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Results for “Monte Carlo Method”

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PubMed2005

Lung cancer screening: simulations of effects of imperfect detection on temporal dynamics.

PURPOSE: To use a mathematic model to demonstrate effects of imperfect detection on temporal dynamics of radiologic lung cancer screening. MATERIALS AND METHODS: Monte Carlo simulations of lung cancer screening programs were performed in subjects at high risk for developing cancer. The effects of detection probabilities, symptomatic presentation of tumors, tumor volume doubling time, and time between screenings were examined. Computed tomography (CT) and chest radiography models were used. RESULTS: For imperfect detection probabilities, the percentage of subjects with cancers detected with repeated screenings decreased to a steady-state value. The transition period was the period during which screenings were performed and detection rates decreased. At steady-state repeat screening, the proportion of subjects with cancers diagnosed at screening or by means of symptomatic presentation was determined by the annual probability of developing cancer and not by the sensitivity of the screening modality. The sensitivity of the screening technique did affect detected cancer size, number of interval cancers, and total number of cancers observed. CT was used to detect more total cancers over the course of the screening program and cancers with a smaller average size; moreover, fewer interval cancers were observed with CT screening than with chest radiography screening. CONCLUSION: Lung cancer screening with imperfect detection has a transition period between baseline screening and steady-state behavior of annual screenings. Advantages of CT screening include a decrease in the average cancer size at detection, a decrease in the number of observed interval cancers, and an increase in the total number of cancers observed. Steady-state behavior indicates that long-term trials of screening may not be necessary.

Humans↗

PubMed2005

Chance mechanisms affecting the burden of metastases.

BACKGROUND: The burden of cancer metastases within an individual is commonly used to clinically characterize a tumor's biological behavior. Assessments like these implicitly assume that spurious effects can be discounted. Here the influence of chance on the burden of metastasis is studied to determine whether or not this assumption is valid. METHODS: Monte Carlo simulations were performed to estimate tumor burdens sustained by individuals with cancer, based upon empirically derived and validated models for the number and size distributions of metastases. Factors related to the intrinsic metastatic potential of tumors and their host microenvironments were kept constant, to more clearly demonstrate the contribution from chance. RESULTS: Under otherwise identical conditions, both the simulated numbers and the sizes of metastases were highly variable. Comparable individuals could sustain anywhere from no metastases to scores of metastases, and the sizes of the metastases ranged from microscopic to macroscopic. Despite the marked variability in the number and sizes of the metastases, their respective growth times were rather more narrowly distributed. In such situations multiple occult metastases could develop into fully overt lesions within a comparatively short time period. CONCLUSION: Chance can have a major effect on the burden of metastases. Random variability can be so great as to make individual assessments of tumor biology unreliable, yet constrained enough to lead to the apparently simultaneous appearance of multiple overt metastases.

Animals↗

PubMed2006

Novel low-kVp beamlet system for choroidal melanoma.

BACKGROUND: Treatment of choroidal melanoma with radiation often involves placement of customized brachytherapy eye-plaques. However, the dosimetric properties inherent in source-based radiotherapy preclude facile dose optimization to critical ocular structures. Consequently, we have constructed a novel system for utilizing small beam low-energy radiation delivery, the Beamlet Low-kVp X-ray, or "BLOKX" system. This technique relies on an isocentric rotational approach to deliver dose to target volumes within the eye, while potentially sparing normal structures. METHODS: Monte Carlo N-Particle (MCNP) transport code version 5.0(14) was used to simulate photon interaction with normal and tumor tissues within modeled right eye phantoms. Five modeled dome-shaped tumors with a diameter and apical height of 8 mm and 6 mm, respectively, were simulated distinct positions with respect to the macula iteratively. A single fixed 9 x 9 mm2 beamlet, and a comparison COMS protocol plaque containing eight I-125 seeds (apparent activity of 8 mCi) placed on the scleral surface of the eye adjacent to the tumor, were utilized to determine dosimetric parameters at tumor and adjacent tissues. After MCNP simulation, comparison of dose distribution at each of the 5 tumor positions for each modality (BLOKX vs. eye-plaque) was performed. RESULTS: Tumor-base doses ranged from 87.1-102.8 Gy for the BLOKX procedure, and from 335.3-338.6 Gy for the eye-plaque procedure. A reduction of dose of at least 69% to tumor base was noted when using the BLOKX. The BLOKX technique showed a significant reduction of dose, 89.8%, to the macula compared to the episcleral plaque. A minimum 71.0 % decrease in dose to the optic nerve occurred when the BLOKX was used. CONCLUSION: The BLOKX technique allows more favorable dose distribution in comparison to standard COMS brachytherapy, as simulated using a Monte Carlo iterative mathematical modeling. Future series to determine clinical utility of such an approach are warranted.

