bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐651: A Novel Non‐Uniformly Distributed Control Points‐Based Algorithm for VMAT Treatment Plan Optimization
Authors: Na Y , Li R , Suh T , Xing L
Published: July 2013

Purpose: To develop a new method for adding control points to volumetric modulated arc therapy (VMAT) optimization in non‐uniformly distributed scheme that improves the delivery efficiency and dose distribution. Methods: The algorithm generates piecewise constant fluence maps using total‐variation regulation (TVR) for each beam direction. The optimized fluence maps of one fluence profile per the initial arc spacing of 6° are distributed as coarse sampling of control points over a single arc, 360°. From the initial control points having an aperture sequence {A(0), …, A(N)}, each association {A(0)‐A(1), …, A(N‐1)‐A(N)} of apertures is assigned a mutual information(MI) score. A new control point (B0) is added to the mid‐point of two apertures {A(0)‐B(0)‐A(1)} in which the {A(0)‐A(1)} association has the lowest MI score having the biggest aperture shape difference. A new shape of aperture at B(0) is created by the interpolation of the adjacent apertures. The aperture shape is then rectified to be a sequencing of arc using the manufacture constraints. The implemented plan optimization is evaluated using clinical cases of head neck, lung, and prostate previously treated with a Varian TrueBeam™STX linac. Results: The propose method with non‐uniformly distributed 120 control points reduced the dose delivery time approximately 32%, 26%, and 30% compared to the uniformly distributed 180 control points for head neck, lung, and prostate plans, respectively. The target dose coverage and critical structure sparing of plans obtained using the proposed method with non‐uniformly distributed 120 control points are almost identical to those obtained using the conventional method with 180 control points uniformly distributed. Conclusion: The results demonstrate that non‐uniformly distributed control points‐based VMAT plan optimization reduced the dose delivery time and slightly improved dose conformity. This new algorithm can substantially provide future station mediated planning parameters. This work was supported in part by Korean grants from the National Research Foundation of Korea (NRFK) of the Korea government (MEST) (No.K20901000001‐09E0100‐00110) and the National Cancer Institute (1R01 CA133474).
SU‐E‐T‐652: GPU‐Based Automatic Treatment Planning Using Previously Delivered Treatment Plans as Prior Knowledge
Authors: Li N , Gautier Q , Zarepisheh M , Graves Y , Tian Z , Zhou L , Jia X , Moore K , Jiang S
Published: July 2013

Purpose: To demonstrate the feasibility of automatically and efficiently developing a clinically optimal treatment plan for a new patient using a GPU‐based automatic planning engine and a library of previously delivered treatment plans. Methods: An in‐house GPU‐based platform called SCORE was used for automatic treatment planning. A library of 65 prostate IMRT plans previously delivered at our institution has been assembled. Leave‐one‐out cross validation was performed whereby each of the 65 patients was selected as a ‘new’ patient and the other 64 patients were treated as reference patients. A treatment plan was automatically generated for the ‘new’ patient using the same beam setup and guided by the DVH of the reference plan. A set of candidates were generated by filtering the 64 plans using institutional criteria for treatment plan quality. A GUI was developed to allow clinicians to navigate through the candidate plans at high efficiency to select the best plan. Results: For each prostate IMRT patient, it took 40–60 minutes to generate 64 new plans without any manual intervention. Compared with the original clinical plan created through the regular planning process, each of the final plans chosen from the candidates had equal or better plan quality in terms of DVH curves and specific plan quality metrics (D95% and Dmax for PTV, V65 and V40 for rectum and bladder, Dmax for femoral heads). Conclusion: We proposed a GPU‐based automatic treatment planning procedure as a solution to the current manual treatment planning process requiring significant human effort, planner experience, and clinician. By using this procedure that leverages prior planning experience, a set of candidate plans can be generated without any manual intervention from which it is easy for the clinician to find a clinically optimal plan using an interactive plan selection GUI.
SU‐E‐T‐654: Direct Aperture Optimization Using An Inverse Form of Back Projection
Authors: Zhu X , Cullip T , Tracton G , Dooley J , Chang S
Published: July 2013

Purpose: Intensity modulated radiotherapy plans using direct aperture optimization (DAO) promise high dosimetric quality and fast treatment delivery. We present a new and fast DAO approach where IMRT and arc therapy optimization are treated as inverse forms of CT reconstruction using multiplicative algebraic reconstruction technique (MART). Methods: We developed a modified MART for DAO, and the multiplicand is termed as ‘index’, indicating how much the current dose should be changed. Index is assigned to each voxel, each beamlet and each segment to optimize fluence map, MLC segment shape, and segment weight, respectively. In addition, a column generation method is developed to add new segments to improve dose conformity. Results: DAO using index‐dose can be completed within a few minutes. Without sacrificing plan quality, the implementation of the DAO in our in‐house treatment planning system reduced average IMRT treatment time from 13 min to 8 min for the prostate, and from 15 min to 9 min for the head and neck. The DAO approach is also applied for arc therapy in a head and neck case using rotational IMRT with burst mode. Conclusion: We have demonstrated a new direct aperture optimization technique using an inverse form of back projection approach. The method effectively reduces the number of segments needed for IMRT plans, and thus reduces treatment time. Our approach also provides a feasible solution for the rotation IMRT with burst mode.
SU‐E‐T‐657: Prostate Robotic Radiotherapy On Conventional C‐Arm Linacs
Authors: Dong P , Nguyen D , Ruan D , Romeijn E , Long T , Kupelian P , Yang Y , Low D , King C , Steinberg M , Sheng K
Published: July 2013

