Optimal Fluence Research Articles

Hydrogen-assisted pulsed KrF-laser irradiation for the in situ photoreduction of graphene oxide films

We report on the use of pulsed KrF-laser irradiation for the in situ reduction of graphene oxide (GO) films under both vacuum and partial hydrogen pressure. By exposing GO films to 500 pulses of a KrF-laser, at a fluence of 10mJ/cm2, their sheet resistance (Rs) is dramatically reduced from highly insulating (∼1010Ω/sq) to conductive values of ∼3kΩ/sq. By increasing the laser fluence, from 10 to 75mJ/cm2, we were able to identify an optimal fluence around 35mJ/cm2 that leads to highly conductive films with Rs values as low as 250Ω/sq and 190Ω/sq, under vacuum (10−5Torr) and 50mTorr of H2, respectively. Raman spectroscopy analyses confirmed the effective reduction of the KrF-laser irradiated GO films through the progressive recovery of the characteristic 2D band of graphene. Furthermore, systematic Fourier-transform infrared spectroscopy analysis has revealed that KrF-laser induced reduction of GO preferentially occurs through photodissociation and removal of carboxyl (COOH) and alcohol (OH) groups. A direct correlation is established between the electrical resistance of photoreduced GO films and their COOH and OH bond densities. The KrF-laser induced reduction of GO films is found to be more efficient under H2 background than under vacuum. It is concluded that our KrF-laser reduced GO films mainly consist of turbostratic graphite built from randomly organized few-layers-graphene building blocks, which contains some residual oxygen atoms and defects. Finally, by monitoring the KrF-laser fluence, it is shown that reduced GO films combining optical transmission as high as ∼80% along with sheet resistance as low as ∼500Ω/sq can be achieved with this room-temperature and on-substrate process. This makes the laser-based reduction process developed here particularly attractive for photovoltaic hybrid devices using silicon substrates.

SU‐E‐T‐250: New IMRT Sequencing Strategy: Towards Intra‐Fraction Plan Adaptation for the MR‐Linac

Purpose:To develop a new sequencer for IMRT planning that during treatment makes the inclusion of external factors possible and by doing so accounts for intra‐fraction anatomy changes. Given a real‐time imaging modality that will provide the updated patient anatomy during delivery, this sequencer is able to take these changes into account during the calculation of subsequent segments.Methods:Pencil beams are generated for each beam angle of the treatment and a fluence optimization is performed. The pencil beams, together with the patient anatomy and the above optimal fluence form the input of our algorithm. During each iteration the following steps are performed: A fluence optimization is done and each beam's fluence is then split to discrete intensity levels. Deliverable segments are calculated for each one of these. Each segment's area multiplied by its intensity describes its efficiency. The most efficient segment among all beams is then chosen to deliver a part of the calculated fluence and the dose that will be delivered by this segment is calculated. This delivered dose is then subtracted from the remaining dose. This loop is repeated until 90% of the dose has been delivered and a final segment weight optimization is performed to reach full convergence.Results:This algorithm was tested in several prostate cases yielding results that meet all clinical constraints. Quality assurance was performed on Delta4 and film phantoms for one of these prostate cases and received clinical acceptance after passing both gamma analyses with the 3%/3mm criteria.Conclusion:A new sequencing algorithm was developed to facilitate the needs of intensity modulated treatment. The first results on static anatomy confirm that it can calculate clinical plans equivalent to those of the commercially available planning systems. We are now working towards 100% dose convergence which will allow us to handle anatomy deformations.This work is financially supported by Elekta AB, Stockholm, Sweden.

