Intensity-modulated Arc Therapy Plans Research Articles

Incorporating geometric ray tracing to generate initial conditions for intensity modulated arc therapy optimization

Intensity modulated arc therapy (IMAT) is a rotational variant of Intensity modulated radiation therapy (IMRT) that is achieved by allowing the multileaf collimator (MLC) positions to vary as the gantry rotates around the patient. This work describes a method to generate an IMAT plan through the use of a fast ray tracing technique based on dosimetric and geometric information for setting initial MLC leaf positions prior to final IMAT optimization. Three steps were used to generate an IMAT plan. The first step was to generate arcs based on anatomical contours. The second step was to generate ray importance factor (RIF) maps by ray tracing the dose distribution inside the planning target volume (PTV) to modify the MLC leaf positions of the anatomical arcs to reduce the maximum dose inside the PTV. The RIF maps were also segmented to create a new set of arcs to improve the dose to low dose voxels within the PTV. In the third step, the MLC leaf positions from all arcs were put through a leaf position optimization (LPO) algorithm and brought into a fast Monte Carlo dose calculation engine for a final dose calculation. The method was applied to two phantom cases, a clinical prostate case and the Radiological Physics Center (RPC)'s head and neck phantom. The authors assessed the plan improvements achieved by each step and compared plans with and without using RIF. They also compared the IMAT plan with an IMRT plan for the RPC phantom. All plans that incorporated RIF and LPO had lower objective function values than those that incorporated LPO only. The objective function value was reduced by about 15% after the generation of RIF arcs and 52% after generation of RIF arcs and leaf position optimization. The IMAT plan for the RPC phantom had similar dose coverage for PTV1 and PTV2 (the same dose volume histogram curves), however, slightly lower dose to the normal tissues compared to a six-field IMRT plan. The use of a ray importance factor can generate initial IMAT arcs efficiently for further MLC leaf position optimization to obtain more favorable IMAT plan.

TH-D-AUD B-02: A Technique For Creating VMAT Plans Using Direct Machine Parameter Optimization

Purpose: The objective of this study was to develop a technique for single rotation volume modulated arc therapy (VMAT) treatment plans using the current clinical version of the Pinnacle Treatment Planning system. The goal was to create treatment plans using only tools currently available in Pinnacle. Method and Materials:Treatment planning for VMAT was performed using the Direct Machine Parameter Optimization (DMPO) algorithm. With DMPO, inverse planning is performed by optimizing MLC leaf positions and segment weights. The treatment delivery was modeled and optimized as 51 equally spaced fixed beams. This beam geometry was selected to generate comparable plans to helical tomotherapy. Upon completion of optimization, the MLC delivery files were exported and converted into a single arc. Fixed‐gantry IMRT plans were also created for comparison by analyzing dose statistics, volume histograms, and differences. Calculated doses and MLC controller files were extracted from Pinnacle and imported into MATLAB for analysis. Gamma analyses were performed between the fixed‐gantry and VMAT plans. In addition, the modulation factor was calculated for the VMAT plans. Results: A total of 11 treatment plans were created for VMAT and fixed‐gantry IMRT. In all cases, the DMPO algorithm was able to optimize a plan resulting in one beam aperture. For prostate and HN and 2.) The VMAT plans have dose distributions similar to fixed‐gantry IMRT.

TU‐EE‐A1‐06: Comparison of Intensity‐Modulated Radiation Therapy, Intensity‐Modulated Arc Therapy and Arc‐Modulated Radiation Therapy

