Collimator Opening Research Articles

Fast, Low-Dose Megavoltage-Topogram Localization in a Helical IGRT Unit: Initial Clinical Experience with Mesothelioma Patients

To present our initial clinical experience with malignant pleural mesothelioma (MPM) patients in Megavoltage (MV)-Topogram trial, for evaluating the potential of MV-Topogram-based alignment as an alternative to Megavoltage Computed Tomography (MVCT) in reducing imaging time and dose. Four mesothelioma patients were enrolled in an on-going Institutional Review Board (IRB)-approved MV-Topograms clinical trial at our institute. Patients were set up using the clinical protocol employing red lasers. Anterior-Posterior (AP) and lateral (LAT) MV-Topograms were acquired using gantry angles of 0°/90° with 1 mm collimator opening, all MLC leafs open, 4 cm/s couch speed, and 12.5 s scanning time. Routine MVCT scans were performed immediately afterwards. MV-Topograms were reconstructed and enhanced using contrast-limited adaptive histogram equalization (CLAHE). AP and LAT kilo voltage (kV) digital reconstructed topograms (DRT) images were reconstructed based on TomoTherapy geometry from CT simulation scans. Registrations between MV-Topograms and kV-DRT images were performed by a physicist manually. Due to image contrast limitations, the registration primarily involved aligning the carina. To be consistent with MV-Topograms, the same physicist re-aligned MVCTs to planning CT based on carina as well. Results were compared between MV-Topograms based shifts and the corresponding MVCT shifts. MV-Topogram and MVCT doses were measured using an ion-chamber in a 28 cm diameter cylindrical water-equivalent phantom at depths between 1-14 cm. The mean and standard deviation of shift discrepancies between MV-Topogram and MVCT were -0.52±1.35 mm, 2.51±2.40 mm, -0.24±4.09 mm in lateral, longitudinal and vertical directions for manual registrations. The dose ratio between MVCT and MV-Topogram ranged from 14.7 to 26.9 for depths between 1 cm and 14 cm, respectively. On average, the time required to acquire 38 cm long MVCT scans were five times more than a pair of 50 cm long MV-Topograms. Lateral and vertical shifts from MV-Topograms showed equivalent alignment results with MVCT. The greatest discrepancy between the MV Topograms and MVCT was in the longitudinal direction. This could be due to the breathing motion of the carina, which has been shown to be approximately 5 mm in the longitudinal direction. Substantial reductions of dose and acquisition time were observed for MV-Topogram. Challenges of employing bony anatomy for Topogram alignment were the relatively poor contrast and image artifacts caused by small dose rate fluctuations. Improving the linear accelerator dose stability would improve the ability to employ MV Topograms for patient alignment.

Dosimetric studies of cadmium free alloy used in compensator based intensity modulated radiotherapy

Aim of this study was to investigate dosimetric properties of cadmium free alloy which is used in compensator based intensity modulated radiotherapy (cIMRT). A mixture of lead, bismuth and tin was used to prepare the alloy whose melting point is 90–95°C. Slabs of different thicknesses ranging from 0.71cm to 6.14cm were prepared. Density of alloy was measured by Archimedes’ principle using water. For six megavolt (6MV) photon beam energy transmission, linear effective attenuation coefficient (µeff), tissue phantom ratio (TPR1020), beam hardening, surface dose (Ds), percentage depth dose (PDD) and effect of scatter has been measured and analyzed for different field sizes and different thickness of compensator. Effect of extended source to detector distance (SDD) on transmissions and µeff was measured. The density of alloy was found to be 9.5456g/cm3. At SDD of 100cm, µeff was observed 0.4253cm−1 for a field size of 10×10cm 2. Calculated TPR1020 was found to be within 3% of experimental TPR1020. It was found to be increasing with increasing thickness of compensator. Ds was found to decrease with thickness of compensator and increase with wider collimator opening due to increased scattered dose. Compensator slabs of 1cm, 1.98cm and 4.16cm decreased surface dose by 4.2%, 6.1% and 9.5% respectively for a field size of 10×10cm2 at 100cm SDD. For small field size of 3×3cm2 and 5×5cm2 PDDs are increased from 3.0% to 5.5% of open beam PDDs as compensator thickness increased from 1cm to 6.14cm at a depth of 10cm in water while variation in PDD is insignificant in for larger field sizes 10×10cm2 to 20×20cm2. A high degree of intensity modulation is essential in cIMRT and it can be achieved with this compensator material. Dosimetric properties analyzed in this study establish this alloy as a reliable, reusable, optimally dense and cost effective compensator material.

