Search

In this study, we have characterized the efficiency of an Elekta linac in the delivery of gated radiotherapy. We have explored techniques to reduce the beam‐on delay and to improve the delivery efficiency, and have investigated the impact of frequent beam interruptions on the dosimetric accuracy of gated deliveries. A newly available gating interface was installed on an Elekta Synergy. Gating signals were generated using a surface mapping system in conjunction with a respiratory motion phantom. A series of gated deliveries were performed using volumetric modulated arc therapy (VMAT) treatment plans previously generated for lung cancer patients treated with stereotactic body radiotherapy. Baseline values were determined for the delivery times. The machine was then tuned in an effort to minimize beam‐on delays and improve delivery efficiency. After that process was completed, the dosimetric accuracy of the gated deliveries was evaluated by comparing the measured and the planned coronal dose distributions using gamma index analyses. Comparison of the gated and the non‐gated deliveries were also performed. The results demonstrated that, with the optimal machine settings, the average beam‐on delay was reduced to less than 0.22 s. High dosimetric accuracy was demonstrated with gamma index passing rates no lower than 99.0% for all tests (3%/3 mm criteria). Consequently, Elekta linacs can provide a practical solution for gated VMAT treatments with high dosimetric accuracy and only a moderate increase in the overall delivery time. PACS numbers: 87.56.bd, 87.55.de, 87.55.ne

Purpose: Intensity modulated arc therapy (IMAT) is a radiation therapy delivery technique that combines the efficiency of arc based delivery with the dose painting capabilities of intensity modulated radiation therapy (IMRT). A key challenge in developing robust inverse planning solutions for IMAT is the need to account for the connectivity of the beam shapes as the gantry rotates from one beam angle to the next. To overcome this challenge, inverse planning solutions typically impose a leaf motion constraint that defines the maximum distance a multileaf collimator (MLC) leaf can travel between adjacent control points. The leaf motion constraint ensures the deliverability of the optimized plan, but it also impacts the plan quality, the delivery accuracy, and the delivery efficiency. In this work, the authors have studied leaf motion constraints in detail and have developed recommendations for optimizing the balance between plan quality and delivery efficiency. Methods: Two steps were used to generate optimized IMAT treatment plans. The first was the direct machine parameter optimization (DMPO) inverse planning module in the Pinnacle{sup 3} planning system. Then, a home-grown arc sequencer was applied to convert the optimized intensity maps into deliverable IMAT arcs. IMAT leaf motion constraints were imposed using limits ofmore » between 1 and 30 mm/deg. Dose distributions were calculated using the convolution/superposition algorithm in the Pinnacle{sup 3} planning system. The IMAT plan dose calculation accuracy was examined using a finer sampling calculation and the quality assurance verification. All plans were delivered on an Elekta Synergy with an 80-leaf MLC and were verified using an IBA MatriXX 2D ion chamber array inserted in a MultiCube solid water phantom. Results: The use of a more restrictive leaf motion constraint (less than 1-2 mm/deg) results in inferior plan quality. A less restrictive leaf motion constraint (greater than 5 mm/deg) results in improved plan quality but can lead to less accurate dose distribution as evidenced by increasing discrepancies between the planned and the delivered doses. For example, the results from our patient-specific quality assurance measurements demonstrated that the average gamma analysis passing rate decreased from 98% to 80% when the allowable leaf motion increased from 3 to 20 mm/deg. Larger leaf motion constraints also led to longer treatment delivery times (2 to 4 folds) due to the additional MLC leaf motion. Conclusions: Leaf motion constraints significantly impact IMAT plans in terms of plan quality, delivery accuracy, and delivery efficiency with the impact magnified for more complex cases. Our studies indicate that a leaf motion constraint of 2 to 3 mm/deg of gantry rotation can provide an optimal balance between plan quality, delivery accuracy, and efficiency.« less

Purpose: Helical tomotherapy (HT) and volumetric modulated arc therapy (VMAT) are arc-based approaches to IMRT delivery. The objective of this study is to compare VMAT to both HT and fixed field IMRT in terms of plan quality, delivery efficiency, and accuracy. Methods: Eighteen cases including six prostate, six head-and-neck, and six lung cases were selected for this study. IMRT plans were developed using direct machine parameter optimization in the Pinnacle{sup 3} treatment planning system. HT plans were developed using a Hi-Art II planning station. VMAT plans were generated using both the Pinnacle{sup 3} SmartArc IMRT module and a home-grown arc sequencing algorithm. VMAT and HT plans were delivered using Elekta's PreciseBeam VMAT linac control system (Elekta AB, Stockholm, Sweden) and a TomoTherapy Hi-Art II system (TomoTherapy Inc., Madison, WI), respectively. Treatment plan quality assurance (QA) for VMAT was performed using the IBA MatriXX system while an ion chamber and films were used for HT plan QA. Results: The results demonstrate that both VMAT and HT are capable of providing more uniform target doses and improved normal tissue sparing as compared with fixed field IMRT. In terms of delivery efficiency, VMAT plan deliveries on average took 2.2 min for prostate andmore » lung cases and 4.6 min for head-and-neck cases. These values increased to 4.7 and 7.0 min for HT plans. Conclusions: Both VMAT and HT plans can be delivered accurately based on their own QA standards. Overall, VMAT was able to provide approximately a 40% reduction in treatment time while maintaining comparable plan quality to that of HT.« less

Intensity-modulated arc therapy (IMAT) is a rotational IMRT technique. It uses a set of overlapping or nonoverlapping arcs to create a prescribed dose distribution. Despite its numerous advantages, IMAT has not gained widespread clinical applications. This is mainly due to the lack of an effective IMAT leaf-sequencing algorithm that can convert the optimized intensity patterns for all beam directions into IMAT treatment arcs. To address this problem, we have developed an IMAT leaf-sequencing algorithm and software using graph algorithms in computer science. The input to our leaf-sequencing software includes (1) a set of (continuous) intensity patterns optimized by a treatment planning system at a sequence of equally spaced beam angles (typically 10 deg. apart), (2) a maximum leaf motion constraint, and (3) the number of desired arcs, k. The output is a set of treatment arcs that best approximates the set of optimized intensity patterns at all beam angles with guaranteed smooth delivery without violating the maximum leaf motion constraint. The new algorithm consists of the following key steps. First, the optimized intensity patterns are segmented into intensity profiles that are aligned with individual MLC leaf pairs. Then each intensity profile is segmented into k MLC leaf openings using amore » k-link shortest path algorithm. The leaf openings for all beam angles are subsequently connected together to form 1D IMAT arcs under the maximum leaf motion constraint using a shortest path algorithm. Finally, the 1D IMAT arcs are combined to form IMAT treatment arcs of MLC apertures. The performance of the implemented leaf-sequencing software has been tested for four treatment sites (prostate, breast, head and neck, and lung). In all cases, our leaf-sequencing algorithm produces efficient and highly conformal IMAT plans that rival their counterpart, the tomotherapy plans, and significantly improve the IMRT plans. Algorithm execution times ranging from a few seconds to 2 min are observed on a laptop computer equipped with a 2.0 GHz Pentium M processor.« less

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.

