75 research outputs found
Effect of Statistical Fluctuation in Monte Carlo Based Photon Beam Dose Calculation on Gamma Index Evaluation
The gamma-index test has been commonly adopted to quantify the degree of
agreement between a reference dose distribution and an evaluation dose
distribution. Monte Carlo (MC) simulation has been widely used for the
radiotherapy dose calculation for both clinical and research purposes. The goal
of this work is to investigate both theoretically and experimentally the impact
of the MC statistical fluctuation on the gamma-index test when the fluctuation
exists in the reference, the evaluation, or both dose distributions. To the
first order approximation, we theoretically demonstrated in a simplified model
that the statistical fluctuation tends to overestimate gamma-index values when
existing in the reference dose distribution and underestimate gamma-index
values when existing in the evaluation dose distribution given the original
gamma-index is relatively large for the statistical fluctuation. Our numerical
experiments using clinical photon radiation therapy cases have shown that 1)
when performing a gamma-index test between an MC reference dose and a non-MC
evaluation dose, the average gamma-index is overestimated and the passing rate
decreases with the increase of the noise level in the reference dose; 2) when
performing a gamma-index test between a non-MC reference dose and an MC
evaluation dose, the average gamma-index is underestimated when they are within
the clinically relevant range and the passing rate increases with the increase
of the noise level in the evaluation dose; 3) when performing a gamma-index
test between an MC reference dose and an MC evaluation dose, the passing rate
is overestimated due to the noise in the evaluation dose and underestimated due
to the noise in the reference dose. We conclude that the gamma-index test
should be used with caution when comparing dose distributions computed with
Monte Carlo simulation
Structure and Stability of an Amorphous Metal
Using molecular dynamics simulations, with a realistic many-body
embedded-atom potential, and a novel method to characterize local order, we
study the structure of pure nickel during the rapid quench of the liquid and in
the resulting glass. In contrast with previous simulations with pair
potentials, we find more crystalline order and fewer icosahedra for slower
quenching rates, resulting in a glass less stable against crystallization. It
is shown that there is not a specific amorphous structure, only the arrest of
the transition from liquid to crystal, resulting in small crystalline clusters
immersed in an amorphous matrix with the same structure of the liquid.Comment: 4 pages, 4 ps figs., to appear in Phys. Rev. Let
GPU-based fast gamma index calcuation
The gamma-index dose comparison tool has been widely used to compare dose
distributions in cancer radiotherapy. The accurate calculation of gamma-index
requires an exhaustive search of the closest Euclidean distance in the
high-resolution dose-distance space. This is a computational intensive task
when dealing with 3D dose distributions. In this work, we combine a geometric
method with a radial pre-sorting technique , and implement them on computer
graphics processing units (GPUs). The developed GPU-based gamma-index
computational tool is evaluated on eight pairs of IMRT dose distributions. The
GPU implementation achieved 20x~30x speedup factor compared to CPU
implementation and gamma-index calculations can be finished within a few
seconds for all 3D testing cases. We further investigated the effect of various
factors on both CPU and GPU computation time. The strategy of pre-sorting
voxels based on their dose difference values speed up the GPU calculation by
about 2-4 times. For n-dimensional dose distributions, gamma-index calculation
time on CPU is proportional to the summation of gamma^n over all voxels, while
that on GPU is effected by gamma^n distributions and is approximately
proportional to the gamma^n summation over all voxels. We found increasing dose
distributions resolution leads to quadratic increase of computation time on
CPU, while less-than-quadratic increase on GPU. The values of dose difference
(DD) and distance-to-agreement (DTA) criteria also have their impact on
gamma-index calculation time.Comment: 13 pages, 2 figures, and 3 table
On the polyamorphism of fullerite-based orientational glasses
The dilatometric investigation in the temperature range of 2-28K shows that a
first-order polyamorphous transition occurs in the orientational glasses based
on C60 doped with H2, D2 and Xe. A polyamorphous transition was also detected
in C60 doped with Kr and He. It is observed that the hysteresis of thermal
expansion caused by the polyamorphous transition (and, hence, the transition
temperature) is essentially dependent on the type of doping gas. Both positive
and negative contributions to the thermal expansion were observed in the low
temperature phase of the glasses. The relaxation time of the negative
contribution occurs to be much longer than that of the positive contribution.