An Analytical Method to Calculate Phantom Scatter Factor for Photon Beam Accelerators.

IntroductionOne of the important input factors in the commissioning of the radiotherapy treatment planning systems is the phantom scatter factor (Sp) which requires the same collimator opening for all radiation fields. In this study, we have proposed an analytical method to overcome this issue.MethodsThe measurements were performed using Siemens Primus Plus with photon energy 6 MV for field sizes from 5×5cm2 to 40×40cm2. Phantom scatter factor was measured through the division of total scatter output factors (Scp), and collimator scatter factor (Sc).ResultsThe mean percent difference between the measured and calculated Sp was 1.00% and -3.11% for 5×5, 40×40 cm2 field size respectively.ConclusionThis method is applicable especially for small fields used in IMRT which, measuring collimator scatter factor is not reliable due to the lateral electron disequilibrium.

Reaching record-low [formula omitted] at the CERN Large Hadron Collider using a novel scheme of collimator settings and optics

The Large Hadron Collider (LHC) at CERN is built to collide intense proton beams with an unprecedented energy of 7TeV. The design stored energy per beam of 362MJ makes the LHC beams highly destructive, so that any beam losses risk to cause quenches of superconducting magnets or damage to accelerator components. Collimators are installed to protect the machine and they define a minimum normalized aperture, below which no other element is allowed. This imposes a limit on the achievable luminosity, since when squeezing β* (the β-function at the collision point) to smaller values for increased luminosity, the β-function in the final focusing system increases. This leads to a smaller normalized aperture that risks to go below the allowed collimation aperture. In the first run of the LHC, this was the main limitation on β*, which was constrained to values above the design specification. In this article, we show through theoretical and experimental studies how tighter collimator openings and a new optics with specific phase-advance constraints allows a β* as small as 40cm, a factor 2 smaller than β*=80cm used in 2015 and significantly below the design value β*=55cm, in spite of a lower beam energy. The proposed configuration with β*=40cm has been successfully put into operation and has been used throughout 2016 as the LHC baseline. The decrease in β* compared to 2015 has been an essential contribution to reaching and surpassing, in 2016, the LHC design luminosity for the first time, and to accumulating a record-high integrated luminosity of around 40 fb−1 in one year, in spite of using less bunches than in the design.

Monte carlo simulation of different source configurations for a new design of rotating Gamma Knife system

Introduction A new design of Gamma Knife consist of 30 60 C0 source capsules, two circular primary collimators (diameter of 6.60 mm and 6.10 mm) and four different changeable collimators. The sources (diameter of 2.8 mm) are distributed in six groups of five sources in the spherical geometry. Each source is individually collimated to obtain four different circular fields at the isocenter (3 mm, 3.5 mm, 6 mm and 8 mm). Purpose To determine the characteristics of the Co 60 beam emerging from a new design of Gamma Knife system and to calculate dose distributions at the isocenter distance for different source configurations and collimator openings Materials and methods We have used the BEAM-Monte Carlo code to realistically model the geometry design, including source capsules, primary and secondary collimators. The shielding of the head was also simulated. The dose distributions at the isocenter distance are calculated using GEPTS in a previous designed spherical component module for the circular field sizes studied. Results The spectra of particles emerging from each source-collimator configuration is calculated.The radial photon fluence does not vary significantly inside the collimator openings. The spectra of particles from different source groups are compared. Conclusion The 60 Co beam emerging from each group source configuration was characterized but our preliminary results do not allow to properly determine the variation of spectra of particles at isocenter as function of source group position. Further investigations are needed.

