Beam Entrance Research Articles

Significance of intra-fractional motion for pancreatic patients treated with charged particles

BackgroundUncertainties associated with the delivery of treatment to moving organs might compromise the accuracy of treatment. This study explores the impact of intra-fractional anatomical changes in pancreatic patients treated with charged particles delivered using a scanning beam. The aim of this paper is to define the potential source of uncertainties, quantify their effect, and to define clinically feasible strategies to reduce them.MethodsThe study included 14 patients treated at our facility with charged particles (protons or 12C) using intensity modulated particle therapy (IMPT). Treatment plans were optimized using the Treatment Planning System (TPS) Syngo® RT Planning. The pre-treatment dose distribution under motion (4D) was simulated using the TPS TRiP4D and the dose delivered for some of the treatment fractions was reconstructed. The volume receiving at least 95% of the prescribed dose (V95CTV) and the target dose homogeneity were evaluated. The results from the 4D dose calculations were compared with dose distributions in the static case and its variation correlated with the internal motion amplitude and plan modulation, through the Pearson correlation coefficient, as well the significant p-value. The concept of the modulation index (MI) was introduced to assess the degree of modulation of IMPT plans, through the quantification of intensity gradients between neighboring pencil beams.ResultsThe induced breathing motion together with dynamic beam delivery results in an interplay effect, which affects the homogeneity and target coverage of the dose distribution. This effect is stronger (∆V95CTV > 10%) for patients with tumor motion amplitude above 5 mm and a highly modulated dose distribution between and within fields. The MI combined with the internal motion amplitude is shown to correlate with the target dose degradation and a lack of plan robustness against range and positioning uncertainties.ConclusionsUnder internal motion the use of inhomogeneous plans results in a decrease in the dose homogeneity and target coverage of dose distributions in comparison to the static case. Plan robustness can be improved by using multiple beams and avoiding beam entrance directions susceptible to density changes. 4D dose calculations support the selection of the most suitable plan for the specific patient’s anatomy.

Impact of dose engine algorithm in pencil beam scanning proton therapy for breast cancer

PurposeProton therapy for the treatment of breast cancer is acquiring increasing interest, due to the potential reduction of radiation-induced side effects such as cardiac and pulmonary toxicity. While several in silico studies demonstrated the gain in plan quality offered by pencil beam scanning (PBS) compared to passive scattering techniques, the related dosimetric uncertainties have been poorly investigated so far. MethodsFive breast cancer patients were planned with Raystation 6 analytical pencil beam (APB) and Monte Carlo (MC) dose calculation algorithms. Plans were optimized with APB and then MC was used to recalculate dose distribution. Movable snout and beam splitting techniques (i.e. using two sub-fields for the same beam entrance, one with and the other without the use of a range shifter) were considered. PTV dose statistics were recorded. The same planning configurations were adopted for the experimental benchmark. Dose distributions were measured with a 2D array of ionization chambers and compared to APB and MC calculated ones by means of a γ analysis (agreement criteria 3%, 3 mm). ResultsOur results indicate that, when using proton PBS for breast cancer treatment, the Raystation 6 APB algorithm does not allow obtaining sufficient accuracy, especially with large air gaps. On the contrary, the MC algorithm resulted into much higher accuracy in all beam configurations tested and has to be recommended. ConclusionsCenters where a MC algorithm is not yet available should consider a careful use of APB, possibly combined with a movable snout system or in any case with strategies aimed at minimizing air gaps.

DIRECTIONAL CHANGES IN TISSUE REFLECTIVITY MAY INFLUENCE FLOW DETECTION ON OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY.

To determine if directional or anatomical changes in tissue reflectivity of Henle fiber layer (HFL) may influence flow detection on optical coherence tomography angiography (OCT-A). Cross-sectional analysis of consecutive cases undergoing OCT-A. Directional changes in visualization of HFL were either produced manually by moving the beam entrance within the pupil or observed occurring naturally because of tilting of the OCT line-scan within a myopic staphyloma. Areas of enhanced HFL visualization were identified on cross-sectional and en face structural OCT scans. Visualization of OCT-A flow within the superficial and the deep capillary plexuses were compared in areas with and without enhanced HFL reflectivity. Fifteen eyes from 9 patients were included, with a mean age of 49.6 ± 17.9 years. On OCT-A, retinal areas with enhanced HFL reflectivity produced manually or by scan tilting within myopic staphylomas showed no changes in the superficial capillary plexus flow. However, deep capillary plexus flow was altered by variable projection of the superficial flow onto the deeper retinal layers. Directional changes in HFL reflectivity can alter the detection of deep capillary plexus flow imaged with OCT-A by influencing the projection of superficial flow onto the deeper retinal layers. We recommend that clinicians minimize scan tilting during scan acquisition and be aware of this phenomenon in eyes with posterior staphyloma.

