Optimal Fluence Research Articles

TH‐C‐137‐02: Robotic Radiotherapy Using Intermediate Beam Energies

Purpose: Intermediate energy (1–2MV) x‐rays have steeper depth dose drop‐off and sharper penumbra than commonly used 6MV x‐rays. Dosimetry benefits of these characteristics are studied on a robotic non‐coplanar planning and delivery platform. Methods: Dose of 1MV and 6MV x‐rays was calculated using the convolution/superposition algorithm with heterogeneity correction and Monte Carlo calculated dose kernels. The X‐ray spectrum was adjusted to match depth dose curves of published data. Thirty noncoplanar beams were selected by a pricing approach from a candidate beam pool, which consisted of 1162 uniformly distributed non‐coplanar beams minus beams leading to collision. The collision model was fit to individual treatment sites. Fluence optimization based on 5 mm MLC was performed after adding each beam. Identical objective functions for PTV and organs‐at‐risk (OARs) were employed in the 3 planning scenarios: 1 MV alone, 6 MV alone and the combination of 1 MV and 6 MV beams (1&6 MV) with the prescription dose covering 95% of the PTV. Four representative cases from the following anatomical sites were included in the study: head and neck, partial breast, lung and liver. Results: 1 MV and 1&6 MV plans provided superior OAR sparing for head, liver, partial breast and lung cases while maintaining the same PTV coverage. Compared with 6 MV plan, 1 MV plans reduced the integral dose by 25%, 23%, 19% and 9% for lung, breast, head and liver cases respectively. The plan quality of 1&6 MV plans, which primarily was slightly superior to that of the 1MV only plans. Conclusion: The dosimetric drawbacks of intermediate energy x‐rays are higher skin doses and shallower penetration when few of them are used on a coplanar platform but these drawbacks were effectively overcome on a highly non‐coplanar treatment planning platform, where its advantages of normal tissue sparing and sharp penumbra are manifested.

Direct pulsed laser crystallization of nanocrystals for absorbent layers in photovoltaics: Multiphysics simulation and experiment

Direct pulsed laser crystallization (DPLC) of nanoparticles of photoactive material—Copper Indium Selenide (nanoCIS) is investigated by multiphysics simulation and experiments. Laser interaction with nanoparticles is fundamentally different from their bulk counterparts. A multiphysics electromagnetic-heat transfer model is built to simulate DPLC of nanoparticles. It is found smaller photoactive nanomaterials (e.g., nanoCIS) require less laser fluence to accomplish the DPLC due to their stronger interactions with incident laser and lower melting point. The simulated optimal laser fluence is validated by experiments observation of ideal microstructure. Selectivity of DPLC process is also confirmed by multiphysics simulation and experiments. The combination effects of pulse numbers and laser intensity to trigger laser ablation are investigated in order to avoid undesired results during multiple laser processing. The number of pulse numbers is inversely proportional to the laser fluence to trigger laser ablation.

Beam angle optimization for intensity-modulated radiation therapy using a guided pattern search method

Generally, the inverse planning of radiation therapy consists mainly of the fluence optimization. The beam angle optimization (BAO) in intensity-modulated radiation therapy (IMRT) consists of selecting appropriate radiation incidence directions and may influence the quality of the IMRT plans, both to enhance better organ sparing and to improve tumor coverage. However, in clinical practice, most of the time, beam directions continue to be manually selected by the treatment planner without objective and rigorous criteria. The goal of this paper is to introduce a novel approach that uses beam’s-eye-view dose ray tracing metrics within a pattern search method framework in the optimization of the highly non-convex BAO problem. Pattern search methods are derivative-free optimization methods that require a few function evaluations to progress and converge and have the ability to better avoid local entrapment. The pattern search method framework is composed of a search step and a poll step at each iteration. The poll step performs a local search in a mesh neighborhood and ensures the convergence to a local minimizer or stationary point. The search step provides the flexibility for a global search since it allows searches away from the neighborhood of the current iterate. Beam’s-eye-view dose metrics assign a score to each radiation beam direction and can be used within the pattern search framework furnishing a priori knowledge of the problem so that directions with larger dosimetric scores are tested first. A set of clinical cases of head-and-neck tumors treated at the Portuguese Institute of Oncology of Coimbra is used to discuss the potential of this approach in the optimization of the BAO problem.

