons. To validate the method, head and neck patient data having a complicated and large target volume is used for 7, 15 and 30 beamsResults: The result shows that the increase in the number of beams results in higher conformation number. In terms of the required number of segments, 7 beams requires the most, while 15 beams and 30 beams need less number of segments to be deliverable. The planning with 30 beams turns out to be somewhat better protection for healthy tissue nearby PTV than that with 15 beams. Significantly, higher number of beams is also beneficial from reducing the beamlet intensity required for treatment. Conclusions: Increasing the number of beams in IMRT inverse planning based upon total‐variation can offer better quality of planning, represented by CN, with similar or less number of beam segments.
SU‐E‐T‐868: Beam Orientation Optimization for Intensity Modulated Radiation Therapy Using Adaptive L1 Minimization
Authors: Jia X , Men C , Lou Y , Jiang S
Published: November 2011

Purpose: Beam orientation optimization (BOO) tries to find a solution which contains nonzero fluence map at only a small number of beam angles to achieve a dosimetric objective. This is equivalent to require that the solution fluence map is sparse at the beam angle level. As such, we develop a BOO algorithm via adaptive L1 minimization, an optimization scheme known to result in a sparse solution. Methods: In addition to the energy function for the dosimetric objective, we introduce a sparsity energy into the total energy function. The sparsity energy is obtained by first taking L2 norm of beamlet intensities within each angle and then taking weighted L1 norm over angles. Such an energy term favors solutions with nonvanishing fluence map at only a few beam angles. During optimization, the weighting factors in the L1 norm are adaptively adjusted. Starting with all candidate angles, the optimization process identifies unimportant orientations gradually and removes them without largely sacrificing the dosimetric objective. The whole process terminates when a target number of beams is achieved. Fluence map optimization (FMO) is then performed based on the optimized configuration and on unoptimized (equiangular) beams. The final FMO energy function values are compared to evaluate plan qualities. Results: In one typical prostate case, the plan quality is compared with that from unoptimized beam angles. We have further systematically validated our algorithm in the contexts of 5–9 coplanar beams for 5 prostate patients and 1 head‐and‐neck patient. It is found that our BOO algorithm can lead to lower FMO objective values in 28 out of 30 combinations of patients and target number of beams. Conclusions: The developed BOO algorithm utilizing adaptive L1 minimization is found to be effective for identifying important beam angles, which leads to better plan qualities than unoptimized beam configurations.
SU‐E‐T‐869: Direct Aperture Optimization Based on Multi‐Objective Evolutionary Algorithm in Intensity Modulated Radiation Therapy
Authors: Cao R , Pei X , Wang J , Wu Y
Published: November 2011

Purpose: Direct Aperture Optimization (DAO) is an Intensity Modulated Radiation Therapy (IMRT) inverse planning technique whereby the shapes and relative weights of the apertures in the plan are optimized directly. As DAO is capable of producing treatment plans that are comparable to plans produced using the traditional inverse planning approach while using significantly fewer segments, DAO is studied more and more in recent years. Inverse planning optimization is a multi‐objective optimization problem whose solution is known as Pareto solution set. So, the multi‐objective direct aperture optimization based on Pareto solution set was studied in this paper.Method: The clinical requirements for a treatment plan were transformed into a multi‐objective optimization problem with multiple constraints, in which the parameters are the shapes and relative weights of the apertures. And then the fast and elitist multi‐objective Non‐dominated Sorting Genetic Algorithm (NSGA‐II) was introduced to optimize the problem. The aim of NSGA‐II based optimization algorithm is to provide a representative set of non‐dominated solutions for problems where many conflicting objectives and many constraints need to be considered simultaneously instead of a single solution. Results: A clinical example was tested using the method. The results showed that a set of non‐dominated solutions obtained distributed uniformly, and the corresponding dose distribution of each solution not only approached to the expected dose distribution but also met the dose‐volume constraints. Conclusions: It was indicated that the clinical requirements were better satisfied by the method and planner could select the optimal treatment plan.