Characterization of Compton-scatter imaging with an analytical simulation method

By collimating the photons scattered when a megavoltage therapy beam interacts with the patient, a Compton-scatter image may be formed without the delivery of an extra dose. To characterize and assess the potential of the technique, an analytical model for simulating scatter images was developed and validated against Monte Carlo (MC). For three phantoms, the scatter images collected during irradiation with a 6 MV flattening-filter-free therapy beam were simulated. Images, profiles, and spectra were compared for different phantoms and different irradiation angles. The proposed analytical method simulates accurate scatter images up to 1000 times faster than MC. Minor differences between MC and analytical simulated images are attributed to limitations in the isotropic superposition/convolution algorithm used to analytically model multiple-order scattering. For a detector placed at 90° relative to the treatment beam, the simulated scattered photon energy spectrum peaks at 140–220 keV, and 40–50% of the photons are the result of multiple scattering. The high energy photons originate at the beam entrance. Increasing the angle between source and detector increases the average energy of the collected photons and decreases the relative contribution of multiple scattered photons. Multiple scattered photons cause blurring in the image. For an ideal 5 mm diameter pinhole collimator placed 18.5 cm from the isocenter, 10 cGy of deposited dose (2 Hz imaging rate for 1200 MU min−1 treatment delivery) is expected to generate an average 1000 photons per mm2 at the detector. For the considered lung tumor CT phantom, the contrast is high enough to clearly identify the lung tumor in the scatter image. Increasing the treatment beam size perpendicular to the detector plane decreases the contrast, although the scatter subject contrast is expected to be greater than the megavoltage transmission image contrast. With the analytical method, real-time tumor tracking may be possible through comparison of simulated and acquired patient images.

Biological and dosimetric characterisation of spatially fractionated proton minibeams

The biological effectiveness of proton beams varies with depth, spot size and lateral distance from the beam central axis. The aim of this work is to incorporate proton relative biological effectiveness (RBE) and equivalent uniform dose (EUD) considerations into comparisons of broad beam and highly modulated proton minibeams. A Monte Carlo model of a small animal proton beamline is presented. Dose and variable RBE is calculated on a per-voxel basis for a range of energies (30–109 MeV). For an open beam, the RBE values at the beam entrance ranged from 1.02–1.04, at the Bragg peak (BP) from 1.3 to 1.6, and at the distal end of the BP from 1.4 to 2.0. For a 50 MeV proton beam, a minibeam collimator designed to produce uniform dose at the depth of the BP peak, had minimal impact on the open beam RBE values at depth. RBE changes were observed near the surface when the collimator was placed flush with the irradiated object, due to a higher neutron contribution derived from proton interactions with the collimator. For proton minibeams, the relative mean RBE weighted entrance dose (RWD) was ~25% lower than the physical mean dose. A strong dependency of the EUD with fraction size was observed. For 20 Gy fractions, the EUD varied widely depending on the radiosensitivity of the cells. For radiosensitive cells, the difference was up to ~50% in mean dose and ~40% in mean RWD and the EUD trended towards the valley dose rather than the mean dose. For comparative studies of uniform dose with spatially fractionated proton minibeams, EUD derived from a per-voxel RWD distribution is recommended for biological assessments of reproductive cell survival and related endpoints.