Purpose: Arc‐modulated radiation therapy (AMRT) is a new technique that delivers comparable dose distributions to IMRT in a single gantry rotation and thereby reduces the delivery times to approximately 2 to 5 minutes. To evaluate the clinical feasibility of AMRT, plan quality and delivery times are compared to conventional IMRT and multi‐arc intensity‐modulated arc therapy (IMAT). Method and Materials: 12 IMRT cases including 3 HN, 3 brain, 3 lung and 3 prostate were selected for this study. For each case, static IMRT plans utilizing 7 to 9 fields were produced in the Pinnacle TPS. IMAT plans with 5 to 11 arcs were generated for each case using a two‐step planning approach: ideal intensity maps for 36 static beams were optimized in Pinnacle using the same optimization objectives as in the corresponding IMRT optimization. The ideal intensity maps were segmented into a deliverable IMAT sequence using the k‐link IMAT leaf sequencer. For AMRT, the same ideal intensity maps were used to translate into a deliverable single‐arc sequence using a graph algorithm. The resultant AMRT sequences were designed to have an equivalent number of segments as in the corresponding IMAT plans. Results: Multi‐arc IMAT plans outperformed AMRT and IMRT for all cases in terms of target coverage, target dose uniformity and sparing of OARs. AMRT attained better or comparable dose distributions as compared with IMRT in all cases even for the challenging HN cases. The delivery times for AMRT were approximately 20% less than IMRT and 50% less than IMAT while the number of MU is 3 times more than IMRT and 1.3 times more than IMAT in general. Conclusion: AMRT is a promising technique that may become a viable alternative to IMRT. The main benefit is the reduced treatment times. A refined planning algorithm may further improve AMRT plan quality.

TH‐C‐350‐01: Planning and Delivery of IMAT: Comparisons of Single and Multiple Arc Approaches

Purpose: To investigate Intensity Modulated Arc Therapy plans (IMAT) with variable dose rate single arc and constant dose rate multiple arc approaches; to provide a formula for the prediction of their delivery times, and a metric to evaluate IMAT treatment plan adaptability. Method and Materials: IMAT plans were generated for localized prostate and oropharynx cancers. For each site, we generated IMAT plans for delivery with variable dose rate single arc and with constant dose rate multiple arcs. Treatment delivery times were modeled mathematically. A maximum leaf motion per unit gantry angle metric is used here to measure the complexity of leaf motions in treatment deliveries, and we proposed to use this metric to assess IMAT plan adaptability. IMAT plans were compared based on dosimetric qualities (target conformality and uniformity indices, dose‐volume indices of critical organs), delivery times, and plan adaptabilities. All IMAT plans were generated on Pinnacle 8.1v (Philips Medical Systems) for Varian linacs. Results: Overall, both IMAT approaches have comparable dosimetric plan qualities, although generating variable dose rate single arc IMAT plans required increased angular sampling to accommodate larger leaf motions, especially when there are two or more target volumes. The delivery time for the single arc is limited by both the gantry rotational speed and the maximum leaf motion between field segments, whereas, the multiple arc approach is generally limited by the gantry rotational speed alone. For the prostate plan, the delivery times are 1.2 minutes and 2 minutes respectively. In the oropharynx case, the single arc and the multiple arc deliveries were similar at approximately 2 minutes. The adaptability indices reflected the complexity of leaf motion in the single arc compared to the multiple arc approach. Conclusion: IMAT plans with variable and constant dose rate deliveries were evaluated under multiple considerations for potential clinical utilizations.

Dose–volume and biological-model based comparison between helical tomotherapy and (inverse-planned) IMAT for prostate tumours

