Study of the photoneutron generation caused by a LinAc Beryllium window with a 6 MeV treatment beam.

In most conventional radiation therapy treatments, special attention is payed for neutron contamination when working with energy beams above 8 MeV and generally it is only considered for shielding requirements, not for dose study in patients or employees. The present work is focused on studying the unwanted generated photoneutrons in a Medical Linear Accelerator (LinAc) Varian TrueBeam using a 6 MeV radiation treatment beam. To that, Monte Carlo (MC) simulation code MCNP6.1.1 was used. This version of the code allows the use of unstructured mesh geometries as a novelty, offering more reliable results and higher speed computation. The particularity of the studied LinAc is the presence of a beryllium filter at the treatment head. Since Beryllium causes photonuclear reactions (γ, n) at energies much lower than other LinAc composing materials, this work aims to analyze if this type of units, when using low energy treatment beams (6 MeV), produce neutron pollution and to ensure that this unwanted radiation can be considered negligible.

Brachytherapy organ dose estimation using Monte Carlo simulations of realistic patient models.

Radiation Therapy Planning Systems (RTPS) currently used in hospitals contain algorithms based on deterministic simplifications that do not properly consider electrons lateral transport in the areas where there are changes of density, and as a result, erroneous dose predictions could be produced. According to this, the present work proposes the use of Monte Carlo method in brachytherapy planning systems, which could affect positively on the radiotherapy treatment planning, since it provides results that are more accurate and takes into account the in homogeneities density variations. This paper presents a Monte Carlo (MC) simulation of a brachytherapy prostate treatment with I-125 seeds, using the latest version of MCNP, v.6.1.1. To that, a 3D model of the anatomy of a real anonymized patient is created from the segmentation of Computed Tomography (CT) images. Treatment over this 3D model is simulated and the dose given to each organ is obtained. These doses are compared with those calculated by deterministic planning system used in hospital demonstrating the effectiveness of MC method in the planning of brachytherapy treatments, because of not only the results precision but also regarding the affordable computing times.

Air radon concentration decrease in a waste water treatment plant.

(222)Rn is a naturally occurring gas created from the decay of (226)Ra. The long-term health risk of breathing radon is lung cancer. One particular place where indoor radon concentrations can exceed national guidelines is in wastewater treatment plants (WWTPs) where treatment processes may contribute to ambient airborne concentrations. The aim of this paper was to study the radon concentration decrease after the application of corrective measures in a Spanish WWTP. According to first measures, air radon concentration exceeded International Commission Radiologica1 Protection (ICRP) normative (recommends intervention between 400 and 1000 Bq m(-3)). Therefore, the WWTP improved mechanical forced ventilation to lower occupational exposure. This measure allowed to increase the administrative controls, since the limitation of workers access to the plant changed from 2 h d(-1) (considering a maximum permissible dose of 20 mSv y(-1) averaged over 5 y) to 7 h d(-1).

Neutron distribution and induced activity inside a Linac treatment room.

Induced radioactivity and photoneutron contamination inside a radiation therapy bunker of a medical linear accelerator (Linac) is investigated in this work. The Linac studied is an Elekta Precise electron accelerator which maximum treatment photon energy is 15 MeV. This energy exceeds the photonuclear reaction threshold (around 7 MeV for high atomic number metals). The Monte Carlo code MCNP6 has been used for quantifying the neutron contamination inside the treatment room for different gantry rotation configuration. Walls activation processes have also been simulated. The approach described in this paper is useful to prevent the overexposure of patients and medical staff.

Comparison between transmission and scattering spectrum reconstruction methods based on EPID images.

Numerous improved physics-based methods for Linac photon spectra reconstruction have been published; some of them are based on transmission data analysis and others on scattering data. In this work, the two spectrum unfolding approaches are compared in order to experimentally validate its robustness and to determine which is the optimal methodology for application on a clinical quality assurance routine. Both studied methods are based on EPID images generated when the incident photon beam impinges onto plastic blocks. The distribution of transmitted/scatter radiation produced by this object centered at the beam field size was measured. Measurements were performed using a 6 MeV photon beam produced by the linear accelerator. The same radiation distribution conditions were also simulated with Monte Carlo code for a series of monoenergetic identical geometry photon beams for both cases. Two systems of linear equations were generated to combine the polyenergetic EPID measurements with the monoenergetic simulation results. Regularization techniques were applied to solve the systems for obtaining the incident photon spectrum. We present a comparison between the well-known photon Spectral Reconstruction based on Transmission Data (Trans-based) technology and the Spectral Reconstruction based on Scattering Data (Scatt-based), which we both developed using EPID images. It is shown that Trans-based reconstruction results display much better agreement with photon spectrum theoretical predictions.