Using direct aperture optimization, we have developed an inverse planning approach that is capable of producing efficient intensity modulated radiotherapy (IMRT) treatment plans that can be delivered without a multileaf collimator. This "jaws-only" approach to IMRT uses a series of rectangular field shapes to achieve a high degree of intensity modulation from each beam direction. Direct aperture optimization is used to directly optimize the jaw positions and the relative weights assigned to each aperture. Because the constraints imposed by the jaws are incorporated into the optimization, the need for leaf sequencing is eliminated. Results are shown for five patient cases covering three treatment sites: pancreas, breast, and prostate. For these cases, between 15 and 20 jaws-only apertures were required per beam direction in order to obtain conformal IMRT treatment plans. Each plan was delivered to a phantom, and absolute and relative dose measurements were recorded. The typical treatment time to deliver these plans was 18 min. The jaws-only approach provides an additional IMRT delivery option for clinics without a multileaf collimator.

A new leaf-sequencing approach has been developed that is designed to reduce the number of required beam segments for step-and-shoot intensity modulated radiation therapy (IMRT). This approach to leaf sequencing is called continuous-intensity-map-optimization (CIMO). Using a simulated annealing algorithm, CIMO seeks to minimize differences between the optimized and sequenced intensity maps. Two distinguishing features of the CIMO algorithm are (1) CIMO does not require that each optimized intensity map be clustered into discrete levels and (2) CIMO is not rule-based but rather simultaneously optimizes both the aperture shapes and weights. To test the CIMO algorithm, ten IMRT patient cases were selected (four head-and-neck, two pancreas, two prostate, one brain, and one pelvis). For each case, the optimized intensity maps were extracted from the Pinnacle{sup 3} treatment planning system. The CIMO algorithm was applied, and the optimized aperture shapes and weights were loaded back into Pinnacle. A final dose calculation was performed using Pinnacle's convolution/superposition based dose calculation. On average, the CIMO algorithm provided a 54% reduction in the number of beam segments as compared with Pinnacle's leaf sequencer. The plans sequenced using the CIMO algorithm also provided improved target dose uniformity and a reduced discrepancy between the optimized and sequencedmore » intensity maps. For ten clinical intensity maps, comparisons were performed between the CIMO algorithm and the power-of-two reduction algorithm of Xia and Verhey [Med. Phys. 25(8), 1424-1434 (1998)]. When the constraints of a Varian Millennium multileaf collimator were applied, the CIMO algorithm resulted in a 26% reduction in the number of segments. For an Elekta multileaf collimator, the CIMO algorithm resulted in a 67% reduction in the number of segments. An average leaf sequencing time of less than one minute per beam was observed.« less

We have studied the degree to which beamlet step-size impacts the quality of intensity modulated radiation therapy (IMRT) treatment plans. Treatment planning for IMRT begins with the application of a grid that divides each beam's-eye-view of the target into a number of smaller beamlets (pencil beams) of radiation. The total dose is computed as a weighted sum of the dose delivered by the individual beamlets. The width of each beamlet is set to match the width of the corresponding leaf of the multileaf collimator (MLC). The length of each beamlet (beamlet step-size) is parallel to the direction of leaf travel. The beamlet step-size represents the minimum stepping distance of the leaves of the MLC and is typically predetermined by the treatment planning system. This selection imposes an artificial constraint because the leaves of the MLC and the jaws can both move continuously. Removing the constraint can potentially improve the IMRT plan quality. In this study, the optimized results were achieved using an aperture-based inverse planning technique called direct aperture optimization (DAO). We have tested the relationship between pencil beam step-size and plan quality using the American College of Radiology's IMRT test case. For this case, a series of IMRT treatment plans were produced using beamlet step-sizes of 1, 2, 5, and 10 mm. Continuous improvements were seen with each reduction in beamlet step size. The maximum dose to the planning target volume (PTV) was reduced from 134.7% to 121.5% and the mean dose to the organ at risk (OAR) was reduced from 38.5% to 28.2% as the beamlet step-size was reduced from 10 to 1 mm. The smaller pencil beam sizes also led to steeper dose gradients at the junction between the target and the critical structure with gradients of 6.0, 7.6, 8.7, and 9.1 dose%/mm achieved for beamlet step sizes of 10, 5, 2, and 1 mm, respectively.

Purpose: Surface guided radiation therapy (SGRT) uses stereoscopic video images in combination with patterns projected onto the patient's surface to dynamically capture and reconstruct a 3D surface map. In this work, we used a C-RAD Catalyst HD system (C-RAD) to evaluate intrafraction motion in the delivery of lung SBRT. Methods: The surface acquired from the 4DCT images from our preliminary cohort of eight lung cancer patients treated with SBRT were matched to the surface images acquired prior to each treatment. Additionally, a CBCT image set was acquired. A linear regression model was established between the external and internal motion of tumor during pretreatment and used to predict the CBCT deviation during treatment. The shifts determined from CBCT and the shifts from surface map imaging were compared and assessed using Bland-Altman method. For intrafraction motion, we assessed the percentage of mean errors that fell outside of the threshold of 2 mm, 3 mm, and 5 mm along the translational directions. The required PTV margin was quantified over the course of treatment. The correlation between intrafraction treatment time and mean error of 3D displacement was evaluated using the Pearson coefficient, r Results: A total of 7971 data points were analyzed. Deviations of 2mm, 3mm, and 5mm were observed less than 7%, 2 %, and 0 % of the time along the translational direction. CBCT and Catalyst showed close agreement during patient positioning. Furthermore, the calculated PTV margins were less than our clinical tolerance of 5 mm. Using the Pearson coefficient r,the mean error of 3D displacement showed significant correlation with treatment time (r=0.69, p= 0.000002). Conclusion: SGRT can be used to ensure accurate patient positioning during treatment without an additional delivery of dose to the patient. This study shows that importance of treatment time as a consideration during the treatment planning process.