Background and purpose Helical tomotherapy (HT) and intensity-modulated arc therapy (IMAT) are two arc-based approaches to the delivery of intensity-modulated radiotherapy (IMRT). Through plan comparisons we have investigated the potential of IMAT, both with constant (conventional or IMAT-C) and variable (non-conventional or IMAT-NC, a theoretical exercise) dose-rate, to serve as an alternative to helical tomotherapy. Materials and methods Six patients with prostate tumours treated by HT with a moderately hypo-fractionated protocol, involving a simultaneous integrated boost, were re-planned as IMAT treatments. A method for IMAT inverse-planning using a commercial module for static IMRT combined with a multi-leaf collimator (MLC) arc-sequencing was developed. IMAT plans were compared to HT plans in terms of dose statistics and radiobiological indices. Results Concerning the planning target volume (PTV), the mean doses for all PTVs were similar for HT and IMAT-C plans with minimum dose, target coverage, equivalent uniform dose (EUD) and tumour control probability (TCP) values being generally higher for HT; maximum dose and degree of heterogeneity were instead higher for IMAT-C. In relation to organs at risk, mean doses and normal tissue complication probability (NTCP) values were similar between the two modalities, except for the penile bulb where IMAT was significantly better. Re-normalizing all plans to the same rectal toxicity (NTCP = 5%), the HT modality yielded higher TCP than IMAT-C but there was no significant difference between HT and IMAT-NC. The integral dose with HT was higher than that for IMAT. Conclusions with regards to the plan analysis, the HT is superior to IMAT-C in terms of target coverage and dose homogeneity within the PTV. Introducing dose-rate variation during arc-rotation, not deliverable with current linac technology, the simulations result in comparable plan indices between (IMAT-NC) and HT.

Leaf‐sequencing for intensity‐modulated arc therapy using graph algorithms

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 degrees 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 a 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.

Comparison of Plan Quality Provided by Intensity-Modulated Arc Therapy and Helical Tomotherapy

Intensity-modulated arc therapy (IMAT) is an arc-based approach to intensity-modulated radiotherapy (IMRT) that can be delivered on a conventional linear accelerator using a conventional multileaf collimator. In a previous work, we demonstrated that our arc-sequencing algorithm can produce highly conformal IMAT plans. Through plan comparisons, we explored the ability of IMAT to serve as an alternative to helical tomotherapy. The IMAT plans were created for 10 patients previously treated with helical tomotherapy. Treatment plan comparisons, according to the target dose coverage and critical structure sparing, were performed to determine whether similar plan quality could be achieved using IMAT. In 8 of 10 patient cases, IMAT was able to provide plan quality comparable to that of helical tomotherapy. In 2 of these 8 cases, the use of non-axial coplanar or non-coplanar arcs in IMAT planning led to significant improvements in normal tissue sparing. The remaining 2 cases posed particular dosimetric challenges. In 1 case, the target was immediately adjacent to a spinal cord that had received previous irradiation. The second case involved multiple target volumes and multiple prescription levels. Both IMAT and tomotherapy were able to produce clinically acceptable plans. Tomotherapy, however, provided a more uniform target dose and improved critical structure sparing. For most cases, IMAT can provide plan qualities comparable to that of helical tomotherapy. For some intracranial tumors, IMAT's ability to deliver non-coplanar arcs led to significant dosimetric improvements. Helical tomotherapy, however, can provide improved dosimetric results in the most complex cases.

IMAT‐SIM: A new method for the clinical dosimetry of intensity‐modulated arc therapy (IMAT)

