Direct Aperture Optimization Research Articles

A novel software and conceptual design of the hardware platform for intensity modulated radiation therapy.

To deliver high quality intensity modulated radiotherapy (IMRT) using a novel generalized sparse orthogonal collimators (SOCs), the authors introduce a novel direct aperture optimization (DAO) approach based on discrete rectangular representation. A total of seven patients-two glioblastoma multiforme, three head & neck (including one with three prescription doses), and two lung-were included. 20 noncoplanar beams were selected using a column generation and pricing optimization method. The SOC is a generalized conventional orthogonal collimators with N leaves in each collimator bank, where N = 1, 2, or 4. SOC degenerates to conventional jaws when N = 1. For SOC-based IMRT, rectangular aperture optimization (RAO) was performed to optimize the fluence maps using rectangular representation, producing fluence maps that can be directly converted into a set of deliverable rectangular apertures. In order to optimize the dose distribution and minimize the number of apertures used, the overall objective was formulated to incorporate an L2 penalty reflecting the difference between the prescription and the projected doses, and an L1 sparsity regularization term to encourage a low number of nonzero rectangular basis coefficients. The optimization problem was solved using the Chambolle-Pock algorithm, a first-order primal-dual algorithm. Performance of RAO was compared to conventional two-step IMRT optimization including fluence map optimization and direct stratification for multileaf collimator (MLC) segmentation (DMS) using the same number of segments. For the RAO plans, segment travel time for SOC delivery was evaluated for the N = 1, N = 2, and N = 4 SOC designs to characterize the improvement in delivery efficiency as a function of N. Comparable PTV dose homogeneity and coverage were observed between the RAO and the DMS plans. The RAO plans were slightly superior to the DMS plans in sparing critical structures. On average, the maximum and mean critical organ doses were reduced by 1.94% and 1.44% of the prescription dose. The average number of delivery segments was 12.68 segments per beam for both the RAO and DMS plans. The N = 2 and N = 4 SOC designs were, on average, 1.56 and 1.80 times more efficient than the N = 1 SOC design to deliver. The mean aperture size produced by the RAO plans was 3.9 times larger than that of the DMS plans. The DAO and dose domain optimization approach enabled high quality IMRT plans using a low-complexity collimator setup. The dosimetric quality is comparable or slightly superior to conventional MLC-based IMRT plans using the same number of delivery segments. The SOC IMRT delivery efficiency can be significantly improved by increasing the leaf numbers, but the number is still significantly lower than the number of leaves in a typical MLC.

WE-AB-303-06: Combining DAO with MV + KV Optimization to Improve Skin Dose Sparing with Real-Time Fluoroscopy

Purpose: Real-time fluoroscopy may allow for improved patient positioning and tumor tracking, particularly in the treatment of lung tumors. In order to mitigate the effects of the imaging dose, previous studies have demonstrated the effect of including both imaging dose and imaging constraints into the inverse treatment planning object function. That method of combined MV+kV optimization may Result in plans with treatment beams chosen to allow for more gentle imaging beam-on times. Direct-aperture optimization (DAO) is also known to produce treatment plans with fluence maps more conducive to lower beam-on times. Therefore, in this work we demonstrate the feasibility of a combination of DAO and MV+kV optimization for further optimized real-time kV imaging. Methods: Therapeutic and imaging beams were modeled in the EGSnrc Monte Carlo environment, and applied to a patient model for a previously treated lung patient to provide dose influence matrices from DOSXYZnrc. An MV + kV IMRT DAO treatment planning system was developed to compare DAO treatment plans with and without MV+kV optimization. The objective function was optimized using simulated annealing. In order to allow for comparisons between different cases of the stochastically optimized plans, the optimization was repeated twenty times. Results: Across twenty optimizations, combined MV+kV IMRT resulted in an average of 12.8% reduction in peak skin dose. Both non-optimized and MV+kV optimized imaging beams delivered, on average, mean dose of approximately 1 cGy per fraction to the target, with peak doses to target of approximately 6 cGy per fraction. Conclusion: When using DAO, MV+kV optimization is shown to Result in improvements to plan quality in terms of skin dose, when compared to the case of MV optimization with non-optimized kV imaging. The combination of DAO and MV+kV optimization may allow for real-time imaging without excessive imaging dose. Financial support for the work has been provided in part by NIH Grant T32 EB002103, ACS RSG-13-313-01-CCE, and NIH S10 RR021039 and P30 CA14599 grants. The contents of this submission do not necessarily represent the official views of any of the supporting organizations.