Uncertainty and sensitivity analysis of the effect of the mean energy and FWHM of the initial electron fluence on the Bremsstrahlung photon spectra of linear accelerators.

A calculation of the correct dose in radiation therapy requires an accurate description of the radiation source because uncertainties in characterization of the linac photon spectrum are propagated through the dose calculations. Unfortunately, detailed knowledge of the initial electron beam parameters is not readily available, and many researchers adjust the initial electron fluence values by trial-and-error methods. The main goal of this work was to develop a methodology to characterize the fluence of initial electrons before they hit the tungsten target of an Elekta Precise medical linear accelerator. To this end, we used a Monte Carlo technique to analyze the influence of the characteristics of the initial electron beam on the distribution of absorbed dose from a 6 MV linac photon beam in a water phantom. The technique is based on calculations with Software for Uncertainty and Sensitivity Analysis (SUSA) and Monte Carlo simulations with the MCNP5 transport code. The free parameters used in the SUSA calculations were the mean energy and full-width-at-half-maximum (FWHM) of the initial electron distribution. A total of 93 combinations of these parameters gave initial electron fluence configurations. The electron spectra thus obtained were used in a simulation of the electron transport through the target of the linear accelerator, which produced different photon (Bremsstrahlung) spectra. The simulated photon spectra were compared with the 6-MV photon spectrum provided by the linac manufacturer (Elekta). This comparison revealed how the mean energy and FWHM of the initial electron fluence affect the spectrum of the generated photons. This study has made it possible to fine-tune the examined electron beam parameters to obtain the resulted absorbed doses with acceptable accuracy (error<1%).

MCNP5 Monte Carlo simulation of amorphous silicon EPID dosimetry from MLC radiation therapy treatment beams.

The present work is focused on a MCNP Monte Carlo (MC) simulation of a multi-leaf collimator (MLC) radiation therapy treatment unit including its corresponding Electronic Portal Imaging Device (EPID). We have developed a methodology to perform a spatial calibration of the EPID signal to obtain dose distribution using MC simulations. This calibration is based on several images acquisition and simulation considering different thicknesses of solid water slabs, using a 6 MeV photon beam and a square field size of 20 cm x 20 cm. The resulting relationship between the EPID response and the MC simulated dose is markedly linear. This signal to dose EPID calibration was used as a dosimetric tool to perform the validation of the MLC linear accelerator MCNP model. Simulation results and measurements agreed within 2% of dose difference. The methodology described in this paper potentially offers an optimal verification of dose received by patients under complex multi-field conformal or intensity-modulated radiation therapy (IMRT).

Pneumonia in military trainees: a comparison study based on adenovirus serotype 14 infection.

BACKGROUND: Adenovirus serotype 14 (Ad-14) recently emerged as a respiratory pathogen in the United States, with studies suggesting higher morbidity and mortality. This study was conducted to determine whether Ad-14 is associated with clinical outcomes in otherwise healthy patients with pneumonia. METHODS: Medical records of military trainees hospitalized with pneumonia during an outbreak of Ad-14 infection were reviewed. Clinical, radiographic, and laboratory parameters were compared on the basis of Ad-14 infection. RESULTS: Two hundred thirty-four trainees received a diagnosis of pneumonia, and 83(35%) were hospitalized. Sixty-one percent of patients with pneumonia were Ad-14 positive; 43% of patients with Ad-14 pneumonia were hospitalized (83% of female patients and 40% of male patients; P = .04), compared with 40% of patients with Ad-14 negative cases. Ad-14 infection was associated with higher admission temperature (38.3°C [interquartile range, (IQR) 37.7, 39.4] vs 37.3°C [IQR (36.7, 38.5)]; P < .01) and lower white blood cell count (8.3 × 1000 cells/µL [IQR, 5.7, 12.4] vs 13 × 1000 cells/µL [IQR, 7.5, 12.9]; P = .01), neutrophil count (6.7 × 1000 cells/µL [IQR, 2.8, 9.7] vs 9.7 × 1000 cells/µL [IQR, 5.6, 12.1]; P = .02), lymphocyte count (0.9 × 1000 cells/µL [IQR, 0.8, 1.1] vs 1.3 × 1000 cells/µL [IQR, 1, 1.9]; P = .001), and platelet count (210 × 1000 cells/µL [IQR, 145, 285] vs 261 × 1000 cells/µL [IQR, 238, 343]; P < .01). Ad-14 pneumonia was not associated with specific radiographic findings, pneumonia severity score, intensive care unit admission, longer hospitalization, or 30-day mortality. CONCLUSIONS: During an outbreak of Ad-14 infection, Ad-14 infection was not associated with excess overall morbidity or mortality. Ad-14 infection was associated with specific laboratory and clinical parameters and higher hospitalization rates in female trainees. These data provide new insight to the epidemiology of Ad-14 infection.