An inverse treatment planning system for Gamma Knife radiosurgery has been developed using nonlinear programming techniques. The system optimizes the shot sizes, locations, and weights for Gamma Knife treatments. In the patient's prescription, the user can specify both the maximum number of shots of radiation and a minimum isodose line that must surround the entire treatment volume. After satisfying all of the constraints included in the prescription, the system maximizes the conformity of the dose distribution. This automated approach to treatment planning has been applied retrospectively to a series of patient cases, and each optimized plan has been compared to the corresponding manual plan produced by an experienced user. The results demonstrate that this tool can often improve the tumor dose homogeneity while using fewer shots than were included in the original plan. Therefore, inverse treatment planning should improve both the quality and the efficiency of Gamma Knife treatments.

Essential for the calculation of photon fluence distributions for intensity modulated radiotherapy (IMRT) is the use of a suitable objective function. The objective function should reflect the clinical aims of tumor control and low side effect probability. Individual radiobiological parameters for patient organs are not yet available with sufficient accuracy. Some of the major drawbacks of some current optimization methods include an inability to converge to a solution for arbitrary input parameters, and/or a need for intensive user input in order to guide the optimization. In this work, a constrained optimization method was implemented and tested. It is closely related to the demanded clinical aims, avoiding the drawbacks mentioned above. In a prototype treatment planning system for IMRT, tumor control was guaranteed by setting a lower boundary for target dose. The aim of low complication is fulfilled by minimizing the dose to organs at risk. If only one type of tissue is involved, there is no absolute need for radiobiological parameters. For different organs, threshold dose, relative seriality of the organs or an upper dose limit could be set. All parameters, however, were optional, and could be omitted. Dose-volume constraints were not used, avoiding the possibility of local minima in the objective function. The approach was benchmarked through the simulation of both a head and neck and a lung case. A cylinder phantom with precalculated dose distributions of individual pencil beams was used. The dose to regions at risk could be significantly reduced using at least seven ports of beam incidence. Increasing the number of ports beyond seven produced only minor further gain. The relative seriality of organs was modeled through the use of an added exponent to the dose. This approach however increased calculation time significantly. The alternative of setting an upper limit is much faster and allows direct control of the maximum dose. Constrained optimization guarantees high tumor control probability, it is computationally more efficient than adding penalty terms to the objective function, and the input parameters are dose limits known in clinical practice.

Convolution/superposition software has been used to produce a library of photon pencil beam dose matrices. This library of pencil beams is designed to serve as a tool for both education and investigation in the field of radiotherapy optimization. The elegance of this pencil beam model stems from its cylindrical symmetry. Because of the symmetry, the dose distribution for a pencil beam from any arbitrary angle can be determined through a simple rotation of a pre-computed dose matrix. Rapid dose calculations can thus be performed while maintaining the accuracy of a convolution/superposition based dose computation. The pencil beam data sets have been made publicly available. It is hoped that the data sets will facilitate a comparison of a variety of optimization and delivery approaches. This paper will present a number of studies designed to demonstrate the usefulness of the pencil beam data sets. These studies include an examination of the extent to which a treatment plan can be improved through either an increase in the number of beam angles and/or a decrease in the collimator size. A few insights into the significance of heterogeneity corrections for treatment planning for intensity modulated radiotherapy will also be presented.

Purpose: In this study, we evaluated the performance of an Elekta linac in the delivery of gated radiotherapy. We examined whether the use of either a short gating window or a long beam hold impacts the accuracy of the delivery Methods: The performance of an Elekta linac in the delivery of gated radiotherapy was assessed using a 20cmX 20cm open field with the radiation delivered using a range of beam-on and beam-off time periods. Two SBRT plans were used to examine the accuracy of gated beam delivery for clinical treatment plans. For the SBRT cases, tests were performed for both free-breathing based gating and for gated delivery with a simulated breath-hold. A MatriXX 2D ion chamber array was used for data collection, and the gating accuracy was evaluated using gamma score. Results: For the 20cmX20cm open field, the gated beam delivery agreed closely with the non-gated delivery results. Discrepancies in the agreement, however, began to appear with a 5-to-1 ratio of the beam-off to beam-on. For these tight gating windows, each beam-on segment delivered a small number of monitor units. This finding was confirmed with dose distribution analysis from the delivery of the two VMAT plans where the gamma score(±1%,2%/1mm) showed passing rates in the range of 95% to 100% for gating windows of 25%, 38%, 50%, 63%, 75%, and 83%. Using a simulated sinusoidal breathing signal with a 4 second period, the gamma score of freebreathing gating and breath-hold gating deliveries were measured in the range of 95.7% to 100%. Conclusion: The results demonstrate that Elekta linacs can be used to accurately deliver respiratory gated treatments for both free-breathing and breath-hold patients. The accuracy of beams delivered in a gated delivery mode at low small MU proved higher than similar deliveries performed in a non-gated (manually interrupted) fashion.

Purpose: Clinical implementation of gated lung SBRT requires tools to verify the accuracy of the target positioning on a daily basis. This is a particular challenge on Elekta linacs where the XVI imaging system does not interface directly to any commercial gating solution. In this study, we used the Elekta's intra-fractional imaging functionality to perform the pretreatment CBCT verifications and evaluated both the image quality and gating accuracy. Methods: To use intrafraction imaging tools for pretreatment verifications, we planned a 360-degree arc with 1mmx5mm MLC opening. This beam was designed to drive the gantry during the gated CBCT data collection. A Catphan phantom was used to evaluate the image quality for the intra-fractional CBCT. A CIRS lung phantom with a 3cm sphereinsert and a moving chest plate were programmed with a simulated breathhold breathing pattern was used to check the gating accuracy. A C-Rad CatalystHD surface mapping system was used to provide the gating signal. Results: The total delivery time of the arc was 90 seconds. The uniformity and low contrast resolution for the intra-fractional CBCT was 1.5% and 3.6%, respectively. The values for the regular CBCT were 1.7% and 2.5%, respectively. The spatial resolution was 7 line-pairs/cm and the 3D spatial integrity was less than 1mm for the intra-fractional CBCT. The gated CBCT clearly demonstrated the accuracy of the gating image acquisition. Conclusion: The intra-fraction CBCT capabilities on an Elekta linac can be used to acquire pre-treatment gated images to verify the accuracy patient positioning. This imaging capability should provide for accurate patient alignments for the delivery of lung SBRT. This research was partially supported by Elekta

The U.S. Coast Guard has been developing a 47′ high-speed, heavy weather motor lifeboat, the 47 MLB, to replace the aging 44′ motor lifeboats. The 47 MLB is designed to be self-righting and operate in up to 20′ breaking waves. The prototype 47 MLB has been in service for four years, and five pre-production boats have been built by Textron Marine Systems in New Orleans, La. If the pre-production boats prove to be successful, plans call for 100 boats to be built in the production run. One of the primary goals in developing a new motor lifeboat was to make major improvements with regard to crew safety, comfort, and efficiency. These human engineering requirements have affected almost every stage of the design, and have been the subject of considerable engineering effort. Throughout the development, a logical human engineering program has been followed. This program will continue through the pre-production evaluation and into the production craft development.

