Primary Photons Research Articles

Clinical simulations of prostate radiotherapy using BOMAB-like phantoms: Results for neutrons

This paper presents in-phantom photoneutron equivalent doses induced by external radiotherapy, these are necessary to assess organ-averaged equivalent doses to derive the risk of secondary cancer development, associated with non-target organ exposures. The measurements were performed by Working Group 9 “Radiation Protection Dosimetry in Medicine” of the European Radiation Dosimetry Group (EURADOS) for simulations of clinical radiotherapy treatments of prostate cancer. The photoneutron dose measurements were carried out in a BOMAB-like water-filled phantom, by means of superheated emulsions (superheated drop detectors SDD and bubble damage BTI® detectors) and Poly-Allyl-Diglicol-Carbonate (PADC) solid state nuclear track detectors. Dose data were acquired in a three-dimensional matrix of reproducible measurement points, which are spaced according to detectors size. Four clinical protocols for the treatment of the prostate tumor were considered and compared, by measuring the doses delivered to the planning target volume (PTV) and to peripheral radiosensitive regions (i.e. colon-rectum and bladder). The clinical irradiations were performed in two clinical radiotherapy facilities based on Varian Clinac 2300 CD accelerator: Santa Chiara University Hospital (Pisa, Italy) and Centre of Oncology M. Skłodowska-Curie Memorial Institute (Krakow, Poland). An additional tomotherapy prostate cancer treatment was also simulated in Campo di Marte Hospital (Lucca, Italy). Radiation qualities of 6, 12, 15, 18 and 20 MV were used; all of these are capable of producing photoneutrons. Data from this work span most of the X-ray beam energies and prostate treatment modalities used in the current clinical practice. These data permit the assessment of doses absorbed by a radiotherapy patient either at the treatment volume or at out-of-field organs. Comparison of different dosimeters, under the same irradiation conditions, showed that dosimeters generally agreed within their 20% 1 SD uncertainty. Comparison of different treatment modalities in the two contributing clinical centers (Pisa and Krakow) were also possible, as well as a comparison of dose profiles resulting from the different treatment techniques, delivered at the same primary photon energy. It was in particular found that photon acceleration energies as low as 6 MV are able to produce a non-negligible photoneutron component, which causes an undue dose to the patient of the order of tens microsievert per unit photon dose delivered at the target volume.

Monte Carlo simulation of photon buildup factors for shielding materials in radiotherapy x‐ray facilities

This paper presents the results of a series of calculations to determine buildup factors for ordinary concrete, baryte concrete, lead, steel, and iron in broad beam geometry for photons energies from 0.125 to 25.125 MeV at 0.250 MeV intervals. Monte Carlo N-particle radiation transport computer code has been used to determine the buildup factors for the studied shielding materials. The computation of the primary broad beams using buildup factors data was done for nine published megavoltage photon beam spectra ranging from 4 to 25 MV in nominal energies, representing linacs made by the three major manufacturers. The first tenth value layer and the equilibrium tenth value layer are calculated from the broad beam transmission for these nine primary megavoltage photon beam spectra. The results, compared with published data, show the ability of these buildup factor data to predict shielding transmission curves for the primary radiation beam. Therefore, the buildup factor data can be combined with primary, scatter, and leakage x-ray spectra to perform computation of broad beam transmission for barriers in radiotherapy shielding x-ray facilities.

WE-G-103-04: X-Ray Scatter in Differential Phase-Contrast Breast Imaging Using Gratings-Based Interferometer