6 and 15 MeV photon spectra reconstruction using an unfolding depth dose gradient methodology.

An accurate knowledge of the spectral distribution emission is essential for precise dose calculations in radiotherapy treatment planning. Reconstruction of photon spectra emitted by medical accelerators from measured depth dose distributions in a water cube is an important tool for commissioning a Monte Carlo treatment planning system. However, the reconstruction problem is an inverse radiation transport function which is poorly conditioned and its solution may become unstable due to small perturbations in the input data. In this paper we present a more stable spectral reconstruction method which can be used to provide an independent confirmation of source models for a given machine without any prior knowledge of the spectral distribution. This technique involves measuring the depth dose curve in a water phantom and applying an unfolding method using Monte Carlo simulated depth dose gradient curves for consecutives mono-energetic beams. We illustrate this theory to calculate a 6 and a 15 MeV photon beam emitted from an Elekta Precise radiotherapy unit using the gradient of depth dose curves in a cube-shaped water tank.

Electron influence on the reconstruction of a linac 6 MeV photon spectra by unfolding methods.

Megavoltage photon sources are normally used for radiotherapy treatments. For these equipments an accurate knowledge of their spectral distribution is essential for accurate dose calculations planning. There are several ways to determine the spectrum of a clinical photon beam: direct measurement, electron source modelling or reconstruction from experimental measures. In this paper we focus on the latter type of spectral reconstruction methods which can be used to provide an independent confirmation of source models for a given machine without any prior knowledge of the spectral distribution. This technique involves measuring the depth dose curve in a water phantom and applying an unfolding method using Monte Carlo simulated depth dose curves for consecutives mono-energetic beams. We illustrate this theory to calculate a 6 MeV photon beam from an Elekta Precise radiotherapy unit using the gradient of depth dose measurements in a cube-shaped water tank.

Comparison of experimental 3D dose curves in a heterogeneous phantom with results obtained by MCNP5 simulation and those extracted from a commercial treatment planning system.

Commercial planning systems used in radiotherapy treatments use determinist correlations to evaluate dose distribution around regions of interest. Estimated dose with this type of planners can be problematic, especially when analyzing heterogeneous zones. The present work is focused in quantifying the dose distribution in a heterogeneous medium irradiated by a 6 MeV photon beam emitted by an Elekta Precise Radiotherapy Unit head. Dose mapping inside the heterogeneous water phantom has been simulated with the photon and electron transport with Monte Carlo computer code MCNP5 and also, using a commercial treatment planning software in the same irradiation conditions. The calculated results were compared with experimental relative dose curves. This comparison shows that inside the heterogeneity region, the commercial algorithms are not able to predict the variation of dose in the heterogeneous zones with the same precision as MCNP5.

3D dose distribution calculation in a voxelized human phantom by means of Monte Carlo method.

The aim of this work is to provide the reconstruction of a real human voxelized phantom by means of a MatLab program and the simulation of the irradiation of such phantom with the photon beam generated in a Theratron 780 (MDS Nordion) (60)Co radiotherapy unit, by using the Monte Carlo transport code MCNP (Monte Carlo N-Particle), version 5. The project results in 3D dose mapping calculations inside the voxelized antropomorphic head phantom. The program provides the voxelization by first processing the CT slices; the process follows a two-dimensional pixel and material identification algorithm on each slice and three-dimensional interpolation in order to describe the phantom geometry via small cubic cells, resulting in an MCNP input deck format output. Dose rates are calculated by using the MCNP5 tool FMESH, superimposed mesh tally, which gives the track length estimation of the particle flux in units of particles/cm(2). Furthermore, the particle flux is converted into dose by using the conversion coefficients extracted from the NIST Physical Reference Data. The voxelization using a three-dimensional interpolation technique in combination with the use of the FMESH tool of the MCNP Monte Carlo code offers an optimal simulation which results in 3D dose mapping calculations inside anthropomorphic phantoms. This tool is very useful in radiation treatment assessments, in which voxelized phantoms are widely utilized.

