Intensity-modulated Fields Research Articles

Research on local sound field intensity control technique in metasurface based on deep neural networks.

The use of tunable metasurface technology to realize the underwater tracking function of submarines, which is one of the hotspots and difficulties in submarine design. The structure-to-sound-field metasurface design approach is a highly iterative process based on trial and error. The process is cumbersome and inefficient. Therefore, an inverse design method was proposed based on parallel deep neural networks. The method took the global and local target sound field feature information as input and the metasurface physical structure parameters as output. The deep neural network was trained using a kernel loss function based on a radial basis kernel function, which established an inverse mapping relationship between the desired sound field to the metasurface physical structure parameters. Finally, the sound field intensity modulation at a localized target range was achieved. The results indicated that within the regulated target range, this method achieved an average prediction error of less than 5 dB for 92.9% of the sample data.

A dynamically switchable multifunctional meta-device in terahertz based on vanadium dioxide and photosensitive silicon

In this study, we propose a dynamically switchable multifunctional device operating in the terahertz range. The device is designed by employing a multilayered planar structure that incorporates vanadium dioxide and photosensitive silicon films. We analyze the transmission characteristics and multifunctional attributes of the device. The device exhibits the capacity to transition among diverse functionalities, including linear polarization conversion, field intensity modulation, frequency selection, and wave absorption. The polarization conversion ratio exceeds 90%, and precise control over the amplitude ratio between cross-polarized and co-polarized fields is achievable. Field intensity modulation can be realized through any of three distinct methods. Upon activation of the frequency selection function, the device can effectively operate as a band-stop filter, yielding a -20dB bandwidth of 0.57 THz. Furthermore, in absorber mode, the device demonstrates a peak absorption efficiency of 90%.

Efficient generation of stationary light pulses due to coupling between two lower levels

A double- $$\varLambda $$ system, the two low levels of which is connected by an additional weak intensity-modulated microwave field as the controlling fields are switched off, is exploited for efficient generation of stationary light pulse (SLP). Analytical and numerical results show that due to the constructive or destructive interference between the introduced microwave field and the atomic spin coherence, the generated SLP and the retrieved signal field can be optimized or degenerated without causing distortion through modulating the intensity and phase of the microwave field.

Cavity resonance tunability of porous silicon microcavities by Ar+ ion irradiation

Present work reports the effect of low-energy Ar+ ion beam irradiation on the optical properties and surface morphology of porous silicon (PSi) based optical microcavities. These microcavities were realized by an active λ/2 spacer layer sandwiched between two Distributed Bragg Reflectors fabricated by galvanostatic electrochemical etching. Low-energy 10 keV Ar+ ions at various fluences were used for this irradiation study for these microcavities and characterized. The peak in the reflectance spectra of resonant cavity shows a red-shift after ion irradiation. These experimental observations are in good agreement with transfer matrix simulations. The Raman studies show that the crystalline Si is modified in nanostructured PSi and subsequently, the structural features of Si-Si and Si-O bonds. The simulated depth profiles suggest that the penetration of Ar+ ion is limited to top two PSi layers and the reduction of effective refractive index of these two layers. These experimental findings, analytical simulations, and cavity modelling provide the evidence of resonent cavity peak tunability, optical field intensity modulation, and photon confinement within the microcavity. This approach could be useful to tune the optical properties of PSi and an effective method for producing widely tunable optical structures in Si-based photonic architectures.

Investigation of subsurface damage density and morphology impact on the laser-induced damage threshold of fused silica.

The laser-induced damage threshold (LIDT) of fused silica is affected by laser field intensity modulation and laser energy absorption. In this paper, the subsurface damage (SSD) density and morphology are detected by the small-angle taper polishing method. The modulation effect of SSD morphology on the incident laser/electric field is analyzed by the finite difference time domain (FDTD) simulation. Finally, the LIDT of the taper polished surface is tested to analyze the relationship among LIDT, SSD density, and SSD morphology, and the results show a high correlation. A reliable regression model is obtained based on the results, which shows that LIDT is inversely proportional to SSD density and the light intensity enhancement factor (LIEF).