Supported by the National Natural Science Foundation under grant No.30900386 and the Anhui Provincial Natural Science Foundation under grant No. 090413095 and 11040606Q55.
SU‐E‐T‐871: Comparison of Treatment Plans in External Beam Radiotherapy
Authors: Nunez L. , Herrero M. A. , Alfonso J.C. L.
Published: November 2011

Purpose: The choice of treatment plan in external beam radiotherapy relies mainly on the personal experience of specialists. The corresponding evaluation processes are highly subjective and prone to inaccuracies. This is why the development of user‐friendly, computer‐assisted decision tools would be quite useful for clinical purposes. Methods: We propose here one such tool, which makes use of data extracted from dose volume histograms DVH (differential and accumulative) of several treatment plans obtained from commercial planners. By means of it we can compare treatment plans taking into account the dose covering of the planning target volume (PTV), organs at risk (OAR) and healthy tissue (HT), as well as the conformity and homogeneity indexes. A weighted cost function is proposed to assess the global relevance of these terms. Several test cases taken from previous clinical treatments have been studied. In any of them a plan is selected out of a number of choices. These were obtained by varying the number, intensity and angles of beams in treatments with IMRT and 3D conformal techniques. Results: We have found a new figure of merit that permits to compare different treatment plans on a quantitative basis. Conclusions: We have obtained a good agreement between choices made according to this figures of merit and actually selected treatments plans.
SU‐E‐T‐872: The Effect of Photon Beam Energy on IMRT Plan for Prostate Cancer: A Planning Study
Authors: Chung J , Lee J , Kim J , Kim I , Suh T
Published: November 2011

Purpose: The effect of beam energy IMRT plans for prostate cancer was studied for competing IMRT plans optimized for delivery with either 6 or 15 MV beams. Methods: This retrospective planning study included 10 patients treated for localized prostate cancer. A dose of 66 Gy was prescribed in 33 daily fractions of 2 Gy. For inverse IMRT treatment planning, we used a 7‐coplanar non‐opposed beam arrangement at 0, 50, 100, 150, 210, 260 and 310 degree angles. To ensure that differences among plans are due only to energy selection, the beam arrangement, number of beam, and dose constraints were kept constant for all plans. The DVHs for the 6 and 15 MV plans were compared for PTV and for OARs such as the rectum, bladder and both femoral heads. Doses received by the 95% and 5% volume of PTV were less than or equal to 1% for 6 MV compared to 15 MV plan for 10 patients.Results: Percentage of doses received by the 10, 30 and 50% volume of bladder were less than or equal to 1%. Percentage of doses received by the 10, 30, and 50% volume of rectum were 1∼2% higher for 6 MV photons. Also, percentage of dosed received by the 10% and 50% volume of femur head were 4∼5% higher for 6 MV Conclusions: There is no greater advantage from 15 MV as compared with 6 MV in 95% volume of PTV coverage. Also, percentage dose received by 5% volume of PTV was no remarkable difference in 6 and 15 MV plan. However, percentage doses received by OARs volume were higher 6 MV. Therefore, we recommend the use of 15 MV for IMRT of prostate cancer to achieve better in target coverage and integral dose which can be reduce by using IMRT of 15 MV.
SU‐E‐T‐876: Comparison of Dose Distributions for Plans Optimized by Using Monte Carlo and Ray‐Tracing Algorithms for the Treatment of Lung Tumors with CyberKnife
Authors: Fan J , Hayes S , Jin L , Wang L , Yang J , Ma C
Published: November 2011

Purpose: To compare dose distributions for plans optimized by using the Monte Carlo algorithm (MC) and the Ray‐Tracing algorithm (effective path lenth method, EPL) for CyberKnife treatments of lung tumors. Methods: In the CyberKnife Multiple planning system, a treatment plan can be created by using the Ray‐Tracing algorithm or by using the Ray‐Tracing algorithm in the first run and switch to the Monte Carlo algorithm for further optimizations. In this study, treatment plans were created using Multiplan 4.0 in both ways with exactly the same planning parameters. Dose was prescribed to the isodose line encompassing 99% of the planning target volume (PTV). The difference in dose distributions was studied by analyzing the dose‐volume histograms (DVHs), isodose distributions and the conformity indexes (CIs). 10 patient plans with different tumor shape, size and locations have been investigated in this preliminary study. Results: The maximum doses and mean doses to target and critical structures between plans generated by the Ray‐tracing (RT plan) and the Monte Carlo (MC plan) shows small differences with no obvious advantages from ether one. However, the largest differences were observed in the target dose conformity. In average, conformity index from the RT plan is about 17% larger than the MC plan. In general, the more irregular shaped target, CI ratio between the RT plan and MC plan becomes the larger. Conclusions: Both plans generated using the Ray‐tracing and re‐calculated, re‐normalized using Monte Carlo and plans optimized using the Monte Carlo directly are clinically acceptable. But the MC plan delivers better target dose conformity. Therefore MC‐based treatment planning is recommended for SBRT of pulmonary targets.
SU‐E‐T‐877: Dosimetric Evaluation of Intracranial Stereotactic Radiotherapy with Helical Tomotherapy
Authors: Sheth N , Cheng J , Jani A , Sim S , Murphy S , Chen Y , Stapleton P , Winant S , Yang C
Published: November 2011