Purpose: To maximize rectum sparing in prostate radiotherapy, both the beam angles and fluence are optimized to be deliverable on a conventional C‐arm linac equipped with robotic couch and gantry. Methods: 12 retrospective low‐risk prostate cancer patients previously treated by volumetric modulated arc therapy (VMAT) were replanned by 4π radiotherapy. Forty Gy in 5 fractions were prescribed to cover 95% of the prostate PTV. To replan, 5×5 mm2 beamlets were precomputed using convolution/superposition. A column generation method was used to optimize beam orientations and fluence in an inter‐leaved fashion. The candidate beam pool consisted of non‐coplanar beams free from collision. The price, defined as the first order inner product, of each additional beam or beamlet was evaluated and the ones with highest price were kept. A total of 30 beams were selected for each patient. The planning target volume coverage, maximum doses (D1cc) to 1 cm3 the rectum and bladder were compared. Results: Both planning methods resulted in PTV coverage according to the prescription. For the centrally located prostate target, the 4pi planning method tends to selected beams confined to non‐coplanar angles between ±30°. Nevertheless, compared with the VMAT plans using two full arcs, rectum D1cc was reduced from 30.2 Gy to 26.9 Gy (p=0.0005), the mean body dose was reduced from 2.07 Gy to 1.75 Gy (p=0.0001). The maximum bladder dose was not significantly changed. Conclusion: By adding beams with moderate non‐coplanar angles and allowing more intensity modulation at each beam orientation, superior prostate plan conformality was achieved compared against state of the planning method; the maximum rectum dose was reduced for more than 10% without compromising target coverage. While prostate SBRT was used as an example, we reasonably expect similar dosimetric improvements for conventionally fractionated prostate treatment. The study supports the development of prostate robotic radiotherapy.
SU‐E‐T‐658: Iterative Optimization Method for IMRT Treatment Planning of Patients with Prosthetic Hardware
Authors: Peskorski D , Lalonde R
Published: July 2013

Purpose: To present a new and improved method of IMRT treatment planning for patients with one or more prosthetic hip implants and compare the results to traditional treatment planning for these patients. Methods: Four patients with unilateral or bilateral hip prostheses were planned for the 6MV beam of a Varian Truebeam linac using the Varian Eclipse planning system (AAA algorithm). Traditional treatment plans were generated with IMRT treatment fields that completely avoided entering through the prosthetics. A second set of plans was generated with 9 evenly spaced fields by iteratively editing the IMRT fluence for each field, so that in the final plan the prosthetics were completely blocked by the MLC during IMRT delivery. All plans were evaluated by normalizing the dose such that 95% of the target volume was treated to 100% of the dose. Results: In the traditional plans, the severe restrictions on beam angle limited the conformality and uniformity of dose delivery to the target volumes, and increased the dose to the organs at risk (bladder, rectum, bowel) anterior and posterior to the target. In the 4 traditional plans, the conformality index (CI)(V100/VPTV) averaged 1.23, while in the iteratively optimized plans the CI average was 1.07. The target dose uniformity index (Dmax/Dprescribed) for the same 4 plans averaged 1.15 in the traditional plans, and 1.12 in the iteratively optimized plans. Average volumes of the organs at risk receiving the prescribed dose were reduced in the iteratively optimized plans by 11.3%, 18.2%, and 41.8% for the rectum, bladder and bowel, respectively. Conclusion: This work presents a new and relatively simple method for IMRT planning of patients with prosthetics. With this method, substantial improvements can be made in target dose coverage and unformity and sparing of organs at risk.
SU‐E‐T‐660: Intermediate IMRT Dose Planning in Lung 4DCT Data Sets
Authors: Lalonde R
Published: July 2013

Purpose: Recently, planning systems have included provision for intermediate IMRT optimization to correct for deficiencies in optimizer dose models, particularly near low‐density heterogeneities. Our purpose is to quantify the effect of intermediate optimization on lung cancer planning, including realistic effects of target motion and setup error. Methods: IMRT plans were generated in the Varian Eclipse system for 5 lung cancer patients, with and without the use of intermediate optimization. PTV volumes were expanded 0.5 cm radially and 1.0 cm S‐I to account for internal target motion. Treatment plan complexity was evaluated by comparing total MUs in each plan, as well as average MLC leaf pair opening (ALPO). PTV dose uniformity index (UI) was calculated as D2%/D98%. Plans were then recalculated for simulated setup errors of up to 5 mm in all directions. For several patients, plans were also recalculated for all phases of a 4DCT data set. Results: Intermediate dose plans generated more uniform doses for PTV volumes, at the expense of increased total MU and plan complexity. The UI decreased from an average of 1.25 in the original plans to 1.15 in the intermediate plans. Average total MU increased by an average of 54.5% in the intermediate plans, while ALPO decreased by an average of 33%. However, the PTV doses did not reflect the actual doses delivered to the CTV as it moved within low density lung tissue. Actual UI for the CTV volumes with motion averaged 1.11 for the original plans, and varied by only 1% with motion. Conclusion: IMRT plans calculated on static CT images in lung give an inaccurate estimate of the true variation of dose with motion and setup errors. Modifying this incorrect dose by intermediate dose planning may be of limited value, and comes at the expense of higher MU and longer delivery times.
SU‐E‐T‐661: Nonlinear Continuous Method for Non‐Negatively Constrained Inverse Problem of IMRT Planning
Authors: Tanaka Y , Fujimoto K , Yoshinaga T
Published: July 2013