Monte Carlo optimization of a Compton suppression system for gamma-ray diagnosis of combustion plasma

The use of Gamma-ray spectrometers provides very important diagnostic tools in large-scale tokamak experiments. In this study, the Geant4 toolkit was used to simulate a Compton suppression system based upon a High-Purity Germanium (HPGe) primary detector in order to gain the optimal sizes for the construction of the system for 0.1 MeV–10 MeV γ-rays which are emitted from the HL-2A tokamak. BGO crystal is selected for the anti-coincidence secondary detector. Simulation results show that when the closer the HPGe primary detector is set to the beam entrance and the smaller the entrance size is, the higher the Compton Suppression Factors (CSFs) are. Moreover, adding a BGO crystal to the back of the HPGe detector can also improve the CSFs, and the CSFs increase more significantly with the higher γ-ray energies. And a BGO crystal with the thickness of 25 mm in front of the HPGe detector can reach a good suppression effect. For the BGO hollow cylinder, the CSFs do not increase obviously after its thickness reaches 60–70 mm. Finally, optimal parameters for the Compton suppression system are suggested. Moreover, for the optimal Compton suppression system, γ-rays produced by neutrons which are also one of fusion products, can be well suppressed.

Influence of a transverse magnetic field on the dose deposited by a 6 MV linear accelerator

Abstract An integrated system of a linear accelerator and a magnetic resonance imaging (MRI) device may provide real-time imaging during radiotherapy treatments. This work investigated changes affecting the dose deposition caused by a magnetic field (B-field) transverse to the beam direction by means of Monte Carlo simulations. Two different phantoms were used: A water phantom (Ph1) and a water-air phantom (Ph2) with a 4-2-4 cm water-air-water cross section. Dose depositions were scored for B-field values of 0 T, 0.35 T, 0.5 T, 1.5 T, 3 T and 5 T. Beams were based on a precalculated photon spectrum taken from an earlier simulated Elekta 6 MV FFF accelerator. All lateral profiles in Ph1 showed a Lorentz force driven shift w.r.t. the B-field strength, presenting a steeper penumbra in the shift's direction. Depositions were shifted up to 0.3 cm for 5 T, showing a constant central axis plateau-dose or an increase by 2.3 % for small fields. Depth-dose curves in Ph1 showed a shift of the dose maximum towards the beam entrance direction for increasing B-field of up to 1.1 cm; the maximum dose was increased by 6.9 %. In Ph2, an asymmetric dose increase by up to 36.9 % was observed for 1.5 T at the water-air boundary, resulting from the electron return effect (ERE). In our scenario, B-field dependent dose shifts and local build-ups were observed, which consequently affect the resulting dose distribution and need to be considered in magnetic resonance guided radiotherapy treatment planning.

Neovascular glaucoma after proton beam therapy of choroidal melanoma: incidence and risk factors.

To analyze the risk factors for the development of neovascular glaucoma (NVG) of patients with choroidal melanoma after proton beam therapy (PBT). Clinical case series, retrospective study. We evaluated 629 consecutive patients receiving proton beam therapy for the treatment of a choroidal melanoma at the oncology service at Charité, Berlin and Helmholtz-Zentrum, Berlin between 05/1998 and 11/2008 regarding the development and risk factors of NVG. Patients with tumor resection, salvage proton beam therapy for recurrent disease and known glaucoma of other origin were excluded from the cohort. Of the 629 patients matching the inclusion criteria, 20.8% developed neovascularization of the iris after a mean time of 2.0years (range 0.45 to 8.4years) after PBT. Forty-seven percent of the patients with a neovascularization of the iris developed NVG after a mean time of 2.0years after PBT, ranging from 5months to 11.6years. Univariate analysis revealed tumor height [p<0.001, hazard ratio (HR): 2.71, 95% confidence interval (CI): 1.36-5.35 for tumors >6mm ≤9mm and 11.32 (4.03-31.73) for tumors >9mm], distance of the tumor to the optic disc (p<0.001, HR: 0.43, 95% CI: 0.24-0.77 for >0mm ≤3mm and HR: 0.13, 95% CI: 0.04-0.37 for >3mm), dose to the ciliary body (p<0.001, HR: 9.21, 95% CI: 5.08-16.71 (21-40 cobalt gray equivalents (CGE), HR 27.23, 95% CI: 6.33-116.97 (41-60 CGE)), dose to the optic disc (p<0.001, HR: 3.53, 95% CI: 1.11-11.27 (21-40CGE), HR: 5.37, 95% CI: 2.72-10.63 (41-60CGE)), the irradiated length of the optic nerve (p<0.001, HR: 4.48, 95% CI: 2.47-8.13) and diabetes mellitus (p<0.05, HR: 2.53, 95% CI: 1.4-4.5) were found to be risk factors for the development of NVG. Multivariate regression analysis identified the dose to the ciliary body [p<0.001, HR: 4.39, 95% CI: 2.28-8.44 (21-40 CGE), HR: 11.04, 95% CI: 1.97-61.69 (41-60 CGE)], the irradiated length of the optic nerve (p<0.001, HR: 3.88, 95% CI: 2.11-7.16), the existence of diabetes mellitus (p<0.01, HR: 1.28, 95% CI: 1.24-4.21) and tumor height [p<0.05, HR: 2.28, 95% CI: 1.17-4.83 (>6mm ≤9mm), HR: 3.74, 95% CI: 1.05-13.23, (>9mm)] to be independent risk factors for the development of NVG. In the present analysis we found tumor height, dose to the ciliary body, irradiated length of the optic nerve and diabetes mellitus to be risk factors for the development of NVG. Whenever possible, critical structures of the anterior and posterior segment should be spared by beam shaping or changing of the beam entry angle.