Purpose: The current SBRT treatment techniques include free breathing (FB) SBRT and gated FB SBRT. Gated FB SBRT has smaller target and less lung toxicity with longer treatment time. The recent development of direct connectivity between the ABC and linac allowing for automated beam gating. In this study, we have examined the feasibility of using ABC system to gate the lung SBRT treatment. Methods: A CIRS lung phantom with a 3cm sphere-insert and a moving chest plate was used in this study. Sinusoidal motion was used for the FB pattern. An ABC signal was imported to simulate breath holds. 4D-CT was taken in FB mode and average-intensity-projection (AIP) was used to create FB and 50% gated FB SBRT planning CT. A manually gated 3D CT scan was acquired for ABC gated SBRT planning.An SBRT plan was created for each treatment option. A surface-mapping system was used for 50% gating and ABC system was used for ABC gating. A manually gated CBCT scan was also performed to verify setup. Results: Among three options, the ABC gated plan has the smallest PTV of 35.94cc, which is 35% smaller comparing to that of the FB plan. Consequently, the V20 of the left lungmore » reduced by 15% and 23% comparing to the 50% gated FB and FB plans, respectively. The FB plan took 4.7 minutes to deliver, while the 50% gated FB plan took 18.5 minutes. The ABC gated plan delivery took only 10.6 minutes. A stationary target with 3cm diameter was also obtained from the manually gated CBCT scan. Conclusion: A strategy for ABC gated lung SBRT was developed. ABC gating can significantly reduce the lung toxicity while maintaining the target coverage. Comparing to the 50% gated FB SBRT, ABC gated treatment can also provide less lung toxicity as well as improved delivery efficiency. This research is funded by Elekta.« less

Purpose: In this work, we have tested the performance of new respiratory gating solutions for Elekta linacs. These solutions include the Response gating and the C-RAD Catalyst surface mapping system.Verification measurements have been performed for a series of clinical cases. We also examined the beam on latency of the system and its impact on delivery efficiency. Methods: To verify the benefits of tighter gating windows, a Quasar Respiratory Motion Platform was used. Its vertical-motion plate acted as a respiration surrogate and was tracked by the Catalyst system to generate gating signals. A MatriXX ion-chamber array was mounted on its longitudinal-moving platform. Clinical plans are delivered to a stationary and moving Matrix array at 100%, 50% and 30% gating windows and gamma scores were calculated comparing moving delivery results to the stationary result. It is important to note that as one moves to tighter gating windows, the delivery efficiency will be impacted by the linac's beam-on latency. Using a specialized software package, we generated beam-on signals of lengths of 1000ms, 600ms, 450ms, 400ms, 350ms and 300ms. As the gating windows get tighter, one can expect to reach a point where the dose rate will fall to nearly zero, indicating that themore » gating window is close to beam-on latency. A clinically useful gating window needs to be significantly longer than the latency for the linac. Results: As expected, the use of tighter gating windows improved delivery accuracy. However, a lower limit of the gating window, largely defined by linac beam-on latency, exists at around 300ms. Conclusion: The Response gating kit, combined with the C-RAD Catalyst, provides an effective solution for respiratorygated treatment delivery. Careful patient selection, gating window design, even visual/audio coaching may be necessary to ensure both delivery quality and efficiency. This research project is funded by Elekta.« less

Purpose: Due to the dynamic nature of the delivery, Volumetric Modulated Arc Therapy (VMAT) demands additional machine specific QA measurements as compared with fixed‐field IMRT. In this study, we will present a comprehensive QA method for VMAT that simultaneously verifies the accuracy of the gantry angle, MLC leaf positions, and dose rate during the delivery of a single VMAT arc. Materials and Methods: A QA plan with specially designed patterns of leaf motion, gantry rotation, and dose rate variation was created and calculated in Pinnacle3. Next, the plan was delivered on an Elekta Synergy equipped with a conventional 80‐leaf MLC. The resulting dose distribution was measured using an IBA MatriXX™ 2D ion chamber array inserted in a MULTICube™ Phantom with a sampling time of 100 ms. A home‐grown software was applied to extract the gantry angle, leaf positions, and dose rate as a function of time. The accuracy of all three parameters was then verified through comparisons with the corresponding theoretical values. Results: A total of 833 frames of data were collected which agrees with the theoretical delivery time of 83.33s. The actual gantry angle was determined by the average leaf projection width on the MatriXX™ detector plane. Our QA results suggest an excellent agreement between the measured and planned gantry angle with a maximum deviation of 2.6° and a standard deviation of 1.0°. The mean deviation of dose rate was 2.7 MU/Minute or 3.2% in relative mode. The mean deviation of leaf positions was 0.2mm with a standard deviation of 1.1mm and a maximum deviation of 3.1mm. Conclusions: The proposed method can be effectively delivered and implemented as a machine specific QA tool for VMAT technique. The three key dynamic variables during VMAT delivery can be checked simultaneously through a single measurement. Research sponsored by Elekta Corporation

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. Elekta's 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: Elekta's 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.

Purpose: To optimize the beam‐on latency of an Elekta linac in delivering gated VMAT. Methods: Step‐wise optimization was performed using a pre‐clinical gating interface developed by Elekta. Phantom studies were performed where surface motion was simulated using a moving chest plate. Two gating windows 77% and 66% around the end of exhalation were used for the gated VMAT deliveries. The treatment plans from three lung patients previously treated with un‐gated 6 MV SBRT were used for this evaluation. Baseline of the average beam‐on latency of the gated deliveries was first established. Then the average beam‐on latencies between a binned dose rate and a continuously variable dose rate (CVDR) deliveries were compared. With the CVDR delivery, further comparison between two different linac gun hold‐on times 1.35 s and 6.50 s was performed. The dosimetric accuracy of the gated deliveries was evaluated by comparing the measured with planned coronal dose distributions using gamma‐index analyses (3 mm/3% passing rate criteria). Results: The average beam‐on latencies of the gated deliveries with the CVDR were greatly reduced as compared with those of the binned dose rate for gating windows higher than 66%. With the CVDR deliveries, longer gun hold‐on times (6.50 s as compared to 1.35 s) made a dramatic improvement on the beam‐on latency for gating windows equal to and lower than 66%. There were insignificant dosimetric differences between all the gated deliveries and the un‐gated deliveries. Conclusion: For gated VMAT deliveries using an Elekta linac with the CVDR and the gun‐hold time of 6.50 s, the average beam‐on latency was reduced to a level (< 0.3 s) that has minimal impact on clinical results. High dosimetric accuracy was demonstrated for as many as 214 beam interruptions during a single 360‐degree‐arc delivery with gamma‐index passing rates were no lower than 99.0% for all tests. The research is partly supported by a grant from Elekta AB.

