PubMed Health⌕ Search

Biomedical subjects

C Sire

Publications and source records attributed to C Sire.

At least 19 recordsLinked to original sources
PubMed2000

Analytical results for random walk persistence

In this paper, we present a detailed calculation of the persistence exponent straight theta for a nearly Markovian Gaussian process X(t), a problem initially introduced elsewhere in [Phys. Rev. Lett. 77, 1420 (1996)], describing the probability that the walker never crosses the origin. Resummed perturbative and nonperturbative expressions for straight theta are derived, which suggest a connection with the result of the alternative independent interval approximation. The perturbation theory is extended to the calculation of straight theta for non-Gaussian processes, by making a strong connection between the problem of persistence and the calculation of the energy eigenfunctions of a quantum mechanical problem. Finally, we give perturbative and nonperturbative expressions for the persistence exponent straight theta(X0), describing the probability that the process remains larger than X(0)sqrt[ ].

Journal Article↗
PubMed2000

Numerical renormalization group of vortex aggregation in two-dimensional decaying turbulence: the role of three-body interactions

We introduce a numerical renormalization group procedure which permits long-time simulations of vortex dynamics and coalescence in a two-dimensional turbulent decaying fluid. The number of vortices decreases as N approximately t(-xi), with xi approximately 1 instead of the value xi=4/3 predicted by a naive kinetic theory. For short time, we find an effective exponent xi approximately 0.7 consistent with previous simulations and experiments. We show that the mean square displacement of surviving vortices grows as approximately t(1+xi/2). Introducing effective dynamics for two- and three-body collisions, we justify that only the latter become relevant at a small vortex area coverage. A kinetic theory consistent with this mechanism leads to xi=1. We find that the theoretical relations between kinetic parameters are all in good agreement with experiments.

Journal Article↗
PubMed2000

Statistics of velocity fluctuations arising from a random distribution of point vortices: the speed of fluctuations and the diffusion coefficient

This paper is devoted to a statistical analysis of the fluctuations of velocity and acceleration produced by a random distribution of point vortices in two-dimensional turbulence. We show that the velocity probability density function PDF behaves in a manner which is intermediate between Gaussian and Levy laws, while the distribution of accelerations is governed by a Cauchy law. Our study accounts properly for a spectrum of circulations among the vortices. In the case of real vortices (with a finite core), we show analytically that the distribution of accelerations makes a smooth transition from Cauchy (for small fluctuations) to Gaussian (for large fluctuations), probably passing through an exponential tail. We introduce a function T(V) which gives the typical duration of a velocity fluctuation V; we show that T(V) behaves like V and V-1 for weak and large velocities, respectively. These results have a simple physical interpretation in the nearest neighbor approximation, and in Smoluchowski theory concerning the persistence of fluctuations. We discuss the analogies with respect to the fluctuations of the gravitational field in stellar systems. As an application of these results, we determine an approximate expression for the diffusion coefficient of point vortices. When applied to the context of freely decaying two-dimensional turbulence, the diffusion becomes anomalous and we establish a relationship nu=1+(xi/2) between the exponent of anomalous diffusion nu and the exponent xi which characterizes the decay of the vortex density.

Journal Article↗
PubMed2000

[Stage III and IV cancers of the oropharynx: results of a randomized study of Gortec comparing radiotherapy alone with concomitant chemotherapy].

The aim of the study was to test whether the addition of three cycles of chemotherapy during standard radiation therapy would improve disease-free survival in patients with stages III and IV oropharynx carcinoma. A total of 226 patients have been entered in a phase III multicentric randomized trial comparing radiotherapy alone (arm A) to radiotherapy with concomitant chemotherapy (arm B). Radiotherapy was identical in the two arms, delivering, with conventional fractionation, 70 Gy in 35 fractions. In arm B patients received simultaneously 3 cycles of a four-day regimen containing carboplatin (70 mg/m2/d) and 5 fluorouracil (600 mg/m2/d) continuous infusion. The two arms were equally balanced regarding to age, gender, stage, performance status, histology, and primary tumor site. Radiotherapy compliance was similar in the two arms regarding to total dose, treatment duration and treatment interruption. Grade 3 and 4 mucositis rate was significantly higher in arm B (67% versus 36%). Skin toxicity was not different. Haematologic toxicity was higher in arm B on neutrophil count and hemoglobin level. Three-year overall actuarial survival and disease-free survival rates were respectively 51% versus 31% and 42% versus 20% for patients treated with combined modality versus radiation alone (p = 0.022 and 0.043). Local and regional control rate has been improved in arm B (66% versus 42%). The statistically significant improvement in overall survival obtained support the use of concomitant chemotherapy as an adjunct to radiotherapy in the management of carcinoma of the oropharynx.