Purpose: To determine the x-ray scatter content in differential phase-contrast imaging of the breast using a gratings-based interferometer. Methods: X-ray scatter during small field-of-view (9.6×5-cm) differential phase-contrast breast imaging with a gratings-based interferometer (25-keV design) was determined using GEANT4 Monte Carlo (MC) toolkit. The duty-cycles of source (G0), phase (G1) and analyzer (G2) gratings were 0.25, 0.5 and 0.5, respectively. All gratings were planar and fabricated from 0.675-mm thick silicon wafers. MC estimated transmission efficiencies of the gratings in parallel-beam geometry were validated with theoretical estimates. After validation, their transmission efficiencies were determined in cone-beam geometry. Position-independent estimates of scatter and primary photons at the detector plane were obtained with 2 to 8 cm thick adipose and fibroglandular breasts and without the breast in the x-ray beam. The recorded x-ray scatter was analyzed after classification as photons that underwent at least one Compton interaction (incoherent scatter) and as photons that underwent Rayleigh interaction alone (coherent scatter). While G2 is needed in practice, simulations were conducted without G2 to determine its role in scatter reduction. Results: In parallel-beam geometry, Monte Carlo estimated transmission efficiencies showed good correspondence (r2>0.99) with theory. In cone-beam geometry, the transmission efficiencies of G0, G1 and G2 gratings were 0.16, 0.7 and 0.25, respectively. Position-independent estimates of scatter-to-primary ratio were 0.08 without the breast, and increased from 0.13 (0.15) to 0.25 (0.31) for 2 to 8 cm thick adipose (fibroglandular) breasts. Coherent scatter was the dominant contributor to total scatter with coherent-to-incoherent scatter ratio ranging from 1.65 (1.94) to 1.8 (2.04) for adipose (fibroglandular) breasts. The G2 grating reduced incoherent scatter by ∼5% to 24% for 2 to 8-cm breasts. Conclusion: The 0.675-mm thick silicon wafer contributed to ∼30% reduction in transmission efficiency for each grating. The G2 grating reduces the incoherent scatter at the detector plane. Supported in part by the National Institutes of Health (NIH) grants R01 CA128906 and R21 CA134128. The contents are solely the responsibility of the authors and do not represent the official views of the NIH or NCI.

SU-E-I-04: An Analytical Model for Fast Computation of Scatter Estimation in KV Cone-Beam CT Images

Purpose: To develop an analytical model for fast and accurate scatter estimation in kV cone‐beam CT images. Methods: With a point source of 100 keV x‐rays we used the Klein‐Nishina (KN) formula for Compton scattering differential cross‐section and integrated the point scatter along an interaction line of tissue‐equivalent medium to obtain a beam‐scatter‐kernel (BSK). We limited our derivation to the case of small imaging field and small scattering angles. The Taylor series of the integrand was approximated by terms up to the 4th order. The integral was then simplified to an analytical form. To calculate attenuation in the medium for scattered photons, we assumed all the scattered photons from a BSK originate from the beam center and have the same energy as the primary photons. We then performed robust calculations using the point scatter theorem for a cubic water phantom (10×10×10 cm3) and treated the results as theoretical expectations for inter‐comparison. Results: Compared to the point‐kernel method, BSK calculations show a +10% systematic difference for either a perpendicular or a tilted beam for the absolute value of scatter fluence. However, the largest difference on the relative scatter distribution is as low as 0.4% and 2.5% for perpendicular and tilted beams, respectively. The largest relative difference for the whole phantom is about ‐ 1.8%. In computation time, our integrated scatter model is faster by over 2 orders than the point kernel method. Conclusion: Our kV scatter model shows a very high computational efficiency in scatter estimation. The expected behavior of scattering distribution is observed and well estimated. We are validating more thoroughly for small imaging fields (∼10×10cm2) and the model is expected to have great potential for imaging small objects and with interior reconstruction algorithms. Future work will extend the frame for multiple photon energies and tissue non‐homogeneity.