Sci‐Thur PM – Brachytherapy 05: Surface Collimation Applied to Superficial Flap High Dose‐Rate Brachytherapy

Purpose:To apply surface collimation for superficial flap HDR skin brachytherapy utilizing common clinical resources and to demonstrate the potential for OAR dose reduction within a clinically relevant setting.Methods:Two phantom setups were used. 3 mm lead collimation was applied to a solid slab phantom to determine appropriate geometries relating to collimation and dwell activation. The same collimation was applied to the temple of an anthropomorphic head phantom to demonstrate lens dose reduction. Each setup was simulated and planned to deliver 400 cGy to a 3 cm circular target to 3 mm depth. The control and collimated irradiations were sequentially measured using calibrated radiochromic films.Results:Collimation for the slab phantom attenuated the dose beyond the collimator opening, decreasing the fall‐off distances by half and reducing the area of healthy skin irradiated. Target coverage can be negatively impacted by a tight collimation margin, with the required margin approximated by the primary beam geometric penumbra.Surface collimation applied to the head phantom similarly attenuated the surrounding normal tissue dose while reducing the lens dose from 84 to 68 cGy. To ensure consistent setup between simulation and treatment, additional QA was performed including collimator markup, accounting for collimator placement uncertainties, standoff distance verification, and in vivo dosimetry.Conclusions:Surface collimation was shown to reduce normal tissue dose without compromising target coverage. Lens dose reduction was demonstrated on an anthropomorphic phantom within a clinical setting. Additional QA is proposed to ensure treatment fidelity.

Technical Note: A treatment plan comparison between dynamic collimation and a fixed aperture during spot scanning proton therapy for brain treatment

To quantitatively assess the advantages of energy-layer specific dynamic collimation system (DCS) versus a per-field fixed aperture for spot scanning proton therapy (SSPT). Five brain cancer patients previously planned and treated with SSPT were replanned using an in-house treatment planning system capable of modeling collimated and uncollimated proton beamlets. The uncollimated plans, which served as a baseline for comparison, reproduced the target coverage and organ-at-risk sparing of the clinically delivered plans. The collimator opening for the fixed aperture-based plans was determined from the combined cross sections of the target in the beam's eye view over all energy layers which included an additional margin equivalent to the maximum beamlet displacement for the respective energy of that energy layer. The DCS-based plans were created by selecting appropriate collimator positions for each row of beam spots during a Raster-style scanning pattern which were optimized to maximize the dose contributions to the target and limited the dose delivered to adjacent normal tissue. The reduction of mean dose to normal tissue adjacent to the target, as defined by a 10 mm ring surrounding the target, averaged 13.65% (range: 11.8%-16.9%) and 5.18% (2.9%-7.1%) for the DCS and fixed aperture plans, respectively. The conformity index, as defined by the ratio of the volume of the 50% isodose line to the target volume, yielded an average improvement of 21.35% (19.4%-22.6%) and 8.38% (4.7%-12.0%) for the DCS and fixed aperture plans, respectively. The ability of the DCS to provide collimation to each energy layer yielded better conformity in comparison to fixed aperture plans.