Brachytherapy↗

PubMed2000

High-diffraction-efficiency pseudorandom encoding.

Pseudorandom encoding (PRE) is a statistics-based procedure in which a pure-phase spatial light modulator (SLM) can yield, on the average, the prescribed diffraction pattern specified by the user. We seek to combine PRE with the optimization of an aperture-based target function. The target function is a fully complex input transmittance, unrealizable by a phase-only SLM, that generates a prescribed light intensity. The optimization is done to increase the diffraction efficiency of the overall process. We compare three optimization methods-Monte Carlo simulation, a genetic algorithm, and a gradient search-for maximizing the diffraction efficiency of a spot-array generator. Calculated solutions are then encoded by PRE, and the resulting diffraction patterns are computer simulated. Details on the complexity of each procedure are furnished, as well as comparisons on the quality, such as uniformity of the output spot array.

Computer Simulation↗

PubMed2005

[Correlation between absorption rates of beta-adrenoreceptor antagonists in rat small intestine and their molecular structures].

OBJECTIVE: To study the correlation between the absorption rate constants of beta-adrenoreceptor antagonists in rat small intestinal segments and their molecular structural parameters. METHODS: The net atomic charges and the molecular volumes of 11 beta-adrenoreceptor antagonists were obtained with the semiempirical self-consistent field molecular orbital calculation CNDO/2 method and Mont Carlo method respectively, using the minimum energy conformation obtained from the optimization of the standard molecular geometry with the molecular mechanics MM+ method. The stepwise multiple regression analysis was used to obtain the correlation equations. RESULTS: The absorption rate constants of beta-adrenoreceptor antagonists in rat jejunum or ileum were well linearly correlated with the sum of the net charges of all hydrogen atoms and the molecular volumes. The beta-adrenoreceptor antagonist with higher lipophilicity, weaker hydrogen-bonding potential,and smaller molecular volume had greater absorption rate constants. CONCLUSION: The absorption rate constants of beta-adrenoreceptor antagonists in rat small intestinal segments are mainly related with their lipophilicity,hydrogen-bonding potential and molecular size.

Adrenergic beta-Antagonists↗

PubMed2006

[Prediction of human intestinal absorption from net polar atomic charges of drug molecules].

OBJECTIVE: To predict human intestinal absorption and permeability coefficients in Caco-2 cell monolayers from net polar atomic charges of drug molecules. METHODS: The net atomic charges and the volumes of drug molecules were obtained with the semiempirical self-consistent field molecular orbital calculation CNDO/2 method and Mont Carlo method respectively, using the minimum energy conformation obtained from the optimization of the standard molecular geometry with the molecular mechanics MM+ method. The stepwise multiple regression analysis was used to obtain the correlation equations. RESULT: Both percent of human intestinal absorption and permeability coefficients in Caco-2 cell monolayers of drug molecules were well correlated with the sum of the net atomic charges of all hydrogen-bonding donors (sigmaQH) and the sum of the net atomic charges of all hydrogen-bonding acceptors (sigmaQN, 0). The more the net positive atomic charges of hydrogen-bonding donors and the net negative atomic charges of hydrogen-bonding acceptors, the less were the percent human intestinal absorption and permeability coefficients in Caco-2 cell monolayers of drug molecules. CONCLUSION: Drug absorption in human intestines is closely related with its hydrogen-bonding potential. The drug molecules with weaker hydrogen-bonding potential have greater percent human intestinal absorption. The net polar atomic charges can be computed simply, so they can be used in high throughput screening of oral drugs.