SU-E-T-555: A Model-Based Dose Calculation Algorithm for Kilovoltage X-Rays

Purpose: Current model-based dose calculation algorithms implemented in commercial radiotherapy treatment planning systems are inaccurate for kilovoltage photon beams. This work presents a new model-based dose calculation algorithm designed to provide accurate, patient-specific dose distributions for kV x-rays. Methods: This algorithm calculates the radiation dose as the sum of primary and scatter dose components. The scatter dose is calculated by convolving an empirically parameterized scatter kernel with the primary photon fluence. Several approximations are introduced to increase calculation speed: (1) the photon energy spectrum is approximated as monoenergetic; (2) density inhomogeneities are accounted for by implementing a global distance scaling factor in the scatter kernel; (3) kernel tilting is ignored. These approximations allow for efficient calculation of the scatter dose convolution with the fast Fourier transform. Medium atomic number dependent effects are accounted for by implementing a Medium-Dependent Correction, which correlates the effects of photoabsorption in bone with a surrogate quantity that is determined for an x-ray source associated with an imaging procedure. Results: The accuracy of this algorithm was tested by comparing dose distributions resulting from kV-CBCT scans and radiographs of real patients with those calculated by the Monte Carlo method. Imaging sites studied include the head-and-neck, thorax, and pelvis. For all patients studied, the differences of mean dose between the Monte Carlo Method and the model-based algorithm for kV-CBCT were within 4% for bone, and 3% for soft tissues. This is in contrast to current model-based methods used in commercial treatment planning systems that Result in underestimations exceeding 100% for bone and overestimations of 8% for soft tissues. Conclusion: This new algorithm overcomes the limitations of current model-based photon beam calculation algorithms and can be implemented in commercial treatment planning systems to extend dose calculations to kV photon beams resulting from imaging guidance procedures or therapeutic orthovoltage beams.

SU‐E‐I‐80: Development of a Compton Camera for Online Range Monitoring of Laser‐Accelerated Proton Beams

Purpose: Development of a versatile detector system primarily designed for online range monitoring of proton beams generated via the novel technique of particle acceleration from high‐power, short‐pulse lasers to be used in bio‐medical applications. Methods: While so far mostly offline diagnostics tools are used in this context, we aim at developing a technique based on the position‐sensitive detection of prompt gamma rays emitted from nuclear reactions between the proton beam and the biological sample. For this purpose, we develop a Compton camera capable to track not only the Compton scattered primary photon, but also the secondary Compton electron. Results: Extensive simulation studies resulted in an optimized design of the Compton camera based on a LaBr3(Ce) scintillation crystal (50×50×30 mm3 block crystal, read out by a multi‐anode PMT) acting as absorber, preceded by a stack of 6 double‐sided silicon strip detectors as scatterers (500 μm thick, 128 strips/side, pitch 390 μm). From the design simulations an angular resolution of <= 2° (at 2–6 MeV) and a source image reconstruction efficiency of 10−3 −10−3 (at 1–5 MeV) can be expected. The LaBr3 crystal has been characterized with laboratory sources, resulting in a time resolution of 540 ps and a position‐dependent energy resolution (best in the central region). Using a collimated source the point‐spread function was determined from the pixelated light distribution, yield a (preliminary and by software optimization improving) spatial resolution of < 3mm. Conclusion: The Compton camera based approach for prompt‐γ detection from nuclear interactions of therapeutic ion beams offers very promising perspectives for online range monitoring. An arrangement of several camera modules could even be used in a γ‐PET mode to additionally detect delayed annihilation radiation from positron emitters in the irradiation interrupts (with improved performance in the presence of an additional third (prompt) γ (as e.g. in 10C, 14O). Funding support via German Science Foundation: DFG Cluster of Excellence MAP (Munich Centre for Advanced Photonics)

Intensity-modulated arc therapy using the gate Monte Carlo simulation platform in a grid environment