The influence of field size and off-axis distance on photoneutron spectra of the 18 MV Siemens Oncor linear accelerator beam

At present, high energy electron linear accelerators (LINACs) producing photons with energies higher than 10 MeV have a wide use in radiotherapy (RT). However, in these beams fast neutrons could be generated, which results in undesired contamination of the therapeutic beams. These neutrons affect the shielding requirements in RT rooms and also increase the out-of-field radiation dose to patients. The neutron flux becomes even more important when high numbers of monitor units are used, as in the intensity modulated radiotherapy. Herein, to evaluate the exposure of patients and medical personnel, it is important to determine the full radiation field correctly. A model of the dual photon beam medical LINAC, Siemens ONCOR, used at the University Hospital Centre of Osijek was built using the MCNP611 code. We tuned the model according to measured photon percentage depth dose curves and profiles. Only 18 MV photon beams were modeled. The dependence of neutron dose equivalent and energy spectrum on field size and off-axis distance in the patient plane was analyzed. The neutron source strength (Q) defined as a number of neutrons coming from the head of the treatment unit per x-ray dose (Gy) delivered at the isocenter was calculated and found to be 1.12 × 1012 neutrons per photon Gy at isocenter. The simulation showed that the neutron flux increases with increasing field size but field size has almost no effect on the shape of neutron dose profiles. The calculated neutron dose equivalent of different field sizes was between 1 and 3 mSv per photon Gy at isocenter. The mean energy changed from 0.21 MeV to 0.63 MeV with collimator opening from 0 × 0 cm2 to 40 × 40 cm2. At the 50 cm off-axis the change was less pronounced. According to the results, it is reasonable to conclude that the neutron dose equivalent to the patient is proportional to the photon beam-on time as suggested before. Since the beam-on time is much higher when advanced radiotherapy techniques are used to fulfill high conformity demands, this makes the neutron flux determination even more important. We also showed that the neutron energy in the patient plane significantly changes with field size. This can introduce significant uncertainty in dosimetry of neutrons due to strong dependence of the neutron detector response on the neutron energy in the interval 0.1–5 MeV.

SU‐F‐J‐181: An Alternative Patient Alignment Tool On TomoTherapy: The First In‐ Human Megavoltage‐Topogram Acquisition

Purpose:To show the first in‐human Megavoltage (MV)‐Topogram acquisition for the evaluation of the potential for MV‐Topogram‐based alignment as an alternative to MVCT for reducing dose and imaging time.Methods:A lung cancer patient was enrolled in an ongoing IRB‐approved clinical trial at our institute. The patient was set up using the clinical protocol employing positioning lasers. 3.2mm diameter tungsten spheres were placed on the patient's skin at their alignment tattoos to check surface‐based marker concordance between topograms and MVCT. Anterior‐Posterior (AP) and lateral (LAT) MV‐Topograms were acquired using gantry angles of 0°/90° with a 1mm collimator opening, all MLC leafs open, 4cm/s couch speed, and 12.5s scanning time. The topogram acquisition was immediately followed by the normal MVCT scan acquisition. MV‐Topograms were reconstructed from the detector exit‐data using in‐house developed software. The topograms were also enhanced using contrast‐limited adaptive histogram equalization (CLAHE). The MV‐Topograms were registered to reference kV‐based digitally reconstructed topograms. The localization results were compared against results obtained comparing the clinical MVCT to the kVCT simulation.Results:The shifts using the unenhanced Topograms, enhanced Topograms, and MVCT were (LAT, LONG, VERT, ROLL) (5.8mm, 2.6mm, −5.6mm, 0.34°), (3.9mm, 2.5mm, −2.2mm, 0.65°) and (2.4mm, 1.5mm, −3.0mm, 0.5°), respectively. The magnitude alignment differences between the enhanced Topograms and MVCT were within 1.5 mm and 0.15°. The average MVCT and total Topogram acquisition times were 272.9s ± 31.5s and 46s, respectively.Conclusion:MV‐Topograms have the potential for providing equivalent performance with less dose and acquisition time than the traditional MVCT technique. We are evaluating other sites as well as adding patients to develop statistically significant analyses regarding the alignment quality differences. MV‐Topograms are likely to be most clinically useful for bony anatomy and radiopaque marker‐based alignments.The study was supported by an Accuray Grant.