Structural and Magnetic Characterizations of 200 MeV Ag<sup>15+</sup> Irradiated Bi<sub>x</sub>Co<sub>2-x</sub>MnO<sub>4</sub> Thin Films

The effect of swift heavy ion (SHI) irradiation on the structural and magnetic properties of thin films of Bi-substituted Co2MnO4 prepared by Pulsed Laser Deposition (PLD) is presented. XRD analysis reveals that the BixCo2-x MnO4 (x = 0.0, 0.1 & 0.3) films grown on amorphous quartz, and crystalline LaAlO3 (LAO) exhibited single phase cubic spinel structure with low strain before and after the irradiation at 200 MeV Ag15+ ions at three fluence values 1 × 1011, 5 × 1011 and 1 × 1012 ions/cm2. DC magnetization hysteresis loop study of the films revealed a ferrimagnetic ordering below the transition temperature ~ 185 K and the saturation magnetization was increased by the irradiation at optimal fluence value 51011 ions/cm2. X-ray Magnetic Circular Dichroism (XMCD) studies showed the antiparallel alignments of Co and Mn magnetic moments.

Laser surface modification of polyethersulfone films: effect of laser wavelength on biocompatibility

In this paper laser ablation of polyethersulfone (PES) films regarding to the change in biocompatibility of the surface is investigated at 3 different wavelengths of 193nm (ArF), 248 nm (KrF) and 308 nm (XeCl). The optimum laser fluence and number of pulses for the improvement of the surface biocompatibility is found by examination of the surface behavior in contact with platelets and fibroblasts cells at 3 wavelengths. These biological modifications are explained by alteration of the surface morphology and chemistry following irradiation. The results show that the KrF laser is the best choice for treatment of PES in biological applications.

Sinogram analysis of aperture optimization by iterative least-squares in volumetric modulated arc therapy

Iterative least-squares (ILS) has been used in CT image reconstruction for many years and has also been successfully applied to fluence optimization in intensity-modulated radiation therapy. However, most optimization schemes use aperture optimization, where it is not so clear how to apply ILS. This study therefore proposes a method of using ILS for aperture optimization in volumetric modulated arc therapy (VMAT) and evaluates the performance by comparing with segment weight optimization. The AutoBeam inverse treatment planning system was used to optimize fluence, sequence the fluence into deliverable apertures and then optimize these apertures and their accompanying segment weights. ILS was used for both the fluence and aperture optimization stages. In the aperture optimization, if the dose beyond a multileaf collimator (MLC) leaf was too high, that leaf was closed up slightly, and if the dose underneath a leaf was too low, that leaf was opened up slightly. To evaluate this heuristic, five prostate and pelvic nodes patients were retrospectively planned. To evaluate the performance of the aperture optimization, these plans were compared to those produced using segment weight optimization alone after sequencing. Sinograms showing effective monitor units delivered by MLC leaf pairs over the course of beam delivery were used to elucidate the comparison. It was found that aperture optimization with ILS produced treatment plans which were conformal, with dose deposition easily controlled by a few clinical objectives. Sinogram analysis showed that aperture optimization with ILS produced less leaf motion and less monitor unit variation between control points than when using segment weight optimization only. By converting fluence corrections to aperture shape corrections, ILS can be used as a powerful method for VMAT aperture optimization. Sinogram analysis provides a clear insight into the behaviour of the heuristic.

Fabrication and characterization of Ag-implantation modificated TiO2 films followed with thermal annealing

TiO2 thin films fabricated by direct-current (DC) reactive magnetron sputtering were implanted with Ag ions at 30kV to fluences of 1×1016, 3×1016 and 5×1016ions/cm2, and subsequently annealed at 500°C for 2h under Ar ambient. Cross-sectional transmission electron microscopy and X-ray photoelectron spectroscopy studies showed that the implanted Ag ions were incorporated in TiO2 to form Ag2O after thermal annealing. The Ag-implanted TiO2 film with fluence of 3×1016ions/cm2 after an annealing showed improved photocatalytic efficiency comparing to TiO2 film. When the implantation fluence increased to the 5×1016ions/cm2, the sample showed poorer phtotcatalytic efficiency. These results indicate that there exists an optimum Ag ions implantation fluence. The photocatalytic behavior can be explained by the effect of Ag+ on the physicochemical properties in terms of electronic structures and film texture.