TH‐EF‐BRD‐06: Rapid Direct Aperture Optimization Via Piecewise Linear Dose Calculation Based On the Dose Influence Matrix

Purpose:Direct aperture optimization (DAO) considers the direct optimization of aperture leaf positions and weights, instead of a two‐step procedure for intensity modulated radiation therapy (IMRT), i.e. first fluence map optimization and then leaf sequencing algorithm. However, the use of the Monte Carlo (MC) method in DAO is often time‐consuming. This work aims to develop a rapid DAO algorithm.Methods:The proposed method is based on the dose influence matrix. That is the leaf position variable is a piecewise linear function of the dose influence matrix. Specifically, the piecewise linear dose calculation model with respect to left and right leaf positions of a aperture consists of three terms: (i) the total dose contribution from the bixels that are fully between the left and right leaf positions; (ii) the partial dose contribution from the leftmost bixel that the left leaf falls into; (iii) the partial dose contribution from the rightmost bixel that the right leaf falls into. As a Result, this model is piecewise linear with respect to the leaf positions.Then instead of MC, this piecewise linear dose calculation engine based on the dose influence matrix is incorporated into the simulated annealing algorithm for DAO. As a Result of rapid dose calculation based on the dose influence matrix, the speed of DAO is improved significantly.Results:The proposed algorithm was tested on the TG119 phantom from the MGH public dataset.Conclusion:Rapid DAO algorithm is developed with the dose calculation engine that replaces MC by the piecewise linear dose calculation based on the dose influence matrix.The authors were partially supported by the NSFC (#11405105), the 973 Program (#2015CB856000) and the Shanghai Pujiang Talent Program (#14PJ1404500).

Optimization approaches to volumetric modulated arc therapy planning

Volumetric modulated arc therapy (VMAT) has found widespread clinical application in recent years. A large number of treatment planning studies have evaluated the potential for VMAT for different disease sites based on the currently available commercial implementations of VMAT planning. In contrast, literature on the underlying mathematical optimization methods used in treatment planning is scarce. VMAT planning represents a challenging large scale optimization problem. In contrast to fluence map optimization in intensity-modulated radiotherapy planning for static beams, VMAT planning represents a nonconvex optimization problem. In this paper, the authors review the state-of-the-art in VMAT planning from an algorithmic perspective. Different approaches to VMAT optimization, including arc sequencing methods, extensions of direct aperture optimization, and direct optimization of leaf trajectories are reviewed. Their advantages and limitations are outlined and recommendations for improvements are discussed.