SU-E-T-234: Modulated Photon Radiotherapy (XMRT):The Impact of Incorporating Energy Modulation Into Intensity Modulated Radiotherapy (IMRT) Optimization

Purpose: To develop a new radiotherapy plan optimization technique that, for a given organ geometry, will find the optimal photon beam energies and fluences to produce a desirable dose distribution. This new modulated (both in energy and fluence) photon radiotherapy (XMRT) was compared with intensity modulated radiotherapy (IMRT) for a simple organ geometry. Methods: The XMRT optimization was formulated using a linear programming approach where the objective function is the mean dose to the healthy organs and dose-point constraints were assigned to each organ of interest. The organ geometry consisted of a target, two organs at risk (OARs), and normal tissue. A seven-equispaced-coplanar beam arrangement was used. For conventional IMRT, only 6 MV beams were available, while XMRT was optimized using 6 and 18 MV beams. A prescribed dose (PD) of 72 GY was assigned to the target, with upper and lower bounds of 110% and 95% of the PD, respectively. Both OARs were assigned a maximum dose of 64 Gy, while the normal tissue was assigned a maximum dose of 66 Gy. A numerical solver, Gurobi, generated solutions for the XMRT and IMRT problems. The dose-volume histograms from IMRT and XMRT solutions were compared. Results: The maximum, minimum, mean, and homogeneity of the dose to the target were comparable between IMRT and XMRT. Though IMRT had improved dose conformity relative to XMRT, XMRT reduced the mean dose to both OARs by more than 1 Gy. For normal tissue, an increase of 5 Gy in mean dose and 27 percent in integral dose was seen for IMRT relative to XMRT. Conclusion: This work demonstrates the benefits of simultaneously modulating photon beam energy and fluence using our XMRT approach in a given phantom geometry. While target coverage was comparable, dose to healthy structures was reduced using XMRT.

A fast beam orientation optimization method that enforces geometric constraints in IMRT for total marrow irradiation

AbstractThe beam orientation optimization (BOO) problem for intensity‐modulated radiation therapy (IMRT) is the selection of beams for radiation delivery. Conventionally, it is desirable for beams to be spatially separated to ensure a homogeneous dose. However, many BOO approaches yield clustered beams. This issue is especially prevalent for total marrow irradiation (TMI), where the target is very large and spread throughout the patient's body. Based on previous set‐cover formulations of the BOO problem for TMI‐IMRT, we propose an extension that enforces geometric beam constraints by iteratively removing beams violating geometric constraints within the set‐cover framework. After beams are selected, they are used as input to a fluence map optimization solver to obtain optimal fluence maps. Results for a clinical TMI case meet clinical guidelines for target coverage and differentiation of organ and target doses.

Thermoelectric properties of Zn4Sb3/CeFe(4−)Co Sb12 nano-layered superlattices modified by MeV Si ion beam

We prepared multilayers of superlattice thin film system with 50 periodic alternating nano-layers of semiconducting half-Heusler β-Zn4Sb3 and skutterudite CeFe2Co2Sb12 compound thin films using ion beam assisted deposition (IBAD) with Au layers deposited on both sides as metal contacts. The deposited multilayer thin films have alternating layers about 5 nm thick. The total thickness of the multilayer system is 275 nm. The superlattices were then bombarded by 5 MeV Si ion at six different fluences to form nano-cluster structures. The film thicknesses and composition were monitored by Rutherford backscattering spectrometry (RBS) before and after MeV ion bombardment. We have measured the thermoelectric efficiency, Figure of Merit ZT, of the fabricated device by measuring the cross plane thermal conductivity by the 3rd harmonic (3ω) method, the cross plane Seebeck coefficient, and the electrical conductivity using the van der Pauw method before and after the MeV ion bombardments. We reached the remarkable thermoelectric Figure of Merit results at optimal fluences.