Purpose: The goals of stereotactic radiotherapy (SRT) are the ablation of target tissue and sparing of critical normal tissue. We evaluate the effect of maximum dose escalation on conformity and gradient to intracranial targets representing brain metastases of varying size with helical tomothearpy SRT (TomoTherapy Inc., Madison, WI). Methods: Spherical targets were generated in the center of a head phantom at the central axis of delivery. The target diameters were 10, 13, 15, 20, 25, 30, 35, and 40 mm. The prescription (Rx) was for a 7 Gy fraction and with maximum dose to Rx dose ratios (MDPD) of 1.025, 1.05, 1.075, 1.1, 1.15, 1.2, 1.25, and 1.3. Plans were run using a consistent approach with a goal of 95% of the target receiving Rx and 90% Rx minimum dose. Conformity of the Rx to the target is evaluated using a ratio of over and under coverage of the target. Dose falloff is analyzed using CGIg: the difference of effective radii of spheres equal to half and full Rx volumes. Results: Plans were developed for each target and MDPD combination expect for the 10 and 13 mm targets with a minimum MDPD of 1.064 and 1.039, respectively. Conformity improved with increased target size, but was mostly consistent for a specific target volume. Improvement in dose falloff increased with MDPD for all targets. The overall difference in effective dose gradient is 0.9, 2.8, 1.9, 1.8, 2.8, 2.6, 2.2, and 2.5 mm corresponding to a relative CGIg improvement of 11.7%, 50.2%, 34.9%, 34.9%, 72.8%, 91.5%, 84.4%, and 156.7% for 10, 13, 15, 20, 25, 30, 35, and 40 mm targets, respectively. Conclusion: Escalation of MDPD improves dose falloff without degradation of conformity. Optimal Rx isodoses should be tailored to the tradeoff between tumor dose heterogeneity and dose falloff with helical tomotherapy SRT.
SU‐E‐T‐878: Accurate Small MLC Field and Penumbra Modeling and Its Clinical Impact
Authors: Wang X , Tailor R , Yang J , Court L , Chung H , Vicic M , Dong L
Published: November 2011

Purpose: To improve the accuracy of small MLC field and penumbra dose calculation by refining 6 MV beam model for IMRT‐based SBRT. Methods: A new dataset of depth and detector size corrected beam profiles of both jaw and MLC‐collimated fields were measured to refine a Pinnacle beam model for a Varian linac. Specific beam modeling sequence was used to tune the output factor for MLC‐collimated fields. In addition, the rounded leaf radius was also used as a free parameter for the new beam model. Previous IMRT patient QA data were also used for the verification of the new beam model and fine turning the MLC modeling parameters. The clinical impact of the refined model was studied by investigating the agreement of QA measurement with the new beam model. Results: Tests in MLC‐collimated fields showed our current beam model consistently over‐estimate the dose by as much as 4% for small MLC fields ranging between 2cm to 5cm with a large jaw opening. A refined new model with detector size corrected profiles required a smaller effective source size and a larger head scatter than the current beam model. We found the new model was able to reduce the IMRT QA discrepancies both in ion chamber and film measurements for SBRT of spine patients. Interestingly, our beam modeling process significantly improved the QA result for small field IMRT without affecting the QA result for large field IMRT cases. Conclusions: A more accurate Pinnacle beam model can be derived from the detector size corrected profiles, MLC‐collimated small field measurements, and the use of large amount of historical QA measurement data. The new beam model will benefit for intensity modulated spinal stereotactic radiosurgery or other small field IMRT applications.
SU‐E‐T‐879: Dosimetric Analysis of Proton Passive‐Scattering Stereotactic Body Radiotherapy (SBRT) of Treated Spine Lesions versus Photon SBRT
Authors: Panahandeh H , Spadea M , Oh K , Seco J
Published: November 2011

