M Afghan

An arc-sequencing algorithm for intensity modulated arc therapy.

Intensity modulated arc therapy (IMAT) is an intensity modulated radiation therapy delivery technique originally proposed as an alternative to tomotherapy. IMAT uses a series of overlapping arcs to deliver optimized intensity patterns from each beam direction. The full potential of IMAT has gone largely unrealized due in part to a lack of robust and commercially available inverse planning tools. To address this, we have implemented an IMAT arc-sequencing algorithm that translates optimized intensity maps into deliverable IMAT plans. The sequencing algorithm uses simulated annealing to simultaneously optimize the aperture shapes and weights throughout each arc. The sequencer enforces the delivery constraints while minimizing the discrepancies between the optimized and sequenced intensity maps. The performance of the algorithm has been tested for ten patient cases (3 prostate, 3 brain, 2 head-and-neck, 1 lung, and 1 pancreas). Seven coplanar IMAT plans were created using an average of 4.6 arcs and 685 monitor units. Additionally, three noncoplanar plans were created using an average of 16 arcs and 498 monitor units. The results demonstrate that the arc sequencer can provide efficient and highly conformal IMAT plans. An average sequencing time of approximately 20 min was observed.

MO‐D‐351‐06: A Generalized Inverse Planning Tool for Arc‐Based IMRT Delivery

Purpose: The recent development of new linaccontrol systems that are capable of delivering Volumetric Modulated Arc Therapy (VMAT) has attracted significant attention. There remains, however, a lack of robust inverse planning tools for VMAT. In this study, we will present a generalized inverse planning tool that can provide highly conformal VMAT solutions using either single‐arc or multiple‐arc deliveries for both Varian and Elekta MLCs.Method and Materials: To generate VMAT plans, we first created optimized multi‐field IMRT plans with equal‐spaced beam angles in Pinnacle3 using direct machine parameter optimization (DMPO). A “deliverable” fluence map was reconstructed using the resulting apertures for each beam. Next, we applied our home‐grown arc sequencer to translate these fluence maps into VMAT plans. Based on the user‐defined requirements, the sequencer can provide either single‐arc or multiple‐arc plans that meet the predefined VMAT leaf‐motion constraints. The obtained VMAT plans were then loaded into Pinnacle3 for a final dose calculation. In this study, 10 cases were tested in this study covering a variety of treatment sites including head‐&‐neck(5), prostate(3), lung(1) and brain(1). Results: A total of 24 VMAT plans were created using these 10 cases. Results demonstrated that highly conformal VMAT dose distributions can be achieved with an average sequencing time of under 8 minutes. On average, the VMAT plans required 513 MUs to deliver between 1 to 3 arcs. The average standard deviation in the target dose was 5.79 cGy/fraction; while the average target volume covered by 95% prescribed dose was 98.1%. Our results show that comparable VMAT plans can be achieved using either the Elekta 80‐leaf MLC or the Varian 120‐leaf Millennium MLC.Conclusion: Our generalized arc‐sequencing algorithm serves as a robust inverse planning solution for VMAT. Highly conformal single‐arc or multiple‐arc VMAT plans can be created for Elekta and Varian MLCs.

SU‐DD‐A1‐05: Study of VMAT Plan QA Using Film, Diode Based, and Ion Chamber Based QA Systems

Purpose: With the commercial introduction of deliverycontrol systems for Volumetric Modulated Arc Therapy (VMAT), the need has arisen for reliable and efficient techniques for performing patient specific VMAT quality assurance (QA). In this work, we have studied three patient specific QA techniques for VMAT: ion chamber with film, 2D diode array, and a 2D ion chamber array. Materials and Methods: The three QA techniques we have utilized are: (1) a stack of solid water slabs with an inserted ion chamber and film sandwiched between two slabs; (2) a 2D diode array (MapCHECK™ device inserted into a MapPHAN™ phantom); and (3) a 2D ion chamber array (MatriXX™ inserted into a MULTICube™ Phantom). Ten VMAT plans were delivered to all three QA systems on an Elekta Synergy linac.Results: With the highest spatial resolution among all three systems, film measurements can provide very good QA results when analyzed in relative mode. Absolute dose comparisons were performed for both the MapCHECK™ and MatriXX™ systems. The average passing rate in gamma analysis were 95.0% and 98.5% using 3%/3mm criteria for the above two systems, respectively. The slightly lower passing rate for MapCHECK™ QA may be attributed to the angular and dose rate dependence of the diode response. It is also observed that the MapCHECK™ QA is more sensitive to the tongue‐&‐groove effect when diodes fall between two leaves. The MatriXX™ system provides slightly higher QA passing rates. However it may be less sensitive to large dose variation within small regions due to its 7.62mm detector grid size. Conclusions: All three systems can be used for VMAT plan QA provided users are attentive about the strengths and limitations of the QA device. Research sponsored by Elekta Corporation.