Clinical Experience of Automated SBRT Paraspinal and Other Metastatic Tumor Planning With Constrained Hierarchical Optimization

PurposeWe report on the clinical performance of a fully automated approach to treatment planning based on a Pareto optimal, constrained hierarchical optimization algorithm, named Expedited Constrained Hierarchical Optimization (ECHO). Methods and materialsFrom April 2017 to October 2018, ECHO produced 640 treated plans for 523 patients who underwent stereotactic body radiation therapy (RT) for paraspinal and other metastatic tumors. A total of 182 plans were for 24 Gy in a single fraction, 387 plans were for 27 Gy in 3 fractions, and the remainder were for other prescriptions or fractionations. Of the plans, 84.5% were for paraspinal tumors, with 69, 302, and 170 in the cervical, thoracic, and lumbosacral spine, respectively. For each case, after contouring, a template plan using 9 intensity modulated RT fields based on disease site and tumor location was sent to ECHO through an application program interface plug-in from the treatment planning system. ECHO returned a plan that satisfied all critical structure hard constraints with optimal target volume coverage and the lowest achievable normal tissue doses. Upon ECHO completion, the planner received an e-mail indicating the plan was ready for review. The plan was accepted if all clinical criteria were met. Otherwise, a limited number of parameters could be adjusted for another ECHO run. ResultsThe median planning target volume size was 84.3 cm3 (range, 6.9-633.2). The median time to produce 1 ECHO plan was 63.5 minutes (range, 11-340 minutes) and was largely dependent on the field sizes. Of the cases, 79.7% required 1 run to produce a clinically accepted plan, 13.3% required 1 additional run with minimal parameter adjustments, and 7.0% required ≥2 additional runs with significant parameter modifications. All plans met or bettered the institutional clinical criteria. ConclusionsWe successfully implemented automated stereotactic body RT paraspinal and other metastatic tumors planning. ECHO produced high-quality plans, improved planning efficiency and robustness, and enabled expedited treatment planning at our clinic.

Isocenter bilateral tangential fields combined with intensity-modulated radiotherapy for synchronous bilateral whole breast irradiation

Objective To investigate the dose characteristics and outcomes of a single isocenter bilateral tangential fields (IBTF) combined with intensity-modulated radiotherapy (IMRT) in bilateral breast radiotherapy (BBR). Methods Fourteen female patients with synchronous bilateral breast cancer (SBBC) after breast-conserving surgery (BCS) were enrolled in this study. All patients received BBR using IBTF combined with IMRT at a conventional (50 Gy/25f) or hypofractionated (43.5 Gy/15f) dose. For patients with invasive cancer, the additional tumor bed boost was given with sequential electron radiation or simultaneously photon IMRT. The coverage, uniformity and short-term clinical efficacy were evaluated. Results The number of the irradiation field was 8-11, including 4-7 intensity-modulated fields. The bilateral breast PTV dose coverage reached 95% in all plans. For the tumor bed, the mean dose coverage was (95.54±1.33)%(left) and (94.19±1.03)%(right) using photon, and (90.25±8.79)%(left) and (85.28±8.35)%(right) using electron. The average V20 of bilateral lungs was (16.69±3.90)%. The cardiac Dmean was 5.48 Gy. Three patients presented with grade Ⅱ acute skin toxicities. No ≥ grade Ⅱ pneumonitis was observed. No recurrence occurred with the median follow-up time of 30.1 months. Eleven patients showed excellent cosmetic results. Conclusion BBR using IBTF combined with IMRT is efficacious and safe for patients with SBBC after BCS. Key words: Breast neoplasm/adjuvant radiotherapy; Radiation technique; Short-term efficacy

Efficient images storage via modulating the atomic spin coherence in a N-type system

A four-level N-type cold atomic system is proposed for optimizing images storage based on the electromagnetically induced transparency (EIT). Both analytical analysis and numerical simulation clearly show that the application, during the storage time, of an additional intensity-modulated signal field and an additional microwave field can impose an intensity and a phase-dependent factors on the atomic spin coherence in a controlled manner, then the amplitude of the retrieved images can be increased or decreased with an enhancement in the visibility. Our results are very promising for the realization of all-optical information processing of images coherently stored in EIT media in the future.

Optimizing radiosurgery with photons for ocular melanoma.