Analysis of wavelength influence on a-Si crystallization processes with nanosecond laser sources

In this work we present a detailed study of the wavelength influence in pulsed laser annealing of amorphous silicon thin films, comparing the results for material modification at different fluence regimes in the three fundamental harmonics of standard DPSS (diode pumped solid state) nanosecond laser sources, UV (355nm), visible (532nm) and IR (1064nm).The crystalline fraction (% crystalline silicon) profiles resulted from irradiation of amorphous silicon thin film samples are characterized with MicroRaman techniques. A finite element numerical model (FEM) is developed in COMSOL to simulate the process. The crystalline fraction results and the local temperature evolution in the irradiated area are presented and analyzed in order to establish relevant correlation between theoretical and experimental results.For UV (355nm) and visible (532nm) wavelengths, the results of the numerical model are presented together with the experimental results, proving that the process can be easily predicted with an essentially physical model based on heat transport at different wavelengths and fluence regimes. The numerical model helps to establish the optimal operation fluence regime for the annealing process.

High Density Cobalt Nanostructures by Ion Beam Induced Dewetting

Ion beam induced dewetting of thin metallic films has emerged as a promising way to grow metallic nanoparticles in a controlled manner. Metal films tend to dewet from non-reactive substrates upon heating, forming islands, due to the high energies of metal surfaces and interfaces. Ion beam irradiation triggers the self organized formation of nanostructures, and allows control over nanoparticle size distributions, since dewetting is initiated by highly localized (in space and time) thermal spikes along the ion track. From 25 nm cobalt films grown by e-beam evaporation, followed by irradiation with 100 keV Ar+ ions, a high density array of nanostructures is formed. AFM scans show their morphological evolution as a function of ion fluence; optimum fluences for narrow particles size distribution have been identified. Glancing angle XRD and Rutherford Backscattering Spectroscopy show evidence of increased substrate exposure as dewetting proceeds. RBS indicates that within the optimal fluence range for nanostructure formation, sputtering and ion beam mixing are not the significant effects in this system. We present ion beam induced dewetting as a well controlled method of forming nanostructured catalysts from metal films. [DOI: 10.1380/ejssnt.2013.99]

Improvement of dose distribution with irregular surface compensator in whole breast radiotherapy.

Aim of this study was to compare the dosimetric aspects of whole breast radiotherapy (WBRT) between an irregular surface compensator (ISC) and a conventional tangential field technique using physical wedges. Treatment plans were produced for 20 patients. The Eclipse treatment planning system (Varian Medical Systems) was used for the dose calculation: For the physical wedge technique, the wedge angle was selected to provide the best dose homogeneity; for the ISC technique, the fluence editor application was used to extend the optimal fluence. These two treatment plans were compared in terms of doses in the planning target volume, the dose homogeneity index, the maximum dose, ipsilateral lung and heart doses for left breast irradiation, and the monitor unit counts required for treatment. Compared with the physical wedge technique, the ISC technique significantly reduced the dose homogeneity index, the maximum dose, the volumes received at 105% of the prescription dose, as well as reducing both the ipsilateral lung and heart doses (P < 0.01 for all comparisons). However, the monitor unit counts were not significantly different between the techniques (P > 0.05). Thus, the ISC technique for WBRT enables significantly better dose distribution in the planning target volume.