Corrosion and wear mechanisms of aluminum alloys surface reinforced by multicharged N-implantation

Abstract Samples of Al-1050 and of Al-2024 aluminum alloys were implanted by means of nitrogen multicharged ion beam provided by an ECR source. Wear and corrosion tests were performed in order to qualify and quantify the surface enhancement created by implantation. The tests performed, respectively, using ball-on-disk set up and linear polarization technique, combined with SEM observations and correlated with microstructural study already published, made possible the identification of damaging mechanisms of nitrogen implanted aluminum surface. The study underlines the importance which has to be given to the implanted fluence and to the initial microstructure, if a consistent surface improvement is targeted. It is demonstrated in this work that the improvement of wear resistance is strongly linked to the intrinsic properties of the nitride protective layer and not to the initial microstructure which only affects optimum fluence. Corrosion tests reveal inverse tendency. The alloy composition is, in this case, of importance, contrarily to implanted fluencies which do not affect the results. This study also shows that if nitrogen implantation is good for surface resistance, a pit (corrosion) or a crack (wear) of implanted surface causes more damage than corrosion or wear of untreated surface.

Ultra-fast fluence optimization for beam angle selection algorithms

Beam angle selection (BAS) including fluence optimization (FO) is among the most extensive computational tasks in radiotherapy. Precomputed dose influence data (DID) of all considered beam orientations (up to 100 GB for complex cases) has to be handled in the main memory and repeated FOs are required for different beam ensembles. In this paper, the authors describe concepts accelerating FO for BAS algorithms using off-the-shelf multiprocessor workstations. The FO runtime is not dominated by the arithmetic load of the CPUs but by the transportation of DID from the RAM to the CPUs. On multiprocessor workstations, however, the speed of data transportation from the main memory to the CPUs is non-uniform across the RAM; every CPU has a dedicated memory location (node) with minimum access time. We apply a thread node binding strategy to ensure that CPUs only access DID from their preferred node. Ideal load balancing for arbitrary beam ensembles is guaranteed by distributing the DID of every candidate beam equally to all nodes. Furthermore we use a custom sorting scheme of the DID to minimize the overall data transportation. The framework is implemented on an AMD Opteron workstation. One FO iteration comprising dose, objective function, and gradient calculation takes between 0.010 s (9 beams, skull, 0.23 GB DID) and 0.070 s (9 beams, abdomen, 1.50 GB DID). Our overall FO time is < 1 s for small cases, larger cases take ~ 4 s. BAS runs including FOs for 1000 different beam ensembles take ~ 15–70 min, depending on the treatment site. This enables an efficient clinical evaluation of different BAS algorithms.

Feasibility of using intermediate x‐ray energies for highly conformal extracranial radiotherapy

To investigate the feasibility of using intermediate energy 2 MV x-rays for extracranial robotic intensity modulated radiation therapy. Two megavolts flattening filter free x-rays were simulated using the Monte Carlo code MCNP (v4c). A convolution/superposition dose calculation program was tuned to match the Monte Carlo calculation. The modeled 2 MV x-rays and actual 6 MV flattened x-rays from existing Varian Linacs were used in integrated beam orientation and fluence optimization for a head and neck, a liver, a lung, and a partial breast treatment. A column generation algorithm was used for the intensity modulation and beam orientation optimization. Identical optimization parameters were applied in three different planning modes for each site: 2, 6 MV, and dual energy 2/6 MV. Excellent agreement was observed between the convolution/superposition and the Monte Carlo calculated percent depth dose profiles. For the patient plans, overall, the 2/6 MV x-ray plans had the best dosimetry followed by 2 MV only and 6 MV only plans. Between the two single energy plans, the PTV coverage was equivalent but 2 MV x-rays improved organs-at-risk sparing. For the head and neck case, the 2 MV plan reduced lips, mandible, tongue, oral cavity, brain, larynx, left and right parotid gland mean doses by 14%, 8%, 4%, 14%, 24%, 6%, 30% and 16%, respectively. For the liver case, the 2 MV plan reduced the liver and body mean doses by 17% and 18%, respectively. For the lung case, lung V 20, V 10, and V5 were reduced by 13%, 25%, and 30%, respectively. V 10 of heart with 2 MV plan was reduced by 59%. For the partial breast treatment, the 2 MV plan reduced the mean dose to the ipsilateral and contralateral lungs by 27% and 47%, respectively. The mean body dose was reduced by 16%. The authors showed the feasibility of using flattening filter free 2 MV x-rays for extracranial treatments as evidenced by equivalent or superior dosimetry compared to 6 MV plans using the same inverse noncoplanar intensity modulated planning method.