Introduction The GATE Monte-Carlo (MC) simulation platform ( www.opengatecollaboration.org ) based on the GEANT4 toolkit has come into widespread use for simulating Positron Emission Tomography, Single Photon Emission Computed Tomography, Computing Tomography (CT) imaging devices and radiation therapy. In this work, we explore the simulation of an intensity-modulated arc therapy (IMAT) treatment using GATE 6.2. To reach this objective, we first performed a complete modeling of the Varian's Novalis Tx linear accelerator (15MV photon mode) using the multileaf collimator (MLC). Then, the simulation of the geometry was validated in water before experiencing a head-and-neck cancer treatment. Material and methods The first stage is to model the Novalis Tx head with the GATE MC platform using manufacturer's data. We simulated this linac in 15 MV mode and compared depth doses, in-plane and cross-plane profiles with reference measurements (performed with ionization chambers (Semiflex 31013, PTW and LA48 Linear Array, PTW) and PTW60012 Diode E) in water for several field sizes ranging from 1 × 1 to 10 × 10 cm 2 . Target and flattening filter geometries and initial electron beamare adjusted in order to fit simulation calculations with dosemeasurements. In a second stage, the MLC geometry implementation enabled the simulation of clinical IMAT treatment plans compared with the iPlan treatment planning software (BrainLab) calculations. In order to reduce the computing time of GATE calculations, the simulations are executed on a distributed infrastructure using the GateLab ( http://vip.creatis.insa-lyon.fr/ ) platform. Results Weshowa good agreement between measurements and simulated depth dose profiles in liquid water. GATE and iPlan normalized treatment plans were compared using agammaindex of ±2%/mm.The ongoing work concerning treatment plan comparisons demonstrates limits of the iPlan Pencil Beam algorithm close to heterogeneities. Using the distributed infrastructure, space phase file was produced in 14 h for 500 million primary photons generated reaching a gain of a factor 50 comparing to calculations performed on a single CPU. Conclusions This study demonstrates that GATE offers efficient tools to make possible IMAT applications. In a subsequent work a microdosimetry study should be performed for a complete validation.

Simultaneous dual-isotope imaging based on an artificial neural network for evaluating myocardial perfusion and fatty acid metabolism

BackgroundWe contrived a scatter correction method based on an artificial neural network (ANN) and applied it to the simultaneous evaluation of myocardial perfusion and fatty acid metabolism in single-photon emission computed tomography (SPECT). MethodsThe count data of three energy windows were used as inputs of the ANN. The count ratios of the estimated primary-to-total photons for 99mTc and 123I, which were used to reconstruct 99mTc and 123I images, were calculated using the ANN. In a phantom study, single- and dual-isotope imaging with 99mTc/123I and 201Tl/123I was performed by means of a cardiac phantom simulating patients with and without obesity. In a human study, five normal volunteers and ten patients with myocardial infarction underwent myocardial perfusion and fatty acid metabolism imaging with single and dual SPECT with combinations of 99mTc-methoxyisobutylisonitrile/123I-beta-methyl(p-iodophenyl)pentadecanoic acid (BMIPP) and 201Tl/123I-BMIPP as tracers. ResultsTechnetium-99m yielded more homogeneous images than 201Tl because of the lower degree of photon attenuation, especially in the condition of obese patients, resulting in clearer visualization of the perfusion-metabolism mismatch. Dual 99mTc/123I SPECT offered comparable images with single SPECT in assessing myocardial damage. ConclusionsThe method effectively separated 99mTc and 123I primary photons and proved applicable to 99mTc/123I dual-isotope myocardial SPECT.

A new methodology to determinate linac photon spectra using the EPID signal

A precise estimation of the photon spectra emitted by radiation therapy Linacs is especially important to accurately estimate the dose received by patients in order to improve treatment plannings. Moreover the knowledge of the spectra is useful to perform routinely Quality Control procedures of these equipments. In this work we propose the use of the Elekta Linac EPID (Electronic Portal Imaging Device) to obtain the fluence spectrum emitted by this unit. To that, several images using different thicknesses of solid water slabs have been taken with the EPID, setting 100UM, a 10cm×10cm field size and a 6MeV energy beam. The relation between the signal response recorded by the EPID with several solid water slabs and the primary photon spectrum emitted is calculated by means of a Response matrix, which has been performed via Monte Carlo simulations. To that, the MCNP5 code has been used to simulate the EPID signal response obtained after irradiation with consecutive monoenergetic beams for different slabs. Each row of the Response Matrix represents the Monte Carlo EPID signal registered using a specific energy bin. Since this matrix is ill conditioned we have applied the Truncated Singular Value Decomposition algorithms to unfold the spectra. Results have been compared with theoretical spectrum, showing that the mixed methodology (experimental-Monte Carlo simulation) can offer an accurate way to determinate the photon spectrum in radiation therapy.