Dynamic-gantry multi-leaf collimator (MLC)-based, intensity-modulated radiotherapy (IMAT) has been proposed as an alternative to tomotherapy. In contrast to fixed-gantry, MLC-based intensity-modulated radiotherapy (IMRT), where commercial treatment planning systems (TPS) or dosimetric analysis software currently provide many automatic tools enabling two-dimensional (2D) detectors (matrix or electronic portal imaging devices) to be used as measurement systems, for the planning and delivery of IMAT these tools are generally not available. A new dosimetric method is proposed to overcome some of these limitations. By converting the MLC files of IMAT beams from arc to fixed gantry-angle modality, while keeping the leaf trajectories equal, IMAT plans can be both simulated in the TPS and executed as fixed-gantry, sliding-window DMLC treatments. In support of this idea, measurements of six IMAT plans, in their double form of original arcs and converted fixed-gantry DMLC beams (IMAT-SIM), have been compared among themselves and with their corresponding IMAT-SIM TPS calculations. Radiographic films and a 2D matrix ionization chamber detector rigidly attached to the accelerator gantry and set into a cubic plastic phantom have been used for these measurements. Finally, the TPS calculation-algorithm implementations of both conformal dynamic MLC arc (CD-ARC) modalities, used for clinical IMAT calculations, and DMLC modalities (IMAT-SIM), proposed as references for validating IMAT plan dose-distributions, have been compared. The comparisons between IMAT and IMAT-SIM delivered beams have shown very good agreement with similar shapes of the measured dose profiles which can achieve a mean deviation (+/-2sigma) of (0.35+/-0.16) mm and (0.37+/-0.14)%, with maximum deviations of 1.5 mm and 3%. Matching the IMAT measurements with their corresponding IMAT-SIM data calculated by the TPS, these deviations remain in the range of (1.01+/-0.28) mm and (-1.76+/-0.42)%, with maximums of 3 mm and 5%, limits generally accepted for IMRT plan dose validation. Differences in the algorithm implementations have been found, but by correcting CD-ARC calculations for the leaf-end transmission offset (LTO) effect the IMAT and IMAT-SIM simulations agree well in terms of final dose distributions. The differences found between IMAT and the IMAT-SIM beam measurements are due to the different controls of leaf motion (via electron gun delay in the latter) that cannot be used in the former to correct possible speed variations in the rotation of the gantry. As the IMAT delivered beams are identical to what the patient will receive during the treatment, and the IMAT-SIM beam calculations made by the TPS reproduce exactly the treatment plans of that patient, the accuracy of this new dosimetric method is comparable to that which is currently used for static IMRT. This new approach of 2D-detector dosimetry, together with the commissioning, quality-assurance, and preclinical dosimetric procedures currently used for IMRT techniques, can be applied and extended to any kind of dynamic-gantry MLC-based treatment modality either CD-ARC or IMAT.

TU‐D‐AUD‐08: Monte Carlo Kernel‐Based Convolution‐Superposition Dose Calculation for Intensity‐Modulated Arc Therapy (IMAT)

Purpose: Dose calculations for intensity‐modulated arc therapy (IMAT) have traditionally been performed by approximating continuous arcs with multiple static beams. Not only that the calculated dose may not truthfully represent the delivered dose, but also the large number of beams used to approximate an arc requires very long calculation time. We propose to use our homegrown Monte Carlo (MC)/superposition dose calculation algorithm to compute doses for rotational delivery. Method and Materials: We first generate static IMRT plans with Pinnacle3 P3 IMRT module utilizing 36 equi‐spaced beams. The resulting intensity maps are inputted to the continuous intensity map optimization (CIMO) IMAT leaf sequencer. CIMO outputs deliverable arcs, each represented by aperture shapes spaced at 10‐degree intervals. The apertures are then interpolated at 1‐degree intervals. Dose calculations for both 36‐field plan and 351‐field plan (which closely represents the actual delivery) are performed using our homegrown MC‐based dose calculator. The time needed for calculating dose for the two plans are compared and the differences in dose distributions are also examined. Results and Conclusion: Four IMAT plans, a prostate, a head and neck, an orbit, and a brain, were selected for the study. Using history‐based sampling, the dose computation time only increased slightly for the 351‐beam plans. Each case took approximately 30 minutes to compute on a single CPU running Mac OSX. No significant differences in plan quality between the 36‐beam and the 351‐beam plans were observed. However, finger‐like artifacts at the lower isodose levels became evident when using 36 beams to approximate an arc. The dose calculation time using our MC‐based dose calculator is largely independent of the number of beam portals. For the cases studied, using 36 beams to approximate an arc delivery is sufficient, but more accurate calculation with finer angular spacing is achievable without a significant increase in computation time.

TH-C-AUD-01: IMAT Leaf Sequencing Using Graph Algorithms

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 sequencing software 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.