Adrenergic beta-Antagonists↗

PubMed2001

Directly observed treatment for multidrug-resistant tuberculosis: an economic evaluation in the United States of America and South Africa.

OBJECTIVE: To estimate the cost-effectiveness of directly observed treatment compared to conventional therapy in reducing the spread of multidrug-resistant tuberculosis, for an industrialised country (represented by the United States of America) and a developing country (South Africa). METHODS: Monte Carlo analysis using published data on probability, cost and health impact. RESULTS: In both countries, directly observed treatment is the dominant strategy, yielding cost savings and improved health outcomes. Cost savings for directly observed treatment relative to conventional therapy become more significant as more expensive second-line drugs are used in treatments. CONCLUSIONS: The cost-effectiveness of directly observed treatment relative to conventional therapy is demonstrated for both the USA and South Africa. Cost savings are more pronounced (especially for South Africa) as the likelihood of multidrug-resistant tuberculosis increases and more expensive second-line therapies are used. Given that health care resources are more severely constrained in developing countries, the data contained in this study are useful in guiding the design of policies for the effective management of multidrug-resistant tuberculosis in settings with limited resources.

Antitubercular Agents↗

PubMed2007

Radiation dose distribution in human kidneys by octreotides in peptide receptor radionuclide therapy.

UNLABELLED: Ex vivo autoradiographs of healthy kidney tissue from patients who received (111)In-DTPA-octreotide (DTPA is diethylenetriaminepentaacetic acid) before nephrectomy showed very heterogeneous radioactivity patterns in the kidneys. The consequences of the reported inhomogeneities have been evaluated for radionuclide therapy with (90)Y- DOTA-Tyr(3)-octreotide (DOTA is 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid), (177)Lu-DOTA-Tyr(3)-octreotate, and (111)In-DTPA-octreotide by calculating dose distributions and dose-volume histograms (DVHs) for the kidneys. METHODS: Monte Carlo radiation transport calculations were performed by using the MCNP code version 5. The autoradiography data were used in a 2-dimensional model of the kidney tissue sections. A voxel structure inside the MIRD Pamphlet 19 multiregion kidney model was developed to generate a 3-dimensional representation of the autoradiographs. Dose distributions were calculated for the beta-emitter (90)Y, the low-energy electron and gamma-emitter (111)In, and the beta- and gamma-emitter (177)Lu. Isodose curves were generated for the 2-dimensional kidney sections and DVHs for the 3-dimensional kidney model. RESULTS: The isodose curves for the high-energy beta-emitter (90)Y did not show a sign of the inhomogeneous activity distribution, apart from the cortex-medulla boundaries. Both (111)In and (177)Lu isodose curves follow the autoradiographic activity distribution exactly. The 2 gamma-rays from (111)In give higher doses to the low-radioactivity regions in the kidney sections. The DVHs show that the inhomogeneous activity distribution creates considerable volumes within the kidney and within the cortex with lower doses than the average kidney dose, together with volumes receiving much higher doses. This effect is most profound for (177)Lu, but also (111)In shows this heterogeneity in the dose distribution. CONCLUSION: Kidney dosimetry for radionuclide therapy can be based on average MIRD-based dose models for high-energy beta-emitters (such as (90)Y). In contrast, low-energy beta-emitters (such as (177)Lu) and Auger-electron-emitting radionuclides (such as (111)In) produce dose distributions in the kidneys that are very dependent on the activity distribution pattern in the kidney or renal cortex. Complication probability models for renal tissue damage after radionuclide therapy with these latter nuclides need to be developed, as the existing models based on average dose to the kidney or cortex from external beam therapy experience are most probably not valid.

Dose-Response Relationship, Radiation↗

PubMed2005

Calculation of eigenfunction fluxes in nuclear systems.