Ion beam modification of the structure and properties of hexagonal boron nitride: An infrared and X-ray diffraction study

The vibrational mode for the cubic symmetry of boron nitride (BN) has been produced by boron ion implantation of hexagonal boron nitride (h-BN). The optimum fluence at 150keV was found to be 5×1014ions/cm2. The presence of the c-BN phase was inferred using glancing incidence XRD (GIXRD) and Fourier Transform Infrared Spectroscopy (FTIR). After implantation, Fourier Transform Infrared Spectroscopy indicated a peak at 1092cm−1 which corresponds to the vibrational mode for nanocrystalline BN (nc-BN). The glancing angle XRD pattern after implantation exhibited c-BN diffraction peaks relative to the implantation depth of 0.4μm.

A study on the pulsed laser printing of liquid-phase exfoliated graphene for organic electronics

The aim of this work is the pulsed laser printing of liquid-phase exfoliated graphene in the nanosecond regime and the optimization of the printing process on Si/SiO2 and flexible polymer substrates (polyethylene naphthalate) via the laser-induced forward transfer technique (LIFT). The laser printing conditions and the optimum energy fluence window for reproducible deposition have been investigated, while the deposited graphene features have been studied morphologically and structurally by means of optical microscopy, micro-Raman spectroscopy and electrical characterization. LIFT experiments were carried out using the fourth harmonic (266 nm) of a pulsed ns Nd:YAG laser combined with a high-power imaging micromachining system to monitor the printing process throughout the experiments. The irradiation of our graphene solution resulted in the deposition of well-resolved patterns on different surfaces, highlighting LIFT as an alternative technique for the printing and patterning of liquid-phase exfoliated graphene for organic electronics applications.

Luminescence of a titanate compound under europium ion implantation

The ability to incorporate europium ions in a near-surface layer of a nonlinear optical material KTiOPO4 by ion implantation is reported here. Europium diffusion as well as surface modification were characterized after annealing using RBS. The photoluminescence of the as-implanted samples indicates that the creation of defects gives rise to green visible emission centered about 550nm. Annealing up to 1000°C does not allow the oxidation to the 3+ valence state of the europium ion. However it is shown that annealing up to such high temperature gives rise to an intense near infra-red photoluminescence in the range 800–1100nm in implanted samples at an optimal fluence of 2×1013 europium ions/cm2.

Surface Structuring with Ultra-short Laser Pulses: Basics, Limitations and Needs for High Throughput

Abstract The interest using ultra-short pulsed laser systems for industrial applications increased remarkably as reliable high power systems with average power exceeding several 10W became available, facilitating the development of high throughput processes. Exhausting the potential of these systems demands an optimized process and an adapted process strategy as well. For surface structuring it can be shown that in case of metals and many other materials, the ablation process itself shows a maximum efficiency at an optimum fluence. The corresponding removal rate directly scales with the average power and depends on the threshold fluence and the energy penetration depth into the material. These parameters are material depending but they change with the pulse duration and are also subject to incubation effects. Working at the optimum point with highest efficiency and keeping the high machining quality sets conditions for the process strategy which can only be fulfilled by either very high marking speeds, not available with common beam guiding systems, or high spot areas as they can e.g. achieved with multi-spots by a diffractive optical element or a spatial light modulator.