IMPACT OF SEGMENTS NUMBER REDUCTION IN IMRT PLANNING

Purpose: To investigate the planning and dosimetric advantages of direct aperture optimization (DAO) over beamlet optimization in IMRT treatment of head and neck (H/N) and prostate cancers. 
 Materials and methods: a brain metastatic case with multiple lesions, five head and neck, as well as five prostate patients were planned using the beamlet optimizer in Elekta-Xio© ver 4.6 IMRT treatment planning system. Based on our experience in beamlet IMRT optimization, PTVs in brain were prescribed to 66Gy using 5 fields, PTVs in H/N plans were prescribed to 70 Gy delivered by 7 fields, and prostate PTVs were prescribed to 76 Gy using nine fields. In all plans, fields were set to be equally spaced. All cases were re-planed using Direct Aperture optimizer (DAO) in Prowess Panther© ver 5.01 IMRT planning system at same configurations and dose constraints. Plans were evaluated according to ICRU criteria, number of segments, number of monitor units and planning time. 
 Results: In brain case, beamlet optimization was better than DAO for both GTVs and PTVs in 95% isodose coverage, and the hot area was about 7% more in beamlet plan than DAO plan. For OAR, results showed an improvement in OAR sparing up to more than 35% in rt. eye, lt. eye, rt. optic nerve and lt. optic nerve when using DAO for planning, while optic chiasma sparing was about 20% also in beamlet optimizer plan. 
 For H/N plans, the near maximum dose (D2) and the dose that covers 95% (D95) of PTV has improved by 4% in DAO. For organs at risk (OAR), DAO reduced the volume covered by 30% (V30) inspinal cord, right parotid, and left parotid by 60%, 54%, and 53% respectively. This considerable dosimetric quality improvement achieved using 25% less planning time and lower number of segments and monitor units by 46% and 51% respectively. 
 In DAO prostate plans, Both D2 and D95 for the PTV wereimproved by only 2%. The V30 of right femur, left femur and bladder were improved by 35%, 15% and 3% respectively. On the contrary, the rectum V30 got even worse by 9%. However, number of monitor units, and number of segments decreased by 20% and 25% respectively. Moreover the planning time reduced significantly too. 
 Conclusion: DAO introduces considerable advantages over beamlet optimization for different sites of cancer, in regards to organ at risk sparing. While no significant improvement occurred in the PTV ICRU reporting dose.

Feasibility of a unified approach to intensity-modulated radiation therapy and volume-modulated arc therapy optimization and delivery.

To study the feasibility of unified intensity-modulated arc therapy (UIMAT) which combines intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) optimization and delivery to produce superior radiation treatment plans, both in terms of dose distribution and efficiency of beam delivery when compared with either VMAT or IMRT alone. An inverse planning algorithm for UIMAT was prototyped within the pinnacle treatment planning system (Philips Healthcare). The IMRT and VMAT deliveries are unified within the same arc, with IMRT being delivered at specific gantry angles within the arc. Optimized gantry angles for the IMRT and VMAT phases are assigned automatically by the inverse optimization algorithm. Optimization of the IMRT and VMAT phases is done simultaneously using a direct aperture optimization algorithm. Five treatment plans each for prostate, head and neck, and lung were generated using a unified optimization technique and compared with clinical IMRT or VMAT plans. Delivery verification was performed with an ArcCheck phantom (Sun Nuclear) on a Varian TrueBeam linear accelerator (Varian Medical Systems). In this prototype implementation, the UIMAT plans offered the same target dose coverage while reducing mean doses to organs at risk by 8.4% for head-and-neck cases, 5.7% for lung cases, and 3.5% for prostate cases, compared with the VMAT or IMRT plans. In addition, UIMAT can be delivered with similar efficiency as VMAT. In this proof-of-concept work, a novel radiation therapy optimization and delivery technique that interlaces VMAT or IMRT delivery within the same arc has been demonstrated. Initial results show that unified VMAT/IMRT has the potential to be superior to either standard IMRT or VMAT.

Direct aperture optimization based on genetic algorithm and conjugate gradient in intensity modulated radiation therapy

Direct aperture optimization based on genetic algorithm and conjugate gradient in intensity modulated radiation therapy

Shared data for intensity modulated radiation therapy (IMRT) optimization research: the CORT dataset.