Forming of Space Charge Wave with Broad Frequency Spectrum in Helical Relativistic Two-Stream Electron Beams**Supported by the Ministry of Education and Science of Ukraine under Grant No 0117U002253.

We elaborate a quadratic nonlinear theory of plural interactions of growing space charge wave (SCW) harmonics during the development of the two-stream instability in helical relativistic electron beams. It is found that in helical two-stream electron beams the growth rate of the two-stream instability increases with the beam entrance angle. An SCW with the broad frequency spectrum, in which higher harmonics have higher amplitudes, forms when the frequency of the first SCW harmonic is much less than the critical frequency of the two-stream instability. For helical electron beams the spectrum expands with the increase of the beam entrance angle. Moreover, we obtain that utilizing helical electron beams in multiharmonic two-stream superheterodyne free-electron lasers leads to the improvement of their amplification characteristics, the frequency spectrum broadening in multiharmonic signal generation mode, and the reduction of the overall system dimensions.

On the orientation effect of the neutron yield increase in d(d, n)3Hе reactions at an energy of 7–12 keV in TiD2 crystals

The orientation dependence of the d(d, n)3He reaction probability in a TiD2 crystal at a deuteron energy of 7–12 keV is investigated. The BCM-1.0 code developed for calculating the trajectories of channeled particles within classical mechanics is used to simulate deuteron trajectories at (200)-planar channeling in a 0.15-μm-thick Ti crystal with the angular divergence of the beam taken into account. The enhancement of the reaction probability in the computer experiments is 2.1 in the case of a parallel deuteron beam and near-zero crystal entry angles relative to the (200) planes. In the case of a deuteron beam with its angular divergence equal to 1/5 of the critical channeling angle, the maximum reaction-probability enhancement is 1.5. The results of calculations agree qualitatively with recent experiments performed at Tomsk Polytechnic University.

Real time radiotherapy verification with Cherenkov imaging: development of a system for beamlet verification

Cherenkov imaging has been shown to allow near real time imaging of the beam entrance and exit on patient tissue, with the appropriate intensified camera and associated image processing. A dedicated system has been developed for research into full torso imaging of whole breast irradiation, where the dual camera system captures the beam shape for all beamlets used in this treatment protocol. Particularly challenging verification measurement exists in dynamic wedge, field in field, and boost delivery, and the system was designed to capture these as they are delivered. Two intensified CMOS (ICMOS) cameras were developed and mounted in a breast treatment room, and pilot studies for intensity and stability were completed. Software tools to contour the treatment area have been developed and are being tested prior to initiation of the full trial. At present, it is possible to record delivery of individual beamlets as small as a single MLC thickness, and readout at 20 frames per second is achieved. Statistical analysis of system repeatibilty and stability is presented, as well as pilot human studies.