Segmentation and leaf sequencing for intensity modulated arc therapy

A common method in generating intensity modulated radiation therapy (IMRT) plans consists of a three step process: an optimized fluence intensity map (IM) for each beam is generated via inverse planning, this IM is then segmented into discrete levels, and finally, the segmented map is translated into a set of MLC apertures via a leaf sequencing algorithm. To date, limited work has been done on this approach as it pertains to intensity modulated arc therapy (IMAT), specifically in regards to the latter two steps. There are two determining factors that separate IMAT segmentation and leaf sequencing from their IMRT equivalents: (1) the intrinsic 3D nature of the intensity maps (standard 2D maps plus the angular component), and (2) that the dynamic multileaf collimator (MLC) constraints be met using a minimum number of arcs. In this work, we illustrate a technique to create an IMAT plan that replicates Tomotherapy deliveries by applying IMAT specific segmentation and leaf-sequencing algorithms to Tomotherapy output sinograms. We propose and compare two alternative segmentation techniques, a clustering method, and a bottom-up segmentation method (BUS). We also introduce a novel IMAT leaf-sequencing algorithm that explicitly takes leaf movement constraints into consideration. These algorithms were tested with 51 angular projections of the output leaf-open sinograms generated on the Hi-ART II treatment planning system (Tomotherapy Inc.). We present two geometric phantoms and 2 clinical scenarios as sample test cases. In each case 12 IMAT plans were created, ranging from 2 to 7 intensity levels. Half were generated using the BUS segmentation and half with the clustering method. We report on the number of arcs produced as well as differences between Tomotherapy output sinograms and segmented IMAT intensity maps. For each case one plan for each segmentation method is chosen for full Monte Carlo dose calculation (NumeriX LLC) and dose volume histograms (DVH) are calculated. In all cases, the BUS method outperformed the clustering, method. We recommend using the BUS algorithm and discuss potential improvements to the clustering algorithms.

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.

228: A Novel Inverse Planning Tool for Intensity Modulated Arc Therapy

Intensity modulated arc therapy (IMAT) is a rotational approach to IMRT delivery that can serve as an alternative to tomotherapy. The key features of IMAT include: (1) it can be delivered on a conventional linear accelerator with an integrated MLC; and (2) IMAT plans can include noncoplanar arcs. A lack of robust inverse planning tools has served as a primary obstacle to the routine clinical implementation of IMAT. In this work, we have developed a new approach to IMAT treatment planning called arc sequencing.

SU‐FF‐T‐07: A Comparison Between Intensity Modulated Arc Therapy (IMAT) and Tomotherapy

Purpose: Intensity Modulated Arc Therapy (IMAT) has been proposed as an alternative to helical tomotherapy. IMAT can be delivered on a conventional linear accelerator and uses overlapping arcs to deliver a modulated intensity pattern from each beam direction. In this study, IMAT treatment plans were developed for ten patients previously treated with helical tomotherapy. The goal was to determine if IMAT could match the dosimetric capabilities of tomotherapy. Method and Materials: The IMAT planning process begins with an IMRT optimization performed using the Pinnacle3 planning system. In Pinnacle3, beams are placed at 10° increments along each arc path. After the optimization, an arc‐sequencing algorithm is applied to the optimized fluence maps to create a deliverable IMAT plan. The treatment sites in this study included lung, prostate, pancreas, brain, and head‐and‐neck. The IMAT plans were created under the assumption that the dose rate can vary from one beam angle to the next in each IMAT arc. Results: For coplanar delivery, the plan comparisons reveal that IMAT can generally provide equivalent plan quality as compared with tomotherapy. An average of 5 arcs and 692 MUs were used for these cases. In some cases improved critical structure sparing was observed in the IMAT plans at the expense of target dose uniformity. For three cases, noncoplanar IMAT plans were developed. The results demonstrate that for select cases the ability to incorporate noncoplanar arcs serves as a distinct advantage for IMAT. For example, in one case IMAT reduced the brainstem mean dose from 1866 to 606 cGy and the mean dose to the optic nerve from 388 to 95 cGy. Conclusion: When only axial coplaner arcs are used, IMAT plan can achieve as conformal dose distributions as tomotherapy plan. The IMAT plan, however, can provide a much better sparing to critical structures with non‐coplaner arcs.

