Beam Weights Research Articles

A continuous arc delivery optimization algorithm for CyberKnife m6.

This study aims to reduce the delivery time of CyberKnife m6 treatments by allowing for noncoplanar continuous arc delivery. To achieve this, a novel noncoplanar continuous arc delivery optimization algorithm was developed for the CyberKnife m6 treatment system (CyberArc-m6). CyberArc-m6 uses a five-step overarching strategy, in which an initial set of beam geometries is determined, the robotic delivery path is calculated, direct aperture optimization is conducted, intermediate MLC configurations are extracted, and the final beam weights are computed for the continuous arc radiation source model. This algorithm was implemented on five prostate and three brain patients, previously planned using a conventional step-and-shoot CyberKnife m6 delivery technique. The dosimetric quality of the CyberArc-m6 plans was assessed using locally confined mutual information (LCMI), conformity index (CI), heterogeneity index (HI), and a variety of common clinical dosimetric objectives. Using conservative optimization tuning parameters, CyberArc-m6 plans were able to achieve an average CI difference of 0.036 ± 0.025, an average HI difference of 0.046 ± 0.038, and an average LCMI of 0.920 ± 0.030 compared with the original CyberKnife m6 plans. Including a 5 s per minute image alignment time and a 5-min setup time, conservative CyberArc-m6 plans achieved an average treatment delivery speed up of 1.545x ± 0.305x compared with step-and-shoot plans. The CyberArc-m6 algorithm was able to achieve dosimetrically similar plans compared to their step-and-shoot CyberKnife m6 counterparts, while simultaneously reducing treatment delivery times.

Physical parameter optimization scheme for radiobiological studies of charged particle therapy

We have developed an easy-to-implement method to optimize the spatial distribution of a desired physical quantity for charged particle therapy. The basic methodology requires finding the optimal solutions for the weights of the constituent particle beams that together form the desired spatial distribution of the specified physical quantity, e.g., dose or dose-averaged linear energy transfer (LETd), within the target region. We selected proton, 4He ion, and 12C ion beams to demonstrate the feasibility and flexibility of our method. The pristine dose Bragg curves in water for all ion beams and the LETd for proton beams were generated from Geant4 Monte Carlo simulations. The optimization algorithms were implemented using the Python programming language. High-accuracy optimization results of the spatial distribution of the desired physical quantity were then obtained for different cases. The relative difference between the real value and the expected value of a given quantity was approximately within ±1.0% in the whole target region. The optimization examples include a flat dose spread-out Bragg peak (SOBP) for the three selected ions, an upslope dose SOBP for protons, and a downslope dose SOBP for protons. The relative difference was approximately within ±2.0% for the case with a flat LETd (target value = 4 keV/µm) distribution for protons. These one-dimensional optimization algorithms can be extended to two or three dimensions if the corresponding physical data are available. In addition, this physical quantity optimization strategy can be conveniently extended to encompass biological dose optimization if appropriate biophysical models are invoked.

Fast Inverse Planning of Beam Directions and Weights for Small Animal Radiotherapy

Current preclinical irradiation systems, such as the small animal radiation research platform, provide many capabilities of clinical systems, including on-board cone beam computed tomography (CBCT) imaging, which motivates the development of treatment planning systems. The time for preclinical treatment planning is often limited, however, due to the large number of subjects and the need for anesthesia for imaging and radiation delivery. This paper presents a 3-D inverse planning solution that optimizes beam directions and weights within a timeframe suitable for small animal radiation research. The system begins with a large number of beams and takes advantage of a graphics processing unit-accelerated implementation of the superposition-convolution method to quickly compute the dose for each beam. Optimization is performed by a linear programming solver, with a minimum bound on dose to the target and a maximum bound on dose to organs at risk (OAR). It is extended to dose shells by using hollow cylinders instead of solid beams. The method is demonstrated on mouse CBCT images with arbitrarily defined targets and OAR. Inverse planning is performed in about 5 min, which enables a preclinical workflow that includes CBCT acquisition, treatment planning, and radiation delivery in a single session.