Purpose: To evaluate the machine‐delivery capabilities and dosimetric accuracy of respiratory‐gated VMAT delivered with an Elekta digital accelerator. Method and Materials: Three lung patients previously treated with SBRT using un‐gated 6 MV VMAT were used for this evaluation. The treatment plans were created in Monaco treatment planning system (ver. 3.0). For each case, one un‐gated and two gated deliveries were delivered to a static ion‐chamber‐array phantom. Surface motion was used as a surrogate for breathing motion and was simulated using a moving chest plate driven by a motor with a constant amplitude range of +/− 3 mm and a period of 4 seconds using a cos6ωt waveform. Two gating windows (GWs), 77% and 66% around the end of exhalation were used for the gated deliveries. An ion chamber array was used and comparisons were made between the measured and the planned coronal dose distributions. The dosimetric accuracy of gated VMAT plans was determined using gamma‐index analyses (3 mm/3% passing rate criteria). Results: There were insignificant differences between the two gated deliveries and the un‐gated delivery. For the planar dose comparisons between the measured and the planned, the gamma passing rates were all higher than 99.0%. For the two gated deliveries, the true duty cycles were calculated as 57% and 39%, as against the corresponding GWs 77% and 66%. This was mainly due to the beam‐on latencies, which were estimated as 0.25 second and 0.40 second for GWs 77% and 66%, respectively. Conclusions: Respiratory‐gated VMAT was demonstrated for the first time using an Elekta digital accelerator with a pre‐clinical gating interface provided by Elekta and a novel surface imaging system as a respiratory input. High dosimetric accuracy was demonstrated in three SBRTlung cases for as many as 213 beam interruptions during a single 360°‐arc delivery. This work is sponsored by a research grant from Elekta AB.

Purpose: VMAT is an arc‐based IMRT technique delivered using conventional linear accelerator. The current application of VMAT has been focused on axial coplanar delivery. However, for certain treatment sites, especially some intracranial targets, non‐coplanar arcs may bring extra dosimetric benefits. This work was carried out to study the non‐coplanar VMAT delivery in terms of both plan quality and delivery efficiency. Materials and Methods: Five GBM cases were selected for this study. For each case, VMAT plans using one axial coplanar arc and multiple non‐coplanar arcs were generated using either Pinnacle3 SmartArc® or a home‐grown arc sequencer. DVH comparisons were made between the two sets of plans. VMAT deliveries using Elekta's PreciseBeam® VMAT control system were performed for each case to compare the delivery efficiency. Results: Both coplanar and non‐coplanar VMAT plans have very similar target dose coverage. For all five cases, the average V95, defined as the target volume receiving at least 95% of the prescribed dose, remained unchanged at 99.8%. As to the critical structure sparing, however, the non‐coplanar plans show significant improvement. For example, on average, the maximum and mean doses to brainstem were reduced from 3277 and 1016 cGy to 2524 and 668 cGy in the non‐coplanar plans. These correspond to 23% and 34% reduction. The average maximum dose to chiasm was also reduced from 876 cGy to 346 cGy in the non‐coplanar plan, a 61% reduction. The average treatment delivery time increased from 1.9 to 4.0 minutes for the non‐coplanar plans. Conclusions: Comparing to the axial coplanar delivery, the VMAT plans using non‐coplanar arcs can significantly improve the critical structure sparing while maintaining the target dose coverage and uniformity. However, these dosimetric benefits come at the cost of a prolonged treatment delivery time. This work was sponsored in part through a grant from Elekta.

Purpose: To simplify the dose calculation process, dynamic delivery techniques such as VMAT are typically approximated using static fields for the dose calculation. A number of techniques have been employed for VMAT dose calculations and delivery. In this work, we have evaluated these different techniques in terms of the accuracy and the agreement between planned and delivered doses. Materials and Methods: Dose calculations for VMAT include static field and Monte‐Carlo simulations. Different methods of setting the control points (CPs) during DICOM export, including MU redistribution, CP insertion, and CP reposition, were tested for dose calculations using static fields. The exported VMAT plans were delivered using the Elekta PreciseBeam® VMAT linaccontrol system. The delivery results were measured using an IBA MatriXX system and were compared with the calculation data to test the delivery accuracy. Results: Using static fields for dose calculation and MU redistribution for VMAT delivery can lead to significant discrepancy between the calculation and delivery. The passing rate of the gamma analysis with 3%/3mm criteria can be as low as 67% when tested using an extreme case. The CP insertion and CP reposition methods can improve the agreement between the planned and measured doses. However, MLC leaf motion between adjacent CPs needs to be limited when static fields are used for VMAT dose calculation. Using Monte‐Carlo dose calculation to simulate the dynamic delivery of VMAT provides excellent agreement between the dose calculations and deliveries regardless of the amount of MLC leaf motion used in the plan with an average gamma analysis passing rate of 98.1% for the 5 selected VMAT plans. Conclusions: Using static fields to simulate VMAT works well as long as CP setting for delivery is handled properly. Monte‐Carlo simulation has a clear advantage in simulating dynamic VMAT delivery. Research partially supported by Elekta

Purpose: Two dimensional diode arrays have been widely adopted in the verification of fixed field IMRT deliveries. With increasing adoption of volume modulated arc therapy (VMAT), an immersed question is whether these diode arrays can be used for quality assurance for rotational therapy. The major concerns are the angular dependence of diode arrays and the detector to detector variance within the array. In this work, we studied the angular dependence of each detector and investigated the tools for correcting this dependency. Material and Methods: A MapCheck‐I (Sun Nuclear Inc.) was used as the diode array for this study. The two dimensional (2D) array was inserted in a multi‐cube solid water phantom and was irradiated on an Elekta Synergy Linac using open square fields from different gantry angles. The measured 2D intensity map was then compared to theoretical map generated in the treatment planning system (TPS), Pinnacle3. A 2D correction ratio map for each gantry angle was produced. Correction was implemented to static beams and VMAT plans of prostate cases. Results: The correction maps for all gantry angles were obtained and the angular response of each individual detector was determined. The correction map was found be dependent on the field size that was used to irradiate the diode array. The correction ratio, as defined as measure/planned, is higher for larger field size. The correction to prostate VMAT cases has no significant improvement. The verification of individual segment reveals that the irregular MLC shapes has impact on the diode response. Conclusion: Cautious should be taken to use 2D diode array for rotational arc therapy quality assurance. More studies need to be done to correct the complex dependence of the correction ratio on field size and aperture shape. Research supported by Elekta.