Comparison of beam angle selection strategies for intracranial IMRT

Various strategies to select beneficial beam ensembles for intensity-modulated radiation therapy (IMRT) have been suggested over the years. These beam angle selection (BAS) strategies are usually evaluated against reference configurations applying equispaced coplanar beams but they are not compared to one another. Here, the authors present a meta analysis of four BAS strategies that incorporates fluence optimization (FO) into BAS by combinatorial optimization (CO) and one BAS strategy that decouples FO from BAS, i.e., spherical cluster analysis (SCA). The underlying parameters of the BAS process are investigated and the dosimetric benefits of the BAS strategies are quantified. For three intracranial lesions in proximity to organs at risk (OARs) the authors compare treatment plans applying equispaced coplanar beam ensembles with treatment plans using five different BAS strategies, i.e., four CO techniques and SCA, to establish coplanar and noncoplanar beam ensembles. Treatment plans applying 5, 7, 9, and 11 beams are investigated. For the CO strategies the authors perform BAS runs with a 5°, 10°, 15°, and 20° angular resolution, which corresponds to a minimum of 18 coplanar and a maximum of 1400 noncoplanar candidate beams. In total 272 treatment plans with different BAS settings are generated for every patient. The quality of the treatment plans is compared based on the protection of OARs yet integral dose, target homogeneity, and target conformity are also considered. It is possible to reduce the average mean and maximum doses in OARs by more than 4 Gy (1 Gy) with optimized noncoplanar (coplanar) beam ensembles found with BAS by CO or SCA. For BAS including FO by CO, the individual algorithm used and the angular resolution in the space of candidate beams does not have a crucial impact on the quality of the resulting treatment plans. All CO algorithms yield similar target conformity and slightly improved target homogeneity in comparison to equispaced coplanar setups. Furthermore, optimized coplanar (noncoplanar) beam ensembles enabled more than a 6% (5%) reduction of the integral dose. For SCA, however, integral dose was increased and target conformity was decreased in comparison to equispaced coplanar setups-especially for a small number of beams. Both BAS strategies incorporating FO by CO and independent BAS strategies excluding FO provide dose savings in OARs for optimized coplanar and especially noncoplanar beam ensembles; they should not be neglected in the clinic.

Characterizing the combinatorial beam angle selection problem

The beam angle selection (BAS) problem in intensity-modulated radiation therapy is often interpreted as a combinatorial optimization problem, i.e. finding the best combination of η beams in a discrete set of candidate beams. It is well established that the combinatorial BAS problem may be solved efficiently with metaheuristics such as simulated annealing or genetic algorithms. However, the underlying parameters of the optimization process, such as the inclusion of non-coplanar candidate beams, the angular resolution in the space of candidate beams, and the number of evaluated beam ensembles as well as the relative performance of different metaheuristics have not yet been systematically investigated. We study these open questions in a meta-analysis of four strategies for combinatorial optimization in order to provide a reference for future research related to the BAS problem in intensity-modulated radiation therapy treatment planning. We introduce a high-performance inverse planning engine for BAS. It performs a full fluence optimization for ≈3600 treatment plans per hour while handling up to 50 GB of dose influence data (≈1400 candidate beams). For three head and neck patients, we compare the relative performance of a genetic, a cross-entropy, a simulated annealing and a naive iterative algorithm. The selection of ensembles with 5, 7, 9 and 11 beams considering either only coplanar or all feasible candidate beams is studied for an angular resolution of 5°, 10°, 15° and 20° in the space of candidate beams. The impact of different convergence criteria is investigated in comparison to a fixed termination after the evaluation of 10 000 beam ensembles. In total, our simulations comprise a full fluence optimization for about 3000 000 treatment plans. All four combinatorial BAS strategies yield significant improvements of the objective function value and of the corresponding dose distributions compared to standard beam configurations with equi-spaced coplanar beams. The genetic and the cross-entropy algorithms showed faster convergence in the very beginning of the optimization but the simulated annealing algorithm eventually arrived at almost the same objective function values. These three strategies typically yield clinically equivalent treatment plans. The iterative algorithm showed the worst convergence properties. The choice of the termination criterion had a stronger influence on the performance of the simulated annealing algorithm than on the performance of the genetic and the cross-entropy algorithms. We advocate to terminate the optimization process after the evaluation of 1000 beam combinations without objective function decrease. For our simulations, this resulted in an average deviation of the objective function from the reference value after 10 000 evaluated beam ensembles of 0.5% for all metaheuristics. On average, there was only a minor improvement when increasing the angular resolution in the space of candidate beam angles from 20° to 5°. However, we observed significant improvements when considering non-coplanar candidate beams for challenging head and neck cases.