SU-FF-T-179: Evaluation of An IMRT Planning Technique to Incorporate Intrafraction Organ Motion Using Rigid-Body Tumor Modeling

Purpose: We have evaluated a technique for incorporating intrafraction organ motion into IMRTtreatment planning using Monte Carlo based dose engine that builds the pattern of respiratory‐induced anatomical displacement into the dose calculation. Method and Materials: For the lungcancer patients included in this study, a patient specific 3D tumor trajectory is derived using 4D CTimages by modeling the tumor as a rigid body and by performing a parametric fit to the center‐of‐mass of the tumor volumes. The patient specific tumor trajectories were then used to create the optimized plans. The optimized plans were compared with plans produced using traditional margin expansion. In addition, measurements were performed with a moving phantom programmed for sinusoidal motion to verify the accuracy of this planning approach. Results: For the three lung patients in this study, significantly improved normal tissue sparing was observed in the plans that incorporated intrafraction organ motion. On average, the motion‐based plans provided a 40.8% reduction in the volume of the involved lung receiving 80% or more of the prescribed dose. Verification measurements performed with a moving phantom demonstrated a significant improvement in the agreement between the planned and measured doses. Conclusion: In contrast to traditional margin expansion, our proposed technique resulted in a significant sparing of critical structures while providing similar target coverage. In addition, the technique obviates the need for gating or tracking.

TH‐C‐303A‐01: Initial Clinical Experience with Electromagnetic Localization and Tracking for External Beam Partial Breast Irradiation

Purpose: The Calypso® 4D Localization System™ (Calypso Medical) uses non‐ionizing ACelectromagnetic radiation to localize and track small wireless devices (called Beacon® transponders) implanted in or near a patients tumor. We report on the first clinical experience with the use of the system for localizing and tracking the lumpectomy cavity during external‐beam accelerated partial breast irradiation (EB APBI). Method and Materials: The study included patients treated receiving EB APBI on an IRB approved protocol. Thirteen patients were implanted with both gold markers (GM) and beacon® transponders and two patients were implanted with beacon® tranponders alone. For patients in whom MRI follow‐up was anticipated, two removable interstitial breast catheters were inserted and afterloaded with gold markers and transponders. The catheters were removed post radiation therapy. Initial alignment was performed using lasers. For patients with gold markers, orthogonal images were used to obtain the necessary shift. The shift values were compared to the shift predicted under electromagnetic guidance. During treatment, Calypso was used to track the target motion. Results: Fifteen patients have been studied, and 93 treatment fractions were analyzed. The catheters and transponders overall showed good stability with inter‐transponder distance changes of less than 2 mm. Calypso based setup can be performed in less than 2 minutes. An average residual setup error of 10.29 mm was determined using gold markers. For the 63 fractions analyzed, the difference between the residual setup error determined by the GM and the Calypso system on average was 1.5 mm. Tracking showed regular motion in the range of 2–3 mm with occasional deeper breaths exceeding 4–5 mm. Conclusion: Results show excellent agreement between gold markers and electromagnetic guidance in EB APBI with electromagnetic guidance providing a more rapid setup and real time tracking during delivery. Research sponsored by Calypso Medical.

