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S C Ferreira

Publications and source records attributed to S C Ferreira.

9 recordsLinked to original sources
PubMed2006

Aggregation in a mixture of Brownian and ballistic wandering particles.

In this paper, we analyze the scaling properties of a model that has as limiting cases the diffusion-limited aggregation (DLA) and the ballistic aggregation (BA) models. This model allows us to control the radial and angular scaling of the patterns, as well as their gap distributions. The particles added to the cluster can follow either ballistic trajectories, with probability Pba, or random ones, with probability Prw=1-Pba. The patterns were characterized through several quantities, including those related to the radial and angular scaling. The fractal dimension as a function of Pba continuously increases from df approximately 1.72 (DLA dimensionality) for Pba=0 to df approximately 2 (BA dimensionality) for Pba. However, the lacunarity and the active zone width exhibit a distinct behavior: they are convex functions of Pba with a maximum at Pba approximately 1/2. Through the analysis of the angular correlation function, we found that the difference between the radial and angular exponents decreases continuously with increasing Pba and rapidly vanishes for Pba>1/2, in agreement with recent results concerning the asymptotic scaling of DLA clusters.

Journal Article↗
PubMed2005

Morphological transition between diffusion-limited and ballistic aggregation growth patterns.

In this work, the transition between diffusion-limited (DLA) and ballistic aggregation (BA) models was reconsidered using a model in which biased random walks simulate the particle trajectories. The bias is controlled by a parameter lambda, which assumes the value lambda=0 (1) for the ballistic (diffusion-limited) aggregation model. Patterns growing from a single seed were considered. In order to simulate large clusters, an efficient algorithm was developed. For lambda (not equal to) 0 , the patterns are fractal on small length scales, but homogeneous on large ones. We evaluated the mean density of particles (-)rho in the region defined by a circle of radius r centered at the initial seed. As a function of r, (-)rho reaches the asymptotic value rho(0)(lambda) following a power law (-)rho = rho(0) +Ar(-gamma) with a universal exponent gamma=0.46 (2) , independent of lambda . The asymptotic value has the behavior rho(0) approximately |1-lambda|(beta) , where beta=0.26 (1) . The characteristic crossover length that determines the transition from DLA- to BA-like scaling regimes is given by xi approximately |1-lambda|(-nu) , where nu=0.61 (1) , while the cluster mass at the crossover follows a power law M(xi) approximately |1-lambda(-alpha) , where alpha=0.97 (2) . We deduce the scaling relations beta=nugamma and beta=2nu-alpha between these exponents.

Journal Article↗
PubMed2005

Critical behavior of the two- and three-dimensional contact replication processes.

The two- and three-dimensional contact replication processes (CRP) for monoclonal reproduction were analyzed through intensive Monte Carlo simulations. In these models, the occupation rates are equally divided among the empty nearest neighbor sites of an occupied site. As the one-dimensional case, the two-dimensional version of the model belongs to the directed percolation universality class and the critical rate defining the absorbing state transition is lambdac = 1.083 20(7). However, the critical exponents of the three-dimensional CRP are those of the four-dimensional original contact process and, consequently, the data suggest that the CRP model has a distinct upper critical dimension dc = 3.

Journal Article↗
PubMed2004

Absorbing state transition in a one-dimensional contact replication process.

In this work, the contact process (CP) is modified in order to model a contact replication process (CRP) for monoclonal reproduction. The occupation rates of an empty site depend on the nearest-neighbor and next-nearest-neighbor sites. The CRP exhibits an absorbing state transition studied through cluster approximations and Monte Carlo simulations. The critical rate obtained from simulations, lambda(c) =2.0263 (4) , is smaller than that for CP. However, the CRP critical exponents are in agreement with those for CP and, consequently, the model belongs to the directed percolation universality class.

Journal Article↗
PubMed2003

Morphology transitions induced by chemotherapy in carcinomas in situ.

Recently, we have proposed a nutrient-limited model for the avascular growth of tumors including cell proliferation, motility, and death [S. C. Ferreira, Jr., M. L. Martins, and M. J. Vilela, Phys. Rev. E 65, 021907 (2002)], which qualitatively reproduces commonly observed morphologies for carcinomas in situ. In the present work, we analyze the effects of distinct chemotherapeutic strategies on the patterns, scaling, and growth laws obtained for the nutrient-limited model. Two kinds of chemotherapeutic strategies were considered, namely, those that kill cancer cells and those that block cell mitosis but allow the cell to survive for some time. Depending on the chemotherapeutic schedule used, the tumors are completely eliminated, reach a stationary size, or grow following power laws. The model suggests that the scaling properties of the tumors are not affected by the mild cytotoxic treatments, although a reduction in growth rates and an increase in invasiveness are observed. For the strategies based on antimitotic drugs, a morphological transition in which compact tumors become more fractal under aggressive treatments was seen.

Antineoplastic Agents↗
PubMed2002

Reaction-diffusion model for the growth of avascular tumor.

A nutrient-limited model for avascular cancer growth including cell proliferation, motility, and death is presented. The model qualitatively reproduces commonly observed morphologies for primary tumors, and the simulated patterns are characterized by its gyration radius, total number of cancer cells, and number of cells on tumor periphery. These very distinct morphological patterns follow Gompertz growth curves, but exhibit different scaling laws for their surfaces. Also, the simulated tumors incorporate a spatial structure composed of a central necrotic core, an inner rim of quiescent cells and a narrow outer shell of proliferating cells in agreement with biological data. Finally, our results indicate that the competition for nutrients among normal and cancer cells may be a determining factor in generating papillary tumor morphology.

Animals↗
PubMed2000

[Cervical cancer screening among indigenous women in the Xingu Indian Reservation, central Brazil].

Although the literature presents worrisome data regarding the incidence of cervical cancer among indigenous populations, in Brazil there is very little information regarding the occurrence of this type of cancer among indigenous peoples. Therefore, the objective of the present descriptive study was to assess the prevalence of cervical cancer and of cervical and vaginal infections among 423 indigenous women living in the Xingu Indian Reservation, in the state of Mato Grosso, Brazil. These women were or had been sexually active. Data were collected between 1989 and 1996. Clinical and gynecological examinations were carried out prior to the collection of cervical specimens and to the performance of cytologic analyses. Upon detection of abnormalities, a colposcopy and a biopsy were also performed. Our results show that 1% of the women studied presented invasive carcinoma and that 3% presented premalignant lesions. In addition, 84% presented inflammatory atypia, resulting from sexually transmitted genital infections. The present findings are in accordance with the results of other international reports regarding the high prevalence of cervical conditions among indigenous populations, and they underscore the need to extend to the indigenous peoples of Brazil programs aiming at the control of sexually transmitted diseases and at the early detection and treatment of cervical cancer.

Adolescent↗