Two-dimensional numerical simulation of a continuous needle-like argon electron-beam plasma

The fluid-Poisson equations coupled with the Monte Carlo method were used to simulate the spatio-temporal behavior of a needle-like argon electron-beam plasma. Based on the Monte Carlo simulation, three coupled parameters characterizing the electron beam propagation for initial energies above several keV were expressed using a universal dimensionless shape function given in terms of the beam range multiplied by a normalized coefficient. Therefore, a single run of the Monte Carlo code was sufficient for the simulations over a wide range of conditions. The spatial potential as a function of space and time was studied from the fluid-Poisson equations. The results indicate that the time evolution of the spatial potential was influenced by the presence of the slowed-down electrons and the flying beam electrons, whereas the potential in quasi-equilibrium was mainly determined from the spatial distribution of the secondary electron. The potential in quasi-equilibrium was positive near the beam entrance and most negative along the tip of the beam range, which was a result of ambipolar diffusion. When the enclosing boundary surfaces were moved within the beam range, the potential was nearly positive everywhere. The calculation on the diffusion-drift flux indicated that the net current of the secondary electrons flowing back to the incident plane in quasi-equilibrium balanced the incident beam current, which was the so-called return current in the three-dimensional space.

90° cylindrical analyzer for the plasma potential fluctuations measurements by heavy ion beam diagnostic on the tokamak ISTTOK

The Heavy Ion Beam Diagnostic of the tokamak ISTTOK is operated with a 20keV Xe+ ion beam and a multiple cell array detector to collect the secondary Xe2+ ions created along the primary beam path by ionizing collisions with plasma electrons. In this paper an improved multi-channel multi-slit 90° cylindrical analyzer for the plasma potential fluctuations measurements in ISTTOK is described. The energy resolution of the ideal 90° cylindrical analyzer indicates the value of minimal detectable change in plasma potential to approximately 5 times higher than the expected plasma potential fluctuation level. In addition, the ideal 90° cylindrical analyzer is characterized by finite first order angular aberration. The SIMION code has been used to investigate the properties of real 90° cylindrical analyzer with inclusion of fringing fields. As a result of numerical simulations, the necessary energy resolution has been obtained in novel approach of operation in combining electric field deflection and deceleration with biased exit electrode. In addition, almost the first order focusing in the range of ±2° has been obtained exploiting the lens effect of fringing field together with optimization of the initial conditions at the beam entrance. A split-plate and a multiple cell detection systems at the analyzer exit are also discussed.

Water equivalent thickness of immobilization devices in proton therapy planning – Modelling at treatment planning and validation by measurements with a multi-layer ionization chamber

PurposeTo investigate the range errors made in treatment planning due to the presence of the immobilization devices along the proton beam path. MethodsThe measured water equivalent thickness (WET) of selected devices was measured by a high-energy spot and a multi-layer ionization chamber and compared with that predicted by treatment planning system (TPS). Two treatment couches, two thermoplastic masks (both un-stretched and stretched) and one headrest were selected. At TPS, every immobilization device was modelled as being part of the patient. The following parameters were assessed: CT acquisition protocol, dose-calculation grid-sizes (1.5 and 3.0mm) and beam-entrance with respect to the devices (coplanar and non-coplanar). Finally, the potential errors produced by a wrong manual separation between treatment couch and the CT table (not present during treatment) were investigated. ResultsIn the thermoplastic mask, there was a clear effect due to beam entrance, a moderate effect due to the CT protocols and almost no effect due to TPS grid-size, with 1mm errors observed only when thick un-stretched portions were crossed by non-coplanar beams. In the treatment couches the WET errors were negligible (<0.3mm) regardless of the grid-size and CT protocol. The potential range errors produced in the manual separation between treatment couch and CT table were small with 1.5mm grid-size, but could be >0.5mm with a 3.0mm grid-size. In the headrest, WET errors were negligible (0.2mm). ConclusionsWith only one exception (un-stretched mask, non-coplanar beams), the WET of all the immobilization devices was properly modelled by the TPS.