Purpose: To investigate the trade‐off between spinal cord sparing and target coverage in spine patients treated with Proton Passive‐scattering Stereotactic Body Radiotherapy (SBRT) compared to Photon SBRT plans with IMRT and 3D‐CRT. Methods: Ten patients with spinal and paraspinal lesions have treated with Proton SBRT in our institution. Afterward, they were re‐planned with photon IMRT and 3D‐CRT SBRT protocol considering sufficient target coverage and sharp dose fall‐off at the margin of target and spinal cord. Dosimetric data obtained from the plans and were compared in target volume, spinal cord and other adjacent critical organs such as thecal sac, Cauda equina, esophagus. To test the validity of dose calculation in heterogeneous tissues for photon and proton, in house developed Monte Carlo has been used. Results: IMRT plans showed better coverage and conformity of the target volume compared to proton plans and 3D‐CRT. Dose fall‐off at the margin of spinal cord and target volume were comparable for IMRT, Proton and 3D‐CRT plans, however IMRT showed a better result. On the other hand, photon plans including IMRT and 3D‐CRT showed more exit dose to the beyond normal tissues. Conclusions: Proton plans showed wider beam penumbra at the margin of spinal cord and target volume due to multiple proton scatter events. The effect can leading to increase of spinal cord dose or decrease of target coverage and therefore risk of treatment failure.
SU‐E‐T‐880: An Investigation of Kernel‐Based Dynamic Dose Painting Treatment Approach
Authors: Ma L , Hu W , Luan S
Published: November 2011

Purpose: Kernel‐based dynamic dose painting was introduced at the 2010 AAPM meeting. The purpose of this study was to investigate a practical treatment planning and delivery approach with current technology and potential dosimetric advantages on 3D patient anatomy for such an approachMethods: For a dynamic dose painting delivery, a dose kernel (i.e., paintbrush) is first formed from a high number of converging beams, and then such a kernel transverses (i.e. paint) continuously throughout a target volume to create a desired dose distribution. In this study, we employed fixed ball‐shaped kernels modeled from the beam geometry similar to those of commercial Gamma Knife (GK) or body GK units such as Gyroknife, and dose painting was realized by shifting the patient (i.e., target) via a dynamic couch with six degrees of freedom. In addition, the size of the kernel was also made changeable along its painting path. We tested such an approach on treatment planning of prostate stereotactic body radiotherapy (SBRT) cases. Results: Conformal prostate SBRT treatment plans were feasibly developed. At the posterior prostate target and the rectum junction area, separation in the isodose lines between 100% and 75% of the prescription dose was less than 2 mm, yielding a dose fall‐off gradient of approximately 13% per mm or higher. Under a nominal dose rate of 600 MU/min, the beam‐on time for such a delivery was approximately 30 minutes for a dose of 10‐12 Gy to the target periphery. The effective number of the beams for such treatment however reached approximately 8,500. Conclusions: Kernel‐based dynamic dose painting offers a new level of dose conformity and normal tissue sparing surrounding a target.
SU‐E‐T‐881: Simulation of a Multileaf Collimator vs. Iris Variable Aperture Collimation for Prostate Robotic Radiosurgery and IMRT
Authors: Stoll A , Nill S , Oelfke U
Published: November 2011