BackgroundWe provide common datasets (which we call the CORT dataset: common optimization for radiation therapy) that researchers can use when developing and contrasting radiation treatment planning optimization algorithms. The datasets allow researchers to make one-to-one comparisons of algorithms in order to solve various instances of the radiation therapy treatment planning problem in intensity modulated radiation therapy (IMRT), including beam angle optimization, volumetric modulated arc therapy and direct aperture optimization.ResultsWe provide datasets for a prostate case, a liver case, a head and neck case, and a standard IMRT phantom. We provide the dose-influence matrix from a variety of beam/couch angle pairs for each dataset. The dose-influence matrix is the main entity needed to perform optimizations: it contains the dose to each patient voxel from each pencil beam. In addition, the original Digital Imaging and Communications in Medicine (DICOM) computed tomography (CT) scan, as well as the DICOM structure file, are provided for each case.ConclusionsHere we present an open dataset – the first of its kind – to the radiation oncology community, which will allow researchers to compare methods for optimizing radiation dose delivery.

Beamlet based direct aperture optimization for MERT using a photon MLC.

A beamlet based direct aperture optimization (DAO) for modulated electron radiotherapy (MERT) using photon multileaf collimator (pMLC) shaped electron fields is developed and investigated. The Swiss Monte Carlo Plan (SMCP) allows the calculation of dose distributions for pMLC shaped electron beams. SMCP is interfaced with the Eclipse TPS (Varian Medical Systems, Palo Alto, CA) which can thus be included into the inverse treatment planning process for MERT. This process starts with the import of a CT-scan into Eclipse, the contouring of the target and the organs at risk (OARs), and the choice of the initial electron beam directions. For each electron beam, the number of apertures, their energy, and initial shape are defined. Furthermore, the DAO requires dose-volume constraints for the structures contoured. In order to carry out the DAO efficiently, the initial electron beams are divided into a grid of beamlets. For each of those, the dose distribution is precalculated using a modified electron beam model, resulting in a dose list for each beamlet and energy. Then the DAO is carried out, leading to a set of optimal apertures and corresponding weights. These optimal apertures are now converted into pMLC shaped segments and the dose calculation for each segment is performed. For these dose distributions, a weight optimization process is launched in order to minimize the differences between the dose distribution using the optimal apertures and the pMLC segments. Finally, a deliverable dose distribution for the MERT plan is obtained and loaded back into Eclipse for evaluation. For an idealized water phantom geometry, a MERT treatment plan is created and compared to the plan obtained using a previously developed forward planning strategy. Further, MERT treatment plans for three clinical situations (breast, chest wall, and parotid metastasis of a squamous cell skin carcinoma) are created using the developed inverse planning strategy. The MERT plans are compared to clinical standard treatment plans using photon beams and the differences between the optimal and the deliverable dose distributions are determined. For the idealized water phantom geometry, the inversely optimized MERT plan is able to obtain the same PTV coverage, but with an improved OAR sparing compared to the forwardly optimized plan. Regarding the right-sided breast case, the MERT plan is able to reduce the lung volume receiving more than 30% of the prescribed dose and the mean lung dose compared to the standard plan. However, the standard plan leads to a better homogeneity within the CTV. The results for the left-sided thorax wall are similar but also the dose to the heart is reduced comparing MERT to the standard treatment plan. For the parotid case, MERT leads to lower doses for almost all OARs but to a less homogeneous dose distribution for the PTV when compared to a standard plan. For all cases, the weight optimization successfully minimized the differences between the optimal and the deliverable dose distribution. A beamlet based DAO using multiple beam angles is implemented and successfully tested for an idealized water phantom geometry and clinical situations.

Poster - Thur Eve - 61: A new framework for MPERT plan optimization using MC-DAO

This work combines the inverse planning technique known as Direct Aperture Optimization (DAO) with Intensity Modulated Radiation Therapy (IMRT) and combined electron and photon therapy plans. In particular, determining conditions under which Modulated Photon/Electron Radiation Therapy (MPERT) produces better dose conformality and sparing of organs at risk than traditional IMRT plans is central to the project. Presented here are the materials and methods used to generate and manipulate the DAO procedure. Included is the introduction of a powerful Java-based toolkit, the Aperture-based Monte Carlo (MC) MPERT Optimizer (AMMO), that serves as a framework for optimization and provides streamlined access to underlying particle transport packages. Comparison of the toolkit's dose calculations to those produced by the Eclipse TPS and the demonstration of a preliminary optimization are presented as first benchmarks. Excellent agreement is illustrated between the Eclipse TPS and AMMO for a 6MV photon field. The results of a simple optimization shows the functioning of the optimization framework, while significant research remains to characterize appropriate constraints.