Purpose: Volume Metric Arc Therapy (VMAT) has received considerable attention due to its ability to provide improved plan quality and delivery efficiency relative to fixed field IMRT. However, due to the limitation of leaf motion and number of control points, the VMAT plan is not always better than fixed field IMRT. This work investigated the cases that IMRT is superior to VMAT on plan quality. Material and Methods: The fixed field IMRT and VMAT plans are produced using CMS XiO and Pinnacle3 SmartArc, respectively. The RT doses are exported to a third party software where the dose volume histogram (DVH) and isodose lines are compared. Results: Fifty clinical cases are investigated. The treatment sites include head‐and‐neck, partial‐brain, lung, pancreas, prostate and pelvis. Three out of total 50 cases show that the IMRT plan quality is better than that of VMAT. The first case is a partial brain. Both IMRT and VMAT plans have similar target coverage. However, the IMRT plan has lower mean doses to the whole brain, which can be attributed to fewer gantry angles used in IMRT. The second case is a mesothelioma, in which large and sharp shrink between target contours are found. The IMRT plan is better on the target conformality. It is more difficult for the VMAT plan to create sharp shrink of isodose line to follow the target contour. The third case is a pelvis case in which again sharp corner exists in target contours. IMRT is superior on the target conformality. Conclusion: Though for most cases, the VMAT plans are better than fixed field IMRT, there are some situations where IMRT can provide better comformality and lower dose to the volume not included in the organ in risks list. Research supported by Elekta.

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.

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 patient's 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.

Purpose: During the delivery of volumetric modulated arc therapy (VMAT), random errors exist in both the MLC leaf positions and the gantry angle. In this work, we investigated the impact of such random errors on VMAT plan quality and delivery accuracy. The impact of system calibration errors was also examined. For comparison purposes, we performed a similar study on step‐and‐shoot IMRT plans. Material and Method: VMAT plans for three treatment sites (prostate, pancreas and head‐&‐neck) were created using a home‐grown arc sequencer. Next, random and systematic leaf position errors were introduced into these plans with the random errors sampled from Gaussian distributions of varying widths ranging from 1 to 3mm. Two types of systematic errors, including MLC bank shifts in the same direction (Type I: leaf gap unchanged) and MLC bank shifts in opposite directions (Type II: leaf gap increase/decrease). The plan quality variations were compared in the Pinnacle3 planning system. Plans with systematic errors were verified using the MatriXX ion chamber array with gamma evaluation criteria of 3%/3mm. Results: The plan degradation observed for VMAT plans was slightly less as compared to that for fixed‐field IMRT plans when random errors up to 3mm to the leaf positions were introduced. With type I systematic errors of 3mm on leaf positions, the average standard deviation of PTV dose increased by 10.2%. This value increased to 18.4% for the corresponding fixed‐field IMRT plans. A larger impact on the IMRT plans was also observed when type II systematic errors were introduced. The above results were confirmed by plan verification measurements with higher gamma passing rates for VMAT plans when systematic errors were applied. Conclusion: The VMAT delivery technique has better tolerance to random and systematic errors in gantry angle and MLC leaf position errors as compared with step‐and‐shoot IMRT. Research supported by Elekta.

Purpose: VMAT has received considerable attention due to its ability to provide improved plan quality and delivery efficiency relative to fixed field IMRT. A key component of VMAT treatment planning is the need to restrict the leaf motion from one control point to the next. In this work, we studied the dependency between restriction on leaf motion and the plan quality, delivery accuracy and efficiency for VMAT. Material and Methods: The leaf motion constraint is the maximum distance one MLC leaf can move between adjacent control points within an arc with a unit of mm per degree of gantry rotation. Three cases with different sites were planed for VMAT treatment using a variety of leaf motion constraints. Final dose calculation for each plan was done with 2‐degree control point spacing. All VMAT plans were delivered on an Elekta Synergy linac with PreciseBeam VMAT® control system, and were verified using IBA MatriXX system. A 2%/2mm passing criteria was used in Gamma Evaluation. Results: The as‐optimized VMAT DVHs looks very similar for different leaf motion constraints. The final dose, however, reveals degradation in plan quality with increasing leaf motion. Such degradation is most pronounced for the Head‐Neck case. Best plan quality was obtained with 2∼3mm/degree leaf motion for Head‐Neck and prostate cases, and 3∼5mm/degree for pancreas case. The delivery time increases monotonically with the increase of leaf motion and can be 2∼3 times different. In terms of delivery accuracy, plans with more leaf motion tend to have lower gamma passing rate (93% for 2mm/deg and 71% for 20mm/deg for the Head‐Neck case). We recommend a 3mm leaf‐motion constrain for creating VMAT plans. Conclusion: The leaf motion constraint in VMAT planning is a critical parameter and has to be considered to ensure plan quality, deliver accuracy and efficiency. Research supported by Elekta.

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 Elekta's 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.

Purpose: The Calypso® 4D Localization System uses electromagnetic fields to localize and track Beacon® transponder implants. In this study, we used the Calypso® System to perform real‐time beam gating based on transponders mounted to a dynamic phantom. The system's latency was studied in conjunction with a Varian linac to determine feasibility of enabling gated radiation therapydelivery for respiratory applications. Method and Materials: An acrylic transponder holder with three embedded Beacon® transponders was placed 6 cm from the center of a rotating disk on a dynamic phantom. Film measurements were made with a 2 cm by 2 cm field delivered in a static, dynamic‐gated, and dynamic‐non‐gated fashion. A linear gating window with a 2 mm width was used for the film demonstration. Latency measurements compared an in‐volume / out‐of‐volume signal obtained directly from the dynamic phantom to the target‐current signal from the linac. The target signal represents the measured current in the MV electron beam, and is directly correlated to the presence (or absence) of the treatment beam. The investigational prototype gating system does not used predictive modeling; the linac beam is enabled when the transponder is within the gating window based solely on the last position estimate of the transponder. Results: Film measurements demonstrated that signals from a prototype electromagnetic gating system can be used to effectively trigger the beam on/off state of a linac. The combined, motion‐triggered latency of the localization system and linac was 65 msec to disable the beam and 75 msec to enable the beam. Conclusion: Use of wireless electromagnetic implanted transponders has the potential to enable real‐time linac beam gating with the latency and update rates required for respiratory applications without the use of predictive algorithms.