Purpose: To study potential advantages and disadvantages of a multileaf collimator (MLC) relative to iris variable aperture collimation for prostate cancer treatment with a CyberKnife robotic radiosurgery system. Methods: Six treatment plans for three prostate cases are generated using target conformal segments with a hypothetical MLC vs. circular beams with an iris variable aperture collimator. Conformality, homogeneity, number of MUs and segments are used as plan quality indicators. Quasi Newton Gradient descent optimization is applied as optimization strategy. MU reduction during the optimization process is achieved with two‐dimensional median fluence smoothing. The final sequencing is performed at sequencing levels 2 and 3 to reduce the number of segments. Iris baseline plans were generated using the vendorˈs TPS. Treatment plans A and B are conventionally fractionated IMRT with homogeneous PTV coverage. Plans C and D are IMRT with homogeneous PTV coverage (−/+ integrated boosts). Plans E and F are hyperfractionated SBRT with homogeneous PTV coverage (−/+ integrated boosts, and stricter OAR dose constraints). Results: With identical number of CyberKnife nodes for an iris and MLC treatment plan, an improvement in dose conformality is achieved with a MLC for all treatment plans and dose homogeneity is only slightly degraded. In all cases, the total number of MUs with multileaf collimated fields is reduced in comparison to circular collimated iris variable aperture fields (avg. reduction 28%). The number of segments for a comparable MLC treatment plan with an iris plan is also reduced for 4 cases and slightly increased for 2 of them (avg. reduction 17%). Conclusions: This investigation shows that a MLC could provide several benefits for intensity modulated prostate radiosurgery with a CyberKnife robotic radiosurgy system. Multileaf collimated beams for radiosurgery allow a highly conformal radiation. Their target conformal segments in addition reduce the required total number of MUs and segments.
SU‐E‐T‐882: Electron Disequilibrium Pitfalls for Small Megavoltage Photon Fields Incident on Lung Tumors
Authors: Disher B , Hajdok G , Gaede S , Battista J
Published: November 2011

Purpose: Stereotactic body radiation therapy (SBRT) of lung uses sub‐centimeter MV x‐ray fields. Under these conditions, lateral electron disequilibrium (LED) can occur in lung tissue, which causes perturbations of the dose distribution near the tumor. This purpose of this work is to characterize the LED effect in lung for clinically relevant ranges of beam energies, field sizes, and lung densities. Methods: The MC code DOSXYZnrc (National Research Council of Canada, Ottawa, ON) was employed to simulate two 20×20×25cm3 water‐lung‐water slab phantoms. The two phantoms were identical in composition except that the second phantom also included a 3×3×3cm3 centrally located water cube to mimic a small lung tumor. To characterize LED, dose calculations were performed using combinations of beam energy (Co‐60 up to 18MV), field sizes (1×1cm2 up to 15×15cm2), and lung densities (0.001g/cm3 up to 1g/cm3) for both phantoms. Results: MC lung slab phantom simulations revealed that for each combination of beam energy and field size, a critical lung density (CLD) could be defined to establish LED. For example, a 6MV 5×5cm2 photon field was subject to LED for lung densities of 0.2g/cm3 or lower. On the contrary, employing an 18MV 5×5cm2 photon field increased the CLD to 0.5g/cm3. With regard to the second lung tumor phantom, the LED effect caused major reductions in the calculated dose near to the tumor. For instance, dose reductions of 24% and 16% were found within the distal and proximal tumor surfaces, respectively. Conclusion: We have fully characterized the LED effect and shown that it causes dose reductions in both lung and tumor tissues. To avoid these dose perturbations, SBRT of lung cancer patients should be optimized to select radiation therapy parameters carefully in accordance with patient lung density.

Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Canadian Institutes of Health Research (CIHR) are gratefully acknowledged.
SU‐E‐T‐885: A Planning Comparison of Dynamic Conformal Arc (DCA), Static Non‐ Coplanar Intensity Modulated Radiotherapy (NCP‐IMRT), Volumetric Modulated Arc Therapy (RapidArc), Robotic Radiosurgery (Cyberknife), and Helical Tomotherapy (HI‐ART TomoTherapy) for SRS
Authors: Thakur V , Ruo R , Doucet R , Soisson E , Seuntjens J , Parker W
Published: November 2011