Channel-Reconstruction-Based Hybrid Precoding for Millimeter-Wave Multi-User MIMO Systems

The focus of this paper is on multi-user MIMO transmissions for millimeter wave systems with a hybrid precoding architecture at the base-station. To enable multi-user transmissions, the base-station uses a cell-specific codebook of beamforming vectors over an initial beam alignment phase. Each user uses a user-specific codebook of beamforming vectors to learn the top-P (where P >= 1) beam pairs in terms of the observed SNR in a single-user setting. The top-P beam indices along with their SNRs are fed back from each user and the base-station leverages this information to generate beam weights for simultaneous transmissions. A typical method to generate the beam weights is to use only the best beam for each user and either steer energy along this beam, or to utilize this information to reduce multi-user interference. The other beams are used as fall back options to address blockage or mobility. Such an approach completely discards information learned about the channel condition(s) even though each user feeds back this information. With this background, this work develops an advanced directional precoding structure for simultaneous transmissions at the cost of an additional marginal feedback overhead. This construction relies on three main innovations: 1) Additional feedback to allow the base-station to reconstruct a rank-P approximation of the channel matrix between it and each user, 2) A zeroforcing structure that leverages this information to combat multi-user interference by remaining agnostic of the receiver beam knowledge in the precoder design, and 3) A hybrid precoding architecture that allows both amplitude and phase control at low-complexity and cost to allow the implementation of the zeroforcing structure. Numerical studies show that the proposed scheme results in a significant sum rate performance improvement over naive schemes even with a coarse initial beam alignment codebook.

A singular value decomposition linear programming (SVDLP) optimization technique for circular cone based robotic radiotherapy

With robot-controlled linac positioning, robotic radiotherapy systems such as CyberKnife significantly increase freedom of radiation beam placement, but also impose more challenges on treatment plan optimization. The resampling mechanism in the vendor-supplied treatment planning system (MultiPlan) cannot fully explore the increased beam direction search space. Besides, a sparse treatment plan (using fewer beams) is desired to improve treatment efficiency. This study proposes a singular value decomposition linear programming (SVDLP) optimization technique for circular collimator based robotic radiotherapy. The SVDLP approach initializes the input beams by simulating the process of covering the entire target volume with equivalent beam tapers. The requirements on dosimetry distribution are modeled as hard and soft constraints, and the sparsity of the treatment plan is achieved by compressive sensing. The proposed linear programming (LP) model optimizes beam weights by minimizing the deviation of soft constraints subject to hard constraints, with a constraint on the l1 norm of the beam weight. A singular value decomposition (SVD) based acceleration technique was developed for the LP model. Based on the degeneracy of the influence matrix, the model is first compressed into lower dimension for optimization, and then back-projected to reconstruct the beam weight. After beam weight optimization, the number of beams is reduced by removing the beams with low weight, and optimizing the weights of the remaining beams using the same model. This beam reduction technique is further validated by a mixed integer programming (MIP) model. The SVDLP approach was tested on a lung case. The results demonstrate that the SVD acceleration technique speeds up the optimization by a factor of 4.8. Furthermore, the beam reduction achieves a similar plan quality to the globally optimal plan obtained by the MIP model, but is one to two orders of magnitude faster. Furthermore, the SVDLP approach is tested and compared with MultiPlan on three clinical cases of varying complexities. In general, the plans generated by the SVDLP achieve steeper dose gradient, better conformity and less damage to normal tissues. In conclusion, the SVDLP approach effectively improves the quality of treatment plan due to the use of the complete beam search space. This challenging optimization problem with the complete beam search space is effectively handled by the proposed SVD acceleration.

Investigation of Temperature Dependence of Polymer Gels for Use with Scanning Magnetic Resonance Imaging

Polymer gels are three-dimensional dosimetric tools. The purpose of the present study was to investigate the temperature dependence of polymer gels during scanning Magnetic Resonance Imaging. Prepared gels were irradiated with a 6MV X-ray beam at intensities ranging from 0 to 20 Gy in order to investigate their dose-R2 and dose-R1 responses. Irradiated gels were evaluated from 1.5-T magnetic resonance R2 and R1 images for each 5°C change in temperature from 5°C to 41°C, and then the four-field box technique irradiation plan was used to deliver a total dose of 4 Gy using the same beam weight in each direction to the prepared gels. The profile of the dose map generated from the four-field irradiated gel data at 20°C was then compared with the planned data. The dose-R2 response curve was linear up to 20 Gy at 20°C, with a slope of 1.17 Gy-1˙s-1. The slopes of the fitted curves of the dose-R2 decreased as gel temperature increased. The slopes of the dose-R1 curves were more parallel than the slopes of the dose-R2 curves between 5 and 41°C. The difference in the full width of half maximum of the gel profile data obtained using the four-field box technique at 20°C and the planned data were below 5% on average. The dose map from the irradiated gels obtained using the dose-R2 curve was the same as that from the planned data under the same temperature conditions. Measurement of difference between various temperatures is significant with dose accuracy. It is suitable to evaluate the gel dosimeter under the thermal equilibrium condition, MRI room temperature from the point of view of the stability of the irradiated gels.