SU-E-T-593: Clinical Evaluation of Direct Aperture Optimization in Head/Neck and Prostate IMRT Treatment

Purpose: To investigate the planning and dosimetric advantages of direct aperture optimization (DAO) over beam-let optimization in IMRT treatment of head and neck (H/N) and prostate cancers. Methods: Five Head and Neck as well as five prostate patients were planned using the beamlet optimizer in Elekta-Xio ver 4.6 IMRT treatment planning system. Based on our experience in beamlet IMRT optimization, PTVs in H/N plans were prescribed to 70 Gy delivered by 7 fields. While prostate PTVs were prescribed to 76 Gy with 9 fields. In all plans, fields were set to be equally spaced. All cases were re-planed using Direct Aperture optimizer in Prowess Panther ver 5.01 IMRT planning system at same configurations and dose constraints. Plans were evaluated according to ICRU criteria, number of segments, number of monitor units and planning time. Results: For H/N plans, the near maximum dose (D2) and the dose that covers 95% D95 of PTV has improved by 4% in DAO. For organs at risk (OAR), DAO reduced the volume covered by 30% (V30) in spinal cord, right parotid, and left parotid by 60%, 54%, and 53% respectively. This considerable dosimetric quality improvement achieved using 25% less planning time and lower number of segments and monitor units by 46% and 51% respectively. In DAO prostate plans, Both D2 and D95 for the PTV were improved by only 2%. The V30 of the right femur, left femur and bladder were improved by 35%, 15% and 3% respectively. On the contrary, the rectum V30 got even worse by 9%. However, number of monitor units, and number of segments decreased by 20% and 25% respectively. Moreover the planning time reduced significantly too. Conclusion: DAO introduces considerable advantages over the beamlet optimization in regards to organs at risk sparing. However, no significant improvement occurred in most studied PTVs.

SU-E-T-214: Intensity Modulated Proton Therapy (IMPT) Based On Passively Scattered Protons and Multi-Leaf Collimation: Prototype TPS and Dosimetry Study

Purpose. Intensity-modulated proton therapy is usually implemented with multi-field optimization of pencil-beam scanning (PBS) proton fields. However, at the view of the experience with photon-IMRT, proton facilities equipped with double-scattering (DS) delivery and multi-leaf collimation (MLC) could produce highly conformal dose distributions (and possibly eliminate the need for patient-specific compensators) with a clever use of their MLC field shaping, provided that an optimal inverse TPS is developed. Methods. A prototype TPS was developed in MATLAB. The dose calculation process was based on a fluence-dose algorithm on an adaptive divergent grid. A database of dose kernels was precalculated in order to allow for fast variations of the field range and modulation during optimization. The inverse planning process was based on the adaptive simulated annealing approach, with direct aperture optimization of the MLC leaves. A dosimetry study was performed on a phantom formed by three concentrical semicylinders separated by 5 mm, of which the inner-most and outer-most were regarded as organs at risk (OARs), and the middle one as the PTV. We chose a concave target (which is not treatable with conventional DS fields) to show the potential of our technique. The optimizer was configured to minimize the mean dose to the OARs while keeping a good coverage of the target. Results. The plan produced by the prototype TPS achieved a conformity index of 1.34, with the mean doses to the OARs below 78% of the prescribed dose. This Result is hardly achievable with traditional conformal DS technique with compensators, and it compares to what can be obtained with PBS. Conclusion. It is certainly feasible to produce IMPT fields with MLC passive scattering fields. With a fully developed treatment planning system, the produced plans can be superior to traditional DS plans in terms of plan conformity and dose to organs at risk.