Prediction and compensation of magnetic beam deflection in MR-integrated proton therapy: a method optimized regarding accuracy, versatility and speed

The integration of magnetic resonance imaging (MRI) and proton therapy for on-line image-guidance is expected to reduce dose delivery uncertainties during treatment. Yet, the proton beam experiences a Lorentz force induced deflection inside the magnetic field of the MRI scanner, and several methods have been proposed to quantify this effect. We analyze their structural differences and compare results of both analytical and Monte Carlo models. We find that existing analytical models are limited in accuracy and applicability due to critical approximations, especially including the assumption of a uniform magnetic field. As Monte Carlo simulations are too time-consuming for routine treatment planning and on-line plan adaption, we introduce a new method to quantify and correct for the beam deflection, which is optimized regarding accuracy, versatility and speed. We use it to predict the trajectory of a mono-energetic proton beam of energy E0 traversing a water phantom behind an air gap within an omnipresent uniform transverse magnetic flux density B0. The magnetic field induced dislocation of the Bragg peak is calculated as function of E0 and B0 and compared to results obtained with existing analytical and Monte Carlo methods. The deviation from the Bragg peak position predicted by Monte Carlo simulations is smaller for the new model than for the analytical models by up to 2 cm. The model is faster than Monte Carlo methods, less assumptive than the analytical models and applicable to realistic magnetic fields. To compensate for the predicted Bragg peak dislocation, a numerical optimization strategy is introduced and evaluated. It includes an adjustment of both the proton beam entrance angle and energy of up to 25° and 5 MeV, depending on E0 and B0. This strategy is shown to effectively reposition the Bragg peak to its intended location in the presence of a magnetic field.

Retarding field energy analyzer for high energy pulsed electron beam measurements.

A retarding field energy analyzer (RFEA) designed specifically for high energy pulsed electron beam measurements is described in this work. By proper design of the entrance grid, attenuation grid, and beam collector, this RFEA is capable of determining the time-resolved energy distribution of high energy pulsed electron beams normally generated under "soft vacuum" environment. The performance of the RFEA is validated by multiple tests of the leakage current, attenuation coefficient, and response time. The test results show that the retarding potential in the RFEA can go up to the same voltage as the electron beam source, which is 20 kV for the maximum in this work. Additionally, an attenuation coefficient of 4.2 is obtained in the RFEA while the percent difference of the rise time of the electron beam pulse before and after attenuation is lower than 10%. When compared with a reference source, the percent difference of the RFEA response time is less than 10% for fall times greater than 35 ns. Finally, the test results of the 10 kV pseudospark-based pulsed electron beam currents collected under varying retarding potentials are presented in this paper.

Recent developments and comprehensive evaluations of a GPU-based Monte Carlo package for proton therapy

Monte Carlo (MC) simulation is commonly considered as the most accurate dose calculation method for proton therapy. Aiming at achieving fast MC dose calculations for clinical applications, we have previously developed a graphics-processing unit (GPU)-based MC tool, gPMC. In this paper, we report our recent updates on gPMC in terms of its accuracy, portability, and functionality, as well as comprehensive tests on this tool. The new version, gPMC v2.0, was developed under the OpenCL environment to enable portability across different computational platforms. Physics models of nuclear interactions were refined to improve calculation accuracy. Scoring functions of gPMC were expanded to enable tallying particle fluence, dose deposited by different particle types, and dose-averaged linear energy transfer (LETd). A multiple counter approach was employed to improve efficiency by reducing the frequency of memory writing conflict at scoring. For dose calculation, accuracy improvements over gPMC v1.0 were observed in both water phantom cases and a patient case. For a prostate cancer case planned using high-energy proton beams, dose discrepancies in beam entrance and target region seen in gPMC v1.0 with respect to the gold standard tool for proton Monte Carlo simulations (TOPAS) results were substantially reduced and gamma test passing rate (1%/1 mm) was improved from 82.7%–93.1%. The average relative difference in LETd between gPMC and TOPAS was 1.7%. The average relative differences in the dose deposited by primary, secondary, and other heavier particles were within 2.3%, 0.4%, and 0.2%. Depending on source proton energy and phantom complexity, it took 8–17 s on an AMD Radeon R9 290x GPU to simulate source protons, achieving less than average statistical uncertainty. As the beam size was reduced from 10 × 10 cm2 to 1 × 1 cm2, the time on scoring was only increased by 4.8% with eight counters, in contrast to a 40% increase using only one counter. With the OpenCL environment, the portability of gPMC v2.0 was enhanced. It was successfully executed on different CPUs and GPUs and its performance on different devices varied depending on processing power and hardware structure.