A new Monte Carlo method is being developed to calculate eigenfunction fluxes in critical or near-critical nuclear systems. The correct estimation of fluxes is essential for radiation protection and shielding near these systems, in addition to isotope production, isotope depletion, nuclear criticality and other applications. The proposed method applies to Monte Carlo criticality eigenvalue calculations in which the fission sites in one generation are used as fission sources in subsequent generations. The usual Monte Carlo power iteration method for such problems often calculates fluxes (eigenfunctions) that are inaccurate and very different in symmetric parts of a problem geometry. The proposed method calculates flux distributions by estimating an approximate fission matrix. The way the fission matrix is estimated and used differs from other recent works. Preliminary results are promising.

Algorithms↗

PubMed2006

On-axis sensitivity and resolution of a slit-slat collimator.

UNLABELLED: A slit-slat collimator combines a slit along the axis of rotation with a set of axial septa, offering both magnification in the transaxial direction and complete sampling with just a circular orbit. This collimator has a sensitivity that increases for points near the aperture slit. The literature treats this collimator as having the same sensitivity as a single-pinhole collimator, ignoring the effect of the axial septa. Herein, the sensitivity and resolution of this collimator are reevaluated. METHODS: Experimental and Monte Carlo methods are used to determine the sensitivity and resolution in both the transaxial and axial directions as a function of distance from the slit (h). Eight configurations are tested, varying the slit width, septal spacing, and septal height. RESULTS: Both the experimental and the Monte Carlo sensitivities agree reasonably with an analytic form that is the geometric mean of the pinhole and parallel-beam formulas, disagreeing with previous literature. Transaxial resolution is consistent with the pinhole-resolution formula. Axial resolution is consistent with the parallel-beam resolution formula. CONCLUSION: The sensitivity of this collimator is proportional to h(-1) and has resolution in the transaxial direction that is consistent with pinhole resolution and in the axial direction that is consistent with parallel-beam resolution.

Algorithms↗

PubMed1997

Orthovoltage radiation therapy treatment planning using Monte Carlo simulation: treatment of neuroendocrine carcinoma of the maxillary sinus.

Dose distributions that result from treating a patient with orthovoltage beams are best determined with a treatment planning system that uses the Monte Carlo method, and such systems are not readily available. In the present work, the Monte Carlo method was used to develop a computer code for determining absorbed dose distributions in orthovoltage radiation therapy. The code was used in planning treatment of a patient with a neuroendocrine carcinoma of the maxillary sinus. Two lateral high-energy photon beams supplemented by an anterior orthovoltage photon beam were utilized in the treatment plan. For the clinical case and radiation beams considered, a reasonably uniform dose distribution (+/- 10%) is achieved within the target volume, while the dose to the lens of each eye is 4-8% of the prescribed dose. Therefore, an orthovoltage photon beam, when properly filtered and optimally combined with megavoltage beams, can be effective in the treatment of cancers below the skin, providing that accurate treatment planning is carried out to establish with accuracy and precision the doses to critical structures.

Humans↗

PubMed2002

Modelling the effect of incorporated halogenated pyrimidine on radiation-induced DNA strand breaks.

PURPOSE: To estimate the enhancement of DNA strand breaks induced by low linear energy transfer (LET) radiation in the presence of halogenated pyrimidines and to examine complexity and clustering properties of damage that could provide a correlation between DNA damage and lethality. MATERIALS AND METHODS: Monte Carlo track structure methods were used to model and estimate the induction of strand breakage by X-ray photons with and without the incorporated Br/I deoxyuridine in cell-mimetic conditions. The increase of DNA strand break induction was modelled by taking into account the direct energy deposition and the reactions of radicals. The yield and spectrum of strand breaks were calculated at various degrees of Br/IdU incorporation. The excess strand breaks due to Br/IdU incorporation was assumed to be induced by highly reactive uracilyl radicals. Four mechanisms were considered for the production of uracilyl radicals classified into three groups, by hydrated electrons, by direct energy deposition, and by both hydrated electrons and direct energy depositions. In total, nine different models were considered to test the excess strand breaks by incorporated Br/IdU assuming different pathways. RESULTS: Model calculations show the following: the yield of strand breaks is enhanced by both the e(aq)(-) reaction and the direct energy deposition on base moiety; there is a significant contribution to the enhancement of yield of strand breaks due to energy transfer about four bases along the DNA to Br/IdU and DNA strand break complexity increases with degree of Br/IdU incorporation. Enhancement ratios of 1.8 and 2.5 for 40% Br/IdU substitution were obtained for single- and double-strand breaks, respectively. CONCLUSIONS: The increase in the yield of strand breaks due to Br/IdU incorporation could be explained by the mechanism of uracilyl radical production by e(aq)(-) and direct energy deposition. The importance of energy transfer along the DNA is demonstrated. It is shown that the incorporation of Br/IdU causes a spectral shift towards a greater complexity of clustered DNA damage.