Purpose: The Axxent® Electronic Brachytherapy system (Xoft Inc.) employs a miniaturized high‐dose‐rate x‐ray source located within an x‐ray catheter. The 50 kV disposable x‐ray source is used in conjunction with a balloon applicator to deliver partial breast irradiation to breast cancer patients following lumpectomy. We report on the acceptance, commissioning, and initial clinical experience with this system. Method and Materials: In addition to the manufacturer's prescribed acceptance tests, a study was performed to address concerns about the potential electrical leakage from the encapsulated tube to the surrounding patient tissues. A system was designed that simulates human body equivalent impedance to measure the electrical leakage. Commissioning was performed in the Plato treatment planning system (Nucletron) following the recommendations of TG‐43. A study has been initiated comparing the dosimetric results for the Axxent® System (x‐ray tube) with the MammoSite® Radiation Therapy System (Ir‐192 HDR source). The study will compare target coverage and normal tissue sparing and determine the required skin bridge for each of these technologies. Our first clinical cases are expected in April 2007 and will involve ten fractions of 3.4Gy delivered twice daily over five days. A total dose of 34Gy will be prescribed 1cm from the surface of the balloon applicator. Results: Acceptance testing and commissioning of the electronic brachytherapy system has been completed. Leakage measurements were performed using a meter consisting of a commercial line powered digital voltmeter, modified with the resistor‐capacitor network suggested by the IEC60601‐1 electrical safety standard. The results showed no measurable leakage currents above the x‐ray ambient noise levels. Initial results in a retrospective comparison demonstrated that it may be possible to improve skin sparing with Xoft through optimized dwell times at multiple dwell positions. Conclusion: An electronic brachytherapy system has been implemented in our clinic. Initial clinical results will be reported.

Purpose: To develop an effective and efficient leaf sequencing algorithm for intensity‐modulated arc therapy (IMAT). Methods: The input to our sequencing algorithm includes: (a) A set of (continuous) intensity patterns optimized by a treatment planning system for a sequence of equally spaced beam angles (10 degrees apart); (b) the IMAT maximum leaf motion constraint; (c) the number of arcs, k, that the user specifies based on the complexity of the problem. The output is a set of k treatment arcs that best approximates the set of input intensity patterns at all beam angles. The MLC shapes for each output arc are interconnected to guarantee a smooth delivery without violating the IMAT maximum leaf motion constraint. Our sequencing algorithm consists of two key steps. First, the intensity profiles aligned with each MLC leaf pair at all beam angles are converted into k MLC leaf openings using a k‐link shortest path algorithm, where k is the specified number of arcs for the delivery. The delivered photon flux using these leaf openings best approximates the desired intensity distribution. Second, the leaf openings are connected into k IMAT treatment arcs under the maximum leaf motion constraint using the minimum‐cost matching and shortest path algorithms. Results: The performance of our leaf sequencingsoftware has been tested for four treatment sites (prostate, breast, head‐and‐neck, and lung). In all cases, our leaf sequencing algorithm provides efficient and highly conformal IMAT plans that rival the counterpart tomotherapy plans and significantly improve the IMRT plans. Execution times of our software that range from a few seconds to 2 minutes are observed on a laptop computer equipped with a Pentium M Processor of 2.0 GHz. Conclusion: This research provided strong evidence that IMAT equipped with an effective leaf sequencing algorithm can provide a feasible and high quality implementation of IMRT.

Purpose: Direct Aperture Optimization (DAO) is a promising new IMRT technique whereby the MLC leaves and relative weights of the apertures are simultaneously optimized. Due to the stochastic nature of the simulated annealingoptimization algorithm, MLC leaves can have positions that are significantly different from their neighboring leaves. Opposed adjacent leaves with these positions can lead to a “ripple” in the dose distribution due to tongue‐and‐groove effect (TGE). In this work, we attempt to quantify the magnitude of the effect and commission the Pinnacle planning system to account for it. Method and Materials: To quantify TGE, we generated a two‐segment beam: one segment with every other leaf on one side extending well past the central axis and the other with every other opposed adjacent leaf extending well past the central axis. Jaws set to 20×20 bound both segments and both had equal monitor units. This beam was delivered to a flat phantom with a film placed at 10cm depth. Newer versions of Pinnacle (7.4 or greater) take into account TGE. This is modeled in the physics tool with a tongue‐and‐groove step parameter. We systematically changed this parameter and compared the resulting dose distributions to the film measurement. We imported DAO IMRT plans into the Pinnacle planning system and compared the dose distributions with and without TGE. These were also compared with verification measurements. Results: The film displayed a 23% variation from minimum to maximum dose. The Pinnacle step parameter of 0.3cm reproduced the variation. If TGE is not accounted for in the planning, an underestimation of the absolute dose is observed for DAO IMRT plans. The magnitude of the underestimation is dependent upon the aperture shapes. Conclusions: TGE affects the dose distribution for DAO plans. Limiting the distance adjacent leaves may travel can reduce the effect of TGE.

Purpose:IMRT can play an important role in the irradiation of head and neck tumors traditionally treated by lateral fields matched with an anterior supraclivicular field. However, due to the complex PTV geometry, these IMRT plans result in large numbers of segments leading to inefficient deliveries. We have developed an alternative IMRT planning technique utilizing Direct Aperture Optimization (DAO) to streamline the planning process and provide significant efficiency gains. Method and Materials: The process begins with the placement of traditional 3D conformal fields (laterals and anerior superclavicular). Next, the dose is calculated with this beam arrangement. The 90% isodose line is converted into a PTV with surrounding critical structures (e.g. spinal cord, parotid glands, posterior medial neck region) excluded from the PTV definition. The resulting PTV serves as the target for IMRT planning. For our planning technique, we have used the DAO algorithm in the Prowess Panther planning system. DAO plans generally result in significantly fewer segments as compared with those generated by traditional IMRT planning techniques. This is of critical importance since traditional IMRT plans for these cases have excessively long treatment times. Using DAO allows practical treatment times without sacrificing plan quality. Results: Fifteen patients were planned and treated with this technique. Seven equispaced beams were used in each. The objectives were PTV dose conformity and low dose to any avoidance regions. The spinal cord limit was 40Gy. For the DAO plans, treatment times ranged from 9 to 17 minutes on an Elekta SL20 acclerator. For corresponding plans produced using Pinnacle3, treatment times ranged from 30 to 45 minutes. Conclusions: An IMRT treatment technique for head and neck cancer has been devised. This technique removes field matching and allows the initial 50Gy to be delivered with a single plan. Using DAO provides significant reductions in treatment times.