Monaco and film dosimetry of 3D CRT, IMRT and VMAT cases in a realistic pelvic prosthetic phantom

The dosimetry of patients with metallic hip implants during irradiation of pelvic lesions is challenging due to dose distortions caused by implants. This work presents a dosimetric comparison of various multi-field photon-beam dose distributions in the presence of unilateral hip titanium prosthesis (UHTiP) embedded in a unique pelvic phantom made out of water-equivalent nylon slices. The impact of the UHTiP on the accuracy of dose calculations from a Monaco TPS (treatment planning system) using the X-ray voxel Monte Carlo (XVMC) algorithm was benchmarked against measured dose data using Gafchromic EBT3 film. Multi-field beam arrangements including a 4-field box, 5-field 3DCRT (three-dimensional conformal radiation therapy), 6-field IMRT (intensity modulated radiation therapy) and a single-arc VMAT (volumetric modulated arc therapy) plan were set up for 6MV and 15MV beams. These plans were generated for the pelvic phantom that contains the prosthesis with film inserted. Compared to Monaco TPS dose calculations, film measurements showed enhanced dose in the prosthesis which was not predicted by Monaco due to its limitation in relative density assignment. The enhanced prosthesis dose increased with increase in beam energy and decreased with the complexity of the treatment plans, with VMAT giving the least escalated dose. The dose increased between 5% and 19% for 6MV and between 6% and 21% for 15MV. A gamma index analysis showed that 70–92% of dose points (excluding the prosthesis) were within 3% discrepancy. Increasing the number of treatment fields increases target dose coverage and improves the agreement between film and Monaco. When the relative electron density (RED) in the prosthesis was varied between 3.72 and 15 the dose discrepancy between film and Monaco increased from 30% to 57% for 6MV and from 30% to 50% for 15MV. The study indicates that beam weights for fields that pass through the prosthesis should be minimised and its RED must be correct for accurate dose calculation on Monaco.

Energy, momentum, and angular momentum of sound pulses.

Pulse solutions of the wave equation can be expressed as superpositions of scalar monochromatic beam wavefunctions (solutions of the Helmholtz equation). This formulation leads to causal (unidirectional) propagation, in contrast to all currently known closed-form solutions of the wave equation. Application is made to the evaluation of the energy, momentum, and angular momentum of acoustic pulses, as integrals over the beam and pulse weight functions. Equivalence is established between integration over space of the energy, momentum, and angular momentum densities, and integration over the wavevector weight function. The inequality linking the total energy and the total momentum is made explicit in terms of the weight function formulation. It is shown that a general pulse can be viewed as a superposition of phonons, each with energy ℏck, z component of momentum ℏq, and z component of angular momentum ℏm. A closed-form solution of the wave equation is found, which is localized and causal, and its energy and momentum are evaluated explicitly.

Abstract ID: 145 Monte Carlo for CyberKnife radiosurgery with the InCise Multileaf Collimator