Base Sequence↗

PubMed1999

On the assessment of statistical significance in disease-gene discovery.

One of the major challenges facing genome-scan studies to discover disease genes is the assessment of the genomewide significance. The assessment becomes particularly challenging if the scan involves a large number of markers collected from a relatively small number of meioses. Typically, this assessment has two objectives: to assess genomewide significance under the null hypothesis of no linkage and to evaluate true-positive and false-positive prediction error rates under alternative hypotheses. The distinction between these goals allows one to formulate the problem in the well-established paradigm of statistical hypothesis testing. Within this paradigm, we evaluate the traditional criterion of LOD score 3.0 and a recent suggestion of LOD score 3.6, using the Monte Carlo simulation method. The Monte Carlo experiments show that the type I error varies with the chromosome length, with the number of markers, and also with sample sizes. For a typical setup with 50 informative meioses on 50 markers uniformly distributed on a chromosome of average length (i.e., 150 cM), the use of LOD score 3.0 entails an estimated chromosomewide type I error rate of.00574, leading to a genomewide significance level >.05. In contrast, the corresponding type I error for LOD score 3.6 is.00191, giving a genomewide significance level of slightly <.05. However, with a larger sample size and a shorter chromosome, a LOD score between 3.0 and 3.6 may be preferred, on the basis of proximity to the targeted type I error. In terms of reliability, these two LOD-score criteria appear not to have appreciable differences. These simulation experiments also identified factors that influence power and reliability, shedding light on the design of genome-scan studies.

Chromosome Mapping↗

PubMed2003

Using Monte Carlo simulations to commission photon beam output factors--a feasibility study.

This study investigates the feasibility of using Monte Carlo methods to assist the commissioning of photon beam output factors from a medical accelerator. The Monte Carlo code, BEAMnrc, was used to model 6 MV and 18 MV photon beams from a Varian linear accelerator. When excellent agreements were obtained between the Monte Carlo simulated and measured dose distributions in a water phantom, the entire geometry including the accelerator head and the water phantom was simulated to calculate the relative output factors. Simulated output factors were compared with measured data, which consist of a typical commission dataset for the output factors. The measurements were done using an ionization chamber in a water phantom at a depth of 10 cm with a source-detector distance of 100 cm. Square fields and rectangular fields with widths and lengths ranging from 4 cm to 40 cm were studied. The result shows a very good agreement (< 1.5%) between the Monte Carlo calculated and the measured relative output factors for a typical commissioning dataset. The Monte Carlo calculated backscatter factors to the beam monitor chamber agree well with measured data in the literature. Monte Carlo simulations have also been shown to be able to accurately predict the collimator exchange effect and its component for rectangular fields. The information obtained is also useful to develop an algorithm for accurate beam modelling. This investigation indicates that Monte Carlo methods can be used to assist commissioning of output factors for photon beams.

Calibration↗

PubMed2006

Monte Carlo simulation of intercalated carbon nanotubes.