Purpose: The intensity‐modulated radiation therapy (IMRT) is an effective therapeutic technique for the treatment of tumors in the human body without inflicting serious damage to normal tissues. IMRT plans include an optimization strategy for minimizing an objective function of radiation beam weights. We present a novel approach for solving non‐negatively constrained inverse problems in IMRT treatment planning, based on the idea of continuous‐time dynamical methods using nonlinear differential equations. Our method can compute a feasible solution without the second derivative of an objective function, which is required for gradient‐based optimization algorithms such as conjugate gradient and steepest descent methods. Methods: We first consider the stability of an equilibrium, which corresponds to the desired planning in the IMRT system, by the use of Lyapunov's stability theorem. Then we perform some phantom experiments using phantom data simulating a clinical setup. Results: We prove theoretically that a Kullback‐Leibler (KL) divergence can be a Lyapunov function for the IMRT planning system, in consistent case. It means that the KL‐divergence measure decreases along the solution to the differential equation. Additionally, we show that the intensity of any radiation beam is not negative. Because the planning system can be created as an analog electronic circuit, its implementation in actual hardware yields dramatically faster planning as a physical phenomenon with real parallel computing, than software‐based iterative methods. Conclusion: The proposed method provides not only the reduction of a computational cost but also no production of a solution with an unphysical negative radiation beam coefficient in solving IMRT planning inverse problems. Moreover the theoretical results on convergence properties of solutions are confirmed by numerical experiments. Based on the results, we found that we can obtain a feasible solution violating the physical dose constraints minimally, even if the system is inconsistent.
SU‐E‐T‐662: Does Optimizing the Placement of Machine Isocenter Affect the Overall Optimized Plan Obtained Using Tomotherapy Treatment Planning System ? A Dosimetric and Analysis Study
Authors: Tewatia D , Tolakanahalli R
Published: July 2013

Purpose: To study the effect of isocenter placement with respect to DICOM center of patient CT images, on IMRT optimization using TomoTherapy Hi‐ART(TM) Methods: The unique feature of TomoTherapy Hi‐ART(TM) machine is the presence of flattening‐filter‐free(FFF) beam, off‐axis IMRT optimization capability and rotational delivery. To cover all possibilities, we choose seven different treatment sites (3 Head and Neck(HN), 1 Hippocampal avoidance, 1 SBRT and 2 brain cases) previously planned and treated on Tomotherapy for this study. For fair comparison planning parameters, used previously for clinical patient treatment were used and left unchanged throughout the study for each case. Four plans were generated for each patient in which the center of patient CT datasets were shifted incrementally from +5 cm to −10 cm with respect to isocenter in the AP direction. Treatment time as a function of planning target volume (PTV) distances from isocenter was studied. Homogeneity index (HI), Target coverage (TC) for PTVs and the mean and max doses of organs at risk (OARs) as a function of the respective distances from the isocenter was also studied. Results: Up to 13% reduction in treatment time (∼100 sec) for HN cases was obtained with PTV coverage by changing the distance of PTV from isocenter by 1.5 cm. Particularly, for SBRT, 3% reduction (80 sec) in treatment time was obtained with identical PTV coverage while decreasing max cord dose by 2 Gy. Conclusion: Optimization of patient position can thus be accomplished in rotational IMRT delivery by decreasing the distance of PTV from the isocenter while maximizing the distance of OARs from the isocenter thus minimizing treatment time and maximizing organ sparing without compromising tumor coverage. This work can be extended for doing fair comparison of plan quality achieved using various emerging technologies based on FFF modes of delivery.
SU‐E‐T‐663: Radiotherapy for Prostate Cancer Using RapidArc: Does Radiation Energy Make a Difference?
Authors: Sidhique H , Kaliyappa S , Kron T , Cramb J , Durai M
Published: July 2013

Purpose: The aim of our investigation was to determine the difference between treatment plans using 6 and 15MV X‐rays when employing RapidArc for radiotherapy of locally advanced prostate cancer Methods: In a retrospective study of five patients 4 plans were developed each (Varian Eclipse V8.9.08). PTV1 consisted of prostate plus seminal vesicles while PTV2 contained bilateral pelvic lymph nodes. RapidArc plans were individually optimized for 6 and 15MV X‐rays to develop the best possible plan in each case. To avoid bias in the comparison two additional plans were created in which the optimization constraints were exchanged to ensure differences between the objectives did not affect the results. Homogeneity Index (HI), Conformity Indices (CI) and several dose/volume parameters were evaluated for PTV and organs at risk. Number of monitor units (mus) was used a surrogate for integral peripheral dose. Results: Plans developed with higher X‐ray energy reduced the dose to rectum and bladder without compromising target coverage. The largest differences between plans was found to be in the medium dose range with a dose reduction of 6, 5.5 and 2.5% of the prescription dose for 50% of the volume of rectum, bladder and intestines, respectively. The exchange of optimization constraints confirmed these findings and resulted as expected in reduced HI in all cases and a small reduction of PTV dose in the case of 6MV. The number of mus was generally smaller for plans employing 15MV. Conclusion: Our study showed that it is possible to demonstrate an advantage for the use of 15MV over 6MV when treating pelvic lesions with RapidArc if optimisation parameters are chosen to maximise the benefit for each energy.
SU‐E‐T‐664: A Comparison of Metaheuristic Techniques for Optimizing Gamma Knife Inverse Treatment Planning
Authors: Langas J
Published: July 2013