Gravity-assisted pulsed laser ablation for fabrication of compositional gradient thin film

A compositional gradient thin film of Fe/Si was fabricated by gravity-assisted pulsed laser ablation under a gravity field of 5.3×104m/s2. Systematic experiments were conducted by varying the values of several parameters including the gravity, distance between the target and the substrate, and laser fluence.The atomic fraction of Fe was measured by scanning electron microscopy/energy dispersive X-ray spectroscopy. We found that the atomic fraction of Fe increases along the gravity direction, i.e., it exhibits an apparent spatial gradient.We also found the optimal laser fluence at which a thin film having the largest possible spatial compositional gradient is obtained. It is shown that the surface energy density on the substrate surface is the key parameter to control the compositional distribution. The relatively high laser fluence as well as the very narrow space between the target and the substrate are found to be essential to sputter the film material. A plausible model is presented to explain the experimental data.

515 nm 피코초 레이저를 이용한 구리 어블레이션 공정의 최적 에너지밀도에 대한 이론적 분석

Ultra-short laser pulses are effective, when high requirements concerning accuracy, surface roughness and heat affected zone are demanded for surface structuring. In particular, picosecond laser systems that are suited to be operated in industrial environments are of great interest for many practical applications. This paper focused on inducing optimum process parameters for higher volume ablation rate by analyzing a relationship between crater diameter and optical spot size. In detail, the dependency of the volume ablation rate, penetration depth and threshold fluence on the pulse duration 8 ps and wavelength of 515 nm was discussed. The experimental results showed that wavelength of 515 nm resulted in less threshold fluence (0.075 J/㎠) on copper than IR wavelength (0.3 J/㎠). As a result, it was possible that optimum fluence for higher volume ablation rate was achieved with 0.28 J/㎠.

Influence of sputtering conditions on the optical and electrical properties of laser-annealed and wet-etched room temperature sputtered ZnO:Al thin films

We explore the influence of the sputtering deposition conditions on the outcome of an excimer laser anneal and chemical etching process with the goal of producing highly textured substrates for thin film silicon solar cells. Aluminum-doped zinc oxide (ZnO:Al) thin films were prepared on glass substrates by radio frequency magnetron sputtering from a ceramic target at room temperature. The effects of the process pressure (0.11–1.2Pa) and oxygen flow (0–2sccm) on the optical and electrical properties of ZnO:Al thin films have been studied both before and after an excimer laser annealing treatment followed by a dilute HCl chemical etch. The as-deposited films varied from completely opaque to yellowish. Thin film laser annealing dramatically improves the optical properties of the most opaque thin films. After laser annealing at the optimum fluence, the average transmittance in the visible wavelength range was around 80% for most films, and reasonable electrical performance was obtained for the films deposited at lower pressures and without oxygen flux (7Ω/□ for films of 1μm). After etching, all films displayed a dramatic improvement in haze, but only the low pressure, low oxygen films retained acceptable electrical properties (<11Ω/□).

Surface morphology and photoelectric properties of fluorine-doped tin oxide thin films irradiated with 532nm nanosecond laser

In order to improve the transparency and conductivity of the commercial fluorine-doped tin oxide (FTO) films deposited on glass substrates, a nanosecond pulsed laser with a wavelength of 532 nm was used to irradiate the surfaces of the films. The effects of laser fluence and scanning speed on the surface morphology, photoelectric property and overall quality of the FTO films were investigated. The FTO films which were subjected to lower laser fluences or higher scanning speeds achieved gentle laser annealing effects, resulting in unapparent changes in surface morphology. These changes caused enhancements in optical transmittance and decrease in sheet resistance. The FTO films irradiated with higher laser fluences or lower scanning speeds melted and ablated, causing the optical transmittance and the electrical conductivity of the films to drop significantly. Experimental results indicated that the optimum irradiation fluence and scanning speed for 532 nm nanosecond laser annealing of FTO films were 1.02 J/ cm2 and 10 mm/ s, respectively. The corresponding film had an obvious increase in crystallite size. The RMS roughness, the average optical transmittance in the waveband of 380–780 nm and the figure of merit were increased from 100.5 nm, 75.4% and 5.82×10−3 Ω−1 to 113.0 nm, 82.7% and 17.00×10−3 Ω−1 respectively, while the sheet resistance was reduced from 10.2 Ω/□ to 8.8 Ω/□.