Comparison between voxelized, volumized and analytical phantoms applied to radiotherapy simulation with Monte Carlo.

The purpose of this paper is to provide a comparison between the different methods utilized for building up anthropomorphic phantoms in Radiotherapy Treatment Plans. A simplified model of the Snyder Head Phantom was used in order to construct an analytical, voxelized and volumized phantom, throughout a segmentation program and different algorithms programmed in Matlab code. The irradiation of the resulting phantoms was simulated with the MCNP5 (Monte Carlo N-Particle) transport code, version 5, and the calculations presented in particle flux maps inside the phantoms by utilizing the FMESH tool, superimposed mesh tally. The different variables involved in the simulation were analyzed, like particle flux, MCNP standard deviation and real simulation CPU time cost. In the end the volumized model resulted to have the largest computer time cost and bigger discrepancies in the particle flux distribution.

Dosimetric capabilities of the Iview GT portal imager using MCNP5 Monte Carlo simulations.

This work is focused on developing a methodology to obtain portal dosimetry with an amorphous silicon Electronic Portal Image Device (a-Si EPID) used in radiotherapy by means of Monte Carlo simulations and experimental measures. Pixel intensity values from portal images have been compared with dose measured from an ionization chamber and dose calculated from Monte Carlo simulations. To this end, several images were acquired with the Elekta iView GT EPID using an attenuator phantom slab (10 cm thickness of solid water) and a 6 MeV photon energy beam with different monitor units settings (MU). The average pixel value in a region of interest (ROI) centered at the beam for each image was extracted and compared to dose measures performed with an ionization chamber. These parameters were found to be linearly related with the number of monitor units. Since MCNP5 simulations allow calculating the deposited dose in the ROI within the phosphor layer of the EPID model, we could compare the portal dose with the simulated transit dose in order to perform a treatment control.

Photon spectra calculation for an Elekta linac beam using experimental scatter measurements and Monte Carlo techniques.

The present work is centered in reconstructing by means of a scatter analysis method the primary beam photon spectrum of a linear accelerator. This technique is based on irradiating the isocenter of a rectangular block made of methacrylate placed at 100 cm distance from surface and measuring scattered particles around the plastic at several specific positions with different scatter angles. The MCNP5 Monte Carlo code has been used to simulate the particles transport of mono-energetic beams to register the scatter measurement after contact the attenuator. Measured ionization values allow calculating the spectrum as the sum of mono-energetic individual energy bins using the Schiff Bremsstrahlung model. The measurements have been made in an Elekta Precise linac using a 6 MeV photon beam. Relative depth and profile dose curves calculated in a water phantom using the reconstructed spectrum agree with experimentally measured dose data to within 3%.

Improving pediatric radiation dose management using Agfa digital radiography DICOM header information.

Dose reduction in pediatric explorations is especially important because of children radiation sensitivity. According to this, with the aim of saving radiation exposure in future clinical practice, we have developed a technique to control delivered dose in pediatric radiographic exams. To that, a computer science program has been developed to calculate entrance skin dose (ESD) provided by AGFA radiology digital system, using the "lgM" parameter exported from Dicom files. ESD values are compared with dose limits established in regulations to detect if children are being exposed to excessive amounts of radiation during their explorations.

Tally and geometry definition influence on the computing time in radiotherapy treatment planning with MCNP Monte Carlo code.

The present work has simulated the photon and electron transport in a Theratron 780 (MDS Nordion) (60)Co radiotherapy unit, using the Monte Carlo transport code, MCNP (Monte Carlo N-Particle), version 5. In order to become computationally more efficient in view of taking part in the practical field of radiotherapy treatment planning, this work is focused mainly on the analysis of dose results and on the required computing time of different tallies applied in the model to speed up calculations.

Considerations of MCNP Monte Carlo code to be used as a radiotherapy treatment planning tool.

The present work has simulated the photon and electron transport in a Theratron 780® (MDS Nordion)60Co radiotherapy unit, using the Monte Carlo transport code, MCNP (Monte Carlo N-Particle). This project explains mainly the different methodologies carried out to speedup calculations in order to apply this code efficiently in radiotherapy treatment planning.