Application of the Monte Carlo codes PENELOPE and MCNP5 to unfold X-ray spectra in the diagnostic energy range

Obtaining primary X-ray spectra is a complex task, mainly due to the high fluence of X-rays. In order to avoid the pile up effect in the detector, an indirect method based on the Compton scattering interactions in a low density rod material can be successfully applied in the diagnostic energy range. In this work, the Monte Carlo codes PENELOPE and MCNP5 have been used to simulate the indirect method based on a simplified Compton spectrometry technique. Both models include the X-ray focus, a poly(mehtylmethacrylate) (PMMA) rod and an HPGe detector. Because the probability that primary photons scattered in the PMMA will be emitted towards the detector is small, it is necessary to consider appropriate values for cut-offs and other simulation parameters. With these models, a response function can be determined, relating the response of the detector to the primary X-ray spectrum. This function can be normally expressed as a matrix, which can be calculated by simulating the response detector to several monochromatic X-ray beams. The main goal of this work is to test the capability of the Monte Carlo codes PENELOPE and MCNP5, together with unfolding methods to estimate the primary spectrum when the response matrix and the response of the detector for a given conditions are known. The reliability of unfolded X-ray spectra is studied by comparing them with theoretical spectra obtained from the IPEM 78 catalog and calculating the Root Mean Squared (RMS) and Quality parameters.

Calculation of the Cherenkov light yield from electromagnetic cascades in ice with Geant4

In this work we investigate and parameterize the amount and angular distribution of Cherenkov photons which are generated by electro-magnetic cascades in water or ice. We simulate electromagnetic cascades with Geant4 for primary electrons, positrons and photons with energies ranging from 1GeV to 10TeV. We parameterize the total Cherenkov-light yield as a function of energy, the longitudinal evolution of the Cherenkov emission along the cascade-axis and the angular distribution of photons. Furthermore, we investigate the fluctuations of the total light yield, the fluctuations in azimuth and changes of the emission with increasing age of the cascade.

Search for ultra-high energy photons and neutrinos using Telescope Array surface detector

We search for ultra-high energy photons by analyzing geometrical properties of shower fronts of events registered by the Telescope Array surface detector. By making use of an event-by-event statistical method, we derive upper limits on the absolute flux of primary photons with energies above 1019 eV, 1019.5 eV and above 1020 eV based on the three years data from Telescope Array surface detector (May 2008 – May 2011). We report the results of down-going neutrino search based on the analysis of very inclined events.

EVIDENCE FOR SECONDARY EMISSION AS THE ORIGIN OF HARD SPECTRA IN TeV BLAZARS

We develop a model for a possible origin of hard very high energy spectra from a distant blazar. In the model, both the primary photons produced in the source and secondary photons produced outside the source contribute to the observed high energy $\gamma$-rays emission. That is, the primary photons are produced in the source through the synchrotron self-Compton (SSC) process, and the secondary photons are produced outside the source through high energy protons interaction with the background photons along the line of sight. We apply the model to a characteristic case was the very high energy (VHE) $\gamma$-ray emissions in distant blazar 1ES 1101-232. Assuming a suitable electron and proton spectra, we obtain excellent fits to observed spectra of distant blazar 1ES 1101-232. This indicated that the surprisingly low attenuation of high energy $\gamma$-rays, especially for the shape of the very high energy $\gamma$-rays tail of the observed spectra, can be explained by secondary $\gamma$-rays produced in interactions of cosmic-ray protons with background photons in the intergalactic space.