Losing focus: how lens position and viewing angle affect the function of multifocal lenses in fishes.

Light rays of different wavelengths are focused at different distances when they pass through a lens (longitudinal chromatic aberration [LCA]). For animals with color vision this can pose a serious problem, because in order to perceive a sharp image the rays must be focused at the shallow plane of the photoreceptor's outer segments in the retina. A variety of fish and tetrapods have been found to possess multifocal lenses, which correct for LCA by assigning concentric zones to correctly focus specific wavelengths. Each zone receives light from a specific beam entrance position (BEP) (the lateral distance between incoming light and the center of the lens). Any occlusion of incoming light at specific BEPs changes the composition of the wavelengths that are correctly focused on the retina. Here, we calculated the effect of lens position relative to the plane of the iris and light entering the eye at oblique angles on how much of the lens was involved in focusing the image on the retina (measured as the availability of BEPs). We used rotational photography of fish eyes and mathematical modeling to quantify the degree of lens occlusion. We found that, at most lens positions and viewing angles, there was a decrease of BEP availability and in some cases complete absence of some BEPs. Given the implications of these effects on image quality, we postulate that three morphological features (aphakic spaces, curvature of the iris, and intraretinal variability in spectral sensitivity) may, in part, be adaptations to mitigate the loss of spectral image quality in the periphery of the eyes of fishes.

Dosimetric comparison of Volumetric Modulated Arc Therapy and intensity-modulated radiotherapy for bilateral hip prostheses prostate cancer patient

Introduction Volumetric Modulated Arc Therapy (VMAT) is a common technique for prostate cancer patients, due to the conformation of the dose distribution. The high atomic number of the hip prostheses, avoiding beam entry through the prostheses, makes this treatment complicated. Purpose Compare the dosimetric quality of intensity-modulated radiotherapy (IMRT) with VMAT for the treatment of prostate cancer in patients with bilateral hip prostheses. Materials and methods Plans for IMRT and VMAT were optimized, prescribing 78 Gy/39 fractions. First a soft-tissue density was overridden to the artifact CT areas, and both prostheses were contoured to avoid them during the treatment. Three IMRT treatment plans were proposed, 7-field, 9-field and 11-field, all optimized with constrained beam inverse planning, fixing jaws during the optimization. Two VMAT treatment plans were optimized with avoiding sectors (AS), one with large AS, and one with small AS, both VMAT plans were optimized with two arcs. Results IMRT technique has better dose distribution than VMAT technique. 11-field IMRT showed the best dose conformation, and in VMAT technique, small AS was the best solution. 11-field IMRT Homogeneity Index was 0.046 against 0.077 of the VMAT small AS. 11-field IMRT Rectum V50 was 28.50% versus 40.90% for VMAT small AS. UM and treatment had better results in VMAT technique. UM were significantly larger for IMRT, a 60.36% for 11-field IMRT compared with VMAT small AS. Conclusion 11-Field IMRT showed better conformation across PTV and lower rectal and bladder dose comparing to the other plans analyzed, despite of its larger treatment time and number of UM.

Improved operation of a microwave pulse compressor with a laser-triggered high-pressure gas plasma switch

The influence of laser beam parameters on the output pulses of a resonant microwave compressor with a laser-triggered plasma switch was investigated. The S-band compressor, consisting of a rectangular waveguide-based cavity and H-plane waveguide tee with a shorted side arm, was filled with pressurized dry air and pumped by 1.8-μs-long microwave pulses of up to 450 kW power. A Nd:YAG laser was used to ignite the gas discharge in the tee side arm for output pulse extraction. The laser beam (at 213 nm or 532 nm) was directed along the RF electric field lines. It was found that the compressor operated most effectively when the laser beam was focused at the center of the switch waveguide cross-section. In this case, the power extraction efficiency reached ∼47% at an output power of ∼14 MW, while when the laser beam was not focused the maximal extraction efficiency was only ∼20% at ∼6 MW output power. Focusing the laser beam resulted also in a dramatic decrease (down to &amp;lt;1 ns) in the delay of the output pulses' appearance with respect to the time of the beam's entrance into the switch, and the jitter of the output pulses' appearance was minimized. In addition, the quality of the output pulses' waveform was significantly improved.