This work investigates the accuracy and efficiency of Monte Carlo dose calculation with the InciseTM 2 Multileaf Collimator (MLC) for the CyberKnife® M6TM System in Accuray PrecisionTM Treatment Planning System 1.1. Phase space above the MLC is derived from a virtual source model calculated from beam commissioning measurements (profiles, TPRs, and output factors). Transport through the MLC is performed by ray tracing, accounting for leaf tip geometry, leaf-bank tilt, beam hardening, and shifts applied by the MLC controller to compensate for variable offset between the radiographic and geometric beam edge at each leaf tip. Patient model transport uses material and density specific photon mean-free-paths in combination with track-repeating of precomputed electron tracks from mono-energetic photon beams in water, modified using local density. Variance reduction techniques include forced interaction, particle splitting, and range rejection. This version improves on previous implementations by linearly propagating electrons below a cutoff energy in some circumstances instead of depositing their remaining energy in one voxel. The algorithm is implemented in C++, multithreaded over 12 CPU cores, and can be used pre- or post- beam weight optimization. Calculations were compared against measurement for single beams and composed plans in homogeneous and inhomogeneous phantoms. Single beam measurements included central axis percent depth dose (PDD) using diode and microdiamond detectors, and planar dose distributions with EBT3 film. For composed plans, point measurements were performed using air-filled micro-ionization chambers and TLD. Calculation speed was characterized by evaluating lung SBRT plans consisting of between 13 and 126 treatment beams, and PTV’s between 14cc and 72cc. The PDD curves showed good agreement in solid water and water-lung-water phantoms between calculation and measurement. Films had ⩾90% pixels meeting a 2%/2 mm gamma criterion for all 12 aperture shapes in similar phantoms. Composite plans had point dose differences ⩽3% in heterogeneous lung phantoms. Calculation speeds at native 512 × 512 CT resolution (approximately 1 mm3 voxels) and 2% requested uncertainty were 9 min on average, reducing to 3 min when medium resolution (approximately 4 mm3 voxels) was used. Methods to reduce calculation times are under investigation. Preliminary results suggest possible 40%–75% reductions without underlying algorithm or hardware changes.

A Randomized Singular Value Decomposition Based Linear Programming Technique for Robotic Radiation Therapy Treatment Plan Optimization

With highly reliable control of linac positioning, the robotic radiotherapy by CyberKnife significantly increases the freedom in radiation beam placement, but also imposes more challenges on treatment plan optimization. The resampling mechanism in vendor supplied treatment planning system (MultiPlan) could not fully explore the increased beam direction search space. This study proposes a randomized singular value decomposition (RSVD) based linear programming (LP) technique for circular collimator robotic radiotherapy treatment plan optimization (CCRTPO). The RSVDLP algorithm initializes beam angle of each available node is to cover the entire target with equivalent beam taper, which fully explores the complete beam space without redundancy. Beam weight is optimized by LP, which minimizes the deviation of relax constraints (RCs) subjected to hard constraints. Based on the degeneracy of dose influence matrix (DIM), of which the column vector is the dose delivered by one beam to all voxels, a RSVD approach is developed to compress the weight variables, and back project the compressed variables to construct beam weights after optimization. The beams with lower weight are removed, and the weight of remaining beams is further optimized using the same method to reduce number of beams. The technique was demonstrated and compared with the MultiPlan on a lung case with a PTV of 15.5cm3. Both algorithms used the same set of 94 nodes, and circular collimators of 15mm & 20mm diameter. The objectives and constraints used in MultiPlan are switched in RSVDLP model. The switched model is more suitable for SBRT, as it guarantees sufficient dose for PTV and avoids cold spot. For spinal cord (SC) and trachea (Tr), RSVDLP model uses stricter RCs to achieve better protection without triggering infeasibility. RSVD is performed to preserve 90% feature of DIM, the compression rate is 24.4% (1670 compressed variables over 6832 initial beams), and the optimization is sped up 4.8 times (1785s over 370s). The table indicates that RSVDLP plan achieves better homogeneity, conformity and OAR protection with lower MU and fewer beams. Similar results were observed on liver and brain cases.Abstract 3639; Table Comparison of optimization models and results (unit in cGy)Objective (O) & constraint (C)DmaxDmeanCI2HIMUBeam #PTVSCEsTrSCEsTrLLRLRSVDLP>4500 C<4650 O<1200 O<2200 O<1200 O1613304913561173491.281.1735745134MultiPlan>4500 O<4650 C<1500 C<4150 C<4050 O<2200 C1681318016591453621.581.2837820136Abbr: esophagus-Es, left lung-LL, right lung-RL, conformity index-CI, homogeneity index-HI. Open table in a new tab Abbr: esophagus-Es, left lung-LL, right lung-RL, conformity index-CI, homogeneity index-HI. We have developed a RSVDLP technique for CCRTPO. The results demonstrate that RSVD is effective in accelerating the optimization. Due to the use of complete beam space and flexible model, the plan generated by RSVDLP achieves better dose distribution than MultiPlan.