Simulation of ultra-high energy photon propagation with PRESHOWER 2.0

In this paper we describe a new release of the PRESHOWER program, a tool for Monte Carlo simulation of propagation of ultra-high energy photons in the magnetic field of the Earth. The PRESHOWER program is designed to calculate magnetic pair production and bremsstrahlung and should be used together with other programs to simulate extensive air showers induced by photons. The main new features of the PRESHOWER code include a much faster algorithm applied in the procedures of simulating the processes of gamma conversion and bremsstrahlung, update of the geomagnetic field model, and a minor correction. The new simulation procedure increases the flexibility of the code so that it can also be applied to other magnetic field configurations such as, for example, encountered in the vicinity of the sun or neutron stars. Program summaryProgram title: PRESHOWER 2.0Catalog identifier: ADWG_v2_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADWG_v2_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 3968No. of bytes in distributed program, including test data, etc.: 37198Distribution format: tar.gzProgramming language: C, FORTRAN 77.Computer: Intel-Pentium based PC.Operating system: Linux or Unix.RAM:< 100 kBClassification: 1.1.Does the new version supercede the previous version?: YesCatalog identifier of previous version: ADWG_v1_0Journal reference of previous version: Comput. Phys. Comm. 173 (2005) 71Nature of problem:Simulation of a cascade of particles initiated by UHE photon in magnetic field.Solution method:The primary photon is tracked until its conversion into an e+ e− pair. If conversion occurs each individual particle in the resultant preshower is checked for either bremsstrahlung radiation (electrons) or secondary gamma conversion (photons).Reasons for new version:1.Slow and outdated algorithm in the old version (a significant speed up is possible);2.Extension of the program to allow simulations also for extraterrestrial magnetic field configurations (e.g. neutron stars) and very long path lengths.Summary of revisions:A veto algorithm was introduced in the gamma conversion and bremsstrahlung tracking procedures. The length of the tracking step is now variable along the track and depends on the probability of the process expected to occur. The new algorithm reduces significantly the number of tracking steps and speeds up the execution of the program. The geomagnetic field model has been updated to IGRF-11, allowing for interpolations up to the year 2015. Numerical Recipes procedures to calculate modified Bessel functions have been replaced with an open source CERN routine DBSKA. One minor bug has been fixed.Restrictions:Gamma conversion into particles other than an electron pair is not considered. Spatial structure of the cascade is neglected.Additional comments:The following routines are supplied in the package, IGRF [1, 2], DBSKA [3], ran2 [4]Running time:100 preshower events with primary energy 1020 eV require a 2.66 GHz CPU time of about 200 sec.; at the energy of 1021 eV, 600 sec.

Search for ultra-high energy photons and neutrinos using Telescope Array surface detector

We search for ultra-high energy photons by analyzing geometrical properties of shower fronts of events registered by the Telescope Array surface detector. By making use of an event-by-event statistical method, we derive upper limits on the absolute flux of primary photons with energies above 1019eV, 1019.5eV and above 1020eV based on the three years data from Telescope Array surface detector (May 2008 – May 2011). We report the results of down-going neutrino search based on the analysis of very inclined events.

Photoproduction of η′ mesons from nuclei and their properties in the nuclear medium

We study the photoproduction of η′ mesons from nuclei near the threshold within the collision model based on the nuclear spectral function. The model properly takes into account both the primary photon–nucleon γN → η′N and the secondary pion–nucleon πN → η′N production processes as well as the effect of the nuclear η′ mean-field potential on these processes. We find that the secondary channel πN → η′N plays an insignificant role in the η′ photoproduction from nuclei. We calculate the transparency ratio for η′ mesons and compare it to the available experimental data. The comparison indicates an inelastic η′N cross section of the order of 6–10 mb. We show that the existing transparency ratio measurements do not allow one to distinguish between two adopted η′ in-medium modification scenarios. Our studies also demonstrate that the momentum distribution and excitation function for η′ production in γA reactions reveal some sensitivity to these scenarios, which means that such observables may be an important tool to get valuable information on the η′ in-medium properties.