SU-FF-T-226: Evaluation of Radiation Therapy Treatment Plans Through Conventional IMRT and Inversely Planned Intensity Modulated ARC Therapy (IMAT), A Clinical Comparison

Purpose: The goals of this study were to evaluate the difference in target dose uniformity and dose distribution through mean DVH and OAR analysis for a given PTV. Dosimetric analysis were also extended to compare sparing of normal tissue and critical structures between two treatment planning techniques. Methods and Material: The ADAC Pinnacle inverse planning system was used to generate conventional IMRT inverse plans for a few head & neck patients. These plans were then compared with treatment plans generated using the IMAT software package manufactured by 3D line USA Medical System Corp. Using axial CT for the same patients, targets, and numerous critical structures were delineated and “best” plans were developed in each environment. In IMAT planning, continuous and dynamically shaped arcs were planned through aperture optimization and leaf sequencing. The dose delivery was accomplished through a specially designed MICRO DMLC with 3 mm leaf width and double focus focalization. Results: For IMRT plans, 5 or 7 co‐planar arcs were used and treatment plans were generally optimized using a predefined set of dose objectives, penalties, min and max dose and percentage of volume required. The IMAT plans were designed with typically two co‐planar arcs of 180 degrees around the patient with similar dosimetry objective parameters. The percentage coverage for V95 in targets 1&2 were 94.9 ± 2.8 and 95.1 ± 2.1 for IMRT and 98.4 ± 2.6 & 98.1 ± 2.8 for IMAT respectively. Sparing of critical structures was generally better in IMAT, a detail of which will be presented at the meeting. Conclusions: In the IMAT technique with direct aperture optimization,dose distribution within the target was found to be better by 8%. Superior critical structure sparing was achieved as compared with similar plan using IMRT technique. The IMAT technique may be considered as an alternative to tomotherapy.

TU‐E‐ValB‐07: A Segmentation and Leaf Sequencing Algorithm for IMAT

Purpose: To develop an intensity segmentation and leaf sequencing algorithm specifically for intensity‐modulated arc therapy (IMAT), which can be applied to optimized intensity patterns derived from existing commercial IMRT inverse planning software. Methods and Materials: Three phantom cases, as well as a clinical case were planned using a Hi‐Art II (Tomotherapy Inc, WI.) planning station. The end of planning sinograms were then extracted and inputted into our IMAT conversion algorithm. The number of required arcs, deliverable MLC segments for each arc and the relative intensity weighting of each arc were outputted. The number of arcs (modulation) could be controlled by a user parameter, α. The resulting MLC segments were then fed into a fast monte‐carlo dose calculation algorithm, NXEGS (NumeriX, LLC) to obtain 3D dose distributions. Dose statistics (max, min, mean) and dose volume histograms of relevant structures were calculated and compared against the results generated by the Hi‐Art II system. Results: Each plan was converted in under three minutes on a typical desktop PC, with the arc numbers varying between 4 and 15 360° arcs. Qualitatively, the dose distributions obtained from the IMAT plans were similar to the tomotherapy results, as well as planned doses. Quantitatively, the IMAT plans were slightly degraded, with the average dose to normal structures being 7.5% higher for IMAT vs. tomotherapy. However, the IMAT plans generally met planned values, being 9.1% below for maximum doses to normal structures. The number of arcs and therefore the resulting dose distribution could be varied according to α. Conclusions: IMAT segmentation and leaf sequencing produced deliverable IMAT MLC segments and relative arc weights directly from Hi‐Art II optimized plans. The algorithm was computationally efficient, and produced similar dose distributions. Additional optimization could improve resulting dose distributions further. IMAT back‐up for tomotherapy is another potential application.