Background and purposePhoton radiotherapy has been established for the treatment of ocular melanoma (OM). Here we investigate the planning qualities of two different planning approaches, a combination of dynamic conformal arcs (DCA) complemented with multiple non-coplanar static intensity-modulated (IMRT) fields (DCA-IMRT), and volumetric modulated arc therapy (VMAT) in combination with automated planning (AP).Materials and methodsThirteen consecutive patients treated for ocular melanoma with curative intent on a Linac-based radiosurgery system were analyzed. Fractionated stereotactic radiosurgery (fSRS) was applied using 50 Gy in 5 fractions using the combination of DCA-IMRT. Plans were reviewed and the thirteen cases were compared to plans obtained with optimized automated VMAT based on a set of 28 distinct patients treated with DCA-IMRT who were selected to generate the AP model for the prediction of dose volume constraints.ResultsOverall, plan quality of DCA-IMRT was superior to AP with VMAT. PTV coverage did not exceed 107% in any case treated with DCA-IMRT, compared to seven patients with VMAT. The median PTV covered by >95% was 98.3% (91.9%–99.7%) with DCA-IMRT, compared to 95.1% (91.5%–97.9%) (p < 0.01) with VMAT. The median mean dose delivered to the treated eye was 22.4 Gy (12.3 Gy–33.3 Gy) with DCA-IMRT compared to 27.2 Gy (15.5 Gy–33.7 Gy) (p < 0.01). Dose to the ipsilateral lacrimal gland and the ipsilateral optic nerve were comparable for DCA-IMRT and VMAT, however, the dose to the lens was lower with DCA-IMRT compared to VMAT.ConclusionsThe combination of multiple arcs complemented with multiple IMRT fields sets the gold standard for fSRS of ocular melanoma for photon therapy.

Quantitative evaluation by measurement and modeling of the variations in dose distributions deposited in mobile targets.

The objective of this study is to quantitatively evaluate variations of dose distributions deposited in mobile target by measurement and modeling. The effects of variation in dose distribution induced by motion on tumor dose coverage and sparing of normal tissues were investigated quantitatively. The dose distributions with motion artifacts were modeled considering different motion patterns that include (a) motion with constant speed and (b) sinusoidal motion. The model predictions of the dose distributions with motion artifacts were verified with measurement where the dose distributions from various plans that included three-dimensional conformal and intensity-modulated fields were measured with a multiple-diode-array detector (MapCheck2), which was mounted on a mobile platform that moves with adjustable motion parameters. For each plan, the dose distributions were then measured with MapCHECK2 using different motion amplitudes from 0-25 mm. In addition, mathematical modeling was developed to predict the variations in the dose distributions and their dependence on the motion parameters that included amplitude, frequency and phase for sinusoidal motions. The dose distributions varied with motion and depended on the motion pattern particularly the sinusoidal motion, which spread out along the direction of motion. Study results showed that in the dose region between isocenter and the 50% isodose line, the dose profile decreased with increase of the motion amplitude. As the range of motion became larger than the field length along the direction of motion, the dose profiles changes overall including the central axis dose and 50% isodose line. If the total dose was delivered over a time much longer than the periodic time of motion, variations in motion frequency and phase do not affect the dose profiles. As a result, the motion dose modeling developed in this study provided quantitative characterization of variation in the dose distributions induced by motion, which can be employed in radiation therapy to quantitatively determine the margins needed for treatment planning considering dose spillage to normal tissue.

Modulation of Visible and Near-Infrared Surface Plasmon Resonance of Au Nanoparticles Based on Highly Doped Graphene