Rapid Access Palliative Radiation Workflow Using MRI-Guided Single-Session Simulation, Online Adaptation, and Treatment

In the palliative setting, priority is placed on streamlining the process of radiation delivery to avoid unnecessary treatment delay and expedite symptom relief. We hypothesized that using magnetic resonance imaging (MRI) guidance, adaptive planning software, and pre-existing diagnostic computed tomography (CT) imaging, we could deliver palliative radiation without the need for a separate CT simulation scan, thus decreasing the time and steps required to start palliative treatment. For patients necessitating urgent palliative radiotherapy, a simple “pre-plan” was created on diagnostic CT imaging prior to patient transport to the treatment machine. For dose calculation purposes, the electron density of the CT was deformed to MRI in the pre-plan or was overridden to water density. In the treatment vault, the patient was imaged in treatment position with volumetric MRI. The pre-plan was then rapidly adapted online to the MRI, updating the target definition, re-defining fields, and re-balancing beam weights. Once the online-adapted plan was finalized and approved, the patient was treated. Eighteen patients with metastatic cancer were treated from October 2015 to November 2016 with urgent palliative radiation for indications such as pain, obstruction, and bleeding. Dose/fractionation regimens included 8 Gy in 1 fraction (8 pts), 25 Gy in 5 fractions (4 pts), and 30 Gy in 10 fractions (6 pts). Median planning target volume (PTV) size was 187cc. Online adaptive MRI-guided radiotherapy enabled plan changes to be made rapidly. Conformal 3D adaptive plans were delivered as quickly as more conventional AP/PA beam arrangements with a median delivery time of 122s. MRI-guided adaptive radiotherapy was planned and delivered in one-hour appointments for each patient, compared with a mean time from CT simulation to delivery of first treatment of 30 hours (95% CI, 24-35) for a similar sample of urgent palliative cases planned and treated with more traditional radiotherapy workflow. Clinical outcomes were similar to historical and sampled controls, with improvement in pain in 11 of 14 patients, relief of obstructive symptoms in 3 of 3 patients, and hemostasis in 1 of 1 patient treated with MRI-guided adaptive radiotherapy. Integrated treatment planning using available MRI-guided online adaptive radiotherapy allows a rapid-access treatment workflow obviating the need for a separate CT simulation. Conformal pre-plans can be created without time pressure and, if necessary, adapted to anatomy at treatment. This streamlined palliative radiation workflow makes efficient use of both staff and patient time and expedites palliative treatment with similarly successful clinical results.

Monitoring the Vela Pulsar with a Phased Array Feed (PAF) Receiver

AbstractWe have observed the Vela pulsar for 1 year using a Phased Array Feed (PAF) receiver on the 12-m antenna of the Parkes Test-Bed Facility (PTF). These observations have allowed us to investigate the stability of the PAF beam weights over time, to demonstrate that pulsars can be timed over long periods using PAF technology and to detect and study the most recent glitch event that occurred on 12 December 2016. The beam weights are shown to be stable to 1% on time scales on the order of three weeks.

Lightweight HPC beam OMEGA

In the design and construction of precast bridge structures, a general goal is to achieve the maximum possible span length. Often, the weight of individual beams makes them difficult to handle, which may be a limiting factor in achieving the desired span. The design of the OMEGA beam aims to solve a part of these problems. It is a thin-walled shell made of prestressed high-performance concrete (HPC) in the shape of inverted Ω character. The concrete shell with prestressed strands is fitted with a non-stressed tendon already in the casting yard and is more easily transported and installed on the site. The shells are subsequently completed with mild steel reinforcement and cores are cast in situ together with the deck. The OMEGA beams can also be used as an alternative to steel - concrete composite bridges. Due to the higher production complexity, OMEGA beam can hardly substitute conventional prestressed beams like T or PETRA completely, but it can be a useful alternative for specific construction needs.

Safety assessment of composite beam under ballistic impact

A novel stochastic design approach is presented for a lightweight composite to sustain ballistic projectile impact. Manufacturing of such lightweight composites is a challenging task. Presently, a significant difference is observed in probabilities of failure amongst the four different arrangements of ply lay ups in composite design. The material properties and initial projectile velocity are considered as random parameters. 3D stochastic finite element method is employed to obtain the stochastic dynamic response using explicit time domain solver. The stochastic stresses obtained at critical locations and strength parameters of random nature; contribute to damage initiation. Fiber failure initiation models adopted to establish the performance functions. The probability of failure (Pf) of anti-symmetric cross ply arrangement is found to be minimum in comparison to other ply lay-ups namely, symmetric cross ply, symmetric angle ply and anti-symmetric angle ply. Sensitivity based design optimization is carried out for symmetric cross ply arrangement based on Pf, significantly influenced by the random parameters. This information may be used effectively for design optimization to achieve better strength and lighter weight of composite beam.

