A PENELOPE-based system for the automated Monte Carlo simulation of clinacs and voxelized geometries-application to far-from-axis fields

Energy spectra considerations for synchrotron radiotherapy trials on the ID17 bio-medical beamline at the European Synchrotron Radiation Facility

The aim of this study was to validate the kilovoltage X-ray energy spectrum on the ID17 beamline at the European Synchrotron Radiation Facility (ESRF). The purpose of such validation was to provide an accurate energy spectrum as the input to a computerized treatment planning system, which will be used in synchrotron microbeam radiotherapy trials at the ESRF. Calculated and measured energy spectra on ID17 have been reported previously but recent additions and safety modifications to the beamline for veterinary trials warranted a fresh investigation. The authors used an established methodology to compare X-ray attenuation measurements in copper sheets (referred to as half value layer measurements in the radiotherapy field) with the predictions of a theoretical model. A cylindrical ionization chamber in air was used to record the relative attenuation of the X-ray beam intensity by increasing thicknesses of high-purity copper sheets. The authors measured the half value layers in copper for two beamline configurations, which corresponded to differing spectral conditions. The authors obtained good agreement between the measured and predicted half value layers for the two beamline configurations. The measured first half value layer was 1.754 ± 0.035 mm Cu and 1.962 ± 0.039 mm Cu for the two spectral conditions, compared with theoretical predictions of 1.763 ± 0.039 mm Cu and 1.984 ± 0.044 mm Cu, respectively. The calculated mean energies for the two conditions were 105 keV and 110 keV and there was not a substantial difference in the calculated percentage depth dose curves in water between the different spectral conditions. The authors observed a difference between their calculated energy spectra and the spectra previously reported by other authors, particularly at energies greater than 100 keV. The validation of the beam spectrum by the copper half value layer measurements means the authors can provide an accurate spectrum as an input to a treatment planning system for the forthcoming veterinary trials of microbeam radiotherapy to spontaneous tumours in cats and dogs.

A geometrical model for the Monte Carlo simulation of the TrueBeam linac

Monte Carlo simulation of linear accelerators (linacs) depends on the accurate geometrical description of the linac head. The geometry of the Varian TrueBeam linac is not available to researchers. Instead, the company distributes phase-space files of the flattening-filter-free (FFF) beams tallied at a plane located just upstream of the jaws. Yet, Monte Carlo simulations based on third-party tallied phase spaces are subject to limitations. In this work, an experimentally based geometry developed for the simulation of the FFF beams of the Varian TrueBeam linac is presented. The Monte Carlo geometrical model of the TrueBeam linac uses information provided by Varian that reveals large similarities between the TrueBeam machine and the Clinac 2100 downstream of the jaws. Thus, the upper part of the TrueBeam linac was modeled by introducing modifications to the Varian Clinac 2100 linac geometry. The most important of these modifications is the replacement of the standard flattening filters by ad hoc thin filters. These filters were modeled by comparing dose measurements and simulations. The experimental dose profiles for the 6 MV and 10 MV FFF beams were obtained from the Varian Golden Data Set and from in-house measurements performed with a diode detector for radiation fields ranging from 3  ×  3 to 40  ×  40 cm(2) at depths of maximum dose of 5 and 10 cm. Indicators of agreement between the experimental data and the simulation results obtained with the proposed geometrical model were the dose differences, the root-mean-square error and the gamma index. The same comparisons were performed for dose profiles obtained from Monte Carlo simulations using the phase-space files distributed by Varian for the TrueBeam linac as the sources of particles. Results of comparisons show a good agreement of the dose for the ansatz geometry similar to that obtained for the simulations with the TrueBeam phase-space files for all fields and depths considered, except for the 40  ×  40 cm(2) field where the ansatz geometry was able to reproduce the measured dose more accurately. Our approach overcomes some of the limitations of using the Varian phase-space files. It makes it possible to: (i) adapt the initial beam parameters to match measured dose profiles; (ii) reduce the statistical uncertainty to arbitrarily low values; and (iii) assess systematic uncertainties (type B) by using different Monte Carlo codes. One limitation of using phase-space files that is retained in our model is the impossibility of performing accurate absolute dosimetry simulations because the geometrical description of the TrueBeam ionization chamber remains unknown.