Inverse planning in the age of digital LINACs: station parameter optimized radiation therapy (SPORT)

The last few years have seen a number of technical and clinical advances which give rise to a need for innovations in dose optimization and delivery strategies. Technically, a new generation of digital linac has become available which offers features such as programmable motion between station parameters and high dose-rate Flattening Filter Free (FFF) beams. Current inverse planning methods are designed for traditional machines and cannot accommodate these features of new generation linacs without compromising either dose conformality and/or delivery efficiency. Furthermore, SBRT is becoming increasingly important, which elevates the need for more efficient delivery, improved dose distribution. Here we will give an overview of our recent work in SPORT designed to harness the digital linacs and highlight the essential components of SPORT. We will summarize the pros and cons of traditional beamlet-based optimization (BBO) and direct aperture optimization (DAO) and introduce a new type of algorithm, compressed sensing (CS)-based inverse planning, that is capable of automatically removing the redundant segments during optimization and providing a plan with high deliverability in the presence of a large number of station control points (potentially non-coplanar, non-isocentric, and even multi-isocenters). We show that CS-approach takes the interplay between planning and delivery into account and allows us to balance the dose optimality and delivery efficiency in a controlled way and, providing a viable framework to address various unmet demands of the new generation linacs. A few specific implementation strategies of SPORT in the forms of fixed-gantry and rotational arc delivery are also presented.

Direct aperture optimization using an inverse form of back-projection.

Direct aperture optimization (DAO) has been used to produce high dosimetric quality intensity‐modulated radiotherapy (IMRT) treatment plans with fast treatment delivery by directly modeling the multileaf collimator segment shapes and weights. To improve plan quality and reduce treatment time for our in‐house treatment planning system, we implemented a new DAO approach without using a global objective function (GFO). An index concept is introduced as an inverse form of back‐projection used in the CT multiplicative algebraic reconstruction technique (MART). The index, introduced for IMRT optimization in this work, is analogous to the multiplicand in MART. The index is defined as the ratio of the optima over the current. It is assigned to each voxel and beamlet to optimize the fluence map. The indices for beamlets and segments are used to optimize multileaf collimator (MLC) segment shapes and segment weights, respectively. Preliminary data show that without sacrificing dosimetric quality, the implementation of the DAO reduced average IMRT treatment time from 13 min to 8 min for the prostate, and from 15 min to 9 min for the head and neck using our in‐house treatment planning system PlanUNC. The DAO approach has also shown promise in optimizing rotational IMRT with burst mode in a head and neck test case.PACS number: 87.55.D‐

Direct aperture optimization using an inverse form of back-projection

Direct aperture optimization using an inverse form of back-projection

Feasibility of using nonflat photon beams for whole‐breast irradiation with breath hold

Removing a flattening filter or replacing it with a thinner filter alters the characteristics of a photon beam, creating a forward peaked intensity profile to make the photon beam nonflat. This study is to investigate the feasibility of applying nonflat photon beams to the whole‐breast irradiation with breath holds for a potential of delivery time reduction during the gated treatment. Photon beams of 6 MV with flat and nonflat intensity profiles were commissioned. Fifteen patients with early‐stage breast cancer, who received whole‐breast radiation without breathing control, were retrospectively selected for this study. For each patient, three plans were created using a commercial treatment planning system: (a) the clinically approved plan using forward planning method (FP); (b) a hybrid intensity‐modulated radiation therapy (IMRT) plan where the flat beam open fields were combined with the nonflat beam IMRT fields using direct aperture optimization method (mixed DAO); (c) a hybrid IMRT plan where both open and IMRT fields were from nonflat beams using direct aperture optimization (nonflat DAO). All plans were prescribed for ≥95% of the breast volume receiving the prescription dose of 50 Gy (2.0 Gy per fraction). In comparison, all plans achieved a similar dosimetric coverage to the targeted volume. The average homogeneity index of the FP, mixed DAO, and nonflat DAO plans were 0.882±0.024, 0.879±0.023, and 0.867±0.027, respectively. The average percentage volume of V105 was 57.66%±5.21%, 34.67%±4.91%, 41.64%±5.32% for the FP, mixed, and nonflat DAO plans, respectively. There was no significant difference (p>0.05) observed for the defined endpoint doses in organs at risk (OARs). In conclusion, both mixed DAO and nonflat DAO plans can achieve similar plan quality as the clinically approved FP plan, measured by plan homogeneity and endpoint doses to the ORAs. Nonflat beam plans may reduce treatment time in breath‐hold treatment, especially for hypofractionated treatment.PACS number: 87.55