TU‐E‐141‐01: Using Respiratory Motion as a Degree of Freedom Through Weighted Phase‐Correlated 4D Planning

Purpose: We investigate a novel 4D treatment planning strategy that exploits respiratory motion as an additional degree of freedom. The key idea is to deliver more (less) fluence when the tumor target is out‐of‐line (in‐line) with an organ‐at‐risk (OAR). We have developed our weighted‐phase‐correlated (WPC) 4D treatment planning paradigm on a commercial platform (Eclipse, Varian) and designed it for an “open‐gate” delivery, where beam is on throughout the respiratory cycle. Methods: 4DCT data were collected from twenty lung SBRT patients who exhibited &gt;= 5mm tumor motion despite the use of abdominal compression. For each patient, (i)f × θ 3D‐conformal plans were created and corresponding 3D dose distributions were calculated, where θ = number of respiratory phases (10) and f = number of fields (7 – 12) (ii) For each beam, the dose from nine phases was registered to the reference phase (end‐exhale) using a GPU‐based B‐spline deformable registration, NiftyReg. (iv) using dynamic‐penalized‐likelihood optimization, fluence weights were optimized over all dose distributions. This key step ensured that for each motion‐induced state of the anatomy, an optimal fluence would be delivered. (v) A summed fluence map was imported back into Eclipse and an MLC leaf sequence was generated. In order to ensure deliverability, a maximum leaf velocity constraint (v &lt;= 3.5 cm/s) was imposed. Each WPC‐4D plan was compared with the corresponding, clinically‐delivered, ITV‐based plan. Results: In all 20 patients, WPC‐4D plans gave PTV conformity comparable to ITV‐based plans. The percent dose‐sparing (maximum, average) achieved using WPC‐4D compared to ITV‐based planning: Spinal Cord Dmax=(66,20); Esophagus Dmax=(94,29), Heart Dmax=(84,30); Heart Dmean=(82,35); Lung V20=(27,7); Lung Dmean=(27,13). Conclusion: We have developed a novel 4D‐planning solution that exploits rather than trying to mitigate respiratory motion. Initial results indicate that this approach achieves significant dose‐sparing in OARs compared to conventional motion‐management (abdominal compression) + ITV‐based planning. This research was partially supported by Varian Medical Systems, Palo Alto, CA

SU‐E‐T‐159: Development of An Independent, Monte Carlo, Dose Calculation, Quality Assurance Tool for Clinical Trials

Purpose: To commission a multiple‐source Monte Carlo model of Elekta linear accelerator beams of nominal energies 6MV and 10MV. Methods: A three source, Monte Carlo model of Elekta 6 and 10MV therapeutic x‐ray beams was developed in a two‐step process. Energy spectra of each of three sources, a primary source corresponding to photons created in the target, an extra‐focal source corresponding to photons originating from scattered events in the linac head, and an electron contamination source, were determined. The two photon sources were determined by an optimization process that fit the relative fluence of 0.25 MeV energy bins to the product of Fatigue‐Life and Fermi functions to match calculated percent depth dose (PDD) data with that measured in water for a 10×10cm2 field. Off‐axis effects were modeled by fitting the off‐axis fluence to a piece‐wise linear function through optimization of relative fluence to match calculated dose profiles with measured dose profiles for a 40×40cm2 field. A 3rd degree polynomial was used to describe the off‐axis half‐value layer as a function of off‐axis angle. The model was then commissioned by comparing calculated PDDs and dose profiles for field sizes ranging from 3×3cm2 to 30×30cm2 to those obtained from measurements. Results: Agreement between calculated and measured data was evaluated using 2%/2mm global gamma criterion for field sizes of 3×3, 5×5, 10×10, 15×15, 20×20, and 30×30cm2. Along the central axis of the beam 99.5% and 99.6% of all data passed the criterion for 6 and 10MV models, respectively. Dose profiles at depths of dmax, 5, 10, 20, and 25cm agreed with measured data for 95.4% and 99.2% of data tested for 6 and 10MV models, respectively. Conclusion: A Monte Carlo multiple‐source model for Elekta 6 and 10MV therapeutic x‐ray beams has been developed as a quality assurance tool for clinical trials. This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.