SU‐FF‐T‐141: Initial Clinical Experience with Elekta VMAT

Purpose: Elekta recently introduced a new deliverycontrol system called PreciseBeam® VMAT that provides dynamic delivery capabilities including volumetric modulated arc therapy (VMAT). VMAT is a rotational approach to the delivery of IMRT that can be delivered on a conventional linear accelerator. In this study, we report on our initial clinical experience with Elekta VMAT. Method and Materials: The PreciseBeam® VMAT control system has been installed on an Elekta Synergy and an Elekta Precise linear accelerator in our clinic. We have tested the delivery system in conjunction with three inverse planning solutions: (1) an in‐house arc sequencing algorithm that translates optimized fluence maps into deliverable VMAT plans, (2) the Ergo++ planning system that utilizes anatomy based inverse planning, and (3) the SmartArc planning module in the Pinnacle3treatment planning system. The optimized treatment plans have been verified using phantom irradiations with the IBA MatriXX 2D ion chamber array embedded in a MULTcube phantom. Results: Using our arc‐sequencing algorithm for VMAT planning, we treated our first Elekta VMAT patient under an institutional review board (IRB) protocol in July 2008. In January 2009, we treated our first Elekta VMAT case planned using Ergo++. Our initial focus was on more simple concave targets. More recently, we have opened up VMAT for all clinical sites. Conclusion: Elektas Precise Beam® VMAT control system can safely and efficiently delivery highly complex VMAT treatment plans. For simple targets such as prostate, pancreatic, and lung cases, single arc VMAT plans are sufficient and be typically delivered in less than 3 minutes. For more complex cases such as head‐and‐neck, the use of multiple arcs provides a dosimetric benefit and plans typically take between 4 and 6 minutes to deliver. Research sponsored in part through a grant from Elekta.

TH‐D‐AUD B‐01: Comparison of Single‐Arc and Multiple‐Arc Approaches for Delivering of Volumetric Modulated Arc Therapy

Purpose:Linear accelerator vendors have recently introduced deliverycontrol systems with the ability to deliver Volumetric Modulated Arc Therapy (VMAT). These systems can deliver VMAT using either a single‐arc or a multi‐arc approach. The goal of this study is to compare single‐arc and multi‐arc delivery techniques in terms of both plan quality and delivery efficiency. Method and Materials: Two prostate cases and three head‐and‐neck cases were included in this study. For each case, single‐arc and three‐arc VMAT plans were generated using our home‐grown arc sequencing algorithm that converts optimized fluence maps into deliverable arcs. Each VMAT plan was evaluated using Pinnacles superposition dose calculation, and DVH comparisons were made between the two sets of VMAT plans. VMAT deliveries using Elektas PreciseBeam Infinity™ control system were performed for each patient to compare the delivery efficiency. Results: For the two prostate cases, the single‐arc and three‐arc VMAT plans resulted in similar target dose coverage and organ at risk (OAR) sparing. For the three head‐and‐neck cases, the three‐arc plans provided improved target dose coverage as compared with the single‐arc plans with the average standard deviation in the target dose reduced from 7.06 to 5.82 cGy. The target volume covered by 95% of the prescribed dose also increased from 97.5% to 98.5% for the three‐arc VMAT plans. For single‐arc VMAT delivery, the average delivery time was approximately 3.2 minutes for prostate cases and 5 minutes for head‐&‐neck cases. While for three‐arc VMAT delivery, these values increased to 4.5 and 5.6 minutes. Conclusion: For simple cases such as prostate, single arc delivery provides comparable plan quality and improved delivery efficiency. However, for more complex cases, multi‐arc VMAT plans are preferable due to the ability to achieve improved dose conformity while maintaining acceptable treatment times. This work was sponsored in part through a grant from Elekta.

TU‐D‐AUD B‐01: Volumetric Modulated Arc Therapy for Head & Neck Cancer

Purpose: Volumetric Modulated Arc Therapy (VMAT) is an arc‐based technique that utilizes dynamically modulated arcs to deliver intensity‐modulated radiation therapy(IMRT)treatments on a conventional linear accelerator. In this work, we evaluate VMAT as a treatment technique for patients with carcinomas of the head‐and‐neck. Method and Materials: Five complex head and neck cancer patients, with multiple prescription levels, were selected for this study. Fully inverse planned VMAT plans were optimized for these patients using our homegrown arc‐sequencing software. The software uses simulated annealing to optimize the aperture shapes and weights while minimizing the differences between the optimized (ideal) and sequenced intensities. The optimized plans were compared with step‐and‐shoot IMRT plans generated using the Pinnacle3treatment planning system. VMAT plan verifications have also been performed using Elektas Precise Beam Infinity™ control system which has been installed on an Elekta Precise linear accelerator in our clinic. Results: Using our arc‐sequencing tool, VMAT can be used to create highly conformal head‐and‐neck treatment plans. As compared with traditional fixed‐field plans, VMAT was able to reduce the average parotid dose from 85.3 cGy to 73.6 cGy per fraction. Additionally, the average number of monitor units was reduced from 1058.3 to 502.3 per fraction. Initial delivery tests demonstrate that using VMAT complex head‐and‐neck deliveries can be completed in under 6 minutes. Conclusion: VMAT should serve as an important tool in the delivery of radiation therapy for head‐and‐neck carcinomas. By utilizing the dosimetric advantages of rotational IMRT, VMAT can provide more uniform target doses and reduced critical structure doses as compared with fixed field IMRT.Conflict of Interest: Research partially sponsored by Elekta Corporation.