SU‐F‐T‐131: No Increase in Biological Effectiveness Through Collimator Scattered Low Energy Protons

Purpose:To reduce the lateral penumbra of low‐energy proton beams, brass collimators are often used in spot‐scanning proton therapy (SSPT). This study investigates the increase in biological effectiveness through collimator scattered protons in SSPT.Methods:The SSPT system of the Hokkaido University Hospital Proton Beam Therapy Center, which consists of a scanning nozzle, a 2‐cm thick brass collimator, and a 4‐cm thick energy absorber, was simulated with our validated Geant4 Monte Carlo code (ver. 9.3). A water phantom was irradiated with proton pencil beams of 76, 110, and 143 MeV. The tested collimator opening areas (COA) were 5×5, 10×10, and 15×15 cm2. Comparisons were made among the dose‐averaged LET values of protons that hit the collimators (LETDColl), protons that did not hit the collimators (LETDNoColl), and all protons (LETDTotal). X‐ray equivalent doses (Deq) were calculated using the linear‐quadratic model with LETDNoColl and LETDTotal, and their maximum difference was determined over regions where the physical dose was greater than 10% of the peak dose of 2 Gy.Results:The ratio of the dose contribution of collimator scattered protons to that of all protons, defined as λ, was large at high proton energies and large COAs. The maximum λ value ranged from 3% (76 MeV, 5×5 cm2) to 29% (143 MeV, 15×15 cm2). Moreover, a large difference between LETDColl and LETDNoColl was only found in regions where λ was below 20% (ΔLETD > 2 keV/µm) and 8% (ΔLETD > 5 keV/µm). Consequently, the maximum difference between LETDNoColl and LETDTotal was as small as 0.8 keV/µm in all simulated voxels, and the difference of Deq reached a maximum of 1.5% that of the peak dose obtained at the water surface with a 76 MeV beam.Conclusion:Although collimator scattered protons have high LET, they only increase the physical dose, not the biological effectiveness.

SU‐F‐T‐533: Study of Dosimetric Properties of Cadmium Free Alloy Used in Compensator Based IMRT

Purpose:To study the dosimetric properties of cadmium free alloy which is used in compensator based IMRT.Methods:A mixture of 30% of lead,52% of bismuth and 18% of tin was used to prepare alloy. We prepared slabs of different thicknesses ranging from 0.71 cm to 6.14 cm. Density of alloy was measured by Archimedes’ principle using SI‐234 Denver instrument and water as buoyant liquid. Transmission, linear attenuation coefficient (µ), tissue phantom ration (TPR), beam hardening, surface dose (Ds), percentage depth dose (PDD) and effect of scatter were measured and analyze for different field size and different thickness of compensator for 6 MV photon beam. Measurements were carried out at 100 cm SSD and 160 cm SSD.Results:Density of alloy was found to be 9.5456 gm/cm3. Melting point of alloy is 90–95 °C. For a field size of 10×10 cm2 µ was 0.4253 cm‐1 at 100 cm SSD. Calculated TPR was found to be within 3 % of measured TPR. Ds was found to be decreasing with increasing thickness of compensator. 1cm, 1.98 cm and 4.16 cm thick compensator slab decreased surface dose by 4.2%, 6.1% and 9.5% respectively for a field size of 10×10cm2 at 100 cm SSD. As field size increases Ds increases for a given compensator thickness. This is due to increase in amount of scattered dose from wider collimator opening. For smaller field size, PDDs are increased from 3.0% to 5.5% of open beam PDDs as compensator thickness increases from 1 cm to 6.14 cm at a depth of 10 cm in water. For larger field size variation in PDDs is not significant.Conclusion:High degree of modulation can be achieved from this compensator material, which is essential in compensator based IMRT. Dosimetric properties analyzed in this study establish this alloy as a reliable, cost effective, reusable compensator material.