We study an active modulation of surface plasmon resonance (SPR) of Au nanoparticles based on highly doped graphene in visible and near-infrared regions. We find that compared to the traditional metal SPR, the SPR of Au nanoparticles based on graphene causes a remarkable blue shift. The field intensity in the gap is redistributed to standing wave. The field intensity of standing wave is about one order of magnitude higher than the traditional model. Moreover, the SPR of Au nanoparticles can be actively modulated by varying the graphene Fermi energy. We find the maximum modulation of field intensity of absorption spectra is more than 21.6 % at λ = 822 nm and the amount of blue shift is 17.4 nm, which is about 2.14 % of the initial wavelength λ 0 = 813.4 nm, with increasing monolayer graphene Fermi energy from 1.0 to 1.5 ev. We find that the SPR sensitivity to the refractive index n of the environment is about 642 nm per refractive index unit (RIU). The SPR wavelengths have a big blue shift, which is about 33 nm, with increasing number of graphene layers from 1 to 3, and some shoulders on the absorption spectra are observed in the models with multilayer graphene. Finally, we study the Au nanorod array based on monolayer graphene. We find that the blue shift caused by the graphene increases from 14 to 24 nm, with increasing gap g y from 10 to 20 nm. Then, it decreases from 24 to 14 nm, with increasing gap g y from 20 to 50 nm. This study provides a new way for actively modulating the optical and optoelectronic devices.

Development and evaluation of whole breast irradiation with volumetric modulated arc therapy-based hybrid intensity-modulated radiotherapy after breast conserving surgery for breast cancer

Objective To develop whole breast irradiation with volumetric-modulated arc therapy (VMAT)-based hybrid intensity-modulated radiotherapy (IMRT) after breast conserving surgery for breast cancer, and to evaluate its value in clinical application. Methods Ten patients with breast cancer undergoing breast conserving surgery were enrolled. Two radiotherapy plans were designed based on hybrid fixed-beam IMRT/IMRT technique and hybrid VMAT/IMRT technique, respectively. One group received whole breast irradiation with a prescribed dose of 50 Gy in 25 fractions. The other group received whole breast irradiation with a prescribed dose of 50 Gy in 25 fractions, as well as simultaneous integrated boost to the tumor bed with 60 Gy in 25 fractions. The dosimetric parameters and delivery efficiency were compared between the two plans using paired t test. Results For patients treated with whole breast irradiation alone, there were no significant differences in conformity index and homogeneity index of target volume between the two plans (P=0.866, 0.056); however, compared with the hybrid fixed-beam IMRT/IMRT technique, the hybrid VMAT/IMRT technique significantly increased the doses to organs at risk (OARs) and the number of monitor units (MUs) in the intensity-modulated field (P=0.000-0.050, P=0.002). For patients treated with whole breast irradiation with a simultaneous integrated boost to the tumor bed; however, the hybrid VMAT/IMRT technique significantly reduced the doses to the lung and spinal cord, number of MUs in intensity-modulated field, and delivery time compared with the hybrid fixed-beam IMRT/IMRT technique (P=0.004, 0.001, 0.000, 0.000). Conclusions For patients treated with whole breast irradiation with a simultaneous integrated boost to the tumor bed, the hybrid VMAT/IMRT technique is highly promising for clinical application due to better OAR sparing and higher delivery efficiency. Key words: Volumetric modulated arc therapy; Intensity-modulated radiotherapy; Hybrid intensity-modulated technique; Breast neoplasms

Dosimetric study of RapidArc plans and conventional intensity modulated radiotherapy for prostate cancer involving seminal vesicles and pelvis lymph nodes

Purpose: The main purpose of this study is to (1) identify the continual diversity between conventional fixed field intensity modulation radiotherapy (IMRT) and RapidArc (RA) for high-risk prostate cancer; and (2) determine potential benefits and drawbacks of using for this type of treatment. Methods: A cohort of 20 prostate cases including prostate, seminal vesicles and pelvic lymph nodes was selected for this study. The primary planning target volume (PTV P ) and boost planning target volume (PTV B ) were contoured. The total prescription dose was 75.6 Gy (45 Gy to PTV P and an additional 21.6 Gy to PTV B ). Two plans were generated for each PTV: multiple 7-fields for IMRT and two arcs for RA. Results: A Sigma index (IMRT: 2.75 ± 0.581; RA: 2.8 ± 0.738) for PTV P and (IMRT: 2.0 ± 0.484; RA: 2.1 ± 0.464) for PTV B indicated similar dose homogeneity inside the PTV. Conformity index (IMRT: 0.96 ± 0.047; RA: 0.95 ± 0.059) for PTV P and (IMRT: 0.97 ± 0.015; RA: 0.96 ± 0.014) for PTV B was comparable for both the techniques. IMRT offered lower mean dose to organ at risks (OARs) compared to RA plans. Normal tissue integral dose in IMRT plan resulted 0.87% lower than RA plans. All the plans displayed significant increase (2.50 times for PTV P and 1.72 for PTB B ) in the average number of necessary monitor units (MUs) with IMRT beam. Treatment delivery time of RA was 2 ‒ 6 minutes shorter than IMRT treatment. Conclusion: For PTV including pelvic lymph nodes, seminal vesicles and prostate, IMRT offered a greater degree of OARs sparing. For PTV including seminal vesicles and prostate, RA with two arcs provided comparable plan with IMRT. RA also improved the treatment efficiency due to smaller number of MUs required.