Purpose: This work compares all linac‐based SRS treatment techniques currently available for single lesion cranial SRS. This study includes the planning comparison of dynamic conformal arc (DCA), static non‐coplanar intensity modulated radiotherapy (NCP‐IMRT), volumetric modulated arc therapy (RapidArc), robotic radiosurgery (Cyberknife), and helical tomotherapy (HI‐ART TomoTherapy) for cranial SRS. Methods: Thirteen target volumes with range 0.23 to 20.76 cc were retrospectively selected and transferred to a CT scan of a phantom designed for end‐to‐end SRS QA (Lucy phantom, Standard Imaging). Plans were developed using, iPlan TPS (v4.1, BrainLAB) for DCA (4 arcs) and NCP‐IMRT (16 beams), meanwhile the Eclipse TPS (v8.6, Varian Medical Systems) was used for the RapidArc (4 arcs) technique. Multiplan TPS (v3.5, Accuray) and TomoTherapy HI‐ART TPS (v3.1.4.23) was used for Cyberknife and TomoTherapy respectively. All plans were evaluated using four criteria, (1) Paddickˈs Conformity Index (CI), (2) Paddickˈs Gradient Index (GI), (3) Homogeneity Index and (4) Wagnerˈs Conformity/Gradient Index (CGI). Results: The average Paddick conformity index, CI was 0.64, 0.72, 0.76, 0.78, and 0.65 for the DCA, NCP‐IMRT, RapidArc, Cyberknife and Tomotherapy techniques respectively.The average Paddick gradient index, GI was 3.3, 3.6, 4.2, 4.4, and 4.9 for the DCA, NCP‐IMRT, RapidArc, Cyberknife and Tomotherapy techniques respectively. The average Wagnerˈs CGI was 71.6, 74.5, 72.2, 71.37, and 62.18 for the DCA, NCP‐IMRT, RapidArc, Cyberknife and Tomotherapy techniques respectively. IMRT‐based techniques and robotic radiosurgery showed better CI and CGI, whereas DCA showed the best dose fall off followed by NCP‐IMRT Conclusions: All methods were able to produce comparable plans for most of the targets tested. More importantly, it is suggested that for future planning studies the plan criteria must be explicit in their goals. In particular, the gradient index should be specified along with the desired dose prescription and conformality.
SU‐E‐T‐886: Exploration of SBRT Lung Treatment Using the First Non‐Commercial VMAT
Authors: Song Y , Obcemea C , Mueller B , Mychalczak B
Published: November 2011

Purpose: The beamlet‐based IMRT‐SBRT has two widely‐recognized deficiencies: an elevated MU and a reduced MU‐to‐cGy coefficient. To address these issues, we developed our own version of VMAT. A thorough literature search indicates that this is the first non‐commercial VMAT in the United States. Our objectives are 1) to minimize MU; 2) to maximize delivery efficiency and 3) to provide IMRT‐SBRT‐comparable plans. Methods: All VMAT‐SBRT plans consisted of a single arc of 180 equally spaced beams. Our VMAT was designed to simultaneously optimize both MLC apertures and aperture MU weights. A novel feature of our VMAT planning system was the inclusion of a normal tissue objective (NTO) function into the total dose‐volume based quadratic objective function. Five VMAT‐SBRT plans were computed and compared with corresponding IMRT‐SBRT plans. Our VMAT‐SBRT plans were delivered in RapidArc mode on a RapidArc‐enabled Varian Trilogy machine. Results: We found that the mean MU for the VMAT‐SBRT plans was 2052±108, while that for the IMRT‐SBRT plans was 3790±63. The MU‐to‐cGy ratio was 1.15±0.06 for the VMAT‐SBRT plans, while the MU‐to‐cGy ratio was 2.11±0.04 for the IMRT‐SBRT plans. The mean delivery time for the IMRT‐SBRT plans was 6.32±1.06 min (600 MU/min), while that for the VMAT‐SBRT plans was only 1.25 min. The mean target dose homogeneity index HI for the IMRT‐SBRT plans was 0.0535±0.0001, while that for the VMAT‐SBRT plans was 0.0366±0.0028. The mean PTV dose conformity PIPTV for the VMAT‐SBRT plans was 1.421±0.0311, while that for the IMRT‐SBRT plans was 1.466 ±0.0028. Conclusions: Based on our preliminary results, we conclude that our own version of VMAT‐SBRT can significantly increase plan delivery efficiency, enhance energy use efficiency, and improve target dose homogeneity and conformity. It is a viable and safe substitute for IMRT‐SBRT.
SU‐E‐T‐888: Dosimetric Evaluation of VMAT Techniques in Lung Stereotactic Body Radiotherapy
Authors: Zhang G , Ku L , Dilling T , Stevens C , Li W , Feygelman V
Published: November 2011