Automatic CBCT-Based Online Adaptive Prostate Therapy

To automatically adapt prostate treatment plans to the daily anatomy as imaged in the pre-treatment cone beam (CBCT) to improve on the current standard of daily patient repositioning. Five IMRT prostate patients with kV CBCT image guidance were chosen. The patients were replanned with Direct Aperture Optimization IMRT, 5 mm margin, prescribed PTV D95 of 76 Gy, prostate D100 of 78 Gy. Our previously reported method of deforming prior days contours and then combining them with STAPLE provided accurate contours for organs and targets on nine CBCTs per patient. The plan CT is deformably registered to each CBCT during the contour generation. A two stage replan is investigated, firstly morphing the segments according to the change of target shape in the beam's eye view (SAM), and then optimizing the weights of each segment (SWO). SAM requires only the contours, SWO requires the electron density from the deformed plan CT. The adapted plans were compared to the current standard of repositioning. All plans were calculated with heterogeneity corrections. Contours and deformed plan CT are produced in ∼1 minute, SAM is instantaneous and SWO and dose calculation ∼3 minutes. CBCT contours were compared to physician contours with differences within inter-observer error. This infers reasonable plan CT deformation to the CBCT, and was also visually confirmed. SAM and SWO both showed significantly better target coverage than repositioning, p < 0.0001. Rectal sparing is also improved, particularly for SWO. Mean bladder dose is lower for repositioning, but only due to considerably reduced target coverage. There is minor correlation of prostate volume to coverage, r2∼0.2, but no other correlation between organ volumes and doses. Our adaptive approach is significantly better than the current standard of repositioning. Other studies have demonstrated SAM/SWO but relied on CT-on-Rails images. While CToR provides superior image quality, it is not common clinically. Our study overcomes the lower image quality of the more common CBCT and so can lead to a high clinical impact. SAM alone shows sufficient improvement over repositioning such that deforming the plan CT for electron density is not necessary, (a potential source of error). Contours can be manually drawn requiring a clinic to obtain only a tested (in the manner described here) aperture morphing scheme for CBCT-based adaptive therapy.Oral Scientific Abstract 337; TablePlan comparison, dose in cGyRepositionedSAMSWOSAM vs rep P valueSWO vs rep P valueProstateD98709677087800<.0001<.0001Mean799181108137<.0001<.0001PTVD95715676327690<.0001<.0001BladderV60(%)11.513.412.2.004.119Mean254727502717<.001<.001RectumV60(%)16.114.413.3.013<.001Mean337033453280.269.020 Open table in a new tab

SU‐E‐T‐654: Direct Aperture Optimization Using An Inverse Form of Back Projection

Purpose: Intensity modulated radiotherapy plans using direct aperture optimization (DAO) promise high dosimetric quality and fast treatment delivery. We present a new and fast DAO approach where IMRT and arc therapy optimization are treated as inverse forms of CT reconstruction using multiplicative algebraic reconstruction technique (MART). Methods: We developed a modified MART for DAO, and the multiplicand is termed as ‘index’, indicating how much the current dose should be changed. Index is assigned to each voxel, each beamlet and each segment to optimize fluence map, MLC segment shape, and segment weight, respectively. In addition, a column generation method is developed to add new segments to improve dose conformity. Results: DAO using index‐dose can be completed within a few minutes. Without sacrificing plan quality, the implementation of the DAO in our in‐house treatment planning system reduced average IMRT treatment time from 13 min to 8 min for the prostate, and from 15 min to 9 min for the head and neck. The DAO approach is also applied for arc therapy in a head and neck case using rotational IMRT with burst mode. Conclusion: We have demonstrated a new direct aperture optimization technique using an inverse form of back projection approach. The method effectively reduces the number of segments needed for IMRT plans, and thus reduces treatment time. Our approach also provides a feasible solution for the rotation IMRT with burst mode.