SU‐FF‐T‐104: Arc Sequencing — An Inverse Planning Technique for Intensity Modulated Arc Therapy

Purpose: Intensity modulated arc therapy (IMAT) is a rotational approach to IMRT in which the leaves of the multileaf collimator move continuously during arced beam delivery. Overlapping arcs are used to deliver optimized intensity patterns from each beam direction. Despite the promising nature of IMAT, its potential has gone largely unrealized due to a lack of robust inverse planning tools. To address this, we have developed an IMAT arc sequencing algorithm that translates optimized intensity maps into deliverable IMAT plans. Material and Methods: The arc‐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 arc sequencer has been tested for ten patient cases. An IMRT plan was developed for each case using the Pinnacle3 treatment planning system, and the arc sequencer translated the optimized plans into deliverable IMAT plans. Results: Ten IMAT patient plans were developed covering the following sites: 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 efficient IMAT delivery plans can be developed that combine the dosimetric advantages of arc therapy with the dose painting capabilities of IMRT. Only modest degradation was seen between the pre and post sequenced plans. Plan accuracy was verified using Monte Carlo dose calculations with each arc approximated as static beams separated by two degrees. An average sequencing time of under 25 minutes was observed. Conclusions: An arc‐sequencing algorithm has been developed that can serve as the first robust inverse planning tool for IMAT.

SU‐FF‐J‐102: Patient Breathing Motion Synchronized IMAT: A New Technique for Compensating Intra‐Fraction Organ Motions

Purpose: To develop an image‐guided radiotherapy technique for compensating intra‐fraction organ motions by taking full advantage of 4‐D CT and motion tracking. Methods and Material: We have developed a new image‐guided radiotherapy technique called patient breathing motion‐synchronized intensity‐modulated arc therapy (IMAT) based on the following observations. (1) If the LINAC gantry rotates x degrees in one breathing cycle during an IMAT arc delivery, then the beam source “sees” the same breathing phase at beam angles spaced x degrees apart. (2) If each breathing cycle is divided into k breathing phases, then we can partition all discrete beam angles used for planning an IMAT treatment into k groups, with each group “seeing” a particular breathing phase. If one can first calculate an intensity map for each group of beam angles using their corresponding snapshot in the 4D CT image set as a gantry‐fixed IMRT plan, then the resulting k groups of intensity maps can be combined and converted into a final set of IMAT treatment arcs. As long as the patient can reproduce his/her own pre‐recorded breathing patterns, the IMAT plan can be delivered and is optimal in 4‐D. (3) To ensure that the patient's breathing is always at the correct breathing phase at the start of the irradiation of each IMAT arc, the beam is not turned on after pressing the “Beam‐On” button until the breathing pattern reaches the predetermined phase through motion tracking. Results: We applied the motion‐synchronized IMAT technique to the dynamic phantom from CIRS Inc., which can simulate patient breathing motions. Our experimental study indicated that this technique can deliver optimal treatments under motions. Conclusion: We have developed a new image‐guided radiotherapy technique called patient breathing motion‐synchronized IMAT. Our prototype study has demonstrated the feasibility and advantage of this novel method that warrants further investigation.

TU-D-ValA-08: Geometric Considerations for Optimizing Beam Directions for IMAT Treatment of Lung Cancer with Mediastinum Nodal Irradiation