Purpose: Volumetric modulated arc therapy (VMAT) is introduced recently as a new lung stereotactic body radiotherapy (SBRT) treatment modality. Because of its shorter delivery time, it reduces the possibility of target misalignment due to patient motion. The dose distributions of VMAT plans are quite different from standard 3D therapy. This study quantitatively evaluates the VMAT plans. Methods: Fifteen lung cancer patients previously treated with 3D or VMAT SBRT were randomly selected. VMAT plans are generated using normal and flattening‐filter‐free (FFF‐VMAT) beams. All plans used the same objectives with 5000 cGy to cover 95% PTV. Two dynamic arcs were utilized in each VMAT plan. The couch was set at ± 5 degrees to the straight position for the two non‐coplanar arcs. SmartArc was used for VMAT planning. The conformity index (CI), which is the ratio of the total volume receiving at least the prescription dose to the target volume receiving at least the prescription dose and the gradient index (GI) which is the ratio of the volume of 50% of the prescription isodose to the volume of the prescription isodose were compared. The dosimetric comparison also includes the V20 (percentage normal lung volume receiving at least 20 Gy) and V5. Results: The average and one standard deviation of CI for the 3D, coplanar, non‐coplanar VMAT and FFF‐VMAT plans was 1.52±0.33, 1.29±0.21, 1.28±0.18 and 1.28±0.19 respectively. GI: 8.41±3.78, 7.42±2.98, 7.33±3.00, 7.87±2.79, indicating sharper dose fall‐off in normal tissues in VMAT plans. V20: 7.8±2.9%, 6.9±2.6%, 7.0± 2.6%, 6.8± 2.3%; V5: 24.8±8.7%, 24.1± 6.5%, 23.7± 6.6%, 22.5±6.5%; PTV mean dose: 5348± 118, 5294±125, 5291±117 and 5279±97 cGy. VMAT plans tend to use more MUs Conclusions: VMAT plans demonstrated better conformity to target, sharper dose fall‐off in normal tissues and smaller V20 and V5 than the 3D plans for lung SBRT while maintaining mean target dose.
MO‐D‐BRC‐02: Image Guidance in Video‐Assisted Thoracoscopic Surgery (VATS) Using a Mobile C‐Arm for Cone‐Beam CT: Image Quality, Dose, and First Experience in Visualizing the Fully Deflated Lung
Authors: Schafer S , Uneri A , Mirota D , Nithiananthan S , Stayman J , Zbijewski W , Kleinszig G , Schmidgunst C , Sussman M , Siewerdsen J
Published: November 2011

Purpose: A growing number of early‐stage lung malignancies identified in low‐dose CT raises a challenge for thoracic surgeons to accurately localize subpalpable nodules, increasing the potential for erroneous resection or unnecessarily large margins. We investigate combining minimally invasive video‐assisted thoracic surgery (VATS) with intraoperative image guidance using a mobile C‐arm for cone‐beam CT (CBCT). Method and Materials:Studies were performed on a C‐arm prototype equipped with a flat‐panel detector enabling high‐quality CBCT. Lung nodule detectability was assessed in three pig models using solid lung nodules (∼50 HU) simulated by ∼6 mm polyethylene spheres inserted into the lung. Scans were performed at inhale, exhale, and complete deflation to assess visualization of tumors within the deflated lung and identify minimum‐dose limits. A cadaveric torso provided first experience in CBCT guidance of VATS in cooperation with a thoracic surgeon. Experiments employed an in‐house guidance system combining pre‐ and intraoperative images, surgical planning, trackers, and thoracoscope video augmentation. Results: Simulated lung nodules were conspicuous in low‐dose scans (0.41 mSv) for the inhale and exhale states by virtue of high contrast within the air‐filled parenchyma. Nodule visibility in the deflated lung required higher dose (1.63 mSv) for confident visualization and suggested a valuable means of intraoperative localization. For the cadaveric torso, preoperative CT and planning were registered to CBCT using deformable registration methods, with error ∼3–10 mm in soft‐tissue registration error. Real‐time tracking of thoracoscopy allowed valuable augmentation of the video stream with image and planning data. Conclusion: C‐arm CBCT integrated with tracking, registration, and video augmentation promises to advance the safety and precision of VATS, showing potential for improved tumor targeting and normal tissue sparing. CBCT image quality was sufficient for direct visualization of solid nodules in the inflated and collapsed lung, and registration with thoracoscopic video provided significant improvement to visualization and workflow.