Inversed-Planned Respiratory Phase Gating in Lung Conformal Radiation Therapy

Toassess whether the optimal gating window for each beam during lung radiation therapy with respiratory gating will be dependent on a variety of patient-specific factors, such as tumor size and location and the extent of relative tumor and organ motion. To create optimal gating treatment plans, we started from an optimized clinical plan, created a plan per respiratory phase using the same beam arrangements, and used an inverse planning optimization approach to determine the optimal gating window for each beam and optimal beam weights (ie, monitor units). Two pieces of information were used for optimization: (1) the state of the anatomy at each phase, extracted from 4-dimensional computed tomography scans; and (2) the time spent in each state, estimated from a 2-minute monitoring of the patient's breathing motion. We retrospectively studied 15 lung cancer patients clinically treated by hypofractionated conformal radiation therapy, for whom 45 to 60Gy was administered over 3 to 15 fractions using 7 to 13 beams. Mean gross tumor volume and respiratory-induced tumor motion were 82.5cm3 and 1.0cm, respectively. Although patients spent most of their respiratory cycle in end-exhalation (EE), our optimal gating plans used EE for only 34% of the beams. Using optimal gating, maximum and mean doses to the esophagus, heart, and spinal cord were reduced by an average of 15% to 26%, and the beam-on times were reduced by an average of 23% compared with equivalent single-phase EE gated plans (P<.034, paired 2-tailed t test). We introduce a personalized respiratory-gating technique in which inverse planning optimization is used to determine patient- and beam-specific gating phases toward enhancing dosimetric quality of radiation therapy treatment plans.

Experimental investigation of the influence of joint connection details between existing buildings and added new frames on failure mode of retrofitted slabs for the vertical expansion of old buildings: Part II

SummaryDismantling entire existing buildings can be circumvented by adding additional frames to provide more residence, thus avoiding the need to reconstruct buildings. The alternatives are to construct new frames on old apartment buildings in order to avoid entirely dismantling existing buildings. The conventional wall frames for super structures added onto existing buildings are replaced by frames consisting of composite beams and steel columns to minimize beam depth and weight of the load from the added frames. This paper studied joint connection types for the rehabilitation of old buildings to withstand lateral loads. Details of the joints between newly added steel columns and existing slabs are presented in this paper. The structural behaviors of anchored joints between the new 3‐story composite frames and an existing 15‐floor building were significantly investigated through extensive experiments. Complex failure modes were identified based on the test of the wall foundation types and the anchor types connecting the base plates to existing slabs. The authors previously reported that the flexural load‐bearing capacity of slabs was the dominant load‐resisting capacity. The column–slab joint connections introduced with experimental demonstrations in this study are expected to contribute to enhancing the lateral capacity of the extended buildings.

Near real-time automated dose restoration in IMPT to compensate for daily tissue density variations in prostate cancer

Proton therapy is very sensitive to daily density changes along the pencil beam paths. The purpose of this study is to develop and evaluate an automated method for adaptation of IMPT plans to compensate for these daily tissue density variations. A two-step restoration method for ‘densities-of-the-day’ was created: (1) restoration of spot positions (Bragg peaks) by adapting the energy of each pencil beam to the new water equivalent path length; and (2) re-optimization of pencil beam weights by minimizing the dosimetric difference with the planned dose distribution, using a fast and exact quadratic solver. The method was developed and evaluated using 8–10 repeat CT scans of 10 prostate cancer patients. Experiments demonstrated that giving a high weight to the PTV in the re-optimization resulted in clinically acceptable restorations. For all scans we obtained V95% ⩾ 98% and V107% ⩽ 2%. For the bladder, the differences between the restored and the intended treatment plan were below +2 Gy and +2%-point. The rectum differences were below +2 Gy and +2%-point for 90% of the scans. In the remaining scans the rectum was filled with air, which partly overlapped with the PTV. The air cavity distorted the Bragg peak resulting in less favorable rectum doses.

Feasibility of robotic stereotactic body radiotherapy of lung tumors with kilovoltage x-ray beams.