Purpose: To determine if the Firefly Algorithm (Yang) or a genetic algorithm would provide viable alternatives to the simulated annealing technique used in Leksell GammaPlan 10 to create potential tumor treatment plans for use with Leksell Gamma Knife. Methods: To determine the viability of each algorithm, separate programs were created to generate treatment plans. The first utilized an annealing algorithm based on Leksell GammaPlan, and was intended to provide an accurate baseline for comparing the performance of the other algorithms. The second utilized an algorithm (Yang) that mimicked the behavior of fireflies in order to search all possible treatment plans to find an acceptable solution. The third mimicked natural selection and breeding among a population of potential solutions to generate acceptable solutions. Each of the programs were then run through a variety of tests to determine its effectiveness, after which each generated treatment was evaluated with the fitness function detailed by Elekta. Results: Of the compared algorithms, the Firefly Algorithm consistently generated the highest quality solutions when all three were run for a set number of generations. The Firefly Algorithm required the least amount of time to generate an acceptable solution for cubic tumors, but the genetic algorithm required the least amount of time for spherical tumors. The genetic algorithm produced the highest quality solutions in tests with shorter maximum run times, while the Firefly Algorithm produced the highest quality solutions in tests with longer maximum run times. Conclusion: Both the Firefly algorithm and genetic algorithm outperformed annealing in almost all test cases. This supports their potential use as alternatives to the existing method used in Leksell GammaPlan. Future studies with a wider variety of more extensive tests (i.e. more tumor varieties, longer tests, etc.) could provide additional support for their potential.
SU‐E‐T‐665: Investigation of Uniform Planning Margin for Intensity Modulated Proton Therapy of Pediatric Brain Tumors
Authors: Beltran C , Fernandez D , Laack N
Published: July 2013

Purpose: Determine if a uniform planning target volume (PTV) margin can be used for intensity modulated proton therapy (IMPT) of centrally located pediatric brain tumors and determine the most appropriate value. Methods: Treatment planning data from three pediatric patients with centrally located brain tumors were used in this study. For each patient, two IMPT plan types, a 3 field multi‐field‐optimized (MFO) and a 3 field single‐field‐optimized (SFO), and a 5 field IMRT plan were created for 7 uniform PTV margins that ranged from 0 to 10mm. The IMPT plans did not use robust optimization. For all plan types, the target coverage was held constant (98% of CTV received 54Gy and 95% of PTV received >53Gy) and normal tissue doses were minimized. Four error types, each with numerous variations, were simulated and their effect were recorded. The four error types were: systematic plan position error, systematic field position error, systematic range error, and random position error. Results: Regarding normal tissues, there was no notable difference between MFO and SFO, both were substantially better than IMRT, e.g. mean cochlea dose was 14Gy for IMRT with 3mm margin and 5Gy for the IMPT plans. In general, smaller margins gave better normal tissue sparring. For both IMPT plan types and IMRT, PTV margins less than 3mm lead to target coverage deficits under the various errors. For position errors, the IMPT and IMRT plans gave similar results as a function of margin size. For density and systematic field errors, the SFO plans maintained target coverage better than MFO. Conclusions: A uniform PTV margin is not suitable for MFO planning. However, a uniform PTV margin of 3mm is appropriate for SFO planning when considering the above uncertainties (excluding large errors). At 3mm, which is also required for IMRT, the SFO plans had superior normal tissue sparing. Varian Medical Systems
SU‐E‐T‐666: Evaluation of Various Spot MU Correction Methods for Range Adaptive Scanning‐Beam Proton Planning
Authors: Cheung J , Court L , Park P , Zhu X , Frank S , Kudchadker R , Dong L
Published: July 2013

Purpose: For CT‐based image‐guided proton therapy, it is possible to adapt to new patient anatomy by correcting the range of protons. In this work we investigated the use of several correction methods to adjust proton spot monitor units (MU) for single field uniform dose for real‐time range adaptive proton therapy. Methods: Spot MU correction factors for range‐adapted plans were calculated based on two different Methods: differences in central‐axis peak dose per MU, and differences in number of protons per MU. These factors were applied either only to the highest energy spot for each ray, or to the entire spot matrix. For each of these methods, the difference between the dose distributions from the verification (non‐adaptive) and adaptive plans to the original treatment plan was assessed. We examined target coverage (V95%), target hot spots (V110%), and dose to the organs‐at‐risk including the spinal cord (Dmax), esophagus (Dmean), contralateral lung (Dmean), and heart (Dmean). A scanning‐beam plan with single‐field optimization was created for a lung cancer patient, simulating a single‐field delivering a uniform target dose pointing directly at organs‐at‐risk. Average change in water equivalent thickness (WET) was used to quantify anatomic change. Results: Dose to normal tissues distal to the target was reduced for the adaptive plans compared to the verification plans for cases where WET decreased, but were similar for cases where WET increased. The adaptive plans almost always improved target coverage when WET increased, but the verification plans tended to have better target coverage when WET decreased. The hot spots in the target were always greater in the adaptive plans compared to the verification plans. Conclusion: The simple proton range correction adaptive scheme shows promise to reduce normal tissue doses without a full reoptimization. However, more sophisticated spot MU correction is warranted to increase dose homogeneity throughout the target volume.
SU‐E‐T‐668: Feasibility Study of Prostate Seed Implant with Intensity Modulated Proton Boost
Authors: Wu J , Cheng C , Wu H
Published: July 2013