TH‐D‐AUD B‐03: Initial Experience with the Delivery of Volumetric Modulated Arc Therapy

Purpose: Recently, there has been a renewed interest in the delivery of arcbased IMRT using conventional linear accelerators. Elekta and Varian have developed linear acceleratorcontrol systems that are capable of delivering rotational IMRT by combining gantry rotation, dynamic MLC leaf motion, and a variable dose rate. Elektas new Precise Beam Infinity™ control system has been installed in our clinic and acceptance testing and plan verifications have been performed. In this study, we will report on our initial experiences with volumetric modulated arc therapy (VMAT) delivered using an Elekta Precise linear accelerator.Method and Materials: VMAT is a radiotherapydelivery technique that combines the dosimetric advantages of rotational delivery with the dose painting capabilities of IMRT. We have developed an arc sequencing algorithm that translates optimized fluence maps into deliverable VMAT treatment plans. In this investigation, one head‐and‐neck and five prostate plans have been delivered in an effort to quantify the efficiency and accuracy of the VMAT delivery system. Results: A complex head‐and‐neck plan involving two targets and a simultaneous boost was delivered in 5 minutes 29 seconds. For this three‐arc head‐and‐neck plan, the point dose agreed within 1.9%. Additional measurements for 5 prostate cases demonstrated an average delivery time of 3 minutes 28 seconds with all ion chamber measurements agreeing within 3%. Film measurements demonstrated close agreement between the predicted and measured isodose curves. Measurements have also been performed to quantify efficiency of single‐arc versus multi‐arc VMAT deliveries.Conclusion: Elektas Precise Beam Infinity control system can safely and efficiently delivery highly complex VMAT treatment plans. In our initial investigation, all plans delivered in less than 5.5 minutes. Numerous additional plan verifications will be performed as we move towards an anticipated clinical implementation of VMAT in May 2008. Research sponsored in part through a grant from Elekta.

SU‐FF‐T‐86: Assessment of Carbon Fiber and Synthetic Fiber Radiation Therapy Tabletops for Kilovoltage Cone Beam CT Imaging

Purpose: The Calypso® System uses electromagnetic fields to localize and continuously track implanted Beacon® transponders. For sub‐millimeter localization accuracy, a non‐conductive surface is required. We studied kilovoltage (kV) cone‐beam CT(CBCT)image quality of a non‐conductive synthetic fiber composite tabletop (Qfix Systems, kVue IGRT™). Material and Method:CBCTimages were acquired with 3 tissue phantoms (CatPhan®) through carbon fiber tabletop (Medical Intelligence) and synthetic fiber tabletop (Kevlar®). Image resolution, low contrast sensitivity, 3D uniformity, circular symmetry and spatial uniformity with carbon and synthetic fiber tabletops were compared. Multiple protocols were used to evaluate image registration accuracy. Impact of side rails incorporated in the synthetic fiber model was evaluated. Results: Both tabletops demonstrated better than 2% of low contrast sensitivity, 3D uniformity was within 2%. Circular symmetry and spatial uniformity were within 1mm. Carbon tabletop and synthetic fiber tabletop with rails outside the kV field achieved equal resolution of 8 1p/cm while the synthetic fiber tabletop with rails inside the field provided a resolution of 7 1p/cm. Compared with the carbon fiber, image pixel values for six materials with relative electron densities ranging from 0.28 to 1.69 were found to be within 4% and 11% for the Kevlar with rails inside and outside of the field respectively. Phantom images of head‐and‐neck, chest, and pelvis were registered to reference images via bony and gray‐value auto‐registration methods. Accuracy measured within 2 mm for both designs. Conclusion:CBCTimaging characteristics using synthetic and carbon fiber tabletops are comparable. Synthetic fiber tabletops are compatible with electromagnetic localization systems and meet image quality performance criteria.