Modifying the conductivity of polypyrrole through low-energy lead ion implantation

Interest lies in the creation of novel nanocomposite materials obtained through mixing, impregnation or incorporation techniques. One such technique is ion implantation which possesses the potential for retaining properties from the base material and implanted material as well as any effects observed from combining the two. To this end low-energy (15 keV) implantation of lead ions of various fluences was performed in conducting polypyrrole films. The presence of lead-rich particles was evidenced through transmission electron microscopy. An interesting trend was observed between fluence and conductivity. Of the fluences tested, the optimum fluences of lead ion implantation in polypyrrole films for enhanced conductivity are 5 × 1014 at. cm−2 and 2 × 1015 at. cm−2. The conductivity and stability appear to result from a combination of effects: polymer degradation arising from ion beam damage, an increase in charge-carriers (dications) present after implantation and precipitation of Pb-rich nanoparticles. Monitoring conductivity over time showed increased retention of conductivity levels after lead implantation. Improvements in stability for polypyrrole open avenues for application and bring polypyrrole one step closer to practical use. A mechanism is suggested for this advantageous retained conductivity.

A study of Roman glass from Mala Barutana/Belgrade Fortress irradiated with pulsed CO2, Nd:YAG and ruby laser — Comparison

Application of non-contact and rapid laser technique, which is minimally invasive, non-contaminant and efficient method, for ancient glass investigation and cleaning is highly desirable for restoration purposes. Irradiation of Roman glass dated from 1st to 4th/5th century AD with TEA CO2 (wavelength 10.6μm; pulse duration tp=100ns), Nd:YAG (wavelength 1064nm and 532nm; tp=150ps) and ruby laser (wavelength 694nm; tp=30ns) in air ambience was studied. For all three lasers, moderate energy densities (15–30J/cm2) induced significant changes of morphology — from superficial exfoliation and occurrence of mosaic structure after few pulses to deep damages and hydrodynamic features after higher number of accumulated shots. Irradiation with moderate energy density, accompanied with plasma appearance in front of the samples, is convenient for numerous potential applications, particularly surface elemental analysis such as laser induced breakdown spectroscopy. On the other hand, lower densities are more suitable for Roman glass cleaning. Calculations of Roman glass surface temperature have shown that pulsed CO2 laser is favorable for surface cleaning and optimal fluence is ~2J/cm2. This was confirmed by additional experiments for fluences 1.5 and 3J/cm2. Morphological changes on the Roman glass surface induced by lasers were studied by optical microscopy (OM) and scanning electron microscopy (SEM). The composition of Roman glass was determined by energy dispersive X-ray analysis (EDX) and inductively coupled plasma (ICP) method. Chemical analysis confirmed that the investigated glass dates from the Roman period.

Combinatorial optimization for an improved transition from fluence optimization to fluence delivery in IMRT treatment planning

The intensity modulated radiation therapy (IMRT) treatment planning problem is usually divided into three smaller problems that are solved sequentially: geometry problem, intensity problem and realization problem. There are many models and algorithms that address each one of the problems in a satisfactory way. However, these problems cannot be seen separately, because strong links exist between them. While the linkage between the geometry problem and the intensity problem is straightforward, the linkage between the intensity problem and the realization problem is all but simple and will determine the quality of the treatment planning. In practice, the linkage between these problems is, most of the times, done in a rather simple way, usually by rounding. This can lead to a significant deterioration of the treatment plan quality. We propose a combinatorial optimization approach to enable an improved transition from optimized to delivery fluence maps in IMRT treatment planning. Two clinical examples of head and neck cancer cases are used, both to present numerical evidences of the resulting deterioration of plan quality if a simplistic approach is used, and also to highlight a combinatorial optimization approach as a valuable alternative when linking the intensity problem and the realization problem.