Purpose: External beam radiation therapy (EBRT) has been widely used to deliver boost doses after permanent prostate seed implant (PSI). In this study, we tested the feasibility of EBRT boost with Intensity Modulated Proton Therapy (IMPT) in the presence of a large number of high Z metallic seeds. The dosimetric effect caused by implanted high density prostate seeds in spot scanned proton beam boost treatment was analyzed. IMPT boost plans were also compared with conventional Intensity Modulated Radiation Therapy (IMRT) boost plans. Method: Post prostate seed implants with seeds of I‐125 and Pd‐103 were tested in this study. The post PSI CT scans were processed at a slice thickness of 1mm. Proton treatment plans were created with two lateral beams in Eclipse treatment planning system. IMRT boost plans were generated with seven co‐planner beams. The calculation grids were set up at 1mm. Extreme cases with prostate seeds close to each other and prostate seed drift were also analyzed. Results: Higher density PSI seeds did not show significant effect on dose distributions in IMPT plans. Compared to conventional IMRT boost plans, IMPT has better sparing of rectum and bladder, while providing the same PTV coverage. The mean rectal dose decreases from 32.5% for IMRT to 22.4% for IMPT of the prescribed doses. The mean bladder dose decreases from 28.3% (IMRT) to 24.9% (IMPT) of the prescribed doses. The spot scanned proton beam provides an improved boost plan to different cold spots of suboptimal dose coverage PSI. Conclusion: PSI with an IMPT boost can be a valuable tool for prostate cancer patient treatment. It delivers better plan in terms of normal tissue sparing compared to IMRT boost plan. The high density seeds have no effect on the proton distribution.
SU‐E‐T‐670: Using Overlap Volume Histogram Analysis of a Prior Plan Dataset to Generate Clinically Acceptable Plans for CyberKnife Robotic Radiosurgery Treatment of Localized Prostate Cancer
Authors: Wu B , Pang D , Gatti J , Lei S , Colin S , McNutt T , Kole T , Collins S.P , Dritschilo A
Published: July 2013

Purpose: CyberKnife offers the potential benefits of non‐isocentric, non‐coplanar treatment delivery. However, its planning is a laborious manual, trial‐and‐error process. This study is to investigate whether an overlap volume histogram (OVH)‐driven planning approach can produce clinically acceptable plans for treatment of localized prostate cancer. Methods: It is assumed that given consistent target coverage, patients with a relevant target‐organ spatial relationship should have similar organ sparing. The OVH is used to characterize the 3‐D spatial relationship between an organ and a target. A database containing the OVH and DVH of prior plans is built to serve as an external reference. During the initial planning, the OVH is used to search through the database to find a prior patient group whose target‐organ relationship is related to that of a new patient. The planning objectives for the new patient are then estimated from the group and input into the CyberKnife TPS for optimization. To demonstrate the effectiveness of the method, the plans of 12 prostate patients (prescription: 36.25Gy in 5 fractions) are generated by the OVH approach and compared to the corresponding clinical plans using the in‐house dosimetric guidelines. Results: Physicians confirm that OVH plans are clinically acceptable. OVH plans: on average, Vcc(37Gy) and V(18.12Gy) to the bladder decrease 1cc and 6% (p<0.05); Vcc(36Gy) and V(18.12Gy) to the rectum decrease 0.02cc and 4.3% (p=0.3); Vcc(40Gy) to the prostatic urethra decreases 0.08cc (p=0.01); V(14.5Gy) to the femur heads decreases 0.58% (p=0.01). V(37Gy) to the membranous urethra increases from 23.1% to 24.5% (p=0.75); V(29.5Gy) to the penile bulb increases from 5.5% to 10.7% (p=0.35); V(36.25Gy) to PTV increases 0.2% (p=0.4); the estimated delivery time deceases 3.5 minutes (p=0.01). Conclusion: With respect to planner‐created clinical plans, this approach offers an alternative way to generate clinically acceptable plans. It advances the possibility of automated CyberKnife planning. C. Sims, employed by Accuray, Inc;S.P Collins, Accuray clinical consultant
SU‐E‐T‐672: Comparisons of Single and Multiple Isocenter Stereotactic IMRT Treatment Planning for Multiple Brain Metastases Treatment
Authors: Hossain S , Algan O , Ali I , Young J , Ahmad S
Published: July 2013