WE‐C‐103‐02: BEST IN PHYSICS (IMAGING) ‐ Strategies for Fluence Field Modulated CT

Purpose: Fluence field modulated computed tomography (FFMCT) proposes using dynamically changing fluence fields during image acquisition for advanced dose reduction and image quality optimization. This work compares the relative effectiveness of FFMCT when the modulator is subject to limiting constraints, such as might be introduced by real physical modulators. Methods: Several tasks were defined for a simulated anthropomorphic chest phantom by defining different regions of high and low image quality, characterized by the signal standard deviation. Optimal modulation fluence patterns were sought for each task using a simulated annealing optimization script, which attempts to achieve the image quality plan under a global dosimetric constraint. Optimization was repeated under different types of modulation constraints representing several different realistic modulator designs (e.g. dynamically varying apertures, discrete fixed apertures, or shaped static filters). Results were compared based on predicted dose outcomes and agreement with the prescribed image quality criteria. Results: Compared to static modulators (e.g. bowtie filter), fluence field modulation approaches utilizing dynamically changing modulators showed improved agreement with the prescribed outcomes, including greater uniformity of image noise over the target regions of interest. For some tasks, highly constrained modulators approached outcomes similar to that of the ideal unconstrained case. Average integral dose reduction compared to a uniform reference field exceeded 20% for all modulation methods. Conclusions: The results support that FFMCT can achieve regionally varying image quality distributions in good agreement with user‐prescribed values, while reducing dose to the patient. The outcomes also support that the benefits of fluence field modulation may be yielded even when highly constrained modulators are used, suggesting practical, feasible implementations of FFMCT may be possible. Research funded in part by the Ontario Graduate Scholarships (OGS), Natural Sciences and Engineering Research Council of Canada (NSERC), Elekta Inc. and the Ontario Research Fund (ORF)

SU‐E‐T‐655: Dose Matrix Sampling Heuristic Applied to Improving IMRT Optimization Efficiency

Purpose: To introduce a novel heuristic that increases the efficiency of IMRT fluence optimization, by employing sampled dose values in evaluation of the objective function. Methods: Pencil beam dose matrices were computed using CERR, for 1‐cm square beamlets. These matrices were then uniformly sampled at various rates (1–10%), and the resulting approximate dose‐volume values compared to the true values for various fractional volumes. By varying the beamlet weights for 10,000 random sets, we estimated the error in these approximations to be &lt;1%. For IMRT optimization, a least squares objective function was used, based on clinical dose‐volume objectives. In the initial stage of optimization, the method utilizes approximate dose values, determined by 1% or 5% dose matrix sampling, in objective function evaluation. This results in a significant reduction in computation time, since the manipulation of large dose matrices is avoided. The optimized beamlet weights are subsequently used as the starting point in the final optimization. This method was evaluated on prostate bed and esophageal patients, by comparing the CPU time required with that of a standard optimization method. The downhill simplex and simulated annealing algorithms were used for the evaluations. Results: The prostate bed patient was optimized with both a 1% and 5% sampling rate resulting in patient treatment plans of equal score, and a time savings of 18% and 42%, respectively. The esophageal patient was optimized with a 5% sampling rate and provided a score improvement of 23%, with a time savings of 17%. Conclusion: We have developed a method to be used in conjunction with existing optimization algorithms, which dramatically decreases the number of standard objective function evaluations and computation time. This technique may be applied to any type of optimization algorithm and set of clinical variables requiring pre‐computed dose matrices and objective functions involving dose‐volume objectives (eg. multi‐criteria optimization).