Phase space determination from measured dose data for intraoperative electron radiation therapy

A procedure to characterize beams of a medical linear accelerator for their use in Monte Carlo (MC) dose calculations for intraoperative electron radiation therapy (IOERT) is presented. The procedure relies on dose measurements in homogeneous media as input, avoiding the need for detailed simulations of the accelerator head. An iterative algorithm (EM-ML) has been employed to extract the relevant details of the phase space (PHSP) of the particles coming from the accelerator, such as energy spectra, spatial distribution and angle of emission of particles. The algorithm can use pre-computed dose volumes in water and/or air, so that the machine-specific tuning with actual data can be performed in a few minutes. To test the procedure, MC simulations of a linear accelerator with typical IOERT applicators and energies, have been performed and taken as reference. A solution PHSP derived from the dose produced by the simulated accelerator has been compared to the reference PHSP. Further, dose delivered by the simulated accelerator for setups not included in the fit of the PHSP were compared to the ones derived from the solution PHSP. The results show that it is possible to derive from dose measurements PHSP accurate for IOERT MC dose estimations.

Web-based, GPU-accelerated, Monte Carlo simulation and visualization of indirect radiation imaging detector performance

PURPOSE

Monte Carlo simulations play a vital role in the understanding of the fundamental limitations, design, and optimization of existing and emerging medical imaging systems. Efforts in this area have resulted in the development of a wide variety of open-source software packages. One such package, hybridmantis, uses a novel hybrid concept to model indirect scintillator detectors by balancing the computational load using dual CPU and graphics processing unit (GPU) processors, obtaining computational efficiency with reasonable accuracy. In this work, the authors describe two open-source visualization interfaces, webmantis and visualmantis to facilitate the setup of computational experiments via hybridmantis.

METHODS

The visualization tools visualmantis and webmantis enable the user to control simulation properties through a user interface. In the case of webmantis, control via a web browser allows access through mobile devices such as smartphones or tablets. webmantis acts as a server back-end and communicates with an NVIDIA GPU computing cluster that can support multiuser environments where users can execute different experiments in parallel.

RESULTS

The output consists of point response and pulse-height spectrum, and optical transport statistics generated by hybridmantis. The users can download the output images and statistics through a zip file for future reference. In addition, webmantis provides a visualization window that displays a few selected optical photon path as they get transported through the detector columns and allows the user to trace the history of the optical photons.

CONCLUSIONS

The visualization tools visualmantis and webmantis provide features such as on the fly generation of pulse-height spectra and response functions for microcolumnar x-ray imagers while allowing users to save simulation parameters and results from prior experiments. The graphical interfaces simplify the simulation setup and allow the user to go directly from specifying input parameters to receiving visual feedback for the model predictions.

Characterization of scatter magnitude and distribution in dedicated breast computed tomography with bowtie filters

Scatter contamination of projection images in cone-beam computed tomography (CT) degrades the image quality. The use of bowtie filters in dedicated breast CT can decrease this scatter contribution. Three bowtie filter designs that compensate for one or more aspects of the beam-modifying effects due to differences in path length in a projection were studied. These designs have been investigated in terms of their ability to reduce the scatter contamination in projection images acquired in a dedicated breast CT geometry. The scatter magnitude was measured as the scatter-to-primary ratio (SPR) using experimental and Monte Carlo techniques for various breast phantom diameters and tube voltages. The results show that a 55% reduction in the center SPR value could be obtained with the bowtie filter designs. On average, the bowtie filters reduced the center SPR by approximately 18% over all breast diameters. The distribution of the scatter was calculated at a range of different locations to produce scatter distribution maps for all three bowtie filter designs. With the inclusion of the bowtie filters, the scatter distribution was more uniform for all breast diameters. The results of this study will be useful in designing scatter correction methods and understanding the benefits of bowtie filters in dedicated breast CT.

Monte Carlo Simulation of the Treatment of Eye Tumors with (106)Ru Plaques: A Study on Maximum Tumor Height and Eccentric Placement

BACKGROUND/AIMS

Ruthenium plaques are used for the treatment of ocular tumors. There is, however, a controversy regarding the maximum treatable tumor height. Some advocate eccentric plaque placement, without a posterior safety margin, to avoid collateral damage to the fovea and optic disc, but this has raised concerns about marginal tumor recurrence. There is a need for quantitative information on the spatial absorbed dose distribution in the tumor and adjacent tissues. We have overcome this obstacle using an approach based on Monte Carlo simulation of radiation transport.