Purpose: To compare quality of single and multiple isocenter stereotactic IMRT treatment plans for multiple intracranial targets. Methods: Five patients (two with three and three with two brain metastatic tumors) treated with multiple isocenters using 9 to 12 non‐coplanar beams per lesion were replanned for single isocenter 10 to 12 non‐coplanar beams. Identical contour sets and dose‐volume constraints were applied. Plans were developed using BrainLab iPlan system for delivery with Varian TrueBeam STx machine. The prescribed dose to each target was 25 Gy in five fractions and minimum 95% dose was required to cover > 95% of target volume. Results: All plans satisfied the dose‐volume constraints, with no significant differences in dose conformity between single and multiple isocenter plans. The mean Paddick conformity index (CI) for two target single and multiple isocenter plans was 0.76 and 0.75 (p = 0.93), and for three target single and multiple isocenter plans was 0.73 and 0.70 (p = 0.37), respectively. Normal brain tissue doses were higher for single isocenter plans compared to multiple isocenter plans. The average normal brain volumes receiving 15 and 10 Gy for two target plans (single vs. multiple isocenter) were: (4.8% vs. 2.1%) and (8.9% vs. 4.5%), respectively. For three target plans these volumes were: (2.0% vs. 1.5%) and (4.6% vs. 3.4%). The estimated total treatment time (patient set‐up, imaging‐verification, beam‐on time at 600 MU/min, etc.) were approximately 1.75 and 2.5 times lower with two and three target single isocenter delivery, respectively. Conclusion: Single isocenter plans were capable of producing quality treatment plans similar to multiple isocenter plans. Treatment delivery times were much higher and the normal brain tissue doses were only slightly lower at 10 and 15 Gy dose levels for multiple isocenter plans compared to single isocenter plans. Clinical consequences of these normal tissue doses are however unknown.
SU‐E‐T‐674: Parameter Sensitivity Analysis of VMAT Planning for Spine SBRT
Authors: Chen H , Winey BA , Daartz J , Gierga DP
Published: July 2013

Purpose: To analyze the sensitivity of VMAT plan quality to various delivery parameters for spine SBRT. Methods: Ten spine patients, previously treated to 18 Gy in 1 fraction with 11 field IMRT plans generated using a Multi‐Criteria Optimization approach in Raystation (RaySearch Laboratories, Stockholm, Sweden) were selected for this study. Optimal VMAT plans, of comparable quality were generated using dual partial arcs and defined as baseline plans. The sensitivity of plan quality on dose rate level (variable dose rates (VDR), discrete dose rates (DDR)), leaf speed constraints (0.1, 0.2, 0.4, 0.6, 0.8 cm/degree and no constraint), was tested. The effect of maximum dose rate, maximum delivery time, and fraction size (18Gy*1, 9Gy*2 and 2Gy*9) was also tested. The resulting VMAT plan quality was analyzed by comparing PTV homogeneity index and conformity number, and D1 of spinal cord. Results: A leaf speed constraint of less than 0.2 cm/degree will degrade PTV conformity and spinal cord sparing, and plan quality of leaf speed more than 0.6 cm/degree is close to that without any leaf speed constraint. The proper selection of VMAT delivery parameters allows VMAT plan quality to match IMRT. In addition, VMAT plan quality of VDR is superior to that of DDR when either maximum delivery time or maximum dose rate is increased. Conclusion: VMAT plan quality for spine SBRT depends on the combination of delivery time, fraction size, and leaf speed. The sensitivity of plan quality depends on dose per fraction and maximum delivery time. Funding support: NCI Federal Share Proton Beam Program Income Grant.

Conflict of Interest:none.
SU‐E‐T‐675: The Effect of Diaphragm Motion to Dose Calculation During Stereotactic Body Radiotherapy (SBRT) Treatment of Spine Lesions Located at the Superior‐Inferior Level of the Diaphragm
Authors: Yao R , Chu J , Diaz A , Turian J
Published: July 2013

Purpose: To investigate the effect of respiratory motion on dose calculation during stereotactic body radiotherapy (SBRT) treatment of spine lesions located at the level of diaphragm. Methods: Helical and four‐dimensional (4D) CT data sets of four patients were used for this study. The 4D sets were binned using 10 respiratory phases (0%–90%) and an average CT was reconstructed from the 4D CT sets. The recorded diaphragm motions were 0.8cm, 1.2cm, 1.4cm, and 1.7cm respectively. The helical CT might represent an arbitrary respiratory phase. The 0% and 50 % CT represent the inhale and exhale phases respectively. One vertebral body (PTV) and the thecal sac at the superior‐inferior level of diaphragm were contoured on the average CT. An SBRT plan, using Rapid Arc, was generated for the average CT. The prescribed dose to the PTV was 24Gy in 3 fractions using V24Gy=95%. The plan was next copied to each of the 10 CT sets for dose calculation. The dose‐volume parameters Dmax and D95% for PTV, and Dmax, D0.25cc, D1.2cc for thecal sac at each respiratory phase CT were compared with those obtained from the average CT. ' Results: The variations of dosimetric parameters for each phase of 4D‐CT relative to the average CT demonstrate similar respiratory shapes. For case #1, all phases, the thecal sac Dmax are higher than that of average CT, and the D0.25cc of thecal sac are lower compared to average CT. For case #4, the thecal sac Dmax for all phases are lower compared with average CT. In this case, the Dmax of thecal sac calculated on the helical CT might be underestimated by 7.3%. Conclusion: Planning on helical CT for spine SBRT at the level of diaphragm might introduce deviations to the thecal sac dose. In such cases, the average CT should be used for dose calculation.
SU‐E‐T‐676: Impact of Carbon Fiber Couchtop On TrueBeam Flattening‐Filter‐Free (FFF) SRS/SBRT Radiation Therapy
Authors: Le A , Huq M , Jang S
Published: July 2013