Measuring number, mass, and size of exhaust particles with diffusion chargers: The dual Pegasor Particle Sensor

The Pegasor Particle Sensor (PPS) is an aerosol measurement instrument based on diffusion charging and electrical detection of the charge acquired by particles as these escape the electrically isolated body of the device. PPS utilizes an ion trap to collect free ions which failed to attach to particles. This escaping current produced is proportional to particle concentration. Apart from free ions, ion trap field intensity modulation may be used to collect a fraction of charged particles, which presents the opportunity to use PPS as a size selective sensor.This study first demonstrates a dual PPS concept capable of simultaneously measuring exhaust particle mean size, number and mass concentrations. Two PPS units were combined at different ion trap voltage, so that each sensor detected a different fraction of the particle size distribution. The difference between the two signals can be related to mean particle size, which allows a more precise estimation of particle number and mass concentrations.The parameters required for the data inversion algorithm, such as charging efficiency and ion trap penetration as a function of size and distribution width, were determined with soot particle calibration for each sensor. The sensitivity of the algorithm to input uncertainties such as distribution width and sample inlet flowrate was examined, with the latter having the larger impact.The application of the dual PPS with polydisperse soot as well as diesel exhaust demonstrates a good agreement with reference instruments for particle size, number and mass. The dual PPS offers significant improvement over the single PPS which is based on a fixed calibration. The prototype concept presented here can likely be optimized by using a single unit with two integrated measuring cells instead of two separate units, along with continuously measuring the inlet flowrate.

SU‐E‐T‐17: A Comparison of Forward and Field in Field Intensity Modulation Radiotherapy Planning for Breast Cancer

Purpose:To compare the dosimetric difference in forward intensity modulation radiotherapy(fIMRT) and field in field IMRT (FIF IMRT)planning for breast cancer.Methods:Ten patients received radiotherapy are selected.For each patient,two treatment plans(fIMRT and FIF IMRT) were designed with Varian Eclipse ver11.0 treatment planning system.Evaluate the dose parameters of targets, organs at risk (OAR), monitor units and treatment time, using dose‐volume histogram (DVH).Results:There were no significant difference were found in conformal index (CI), homogeneity index (HI) of PTV,V5,V10,V20,V30,V40,V50 of heart, lung and monitor unit(MU)(P&gt;0.05).The differences were significant in the treatment time(fIMRT=8.3min,FIF IMRT=2.5,p&lt;0.05).Conclusion:FIF IMRT is equal to fIMRT in dosimetril evaluation. Due to much less delivery time,FIF IMRT is an efficient technique in treating patients by reduceing the uncomfortable influnce which could effect the treatment.

SU-E-T-286: Dose Verification of Spot-Scanning Proton Beam Using GafChromic EBT3 Film