Monte Carlo simulations of the single- and double-walled carbon nanotubes (CNT) intercalated with different metals have been carried out. The interrelation between the length of a CNT, the number and type of metal atoms has also been established. This research is aimed at studying intercalated systems based on CNTs and d-metals such as Fe and Co. Factors influencing the stability of these composites have been determined theoretically by the Monte Carlo method with the Tersoff potential. The modeling of CNTs intercalated with metals by the Monte Carlo method has proved that there is a correlation between the length of a CNT and the number of endo-atoms of specific type. Thus, in the case of a metallic CNT (9,0) with length 17 bands (3.60 nm), in contrast to Co atoms, Fe atoms are extruded out of the CNT if the number of atoms in the CNT is not less than eight. Thus, this paper shows that a CNT of a certain size can be intercalated with no more than eight Fe atoms. The systems investigated are stabilized by coordination of 3d-atoms close to the CNT wall with a radius-vector of (0.18-0.20) nm. Another characteristic feature is that, within the temperature range of (400-700) K, small systems exhibit ground-state stabilization which is not characteristic of the higher ones. The behavior of Fe and Co endo-atoms between the walls of a double-walled carbon nanotube (DW CNT) is explained by a dominating van der Waals interaction between the Co atoms themselves, which is not true for the Fe atoms.

Cobalt↗

PubMed1997

Implementation of a Monte Carlo dosimetry method for patient-specific internal emitter therapy.

In internal emitter therapy, an accurate description of the absorbed dose distribution is necessary to establish an administered dose-response relationship, as well as to avoid critical organ toxicity. This work describes the implementation of a dosimetry method that accounts for the radionuclide decay spectrum, and patient-specific activity and density distributions. The dosimetry algorithm is based on a Monte Carlo procedure that simulates photon and electron transport and scores energy depositions within the patient. The necessary input information may be obtained from a registered set of CT and SPECT or PET images. The algorithm provides the absorbed dose rate for the radioactivity distribution provided by the SPECT or PET image. The algorithm was benchmarked by reproducing dosimetric quantities using the Medical Internal Radionuclide Dose (MIRD) Committee's Standard Man phantom and was used to calculate absorbed dose distributions for representative case studies.

Humans↗

PubMed2002

Experimental and numerical study of the colour appearance of tattoo models.

The colour of tattooed skin has been predicted by a Monte Carlo method based on the optical coefficient spectra of the skin and tattoo dyes. Slices of pig skin, a tattoo phantom and skin phantoms with different thickness were prepared, and their reflectance and transmittance spectra were measured using an integrating sphere at wavelengths varying from 400nm to 700nm. The absorption and scattering coefficient spectra of skin phantoms, pig skins and the tattoo phantom were each calculated using the inverse Monte Carlo method. The skin phantoms and pig skins were overlaid on the tattoo phantom, and the reflectance spectra of the two-layered structures were measured. The reflectance spectra of the two-layered structures were calculated from the optical coefficient spectra using the Monte Carlo method. They agreed well with the measured spectra. The colour differences between the calculated and measured spectra were also evaluated by the L*a*b* colour space distances and showed good agreement, with 3.49 for the skin phantoms and 8.27 for the pig skins.

Animals↗

PubMed1994

Comparison of four scatter correction methods using Monte Carlo simulated source distributions.

UNLABELLED: Scatter correction in SPECT is important for improving image quality, boundary detection and the quantification of activity in different regions. This paper presents a comparison of four scatter correction methods, three using more than one energy window and one convolution-subtraction correction method using spatial variant scatter line-spread functions. METHODS: The comparison is based on Monte Carlo simulated data for point sources on- and off-axis, hot and cold spheres of different diameters, and a clinically realistic source distribution simulating brain imaging. All studies were made for a uniform cylindrical water phantom. Since the nature of the detected photon is known with Monte Carlo simulation, separate images of primary and scattered photons can be recorded. These can then be compared with estimated scatter and primary images obtained from the different scatter correction methods. The criteria for comparison were the normalized mean square error, scatter fraction, % recovery and image contrast. RESULTS: All correction methods significantly improved image quality and quantification compared to those obtained with no correction. Quantitatively, no single method was observed to be the best by all criteria for all the source distributions. Three of the methods were observed to perform the best by at least one of the criteria for one of the source distributions. For brain imaging, the differences between all the methods were much less than the difference between them and no correction at all. CONCLUSION: It is concluded that performing scatter correction is essential for accurate quantification, and that all four methods yield a good, but not perfect, scatter correction. Since it is hard to distinguish the methods consistently in terms of their performance, it may be that the choice should be made on the basis of ease of implementation.

Brain↗