The positive contribution is found to be due to phonon and libron modes, whilst
the negative contribution is attributed to tunneling states of the C60
molecules. The characteristic time of the phase transformation from the low-T
phase to the high-T phase has been found for the C60-H2 system at 12K. A
theoretical model is proposed to interpret these observed phenomena. The
theoretical model proposed, includes a consideration of the nature of
polyamorphism in glasses, as well as the thermodynamics and kinetics of the
transition. A model of non-interacting tunneling states is used to explain the
negative contribution to the thermal expansion. The experimental data obtained
is considered within the framework of the theoretical model. From the
theoretical model the order of magnitude of the polyamorphous transition
temperature has been estimated. It is found that the late stage of the
polyamorphous transformation is described well by the Kolmogorov law with an
exponent of n=1. At this stage of the transformation, the two-dimensional phase
boundary moves along the normal, and the nucleation is not important.Comment: 29 pages, 14 figures, added references, corrected typo
Risk Related to Pre-Diabetes Mellitus and Diabetes Mellitus in Heart Failure With Reduced Ejection Fraction: Insights From Prospective Comparison of ARNI With ACEI to Determine Impact on Global Mortality and Morbidity in Heart Failure Trial
BACKGROUND: The prevalence of pre-diabetes mellitus and its consequences in patients with heart failure and reduced ejection fraction are not known. We investigated these in the Prospective Comparison of ARNI With ACEI to Determine Impact on Global Mortality and Morbidity in Heart Failure (PARADIGM-HF) trial. METHODS AND RESULTS: We examined clinical outcomes in 8399 patients with heart failure and reduced ejection fraction according to history of diabetes mellitus and glycemic status (baseline hemoglobin A1c [HbA1c]: /=6.5% [>/=48 mmol/mol; diabetes mellitus]), in Cox regression models adjusted for known predictors of poor outcome. Patients with a history of diabetes mellitus (n=2907 [35%]) had a higher risk of the primary composite outcome of heart failure hospitalization or cardiovascular mortality compared with those without a history of diabetes mellitus: adjusted hazard ratio, 1.38; 95% confidence interval, 1.25 to 1.52; P6.5%) and known diabetes mellitus compared with those with HbA1c<6.0% was 1.39 (1.17-1.64); P<0.001 and 1.64 (1.43-1.87); P<0.001, respectively. Patients with pre-diabetes mellitus were also at higher risk (hazard ratio, 1.27 [1.10-1.47]; P<0.001) compared with those with HbA1c<6.0%. The benefit of LCZ696 (sacubitril/valsartan) compared with enalapril was consistent across the range of HbA1c in the trial. CONCLUSIONS: In patients with heart failure and reduced ejection fraction, dysglycemia is common and pre-diabetes mellitus is associated with a higher risk of adverse cardiovascular outcomes (compared with patients with no diabetes mellitus and HbA1c <6.0%). LCZ696 was beneficial compared with enalapril, irrespective of glycemic status. CLINICAL TRIAL REGISTRATION: URL: http://www.clinicaltrials.gov. Unique identifier: NCT01035255
Angiotensin Receptor Neprilysin Inhibition Compared With Enalapril on the Risk of Clinical Progression in Surviving Patients With Heart Failure
BACKGROUND: -Clinical trials in heart failure have focused on the improvement in symptoms or decreases in the risk of death and other cardiovascular events. Little is known about the effect of drugs on the risk of clinical deterioration in surviving patients. METHODS AND RESULTS: -We compared the angiotensin-neprilysin inhibitor LCZ696 (400 mg daily) with the angiotensinconverting enzyme inhibitor enalapril (20 mg daily) in 8399 patients with heart failure and reduced ejection fraction in a double-blind trial. The analyses focused on prespecified measures of nonfatal clinical deterioration. In comparison with the enalapril group, fewer LCZ696-treated patients required intensification of medical treatment for heart failure (520 versus 604; hazard ratio, 0.84; 95% confidence interval, 0.74-0.94; P=0.003) or an emergency department visit for worsening heart failure (hazard ratio, 0.66; 95% confidence interval, 0.52-0.85; P=0.001). The patients in the LCZ696 group had 23% fewer hospitalizations for worsening heart failure (851 versus 1079; P<0.001) and were less likely to require intensive care (768 versus 879; 18% rate reduction, P=0.005), to receive intravenous positive inotropic agents (31% risk reduction, P<0.001), and to have implantation of a heart failure device or cardiac transplantation (22% risk reduction, P=0.07). The reduction in heart failure hospitalization with LCZ696 was evident within the first 30 days after randomization. Worsening of symptom scores in surviving patients was consistently more common in the enalapril group. LCZ696 led to an early and sustained reduction in biomarkers of myocardial wall stress and injury (N-terminal pro-Btype natriuretic peptide and troponin) versus enalapril. CONCLUSIONS: -Angiotensin-neprilysin inhibition prevents the clinical progression of surviving patients with heart failure more effectively than angiotensin-converting enzyme inhibition. Clinical Trial Registration-URL: http://www.clinicaltrials.gov. Unique identifier: NCT01035255
Specific features of thermal expansion and polyamorphism in CH4–C60 solutions at low temperatures
The temperature dependence of the linear thermal expansion coefficient α(T) has been investigated in the temperature range of 2.5 to 23 K for two different CH4–C60 solutions in which CH4 molecules occupied 24 and 50% of the octahedral interstitial sites of the C60 lattice. In both cases, α(T) exhibits hysteresis, suggesting the existence of two types of orientational glass associated with these solutions. The temperature of the first-order phase transition between these two glasses was estimated and the behavior of these two glasses compared. The characteristic times of thermalization τ1, reorientation of the C60 molecules τ2, and of the phase transformation between the glasses τ', have been estimated for these solutions. Both the temperature dependence of α(T) and the characteristic thermalization time τ1are found to have features near the phase transition temperature and an explanation has been put forward to explain these observed features
Low temperature thermal expansion of pure and inert gas-doped Fullerite C60
The low temperature (2-24 K) thermal expansion of pure (single crystal and
polycrystalline) C60 and polycrystalline C60 intercalated with He, Ne, Ar, and
Kr has been investigated using high-resolution capacitance dilatometer. The
investigation of the time dependence of the sample length variations on heating
shows that the thermal expansion is determined by the sum of positive and
negative contributions, which have different relaxation times. The negative
thermal expansion usually prevails at helium temperatures. The positive
expansion is connected with the phonon thermalization of the system. The
negative expansion is caused by reorientation of the C60 molecules. It is
assumed that the reorientation is of quantum character. The inert gas
impurities affect very strongly the reorientation of the C60 molecules
especially at liquid helium temperatures. A temperature hysteresis of the
thermal expansion coefficient of Kr- and He- C60 solutions has been revealed.
The hysteresis is attributed to orientational polyamorphous transformation in
these systems.Comment: 18 pages, 12 figure
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