Purpose: Dose verification of spot-scanning proton pencil beam is performed via planar dose measurements at several depths using an ionization-chamber array, requiring repeat irradiations of each field for each depth. Here we investigate film dosimetry which has two advantages: higher resolution and efficiency from one-shot irradiation for multiple depths. Methods: Film calibration was performed using an EBT3 film at 20-cm depth of Plastic Water (CIRS, Norfolk, VA) exposed by a 10-level step wedge on a Proteus Plus proton system (IBA, Belgium). The calibration doses ranged from 25–250 cGy(RBE) for proton energies of 170–200 MeV. A uniform 1000 cm3 dose cube and a clinical prostate combined with seminal-vesicle and pelvic-nodes plan were used for this study. All treatment plans were generated in the RayStation (RaySearch Lab, Sweden). The planar doses at different depths for both cases were measured with film using triple-channel dosimetry and the MatriXX PT (IBA Dosimetry, Germany). The Gamma passing rates, dose-difference maps, and profiles of 2D planar doses measured with EBT3 film and MatriXX, versus treatment planning system (TPS) calculations were analyzed and compared using the FilmQA Pro (Ashland Inc., Bridgewater, NJ). Results: The EBT3 film measurement results matched well with the TPS calculation data with an average passing rate >95% for 2%/2mm and are comparable with the MatriXX measurements (0.7%, 1.8%, 3.8% mean differences corresponding to 3%/3mm, 3%/2mm, 2%/2mm, respectively). Overall passing rates for EBT3 films appear higher than those with MatriXX detectors. Conclusion: The energy dependence of the film response could be minimized by calibration using proton beam with mixed energies. The greater efficiency of the dose verification using GafChromic EBT3 results in a potential cost trade-off between room capacity and film cost. EBT3 film may offer distinct advantages in highly intensity-modulated fields due to its higher resolution compared to 2D ionization-chamber array detectors. One of the authors has a research grant from Ashland Inc., the manufacturer of GafChromic film

Dosimetric comparison and time consuming evaluations for neoadjuvant 5-fields versus 7-fields Intensity-Modulated Radiation Therapy for locally advanced rectal cancer.

Aims and background. Preoperative – multidrug - chemoradiotherapy schedule (PCR) is proven to be the standard approach in patients with locally advanced rectal cancer (LARC). The main limiting factor of PCR delivery is severe acute gastrointestinal (GI) toxicity due to the small bowel irradiated volume. Compared to 3-dimensional conformal radiation therapy (3DCRT), intensity-modulated radiotherapy (IMRT) plans, with 7 or more fields, by highly conformal dose distributions, will minimize the dose received by small bowel, but required an increase in overall treatment time. The purpose of this study was to investigate if simple IMRT plans were comparable to conventional 7-fields IMRT in terms of dose distribution, with the advantage of reducing time delivery. Methods. Twelve patients with histologically proved LARC underwent PCR with concurrent chemotherapy with Oxaliplatin 50 mg/m2 intravenously weekly x 6 weeks + Capecitabine 1500 mg/mq day (on days 1,14 every 21 days) and 7-field IMRT step-and -shoot technique at the cumulative dose of 50.4 Gy to the target volume, with conventional fractionation schedule of 1.8 Gy each day. Dose calculation for selected Organ at Risks (OARs) such as femoral heads, bladder and small bowel, were performed. Without exceeding OARs dose constraints or compromising target coverage, for each treated patients, a “simple” IMRT plan with 5 fields was computed and retrospectively compared with 7-field IMRT. Dosimetric parameters and treatment time data were analized. Results. As expected, homogeneity of dose distribution was significantly better for 7F-IMRT than 5F-IMRT; while in terms of Conformation Number, as proposed by van’t Riet et. Al, we registered no statistically significant differences between the two IMRT techniques resulting in equal sparing of healthy tissues. We recorded a median delivering treatment time of 7.9 minutes for 5F-IMRT and of 12.3 minutes for 7F-IMRT, with a difference statistically significant. Conclusions.This preliminary study demonstrated that the reduction in the number of fields obtained with the 5F-IMRT is a strategy to improve treatment time delivery without a detriment in target coverage and normal tissue sparing in PCR for LARC patients.

Isocentric integration of intensity-modulated radiotherapy with electron fields improves field junction dose uniformity in postmastectomy radiotherapy

Background. In postmastectomy radiotherapy (PMRT), the dose coverage of the planning target volume (PTV) with additional margins, including the chest wall, supraclavicular, interpectoral, internal mammary and axillar level I–III lymph nodes, is often compromised. Electron fields may improve the medial dose coverage while maintaining organ at risk (OAR) doses at an acceptable level, but at the cost of hot and cold spots at the electron and photon field junction. To improve PMRT dose coverage and uniformity, an isocentric technique combining tangential intensity-modulated (IM)RT fields with one medial electron field was implemented.Material and methods. For 10 postmastectomy patients isocentric IMRT with electron plans were created and compared with a standard electron/photon mix and a standard tangent technique. PTV dose uniformity was evaluated based on the tolerance range (TR), i.e. the ratio of the standard deviation to the mean dose, a dice similarity coefficient (DSC) and the 90% isodose coverage and the hot spot volumes. OAR and contralateral breast doses were also recorded.Results. IMRT with electrons significantly improved the PTV dose homogeneity and conformity based on the TR and DSC values when compared with the standard electron/photon and tangent technique (p < 0.02). The 90% isodose coverage improved to 86% compared with 82% and 80% for the standard techniques (p < 0.02). Compared with the standard electron/photon mix, IMRT smoothed the dose gradient in the electron and photon field junction and the volumes receiving a dose of 110% or more were reduced by a third. For all three strategies, the OAR and contralateral breast doses were within clinically tolerable limits.Conclusion. Based on these results two-field IMRT combined with an electron field is a suitable strategy for PMRT.