SU‐E‐T‐187: Collimation Methods in Spot Scanning Proton Therapy: A Treatment Plan Comparison Between a Fixed Aperture and a Dynamic Collimation System

Purpose:Low‐energy treatments during spot scanning proton therapy (SSPT) suffer from poor conformity due to increased spot size. Collimation devices can reduce the lateral penumbra of a proton therapy dose distribution and improve the overall plan quality. The purpose of this work was to study the advantages of individual energy‐layer collimation, which is unique to a recently proposed Dynamic Collimation System (DCS), in comparison to a standard, fixed aperture that allows only a single shape for all energy layers.Methods:Three brain patients previously planned and treated with SSPT were re‐planned using an in‐house treatment planning system capable of modeling collimated and un‐collimated proton beamlets. The un‐collimated plans, which served as a baseline for comparison, reproduced the target coverage of the clinically delivered plans. The collimator opening for the aperture based plans included a 0.6 cm expansion of the largest cross section of the target in the Beam's Eye View, while the DCS based plans were created by optimizing the collimator position for beam spots near the periphery of the target in each energy layer.Results:The reduction of mean dose to normal tissue adjacent to the target, as defined by a 10 mm ring, averaged 9.13% and 3.48% for the DCS and aperture plans, respectively. The conformity index, as defined by the ratio of the volume of the 50% isodose line to the target volume, yielded an average improvement of 16.42% and 8.16% for the DCS and aperture plans, respectively.Conclusion:Collimation reduces the dose to normal tissue adjacent to the target and increases dose conformity to the target region for low‐energy SSPT. The ability of the DCS to provide collimation to each energy layer yields better conformity in comparison to fixed aperture plans.This work was partially funded by IBA (Ion Beam Applications S.A.).

SU-E-T-605: A New Design for a Rotating Gamma Knife. Monte Carlo Simulation

Purpose: to determine the characteristics of the 60Co beam emerging from a new design of a rotating Gamma Knife system and to calculate 3D dose distributions at the isocenter for different source configurations and collimator openings. Methods: We employed the BEAM-Monte Carlo code to realistically model the geometry design, including 30 60Co source capsules, two circular primary collimators (diameter of 6.6mm and 6.1mm) and four different changeable collimators. The shielding of the head design was also simulated. The sources (2.8mm diameter) are distributed in six groups in the spherical geometry. Each source is individually collimated to obtained four different circular fields at the isocenter (3mm, 3.5mm, 6mm and 8mm).The phase-space particles reaching the scoring plane below the primary collimation assembly were recorded and the BEAMDP code was used to determine the fluence and energy spectra of the particles emerging from each source-collimator configuration. The dose distributions at the isocenter plane (397.6mm from the source) were calculated in a spherical component module for the circular field sizes studied. Results: The energy spectra below the head assembly and primary collimators have been obtained, which exhibited the typical 60Co peaks and a small low-energy tail due to scattered photons (from about 200keV to 1MeV). The scattered component of the spectra represents about 8 % of the total number of photons reaching the scoring plane. The radial photon fluence does not vary significantly inside the collimator openings. The spectra of particles from different source groups are compared. Conclusion: The 60Co beam emerging from each source configuration was characterized, which can be used to establish a generic source model for all the sources for fast MC dose calculation. Further investigations are needed to determine the dose variations as a result of partial switching on/off different groups of sources for advanced Gamma Knife SRS/SBRT planning.