Purpose: To evaluate the attenuation of Flattening‐Filter‐Free (FFF) SRS/SBRT beams resulting from transmission through the Exac Trac carbon fiber couchtop and to validate the couchtop model in Eclipse Ver.11. Methods: A cylindrical phantom of 8cm‐diameter was scanned with an ion chamber placed in the center. Radiation doses of static fields (3×3, 5×5, and 10×10 cm2) with gantry angles ranging from 100 to 180 degrees were computed for 6X, 6XFFF, 10X, 10XFFF, and 15 MV photon beams with and without ExacTrac couchtop. The calculated attenuation of FFF beams with small fields was compared with measured data. Skin‐sparing effect with a posterior‐anterior FFF beam was evaluated with TLDs placed underneath a rectangular solid‐water phantom on top of the carbon couchtop. Results: The largest measured couch attenuation was 17.7 %for a gantry angle of 100 degree, 6MV‐FFF mode and 3×3 cm2 field size, in comparison to 6.7 % calculated by Eclipse TPS. The Eclipse TPS underestimates the couch attenuation with the largest difference between measured and calculated dose of 10%. The effect of couch attenuation is more prominent at gantry angle close to 100 degree, FFF beams and smaller field size. Conclusion: There will be a significant dose difference due to high couch attenuation of EXACTRAC carbon fiber couchtop. Furthermore, the virtual couch modeled in TPS (Eclipse v11, Varian) underestimates the couch attenuation for this couch. Therefore, precaution and careful QA should be considered, especially when treating patients with posterior target using small field SBRT techniques and FFF mode for low energy photon beam.
SU‐E‐T‐677: The Effects of Respiratory Motion in RapidArc Spinal Stereotactic Radiosurgery
Authors: Pursley J , Manning M , Sintay B , Terrell J , Maurer J , Yount C , Wiant D
Published: July 2013

Purpose: Spinal radiosurgery is planned to strict tolerances. Unaccounted for dosimetric changes resulting from respiratory motion might lead to delivery of a sub‐optimal plan. We examine these effects for RapidArc spine radiosurgery. Methods: Plans were created for three patients that received 4D‐CT scans in a spinal radiosurgery set‐up. A target covering a single vertebral body and pedicles was defined and the spinal canal was contoured for each patient. Plans that delivered 18 Gy to the targets were created on the free breathing scans (FB). The maximum inspiration and expiration 4D‐CT phases were fused to the FB based on target and spinal canal position. The FB plans were copied onto these images and doses were calculated. Point doses, DVHs, and plan subtractions were used to evaluate plan differences. Motion was measured at fixed points on the 4D‐CTs. Results: The target for case 1 was defined at T4, case two at T7, and case three at T9. For all cases the target and the spinal canal moved < 1 mm, while the skin surface moved 1–2.5 mm. For the T4 plan the aortic arch moved about 6 mm. For the T7 and T9 cases the diaphragm moved 5 mm and 12.3 mm, respectively. Point dose changes in the targets between the maximum inspiration and exhalation plans were 8.1 cGy, 69.0 cGy, and 58.1 cGy for the T4, T7, and T9 plans. Point changes in the spinal canal were 13.8 cGy, 34.0 cGy, and 34.1 cGy for the T4, T7, and T9 plans. Spinal canal DVHs changes were negligible in all cases. Changes in target coverage at doses >= 18 Gy were approximately 5% for the T7 and T9 plans. Conclusion: Respiratory motion may alter the dosimetry of spinal radiosurgery plans. The clinical impact of these changes appears to be small.
SU‐E‐T‐678: Normal Tissue Dose‐Volume Constrains for Inverse Planning of Acoustic Neuroma Stereotactic Radiosurgery (SRS)
Authors: Liu H , Andrews D , Werner‐Wasik M , Xiao Y , Yu Y , Shi W
Published: July 2013

Purpose: Stereotacic radiosurgery (SRS) of acoustic neuroma was traditionally planned using forward planning techniques, either with Elekta GammaKnife ball‐packing technique, or with multiple conformal dynamic arcs technique using micro‐MLC and Linac (BrainLAB Novalis). As the recent development of Volumetric Modulated Arc Therapy (VMAT), inverse planning using multiple‐arcs VMAT technique for SRS becomes commercially available, and it has the advantage of faster delivery with comparable dosimetry. However, planners need proper dose volume constrains to drive the optimizer to achieve the dosimetric goals, both clinically acceptable and technically achievable. This study is investigating how to set up the proper dose volume constraints for SRS plans of acoustic neuroma. Methods: Twenty cases were selected with their volumes cover the general range of acoustic neuroma SRS (0.1 cc to 5cc). Each case was planned using Gammaknife Perfexion with sector optimization. Prescription is 12Gy to 50% isodose, with 100% target coverage. The total tissue volumes that receive greater than 12, 8, and 6 Gy (V12, V8, and V6) were recorded. The relationship between the target volumes (Vtarget) and V12, V8, and V6 were plotted and linear regressions were fitted for each parameter. Results: A set of linear relationships between the target volume and the volumes that receive greater than 12, 8, and 6Gy was constructed as the following: V12 = 1.476Vtarget (R2 = 0.995), V8 = 3.04Vtarget +0.16(R2 = 0.990), and V6 = 4.64 Vtarget + 0.35 (R2 = 0.982). Conclusions: Traditional plan quality indices such as conformity index, selectivity, etc. are good for evaluating forward plans, but cannot be used to drive the inverse planning engine to optimize inverse plans. The normal tissue dose volume constraints obtained in this study, combined with clinical constraints for brainstem and cochlea, can provide inverse planner with objectives to obtain an inverse plan for acoustic neuroma SRS.
SU‐E‐T‐679: Implications of Radiobiological Bath and Shower Effects On Brainstem Dose Tolerance
Authors: Grimm J , Chen A , Ying W , Xue J
Published: July 2013