Phase I study of photodynamic therapy using talaporfin sodium and diode laser for local failure after chemoradiotherapy for esophageal cancer

BackgroundPhotodynamic therapy (PDT) is a less invasive and effective salvage treatment for local failure after chemoradiotherapy (CRT) for esophageal cancer, however it causes a high rate of skin phototoxicity and requires a long sun shade period. Talaporfin sodium is a rapidly cleared photosensitizer that is expected to have less phototoxicity. This study was undertaken to clarify the optimum laser fluence rate of PDT using talaporfin sodium and a diode laser for patients with local failure after CRT or radiotherapy (RT) for esophageal cancer.MethodsThis phase I, laser dose escalation study used a fixed dose (40 mg/m2) of intravenous talaporfin sodium administered 4 to 6 hours before irradiation in patients with local failure limited to T2 after CRT or RT (≥ 50 Gy). The primary endpoint was to assess the dose limiting toxicity (DLT) of PDT, and the secondary endpoints were to evaluate the adverse events and toxicity related to PDT. The starting fluence of the 664 nm diode laser was 50 J/cm2, with an escalation plan to 75 J/cm2 and 100 J/cm2.Results9 patients with local failure after CRT or RT for ESCC were enrolled and treated in groups of 3 individuals to the third fluence level. No DLT was observed at any fluence level. Phototoxicity was not observed, but one subject had grade 1 fever, three had grade 1 esophageal pain, and 1 had grade 1 dysphagia. Five of 9 patients (55.6%) achieved a complete response after PDT.ConclusionsPDT using talaporfin sodium and a diode laser was safe for local failure after RT in patients with esophageal cancer. The recommended fluence for the following phase II study is 100 J/cm2.

Sci-Thur AM: Planning - 04: Evaluation of the fluence complexity, solution quality, and run efficiency produced by five fluence parameterizations implemented in PARETO multiobjective radiotherapy treatment planning software.

PARETO (Pareto-Aware Radiotherapy Evolutionary Treatment Optimization) is a novel multiobjective treatment planning system that performs beam orientation and fluence optimization simultaneously using an advanced evolutionary algorithm. In order to reduce the number of parameters involved in this enormous search space, we present several methods for modeling the beam fluence. The parameterizations are compared using innovative tools that evaluate fluence complexity, solution quality, and run efficiency. A PARETO run is performed using the basic weight (BW), linear gradient (LG), cosine transform (CT), beam group (BG), and isodose-projection (IP) methods for applying fluence modulation over the projection of the Planning Target Volume in the beam's-eye-view plane. The solutions of each run are non-dominated with respect to other trial solutions encountered during the run. However, to compare the solution quality of independent runs, each run competes against every other run in a round robin fashion. Score is assigned based on the fraction of solutions that survive when a tournament selection operator is applied to the solutions of the two competitors. To compare fluence complexity, a modulation index, fractal dimension, and image gradient entropy are calculated for the fluence maps of each optimal plan. We have found that the LG method results in superior solution quality for a spine phantom, lung patient, and cauda equina patient. The BG method produces solutions with the highest degree of fluence complexity. Most methods result in comparable run times. The LG method produces superior solution quality using a moderate degree of fluence modulation.

Efficient IMRT inverse planning with a new L1-solver: template for first-order conic solver

Intensity modulated radiation therapy (IMRT) inverse planning using total-variation (TV) regularization has been proposed to reduce the complexity of fluence maps and facilitate dose delivery. Conventionally, the optimization problem with L-1 norm is solved with quadratic programming (QP), which is time consuming and memory expensive due to the second-order Newton update. This study proposes to use a new algorithm, template for first-order conic solver (TFOCS), for fast and memory-efficient optimization in IMRT inverse planning. The TFOCS utilizes dual-variable updates and first-order approaches for TV minimization without the need to compute and store the enlarged Hessian matrix required for Newton update in the QP technique. To evaluate the effectiveness and efficiency of the proposed method, two clinical cases were used for IMRT inverse planning: a head and neck case and a prostate case. For comparison, the conventional QP-based method for the TV form was adopted to solve the fluence map optimization problem in the above two cases. The convergence criteria and algorithm parameters were selected to achieve similar dose conformity for a fair comparison between the two methods. Compared with conventional QP-based approach, the proposed TFOCS-based method shows a remarkable improvement in computational efficiency for fluence map optimization, while maintaining the conformal dose distribution. Compared with QP-based algorithms, the computational speed using TFOCS for fluence optimization is increased by a factor of 4 to 6, and at the same time the memory requirement is reduced by a factor of 3 to 4. Therefore, TFOCS provides an effective, fast and memory-efficient method for IMRT inverse planning. The unique features of the approach should be particularly important in inverse planning involving a large number of beams, such as in VMAT and dense angularly sampled and sparse intensity modulated radiation therapy (DASSIM-RT).