SU‐E‐T‐240: Monte Carlo Modelling of SMC Proton Nozzles Using TOPAS

Purpose:To simulate the 60Co beam from a novel Gamma—Tomo SBRT system and to calculate the output factors and dose rates for different source configurations and collimator sizes.Methods:The BEAM‐Monte Carlo code is used to realistically model the system geometry, including 32 source capsules, source housing, primary collimator and 4 different changeable collimators. The sources (6.22 mm of diameter) are located in 2 rows with different angles and distances to the longitudinal axis and the beams can be collimated to obtain four different circular field sizes at the isocenter (35 mm, 16 mm, 7mm, and 3.5mm). Since the dose distributions will be determined in a spherical polystyrene phantom of 8cm radius centered at the isocenter, a new geometry module is designed for the dose calculation.Results:: The caracteristics of the particle spectra emerging from each source are determined and the influence of the source position on the spectra reaching the isocenter is found to be minimal (<2%). We found that the scattered photons represent about 15% of the total number of photons reaching the scoring plane immediately (1 mm) below the source and primary collimator. The radial photon fluence does not vary significantly inside the collimator opening and decreases quickly beyond the 3cm radius (projected at the isocenter plane).Conclusion:Our preliminary results indicate that the position of the source and primary collimator assembly does not affect the energy spectra and fluence distributions reaching the isocenter significantly. This allows the use of a well‐represented source model for all the sources to perform the dose calculations for all the changeable collimators for this novel Gamma‐Tomo SBRT system.

Energy Dependence of Parameters Characterizing Multiply Backscattering of Gamma Photons

The present studies aimed to investigate the effects of energy dependence of parameters characterizing multiply backscattering of gamma photons. The numbers of multiply backscattered events are found to be increasing with thickness of copper target, and saturate for a particular thickness known as saturation thickness. The saturation thickness is found to be decreasing with increase in incident gamma photon energy, and also is not altered by the variation in collimator opening. The number, energy and dose albedos, characterizing the reflection probability of a material, are also evaluated. For each of the incident gamma photon energy, the number and energy albedos show an increase with increasing target thickness, and finally saturate. Monte Carlo calculations support the results of present experimental work.

MO‐F‐108‐08: A Revolving Collimator for Proton Stereotactic Radiosurgery (pSRS)

Purpose: Stereotactic radiosurgery (SRS) is a non‐invasive treatment modality appropriate for a wide array of tumors. Particularly in patients with benign tumors receiving proton SRS (pSRS), the generation of secondary neutrons from brass apertures may be associated with higher risk of secondary radiation‐induced tumors. A specially designed revolving titanium alloy collimator system with multiple sized collimator openings in circular and elliptical shapes is proposed and investigated in this study. Methods: Simulations of the passage of a proton beam through the proposed collimator system was performed using the FLUKA Monte Carlo general purpose code with treatment nozzle model implemented. The set of materials potentially applicable for the collimator fabrication, i.e. brass, stainless steel (SS), tungsten alloy (WA) and titanium alloy (TiA) (Ti‐6A1‐4V Grade 5) were simulated. The neutron fluence, dose equivalent, activation of the collimator material, the cooling times and beam shaping properties are analyzed. Results: Utilization of TiA results in less neutron dose equivalent at the patient compared to brass, WA or SS. The neutron flux spectra for TiA is an order of magnitude lower than those for the other materials in the energy interval with the maximum neutron weighting factor. The activation of TiA is lower and the cooling time 10 times shorter compared to WA. Dose shaping with the pSRS collimator is similar to a brass aperture. Dose enhancement at shallow depths for small proton fields limits the practical collimator opening size. Conclusion: The specially designed revolving pSRS titanium alloy collimator proposed in this study may reduce secondary neutron dose to the patient compared to other collimator materials investigated. This may reduce the risk of secondary tumors and malignancies and reduce late effects to the patient. The pSRS collimator allows the establishment of a pSRS system and methodology without the need for a dedicated pSRS treatment vault.

Modeling in the leakage radiation of a medical linear accelerator a function of the collimator opening: Analytical extrapolation of the leak field opened by a method semi-empirical based measurement at zero field