Purpose: For treatment planning, clinicians typically compensate for respiratory motion by applying treatment margins that ensure the clinical target volume (CTV) always remains in the treatment field. We have investigated a more sophisticated approach where the pattern of organ motion is directly incorporated into both the dose calculation process and the optimization of IMRT treatment plans, and we have evaluated the degree to which plan quality can be improved by accounting for intra‐fraction organ motion during IMRT optimization. Method and Materials: In this study, we employed a convolution/superposition based dose calculation that incorporates organ motion. For each photon history, the isocenter is randomly sampled from a probability distribution defined by the respiration‐induced anatomical displacement. The resulting blurred pencil beam dose distributions have been incorporated into our IMRT plan optimizations. Comparisons were made between plans optimized using a motion‐based dose calculation and those optimized using a static dose calculation. Tests were performed for both a concave target in a phantom and a lungcancer patient. Results: For the phantom case, the mean dose to the adjacent organ‐at‐risk was reduced from 67.3% to 48.4% of the prescribed dose when motion was included in the optimization. For the lung patient, the volume of the right lung receiving greater than 80% of the prescribed dose was reduced from 14.1% to 7.2% when organ motion was included in the optimization. Conclusion: The motion‐based optimization accounts for the fact that the CTV will not be at all locations with equal probability and is thus more sophisticated than the application of patient specific margins. Consequently, significant additional sparing of critical structures can be achieved by incorporating intra‐fraction organ motion into IMRT optimization. Additionally, the agreement between the predicted and delivered doses is improved because the impact of organ motion has been accounted for in the dose calculation.

Purpose: We have implemented clinically an automated planning tool for 3DCRT that simultaneously optimizes the beam angles, beam weights, wedge angles, and wedge orientations. Method and Materials: An automated planning tool for 3DCRT has been developed that interfaces to the Pinnacle 3 treatment planning system through its hotscripts utilities. Pinnacle3 is used for dose calculation, plan evaluation, and RT export. A commercial optimizationsoftware package (GAMS) is used for plan optimization. For each planning problem, the optimized beam angles and their weights were chosen from either 36 or 72 candidate beams along with their corresponding wedged fields. Because dose contributions from all candidate beams are needed for the optimization, the amount of data is very large. We developed a three‐phases sampling technique to effectively handle the large data set and reduce the optimization time. Results: The tool has been applied to several cases including pancreas, head‐and‐neck, and lung patients. All planning parameters for a 3DCRT plan can be optimized within 20 minutes, and the optimized plans are comparable to those of experience dosimetrists. By using a three‐phase approach, the optimization time can be reduced significantly. During initial angle selection phase, optimizations were performed on randomly selected subsets of the dose calculation points representing about 1% of the points. During the second phase, the gantry and wedge orientation can be obtained from the previous phase with about 10% of original data, and the weight of the selected gantry and wedge fields can be optimized with more data points in the final phase. Conclusion: We have developed an automated planning tool for 3DCRT. The tool has been evaluated using several treatment sites. Optimized plans can be obtained within 20 minutes and are comparable to those of experience dosimetrists.

Purpose: Inverse planning for breast IMRT can be accomplished with Direct Aperture Optimization (DAO) where the user defines the traditional tangential fields within which the shapes and weights of multiple segments, including an open segment as ‘flash’, are optimized. The purpose of this study is to evaluate the technique and examine the effects of relative weights of the open segments to other segments on the impact of breathing motion. Method and Materials:IMRT plans were generated using DAO inverse planning with manually fixed relative weights of the open segments. To simulate breathing motion, the same set of optimized segments was reapplied with the isocenter shifted up and down by up to 1cm and the resulting dose distributions were added. The quality of both the static plans and the composite plans were compared among different open segment weights. Results: Five cases with varying breast sizes were examined. For the static plans, we found that uniform dose distributions could be generated with relative weights of the open segments in the range of 45–85%. Unacceptable hot spots were produced when the weights of the open segments were larger than 85%. For composite plans simulating breathing motion, the most uniform dose distribution happens when the open segment weights were in the range of 75–85%, below which the dose uniformity degrades. We observed that in the composite plans, for each 1% decrease in the open segment weighting, the volume covered by the 98% dose decreased by approximately 0.06%, independent of breast size. Conclusion: DAO provides a robust and efficient technique for breast IMRT planning. The proper open segment weight was found to be between 75% and 85%, and is not significantly dependent of breast size and laterality. Within this range, high‐quality IMRT plans were produced for each case.

Purpose: To demonstrate the clinical feasibility of delivering IMRT treatment plans using only independent collimators.Method and Materials: The Direct Aperture Optimization (DAO) technique is used to optimize the jaw positions and the relative weights assigned to each aperture. Since all of the delivery constraints imposed by the jaws are incorporated into the optimization, the need for leaf sequencing is eliminated. This allows for “jaws‐only” IMRT plans with a significantly reduced number of segments as compared with “jaws‐only” plans produced with the traditional two‐step IMRT approach. We applied the DAO “jaws‐only” technique to three clinical cases: an abdomen, a prostate, and a head and neck. For each case, “jaws‐only” DAO (JODAO) plans were produced with 5, 10, 15, 20, and 25 apertures. For comparison, a DAO plan was created that utilized an MLC (MLCDAO). The resulting JODAO and MLCDAO plans were delivered to a phantom using an Elekta Precise linear accelerator.Results: The results demonstrate that between 15 and 25 “jaws‐only” apertures are required per beam direction to obtain conformal IMRT treatment plans that are comparable to the MLCDAO plans. The delivery times for the JODAO plans were between 15 and 20 minutes. This compares to the delivery times of 7 to 12 minutes for the MLCDAO plans. Conclusion: Using DAO, it is possible to create IMRT treatment plans that utilize only independent collimators. In addition, these “jaws‐only” plans can be delivered in a reasonable amount of time. This can make IMRT feasible in clinics which have linear accelerators not equipped with an MLC.