Robotic Stereotactic body radiation therapy (SBRT) for lung tumors is treatment modality that, for cases of inoperable lung tumors, has shown excellent treatment outcomes. The typical photon energy when delivering this type of treatments is 6 MeV. In this work, using Monte Carlo simulation and realistic patient models we evaluate the characteristics of the absorbed dose distributions that result when x-ray beams of peak spectral energy of 220 keV are used to irradiate lung tumors assuming a robotic SBRT delivery mode. Both male and female patient models, based on voxelized phantoms, are used in our study. Two types of tumors are considered: centrally and peripherally located lung tumors. The Monte Carlo code PENELOPE was used to calculate absorbed dose distributions for each of the beams used in the treatments which were assumed circular with diameter ranging from 1 cm-3 cm. An optimization algorithm was then applied to determine the appropriate beam weight necessary to accomplish the treatment objectives. The feasibility of our proposed approach is determined based on the guidelines set by Radiation Therapy Oncology Group (RTOG) 0813 for central tumors and RTOG 0915 for peripheral tumors. While the dose to the skin and bony structures is higher for the kilovoltage treatment, they are within the safe limits established by both RTOG 0813 and 0915. Conversely, the maximum dose to distant structures, such as the heart wall and esophagus, are up to 10 Gy higher for some of the megavoltage treatments but, again, within the limits recommended by the aforementioned clinical protocols. We have shown that robotic SBRT of lung tumors using kilovoltage beams is feasible and that, therefore, it may represent an attractive alternative to the use of more expensive megavoltage linear accelerators, particularly in developing countries, where the high cost of such equipment poses an increasing economic burden.

4D VMAT planning and verification technique for dynamic tracking using a direct aperture deformation (DAD) method.

We developed a four‐dimensional volumetric modulated arc therapy (4D VMAT) planning technique for moving targets using a direct aperture deformation (DAD) method and investigated its feasibility for clinical use. A 3D VMAT plan was generated on a reference phase of a 4D CT dataset. The plan was composed of a set of control points including the beam angle, MLC apertures and weights. To generate the 4D VMAT plan, these control points were assigned to the closest respiratory phases using the temporal information of the gantry angle and respiratory curve. Then, a DAD algorithm was used to deform the beam apertures at each control point to the corresponding phase to compensate for the tumor motion and shape changes. Plans for a phantom and five lung cases were included in this study to evaluate the proposed technique. Dosimetric comparisons were performed between 4D and 3D VMAT plans. Plan verification was implemented by delivering the 4D VMAT plans on a moving QUASAR™ phantom driven with patient‐specific respiratory curves. The phantom study showed that the 4D VMAT plan generated with the DAD method was comparable to the ideal 3D VMAT plan. DVH comparisons indicated that the planning target volume (PTV) coverages and minimum doses were nearly invariant, and no significant difference in lung dosimetry was observed. Patient studies revealed that the GTV coverage was nearly the same; although the PTV coverage dropped from 98.8% to 94.7%, and the mean dose decreased from 64.3 to 63.8 Gy on average. For the verification measurements, the average gamma index pass rate was 98.6% and 96.5% for phantom 3D and 4D VMAT plans with 3%/3 mm criteria. For patient plans, the average gamma pass rate was 96.5% (range 94.5–98.5%) and 95.2% (range 94.1–96.1%) for 3D and 4D VMAT plans. The proposed 4D VMAT planning technique using the DAD method is feasible to incorporate the intra‐fraction organ motion and shape change into a 4D VMAT planning. It has great potential to provide high plan quality and delivery efficiency for moving targets.

Spatial light structuring using a combination of multiple orthogonal orbital angular momentum beams with complex coefficients.

Analogous to time signals that can be composed of multiple frequency functions, we use uniquely structured orthogonal spatial modes to create different beam shapes. We tailor the spatial structure by judiciously choosing a weighted combination of multiple modal states within an orthogonal orbital angular momentum (OAM) basis set, creating desired beam intensity "shapes." The weights of the OAM beams to be combined forms a Fourier pair with the spatial intensity distribution in the azimuthal direction of the resultant beam. As an example, we simulate and experimentally create various beam shapes by designing the weights of the combined OAM beams. We also find that 6× higher localized power, as compared to traditional beam combining, could be achieved by coherently combining nine orthogonal OAM beams.