Introduction The optimization of radiotherapy requires precise knowledge of all sources at the origin of the radiation, even at low doses to healthy tissues of the patient. Currently, the developed analytical models for estimating the doses to healthy tissue based on the assumption that the radiation leakage from a medical linear accelerator would be independent of the collimator opening. However, this hypothesis does not account for a very large number of physical and geometrical aspects of each accelerator. The objective of this work is to propose a more realistic model for accelerators leaks. Materials and methods Themeasurements are performed on a linear accelerator Novalis Tx TM (Varian Medical Systems, Palo Alto, CA, USA). Detectors are used thermoluminescent powder type 7 (TLD700) compounds of powder LiF doped with Mg et Ti provided by the company Equal-Estro Laboratory. Development of computer codes using the libraries: CodeBlocks ® (libraries: OpenGL), Matlab ® et Gnuplot ® . Results Concerning theoretical modeling, the early results give encouraging theoretical distributions. The model is the end of development measures analysis is ongoing and results will be presented and discussed. Overall we see a gradual decrease in leakage with increasing the opening of the collimator. This decrease is even more important that we be near the field. Conclusion Our model can calculate the dose from accelerator of the leakage for any location in the patient and configuration of irradiation.

Microdosimetric calculation of penumbra for biological dose in wobbled carbon-ion beams with Monte Carlo Method

In carbon-ion radiotherapy, it is important to evaluate the biological dose because the relative biological effectiveness values vary greatly in a patient's body. The microdosimetric kinetic model (MKM) is a method of estimating the biological effect of radiation by use of microdosimetry. The lateral biological dose distributions were estimated with a modified MKM, in which we considered the overkilling effect in the high linear-energy-transfer region. In this study, we used the Monte Carlo calculation of the Geant4 code to simulate a horizontal port at the Heavy Ion Medical Accelerator in Chiba of the National Institute of Radiological Sciences. The lateral biological dose distributions calculated by Geant4 were almost flat as the lateral absorbed dose in the flattened area. However, in the penumbra region, the lateral biological dose distributions were sharper than the lateral absorbed dose distributions. Furthermore, the differences between the lateral absorbed dose and biological dose distributions were dependent on the depth for each multi-leaf collimator opening size. We expect that the lateral biological dose distribution presented here will enable high-precision calculations for a treatment-planning system.

Quantitative analysis of in-air output ratio

Output factor (Scp) is one of the important factors required to calculate monitor unit (MU), and is divided into two components: phantom scatter factor (Sp) and in-air output ratio (Sc). Generally, Sc for arbitrary fields are calculated using several methods based on Sc determined by the absorbed dose measurement for several square fields. However, there are calculation errors when the treatment field has a large aspect ratio and the opening of upper and lower collimator are exchanged. To determine Sc accurately, scattered photons from the treatment head and backscattered particles into the monitor chamber must be analyzed individually. In this report, a simulation model that agreed well with measured Sc was constructed and dose variation by scattered photons from the treatment head and by backscattered particles into the monitor chamber was analyzed quantitatively. The results showed that the contribution of scattered photons from the primary collimator was larger than that of the flattening filter, and backscattered particles were affected by not only the upper jaw but also the lower jaw. In future work, a new Sc determination algorism based on the result of this report will be proposed.

Http://www.omicsonline.org/stem-cell-research-therapy.php

Patients who suffer from tremors are initially treated with pharmecuticals. Although medication has proven to be effective in achieving tremor control for some patients, there is still a fraction of patients who seek neurosurgical alternatives due to unacceptable tremor relief or side-effects from the prescribed drugs. Invasive neurosurgical procedures include radiofrequency thalamotomy and in recent years deep brain stimulation. However, patients who have advanced cardiac or respiratory disease, patients who use anticoagulants and patients who are of advanced age are not qualified candidates for neurosurgery. An alternative modality for lesioning intracranial structures is stereotactic radiosurgery using the Gamma Knife. The Gamma Knife is a Cobalt-60 based machine, with 201 separate 4 to18 mm collimator openings that emits multiple gamma rays that converge on a focal point in the brain specified by computer planning. Since its introduction by Leksell, the role of Gamma Knife radiosurgery as a management approach for patients diagnosed with tremors is continuously increasing. This article will review the efficacy of GKRS in the management of tremors, as well as describe the treatment planning and methods associated with this evolving treatment strategy.