Importance of upgraded energy reconstruction for direct dark matter searches with liquid xenon detectors

The usual nuclear recoil energy reconstruction employed by liquid xenon dark matter search experiments relies only on the primary scintillation photon signal. Energy reconstruction based on both the photon and electron signals yields a more accurate representation of search results. For a dark matter particle of mass ${m}_{\ensuremath{\chi}}\ensuremath{\sim}10\text{ }\text{ }\mathrm{GeV}$, a nuclear recoil from a scattering event is more likely to be observed in the lower-left corner of the typical search box, rather than near the nuclear recoil calibration centroid. In this region of the search box, the actual nuclear recoil energies are smaller than the usual energy scale suggests, by about a factor of 2. Recent search results from the XENON100 experiment are discussed in light of these considerations.

TIME STRUCTURE OF GAMMA-RAY SIGNALS GENERATED IN LINE-OF-SIGHT INTERACTIONS OF COSMIC RAYS FROM DISTANT BLAZARS

Blazars are expected to produce both gamma rays and cosmic rays. Therefore, observed high-energy gamma rays from distant blazars may contain a significant contribution from secondary gamma rays produced along the line of sight by the interactions of cosmic-ray protons with background photons. Unlike the standard models of blazars that consider only the primary photons emitted at the source, models which include the cosmic-ray contribution predict that even ~10 TeV photons should be detectable from distant objects with redshifts as high as z> 0.1. Secondary photons contribute to signals of point sources only if the intergalactic magnetic fields are very small, below ~10 femtogauss, and their detection can be used to set upper bounds on magnetic fields along the line of sight. Secondary gamma rays have distinct spectral and temporal features. We explore the temporal properties of such signals using a semi-analytical formalism and detailed numerical simulations, which account for all the relevant processes, including magnetic deflections. In particular, we elucidate the interplay of time delays coming from the proton deflections and from the electromagnetic cascade, and we find that, at multi-TeV energies, secondary gamma-rays can show variability on timescales of years for femtogauss magnetic fields.

A novel 2D binary collimator for IMRT dose delivery: dosimetric characterization using Monte Carlo simulations

We present a novel technical concept of a two-dimensional binary multileaf collimator (2D-bMLC) especially designed for fast dose delivery in rotational IMRT. The 2D-bMLC consists of individually controlled absorber channels, which are arranged side by side forming a 2D collimator aperture. In each channel three separate tungsten modules are arranged behind each other. To open and close an element, the central module is shifted between two positions. The purpose of this work is the presentation of the 2D-bMLC concept and its dosimetric evaluation. To determine the dosimetric properties, we designed a Monte Carlo model of an exemplary 2D-bMLC, consisting of 30 × 30 elements. A virtual source model of a flattening filter-free 7 MV linac was used to characterize the linac phase space. A primary radiation efficiency factor of 43% was calculated for the open 2D-bMLC by dividing the integral dose scored for a 2D-bMLC field by the integral dose scored for an open field with the same dimensions. The leakage calculated for the closed collimator was below 0.5%. Following the primary photon fluence distribution, the bixel intensity decreases with the distance of the element to the central axis of the treatment machine. From the collimator field’s center toward its borders, the geometric bixel widths increase in a symmetric and predictable manner by up to 4%. The increase is explained by the specific design of the 2D-bMLC. Abutting element beams exhibit a slight tongue-and-groove effect if opened sequentially. This effect as well as the primary radiation efficiency is basically affected by the source size and the dimensions of the collimator elements. We successfully established and evaluated a dosimetric model of the 2D-bMLC. The results are promising, and we will therefore investigate on real patient plans, if the concept could be advantageous for fast rotational IMRT treatments.

Monte Carlo analyses of multiple backscattering of gamma rays

We have written a Monte Carlo code to simulate the experimental results of a previously reported study. We were able to analyse the energy distributions of photons that reached the detector system after suffering several successive Compton scatterings in the target. We have also investigated how the number of multiply backscattered events depends on the target thickness and the energy of the primary photons.