METHODS

CCA and CCB (106)Ru plaques were modeled and their geometry embedded in a computerized tomography scan of the eye of a patient. Different tumor sizes and locations were simulated with the general-purpose Monte Carlo code PENELOPE.

RESULTS

Cumulative dose-volume histograms were obtained for the tumors and the tissues at risk considered. Plots of isodose lines for both plaques were obtained in a computerized tomography study.

CONCLUSIONS

Ruthenium eye plaques are an adequate treatment option for tumors up to around 5 mm in height. According to our results, assuming a correct placement of the plaque, a tumor of 6.5 mm apical height is about the maximum size that can be treated safely with the large CCB plaque.

Monte Carlo study for designing a dedicated "D"-shaped collimator used in the external beam radiotherapy of retinoblastoma patients

PURPOSE

Retinoblastoma is the most common intraocular malignancy in the early childhood. Patients treated with external beam radiotherapy respond very well to the treatment. However, owing to the genotype of children suffering hereditary retinoblastoma, the risk of secondary radio-induced malignancies is high. The University Hospital of Essen has successfully treated these patients on a daily basis during nearly 30 years using a dedicated "D"-shaped collimator. The use of this collimator that delivers a highly conformed small radiation field, gives very good results in the control of the primary tumor as well as in preserving visual function, while it avoids the devastating side effects of deformation of midface bones. The purpose of the present paper is to propose a modified version of the "D"-shaped collimator that reduces even further the irradiation field with the scope to reduce as well the risk of radio-induced secondary malignancies. Concurrently, the new dedicated "D"-shaped collimator must be easier to build and at the same time produces dose distributions that only differ on the field size with respect to the dose distributions obtained by the current collimator in use. The scope of the former requirement is to facilitate the employment of the authors' irradiation technique both at the authors' and at other hospitals. The fulfillment of the latter allows the authors to continue using the clinical experience gained in more than 30 years.

METHODS

The Monte Carlo code PENELOPE was used to study the effect that the different structural elements of the dedicated "D"-shaped collimator have on the absorbed dose distribution. To perform this study, the radiation transport through a Varian Clinac 2100 C/D operating at 6 MV was simulated in order to tally phase-space files which were then used as radiation sources to simulate the considered collimators and the subsequent dose distributions. With the knowledge gained in that study, a new, simpler, "D"-shaped collimator is proposed.

RESULTS

The proposed collimator delivers a dose distribution which is 2.4 cm wide along the inferior-superior direction of the eyeball. This width is 0.3 cm narrower than that of the dose distribution obtained with the collimator currently in clinical use. The other relevant characteristics of the dose distribution obtained with the new collimator, namely, depth doses at clinically relevant positions, penumbrae width, and shape of the lateral profiles, are statistically compatible with the results obtained for the collimator currently in use.

CONCLUSIONS

The smaller field size delivered by the proposed collimator still fully covers the planning target volume with at least 95% of the maximum dose at a depth of 2 cm and provides a safety margin of 0.2 cm, so ensuring an adequate treatment while reducing the irradiated volume.

A virtual trial framework for quantifying the detectability of masses in breast tomosynthesis projection data

PURPOSE

Digital breast tomosynthesis (DBT) is a promising breast cancer screening tool that has already begun making inroads into clinical practice. However, there is ongoing debate over how to quantitatively evaluate and optimize these systems, because different definitions of image quality can lead to different optimal design strategies. Powerful and accurate tools are desired to extend our understanding of DBT system optimization and validate published design principles.

METHODS

The authors developed a virtual trial framework for task-specific DBT assessment that uses digital phantoms, open-source x-ray transport codes, and a projection-space, spatial-domain observer model for quantitative system evaluation. The authors considered evaluation of reconstruction algorithms as a separate problem and focused on the information content in the raw, unfiltered projection images. Specifically, the authors investigated the effects of scan angle and number of angular projections on detectability of a small (3 mm diameter) signal embedded in randomly-varying anatomical backgrounds. Detectability was measured by the area under the receiver-operating characteristic curve (AUC). Experiments were repeated for three test cases where the detectability-limiting factor was anatomical variability, quantum noise, or electronic noise. The authors also juxtaposed the virtual trial framework with other published studies to illustrate its advantages and disadvantages.