Purpose: Based on geometric considerations, we optimized beam directions and arc ranges for intensity‐modulated arc therapy (IMAT) to maximize the sparing of lung and spinal cord while treating the mediastinum nodes and gross disease. Methods and Materials: A phantom with multiple planning target volumes (PTV) and lung with varying geometric shapes was created on Pinnacle 7.6c (Philips Medical Systems). A spinal cord was placed posteriorly. Mean lung and maximum cord dose were obtained for the various geometries for AP/PA and IMAT fields that were designed to spare the cord. We combined AP/PA and IMAT fields in order to achieve an optimal plan for each geometry, and seek correlations between patient geometry and their optimal relative weights. This combination of AP/PA and the IMAT fields were used to treat a patient in 2004 diagnosed with non‐small cell lungcancer with concurrent chemotherapy.Results: For a circular PTV in the phantom, the mean lungdose varied for AP/PA between 50% and 29% of the mean PTV dose, depending on the lung geometry, and for IMAT, between 50% and 46%. The maximum cord dose was 107% for AP/PA versus 33% for IMAT. Optimized composite plan of AP/PA and IMAT were created for each phantom geometry, balancing PTV dose,lung and cord tolerance. For the patient plan, the prescription dose was 60Gy, and the composite AP/PA‐IMAT plan gave a maximum cord dose of 45Gy with mean lungdose =16.1Gy and V20Gy =23%. Patient completed treatment and had no acute or late lung or spinal cord toxicities. Conclusions: IMAT was found to be useful in sparing the cord but not the lung, while AP/PA maximizes lung sparing but not the cord. A combination of AP/PA and IMAT provides an optimal class solution for treatment of lungcancer where mediastinum nodal irradiation is indicated.

Clinical implementation of intensity-modulated arc therapy (IMAT) for rectal cancer

Purpose In rectal cancer, combined radiotherapy and chemotherapy, either pre- or postoperatively, is an accepted treatment. Late small bowel (SB) toxicity is a feared side effect and limits radiation-dose escalation in a volume-dependent way. A planning strategy for intensity- modulated arc therapy (IMAT) was developed, and IMAT was clinically implemented with the aim to reduce the volume of SB irradiated at high doses and thus reduce SB toxicity. We report on the treatment plans of the first 7 patients, on the comparison of IMAT with conventional 3D planning (3D), and on the feasibility of IMAT delivery. Methods and materials Seven patients, who were referred to our department for preoperative ( n = 4) or postoperative ( n = 3) radiotherapy for rectal cancer, gave written consent for IMAT treatment. All patients had a planning CT in prone position. The delineation of the clinical target volume was done after fusion of CT and MRI, with the help of a radiologist. For the IMAT plan, arcs were generated using an anatomy-based segmentation tool. The optimization of the arcs was done by weight optimization (WO) and leaf position optimization (LPO), both of which were adapted for IMAT purposes. The 3D plans used one posterior and two lateral wedged beams, of which the outlines were shaped to the beam's-eye view projection of the planning target volume (PTV). Beam WO was done by constrained matrix inversion. For dose–volume histogram analysis, all plans were normalized to 45 Gy as median PTV dose. Polymer gel dosimetry (PGD) on a humanoid phantom was used for the validation of the total chain (planning to delivery). IMAT treatments were delivered by an Elekta Sl i Plus linear accelerator using prototype software with the same interlock class as in clinical mode. Results The IMAT plan resulted in 3 to 6 arcs, with a mean delivery time of 6.3 min and a mean of 456 monitor units (MU) for a 180 cGy fraction. The minimal dose in the PTV was not significantly different between 3D and IMAT plans. Inhomogeneity was highest for the IMAT plans (14.1%) and lowest for the 3D plans (9.9%). Mean dose to the SB was significantly lower for the IMAT plans (12.4 Gy) than for the 3D plans (17.0 Gy). The volume of SB receiving less than any dose level was lower for the IMAT plans than for 3D plans. Integral dose was lower in the IMAT plans than for the 3D plans (respectively 244 J and 262 J to deliver 45 Gy). Differences between the PGD measured dose and the calculated dose were as small for IMAT as for 3D treatments. Conclusion IMAT plans are deliverable within a 5–10-minute time slot, and result in a lower dose to the SB than 3D plans, without creating significant underdosages in the PTV. PGD showed that IMAT delivery is as accurate as 3D delivery.