TH‐A‐116‐05: A Robust Direct Aperture Optimization Approach for Left‐Sided Breast IMRT

Purpose: To test the feasibility of a robust direct aperture optimization (DAO) model for cardiac sparing IMRT planning of left‐sided breast cancer patients. Methods: We extended a previously developed robust optimization method to obtain a DAO approach for tangential breast IMRT that is robust against uncertainty in the patient's breathing pattern. The robust DAO model contains additional constraints on the segment shapes and weights in order to reduce the complexity of delivery and ensure that high weight segments cover the whole breast volume. A 4DCT dataset was used to test the model, and 100 simulated breathing patterns were generated. Results were compared with the previously developed robust fluence map optimization (FMO) model as well as the clinical treatment methods for breast cancer (inhale breath‐hold with active breathing control (ABC) and free breathing). Results: On average, the robust DAO approach reduced the mean heart dose by 15.1% compared to the conventional IMRT method for breast cancer (with free breathing) using an equal number of segments. The maximum dose to 10cc of the heart was reduced by 19.7%. Compared to the previous robust FMO method, the robust DAO method delivered less than 1% higher dose to the heart and 1% lower dose to the CTV on average. The dose‐volume criteria for the heart and CTV were met in all of the breathing scenarios. The robust DAO approach resulted in a 20.8% reduction in the number of cases that would have required ABC treatment in comparison with the clinical method with free breathing. Conclusions: The proposed DAO approach results in lower heart dose compared to the conventional method with free breathing and shows low sensitivity to breathing pattern uncertainty. The robust DAO method is able to derive simpler IMRT segments during free breathing and can potentially reduce the need for breath‐hold techniques. This research was supported in part by the Ontario Ministry of Economic Development and Innovation, the Canadian Breast Cancer Foundation ‐ Ontario Region, and the Ontario Consortium for Adaptive Interventions in Radiation Oncology (OCAIRO) funded by the Ontario Research Fund (ORF).

SU‐E‐T‐656: Multi‐Criteria Direct Aperture Optimization

Purpose: Multi‐criteria optimization has been applied to IMRT planning, mainly in the fluence‐map optimization stage, to interactively explore the trade‐off between conflicting treatment objectives. However, the subsequent leaf‐sequencing step may degrade the plan quality due to dose discrepancies. To circumvent this issue, this study aims at developing multi‐criteria optimization approaches to IMRT planning using direct aperture optimization methods. Methods: The direct aperture optimization problem is formulated and solved using a multi‐criteria optimization approach. In particular, a solution method is developed to obtain a collection of apertures that can adequately span the entire Pareto surface. To that end, the column generation method for direct aperture optimization is extended to a multi‐criteria setting in which apertures that can improve the entire Pareto surface are sequentially identified and added to the aperture collection. More specifically, we approximate the improvement rate of an aperture in the Pareto surface by sampling Pareto‐optimal points and measuring the cumulative improvement rate of the aperture at those sampling points. Results: Our solution method is tested on a paraspinal case in which the trade‐off between target coverage and spinal cord sparing is studied. We compare the trade‐off curves obtained using our solution method and two other related heuristics. The computational results validate that the proposed method obtains a balanced approximation of the Pareto surface over a wide range of clinically relevant plans. Conclusion: We have developed a multi‐criteria direct aperture optimization method that allows for conducting the trade‐off exploration directly in the space of deliverable plans, thereby avoiding dose discrepancies caused by the leaf‐sequencing stage.