RESULTS

The large number of variables in a virtual DBT study make it difficult to directly compare different authors' results, so each result must be interpreted within the context of the specific virtual trial framework. The following results apply to 25% density phantoms with 5.15 cm compressed thickness and 500 μm(3) voxels (larger 500 μm(2) detector pixels were used to avoid voxel-edge artifacts): 1. For raw, unfiltered projection images in the anatomical-variability-limited regime, AUC appeared to remain constant or increase slightly with scan angle. 2. In the same regime, when the authors fixed the scan angle, AUC increased asymptotically with the number of projections. The threshold number of projections for asymptotic AUC performance depended on the scan angle. In the quantum- and electronic-noise dominant regimes, AUC behaviors as a function of scan angle and number of projections sometimes differed from the anatomy-limited regime. For example, with a fixed scan angle, AUC generally decreased with the number of projections in the electronic-noise dominant regime. These results are intended to demonstrate the capabilities of the virtual trial framework, not to be used as optimization rules for DBT.

CONCLUSIONS

The authors have demonstrated a novel simulation framework and tools for evaluating DBT systems in an objective, task-specific manner. This framework facilitates further investigation of image quality tradeoffs in DBT.

Geometrical splitting technique to improve the computational efficiency in Monte Carlo calculations for proton therapy

PURPOSE

To present the implementation and validation of a geometrical based variance reduction technique for the calculation of phase space data for proton therapy dose calculation.

METHODS

The treatment heads at the Francis H Burr Proton Therapy Center were modeled with a new Monte Carlo tool (TOPAS based on Geant4). For variance reduction purposes, two particle-splitting planes were implemented. First, the particles were split upstream of the second scatterer or at the second ionization chamber. Then, particles reaching another plane immediately upstream of the field specific aperture were split again. In each case, particles were split by a factor of 8. At the second ionization chamber and at the latter plane, the cylindrical symmetry of the proton beam was exploited to position the split particles at randomly spaced locations rotated around the beam axis. Phase space data in IAEA format were recorded at the treatment head exit and the computational efficiency was calculated. Depth-dose curves and beam profiles were analyzed. Dose distributions were compared for a voxelized water phantom for different treatment fields for both the reference and optimized simulations. In addition, dose in two patients was simulated with and without particle splitting to compare the efficiency and accuracy of the technique.

RESULTS

A normalized computational efficiency gain of a factor of 10-20.3 was reached for phase space calculations for the different treatment head options simulated. Depth-dose curves and beam profiles were in reasonable agreement with the simulation done without splitting: within 1% for depth-dose with an average difference of (0.2 ± 0.4)%, 1 standard deviation, and a 0.3% statistical uncertainty of the simulations in the high dose region; 1.6% for planar fluence with an average difference of (0.4 ± 0.5)% and a statistical uncertainty of 0.3% in the high fluence region. The percentage differences between dose distributions in water for simulations done with and without particle splitting were within the accepted clinical tolerance of 2%, with a 0.4% statistical uncertainty. For the two patient geometries considered, head and prostate, the efficiency gain was 20.9 and 14.7, respectively, with the percentages of voxels with gamma indices lower than unity 98.9% and 99.7%, respectively, using 2% and 2 mm criteria.

CONCLUSIONS

The authors have implemented an efficient variance reduction technique with significant speed improvements for proton Monte Carlo simulations. The method can be transferred to other codes and other treatment heads.

PRIMO: a graphical environment for the Monte Carlo simulation of Varian and Elekta linacs

BACKGROUND

The accurate Monte Carlo simulation of a linac requires a detailed description of its geometry and the application of elaborate variance-reduction techniques for radiation transport. Both tasks entail a substantial coding effort and demand advanced knowledge of the intricacies of the Monte Carlo system being used.

METHODS

PRIMO, a new Monte Carlo system that allows the effortless simulation of most Varian and Elekta linacs, including their multileaf collimators and electron applicators, is introduced. PRIMO combines (1) accurate physics from the PENELOPE code, (2) dedicated variance-reduction techniques that significantly reduce the computation time, and (3) a user-friendly graphical interface with tools for the analysis of the generated data. PRIMO can tally dose distributions in phantoms and computerized tomographies, handle phase-space files in IAEA format, and import structures (planning target volumes, organs at risk) in the DICOM RT-STRUCT standard.