Research supported by National Institutes of Health R01‐CA‐127444 and collaboration with Siemens Healthcare (Erlangen Germany).
MO‐D‐BRC‐03: Concomitant RapidArc® and Acquisition of KV‐CBCT with Minimal MV Scatter
Authors: Ling C , Zhang P , Sun M , Hunt M
Published: November 2011

Purpose: To develop a novel method to simultaneously deliver RapidArc® and acquire kV CBCT with minimal MV scatter using the TrueBeam®. Methods and Materials: A standard RapidArc® plan consists of hundreds of control points (CP) for dose delivery with designed MLC aperture, dose rate, and gantry rotation. In our design concept each original CP is converted into two: one for MV dose delivery, and the other for kV imaging. Gantry intervals of each CP and gantry speed are chosen to acquire sufficient CBCT projections in the imaging CP, while minimizing the process time. kV projections with minimal‐MV scatter are selected, based on the recorded gantry angle, and sCBCT (s: without MV‐scatter) is reconstructed. For comparison, a reference rCBCT image is acquired without MV dose delivery, and a cCBCT (c: with MV‐Scatter) acquired by running the same RapidArc®/CBCT plan but without the imaging CP. CBCT are acquired in both full‐fan and half‐fan modes. Results: The image quality of sCBCT and rCBCT are comparable, whereas MV scatter significantly degrades the cCBCT image and contributes artifacts. Image analysis results show that the contrast‐to‐noise ratio of the sCBCT and rCBCT are comparable, and is twice that of the cCBCT. Likewise, the image homogeneity (defined as the normalized standard deviation) of the sCBCT and rCBCT are also superior to cCBCT. Visually, an object of ∼2% density difference from the background can be discerned in the sCBCT and rCBCT image, but not in the cCBCT image. In this proof‐of‐principle study, a 350 MU RapidArc®/CBCT plan was delivered in 2.2 minutes. With further optimization, it should be possible to shorten this to ∼1.5 minutes. Conclusion: Our method truly integrates imaging and dose delivery. kV‐CBCT with minimal MV scatter can be acquired concomitantly with the RapidArc® with little overhead. Engineering development is needed to further improve the process.

Research agreement with Varian Medical Systems.
MO‐D‐BRC‐09: Dose Reduction for CBCT by Incorporating Prior Volumetric Image Information
Authors: Ouyang L , Chance W , Solberg T , Wang J
Published: November 2011

Purpose: Repeated use of CBCT in radiotherapy for patient setup and plan verification introduces non‐negligible dose to patients. In this work, we propose a method to effectively reduce the imaging dose by incorporating previously acquired CBCT into an on‐treatment CBCT reconstruction process. Methods: Three CBCT scans were acquired using a low dose protocol, 10mA/10ms per projection view, on a CatPhan 600 phantom. The first two scans represent prior existing scans while the third scan represents an on‐treatment low‐dose scan. In this study, the Karhunen‐Loève (KL) transform is used to consider the correlation among the on‐treatment low dose CBCT and prior CBCTs in the projection domain. After the KL transform, the selected CBCT projection data are decomposed into un‐ correlated, ordered principle components. Then, a penalized weighted least‐ squares (PWLS) criterion is applied to restore each KL component. Different penalty strengths of the PWLS criterion are applied for restoring each KL component, which is inversely proportional to its corresponding KL eigenvalue. In such a way, a higher smoothing strength is applied on the component associated with a lower signal‐to‐noise ratio. After the inverse KL transform on the processed data, an FDK algorithm is used to reconstruct the on‐treatment CBCT image. Results: The proposed algorithm is able to suppress noise while preserving edge information. The image quality of low‐dose CBCT processed using the proposed strategy is similar to that of a high‐dose image, which is acquired with a protocol of 10 times dose. Furthermore, this new strategy outperforms the PWLS algorithm without considering prior information based on the noise‐resolution tradeoff measure. Conclusions: Information extracted from previously acquired CBCT can be effectively utilized to suppress noise in on‐treatment low‐dose CBCT. The presented strategy can significantly lower the patient CBCT radiation dose without compromising the quality of the images.