Combined Modality Therapy↗
PubMed1999

Randomized trial of radiation therapy versus concomitant chemotherapy and radiation therapy for advanced-stage oropharynx carcinoma.

BACKGROUND: We designed a randomized clinical trial to test whether the addition of three cycles of chemotherapy during standard radiation therapy would improve disease-free survival in patients with stages III and IV (i.e., advanced oropharynx carcinoma). METHODS: A total of 226 patients have been entered in a phase III multicenter, randomized trial comparing radiotherapy alone (arm A) with radiotherapy with concomitant chemotherapy (arm B). Radiotherapy was identical in the two arms, delivering, with conventional fractionation, 70 Gy in 35 fractions. In arm B, patients received during the period of radiotherapy three cycles of a 4-day regimen containing carboplatin (70 mg/m(2) per day) and 5-fluorouracil (600 mg/m(2) per day) by continuous infusion. The two arms were equally balanced with regard to age, sex, stage, performance status, histology, and primary tumor site. RESULTS: Radiotherapy compliance was similar in the two arms with respect to total dose, treatment duration, and treatment interruption. The rate of grades 3 and 4 mucositis was statistically significantly higher in arm B (71%; 95% confidence interval [CI] = 54%-85%) than in arm A (39%; 95% CI = 29%-56%). Skin toxicity was not different between the two arms. Hematologic toxicity was higher in arm B as measured by neutrophil count and hemoglobin level. Three-year overall actuarial survival and disease-free survival rates were, respectively, 51% (95% CI = 39%-68%) versus 31% (95% CI = 18%-49%) and 42% (95% CI = 30%-57%) versus 20% (95% CI = 10%-33%) for patients treated with combined modality versus radiation therapy alone (P =.02 and.04, respectively). The locoregional control rate was improved in arm B (66%; 95% CI = 51%-78%) versus arm A (42%; 95% CI = 31%-56%). CONCLUSION: The statistically significant improvement in overall survival that was obtained supports the use of concomitant chemotherapy as an adjunct to radiotherapy in the management of carcinoma of the oropharynx.

Adult↗
PubMed1999

Genetic diversity and recruitment pattern of Schistosoma mansoni in a Biomphalaria glabrata snail population: a field study using random-amplified polymorphic DNA markers.

Random-amplified polymorphic DNA markers have been used to assess the amount and the distribution of the genetic diversity of Schistosoma mansoni within a natural population of Biomphalaria glabrata at a transmission site of the murine schistosomiasis focus of Guadeloupe. Despite high infection rate and heavy schistosome load within the definitive hosts (Ratus rattus), prevalences within intermediate snails ranged from 0.2 to 4.8%. Whatever the transmission season may be (rainy vs. dry), most of the infected snails were spatially aggregated and 88.4% of them harbored a single parasite genotype indicative of a monomiracidial infection; 4.7% had dual sex infections and a parasite intensity not exceeding 3 miracidia per snail. A substantial resistance level toward the parasite and recruitment regulatory process within snails may explain in part the observed low parasite prevalences and intensities. Considering such a distribution pattern of larval S. mansoni genetic diversity among B. glabrata, mobility of the definitive hosts, or rapid turnover of infected snails, or both, are required to maintain genetic heterogeneity within adult schistosome populations.

Animals↗
PubMed1998

Failure of Schistosoma mansoni to reinfect Biomphalaria glabrata snails: acquired humoral resistance or intra-specific larval antagonism?

Failure of snail reinfection by Schistosoma mansoni has been demonstrated in susceptible Biomphalaria glabrata infected with 1 miracidium and subsequently re-exposed to 1 or 5 homologous parasite larvae. The acquisition of 'resistance' to secondary parasite infection was time dependent since complete inhibition was observed at 2 weeks and longer following monomiracidial exposure. This phenomenon was still observed in snails challenged 8 weeks after primary infection. Histological observations revealed that sporocysts from the challenge infection were free of encapsulation, their development was stopped and they degenerated slowly in the absence of haemocytic reaction of the host. Under the hypothesis of an acquired homologous resistance mechanism, this strongly suggests that 1 or several unidentified humoral factors are responsible for the non-development of the sporocysts from the challenge infection. However, considering the time-dependent nature of the phenomenon, an intraspecific larval antagonism process between sporocysts resulting from the primary infection and those from the challenge infection may be involved.

Animals↗