TH‐A‐213AB‐01: A Novel Method to Accelerate Optimization by Employing Approximate Dose Values

Purpose: To introduce a novel method that increases the efficiency of beam and beam let weight and fluence optimization, by employing approximate dose values in the objective function. Methods: A least squares objective function was used, based on clinical dose‐volume objectives. In the initial stage of optimization, the method utilizes dose values determined by the additive dose approximation (ADA) to evaluate the objective function. These dose values are based on pre‐computed dose indices, along with a novel correction scheme. During this stage, no calculation of the true dose is required, resulting in a significant reduction in computation time, since the manipulation of large dose matrices is avoided. The resulting optimized parameters are subsequently used as the starting point in the final optimization step. This method was evaluated on prostate bed, esophageal, and brain patients with varying numbers of treatment fields, by comparing the CPU time required with that of a standard optimization method. The downhill simplex algorithm was used for the evaluations. Results: Nine prostate bed patients with three different beam configurations were used: 4 field 3D‐CRT plans, 16 field 3D‐CRT plans and a 64‐beamlet IMRT‐like plan. These demonstrated an average reduction in optimization time of 33%, 54%, and 46%, respectively. Two 3D‐CRT esophageal patients were evaluated, one with 14 fields and one with 16 fields, resulting in an average improvement of 29%. Finally, a 3D‐CRT 16 field brain patient was studied and exhibited a 57% decrease in CPU time. Conclusions: The proposed method, when used in conjunction with existing optimization algorithms, dramatically decreases the number of standard objective function evaluations and, as a result, computation time. This technique may be applied to any type of optimization algorithm and set of clinical variables requiring pre‐computed dose matrices and an objective function involving dose‐volume objectives (eg. multi‐criteria optimization).

SU-E-T-469: The Effects of Patient Anatomy and Parallel Magnetic Fields on Beamlet Dose Distributions.

Beamlets are generated in a patient geometry in the presence of a magnetic field to investigate the effects of tissue density and magnetic field on beamlet dose distributions, which is important for the optimization of photon fluence to be delivered by a linac-MR system. 50×50 mm2 fields were placed with isocenter in the middle of a patient's right lung. Each treatment field was decomposed into 100 beamlets (each 5×5 mm2 ). BEAMnrc scored the particle phase space at 100.2 cm from the source in the linac-MR geometry (isocentre at 126 cm) with parallel magnetic fields of 0, 0.56, and 3T. DOSXYZnrc was modified to score the energy deposited by particles from this phase space as a function of the beamlet the particle passed through. The calculation volume of 70×46×64 voxels encompassed the patient with a voxel size of 3×3×3 mm3 . Each beamlet was normalized to the dose calculated to a 3×3×3 mm3 voxel with isocenter at 5cm depth in a flat water tank without a magnetic field. Beamlet files were calculated on Western Canada's high performance computing cluster (Westgrid) using 100 processors, enabling simulation of 109 histories in less than 3 hours. The resulting files, which contained 3D dose distributions for all 100 beamlets, were 81 MB per field. The Monte Carlo uncertainty was also stored. The gyroradii for 1 MeV electron traversing field lines at 20 degrees are 2.9mm and 0.5mm for 0.56 and 3T fields respectively. The 0.56T parallel magnetic field has a small effect compared to the distortion of the beamlet introduced by the presence of lung. The effect of tissue heterogeneities is more significant than the effect of a 0.56T parallel magnetic field. A 3T field refocuses the dose in lung to the beamlet path and significantly reduces the lateral electron scatter.