RESULTS

A prostate treatment, conformed with a high definition Millenium multileaf collimator (MLC 120HD) from a Varian Clinac 2100 C/D, is presented as an example. The computation of the dose distribution in 1.86×3.00×1.86 mm3 voxels with an average 2% standard statistical uncertainty, performed on an eight-core Intel Xeon at 2.67 GHz, took 1.8 h-excluding the patient-independent part of the linac, which required 3.8 h but it is simulated only once.

CONCLUSION

PRIMO is a self-contained user-friendly system that facilitates the Monte Carlo simulation of dose distributions produced by most currently available linacs. This opens the door for routine use of Monte Carlo in clinical research and quality assurance purposes. It is free software that can be downloaded from http://www.primoproject.net.

Monte Carlo-based dose calculation engine for minibeam radiation therapy

Minibeam radiation therapy (MBRT) is an innovative radiotherapy approach based on the well-established tissue sparing effect of arrays of quasi-parallel micrometre-sized beams. In order to guide the preclinical trials in progress at the European Synchrotron Radiation Facility (ESRF), a Monte Carlo-based dose calculation engine has been developed and successfully benchmarked with experimental data in anthropomorphic phantoms. Additionally, a realistic example of treatment plan is presented. Despite the micron scale of the voxels used to tally dose distributions in MBRT, the combination of several efficiency optimisation methods allowed to achieve acceptable computation times for clinical settings (approximately 2 h). The calculation engine can be easily adapted with little or no programming effort to other synchrotron sources or for dose calculations in presence of contrast agents.

hybridMANTIS: a CPU-GPU Monte Carlo method for modeling indirect x-ray detectors with columnar scintillators

The computational modeling of medical imaging systems often requires obtaining a large number of simulated images with low statistical uncertainty which translates into prohibitive computing times. We describe a novel hybrid approach for Monte Carlo simulations that maximizes utilization of CPUs and GPUs in modern workstations. We apply the method to the modeling of indirect x-ray detectors using a new and improved version of the code MANTIS, an open source software tool used for the Monte Carlo simulations of indirect x-ray imagers. We first describe a GPU implementation of the physics and geometry models in fastDETECT2 (the optical transport model) and a serial CPU version of the same code. We discuss its new features like on-the-fly column geometry and columnar crosstalk in relation to the MANTIS code, and point out areas where our model provides more flexibility for the modeling of realistic columnar structures in large area detectors. Second, we modify PENELOPE (the open source software package that handles the x-ray and electron transport in MANTIS) to allow direct output of location and energy deposited during x-ray and electron interactions occurring within the scintillator. This information is then handled by optical transport routines in fastDETECT2. A load balancer dynamically allocates optical transport showers to the GPU and CPU computing cores. Our hybridMANTIS approach achieves a significant speed-up factor of 627 when compared to MANTIS and of 35 when compared to the same code running only in a CPU instead of a GPU. Using hybridMANTIS, we successfully hide hours of optical transport time by running it in parallel with the x-ray and electron transport, thus shifting the computational bottleneck from optical tox-ray transport. The new code requires much less memory than MANTIS and, asa result, allows us to efficiently simulate large area detectors.

Long-term follow-up of solitary intracerebral juvenile xanthogranuloma. Case report and review of the literature

Juvenile xanthogranuloma is a benign, non-Langerhans-cell histiocytic infiltrate that typically presents as a solitary cutaneous lesion in childhood. There are reports of extracutanous involvement, including tumors in the central nervous system. A solitary, intraparenchymal tumor without skin manifestations is a rare event, with only 3 prior cases reported in the literature. Cerebral lesions have been associated with multifocal or systemic forms of the disease, with an occasionally fulminate clinical course. Considering the rarity of this tumor, it is unclear whether patients need adjuvant therapy after excision of a solitary intraparenchymal tumor. Previous reports suggested that complete excision of the lesion was curative; however long-term follow-up was not provided. This report illustrates a case of surgical excision of a solitary juvenile xanthogranuloma in an 8-year-old male with a 3-year follow-up period.