Purpose: To gain insight into radiosurgical brainstem dose tolerance by considering the radiobiological implications of the bath and shower experiments. Methods: Clinical brainstem dose tolerance for radiosurgery is compared to the two extreme cases of hypofractionated whole brain irradiation and trigeminal neuralgia radiosurgery. In 2003 and 2009, experimental results were published wherein a low dose to a large volume (bath) was combined with a high dose to a small volume (shower), for the cases of rat cervical spinal cord and rat parotid gland. Clinically the uniqueness of each patients' tumor requires a customized mixture of bath and shower doses to the adjacent critical structures, whereas in the animal experiments it was possible to systematically vary the bath dose and the shower dose and to statistically analyze the outcomes to study the interactions in detail, although human response may differ. Results: Hypofractionated whole brain irradiation results in essentially uniform bath dose to the brainstem at a single fraction dose equivalent of about 10Gy, according to the LQ model. For radiosurgery, Timmerman's 2008 limit allows 1cc of brainstem to exceed 10Gy in a single fraction. Therefore since the volume of brainstem receiving the bath dose is reduced, it allows a higher shower dose to be received by a small volume, and the corresponding TG101 limit allows a max dose of 15Gy. In the case of trigeminal neuralgia, the bath dose to brainstem is much lower, and some institutions have allowed an extremely high shower dose of 45Gy in a single fraction to the maximum point dose in brainstem, with surprisingly infrequent side effects under conditions of meticulous targeting and dosimetric accuracy. Conclusion: Clinically the tradeoff between doses to large and small volumes is already implicitly taken into account, but comparison to the radiobiological research can help provide insight. Dr. Grimm developed and holds intellectual property rights to the DVH Evaluator software tool which is an FDA‐cleared product in commercial use
SU‐E‐T‐682: The Dosimetric Consequences of 4DCT‐Based Motion Margin Estimation for Proton Radiotherapy
Authors: Koybasi O , Lewis J , Mishra P , James S , Seco J
Published: July 2013

Purpose: Motion management in standard radiotherapy uses ITV margins obtained from pre‐treatment 4DCT. The purpose of this study is to investigate the dosimetric effects of inaccuracies in motion margins obtained from pre‐treatment 4DCT in proton radiotherapy. Methods: We use the locations of gold fiducial markers implanted in lung tumors of patients to track the tumor. A 20 mm diameter spherical tumor is inserted into a realistic digital respiratory phantom, where the tumor motion is based on real patient lung tumor trajectories recorded over multiple days. Using“Day 1” patient data, 100 ITVs were generated with 1 s interval between consecutive scan start times. Each ITV was made up by the union of 10 tumor positions obtained from 6 s scan time. Two ITV volumes were chosen for treatment planning: ITVmean−σ and ITVmean+σ. The delivered dose was computed on i) 10 phases forming the planning ITV (“10‐phase” ‐ simulating dose calculation based on 4D‐CT) and ii) 50 phantoms produced from 100 s of data from any other day with tumor positions sampled every 2 s (“dynamic” ‐ simulating the dose that would actually be delivered). Results: For a patient with similar breathing patterns on “Day 1” and the “other day”, the 95% volume coverage (D95) for “dynamic” case was 8.13% lower than the “10‐phase” case for ITVmean+σ. For a patient with relatively different breathing patterns on “Day 1” and the “other day”, this difference was as high as 24.5% for ITVmean−σ. ' Conclusion: Our preliminary results have shown that proton treatment planning based on 4DCT can lead to under‐dosage of the tumor and over‐dosage of the surrounding tissues. This is more severe in proton therapy due to possible shift of the Bragg peak compared to photon therapy in which the tumor is surrounded by an electron bath.
SU‐E‐T‐683: Prediction of Sensitivity of IMPT Plans to Anatomical Change Due to Plan Complexity
Authors: Cheung J , Dong L , Park P , Zhu X , Kudchadker R , Frank S , Palmer M , Liu W , Court L
Published: July 2013

Purpose: To assess the significance of the treatment plan complexity on the sensitivity of multifield optimized intensity modulated proton therapy (IMPT) plans to inter‐fraction anatomical changes. Methods: Eight lung cancer patients were planned with IMPT to meet clinical criteria. These plans were retrospectively analyzed for this study. A complexity score based on the in‐field dose heterogeneity within the target weighted by the number of regions of dose homogeneity was devised to evaluate the complexity of IMPT plans. The complexity score was calculated for each of the treatment plans and the change in target coverage over the course of treatment was assessed for each patient based on weekly CT imaging. The change in proton range to the distal edge of the clinical target volume was used as a quantitative metric for anatomical change. Greater than 5% drop to the target D99% was chosen as an action level to signal a clinically significant drop in target coverage that would require adaptive replanning. Results: For plans with a low complexity score, target coverage was well preserved over the course of treatment for even a large change in proton range to the target distal edge. Plans with high complexity scores, however, were sensitive to changes in anatomy, and even a small WET change resulted in a clinically significant drop in target coverage. Plans with complexity score below 1 preserved target coverage in 16/18 cases, while plans with complexity score above 1 preserved target coverage in only 3/25 cases. Conclusion: IMPT plan with highly modulated field‐to‐field dose distribution can suffer clinically significant target coverage changes due to anatomical change. The complexity score function devised in this work can be a good predictor of robustness to anatomical change and could potentially be a part of routine plan check.