Direct leaf trajectory optimization for volumetric modulated arc therapy planning with sliding window delivery

The authors propose a novel optimization model for volumetric modulated arc therapy (VMAT) planning that directly optimizes deliverable leaf trajectories in the treatment plan optimization problem, and eliminates the need for a separate arc-sequencing step. In this model, a 360° arc is divided into a given number of arc segments in which the leaves move unidirectionally. This facilitates an algorithm that determines the optimal piecewise linear leaf trajectories for each arc segment, which are deliverable in a given treatment time. Multileaf collimator constraints, including maximum leaf speed and interdigitation, are accounted for explicitly. The algorithm is customized to allow for VMAT delivery using constant gantry speed and dose rate, however, the algorithm generalizes to variable gantry speed if beneficial. The authors demonstrate the method for three different tumor sites: a head-and-neck case, a prostate case, and a paraspinal case. The authors first obtain a reference plan for intensity modulated radiotherapy (IMRT) using fluence map optimization and 20 intensity-modulated fields in equally spaced beam directions, which is beyond the standard of care. Modeling the typical clinical setup for the treatment sites considered, IMRT plans using seven or nine beams are also computed. Subsequently, VMAT plans are optimized by dividing the 360° arc into 20 corresponding arc segments. Assuming typical machine parameters (a dose rate of 600 MU/min, and a maximum leaf speed of 3 cm/s), it is demonstrated that the optimized VMAT plans with 2-3 min delivery time are of noticeably better quality than the 7-9 beam IMRT plans. The VMAT plan quality approaches the quality of the 20-beam IMRT benchmark plan for delivery times between 3 and 4 min. The results indicate that high quality treatments can be delivered in a single arc with 20 arc segments if sufficient time is allowed for modulation in each segment.

SU-E-T-391: Quantitative Measurement and Modeling of the Variations in Dose Distributions Induced by Cyclic Motion

Purpose: To assess by measurement the quantitative variations in dose distributions for different motion patterns. Mathematical modeling of dose variations induced by motion was developed.Materials and Methods: The dose distributions from various plans that included open and intensity-modulated fields were measured using a multiple-diode-array detector (MapCHECK2). The MapCHECK2 phantom was mounted on a mobile platform that moves with adjustable calibrated motion patterns in one-dimension (Y-Axis). For each plan, the dose distributions were measured with MapCHECK2 using different ranges of motion (ROM) from 0–40 mm and a frequency of 15 cycles per minute. Mathematical modeling was developed to predict the variations in the dose distributions and their dependence on the motion parameters. Results: The central dose remains unchanged and agrees (< 3%) with that for the static phantom. At periphery of dose distributions along the motion direction, hot spots resulted in low dose regions while cold dose spots were induced close to penumbra due to phantom motion. The difference in dose outside the fields increased significantly as the ROM increased where maximal dose reached up to a factor of 5 at ROM = 25 mm. The modeling showed that maximal dose and spread-out position at the edges of the field along motion direction increased linearly with ROM. The dose profile at the edge of field decreased linearly with ROM. The dose distributions from IMRT fields were smeared and optimization features were reduced due to motion. Conclusion: The dose distributions at the edges of the open or IMRT fields varied considerably with motion. Maximal and minimal dose variations, spatial position of maximal dose and dose drop rate in the dose distributions of the mobile phantom were modeled. This modeling provides quantitative characterization of dose variations induced by motion which might be employed in IMRT optimization to compensate of these dose artifacts.