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

An Ergonomic Assessment of Different Postures and Children Risk during Evacuations

Crawling is recommended for avoiding high heat and toxic fumes and for obtaining more breathable air during evacuations. Few studies have evaluated the effects of crawling on physical joints and velocity, especially in children. Based on motion capture technology, this study proposes a novel method of using wearable sensors to collect exposure (e.g., mean duration, frequency) on children’s joints to objectively quantify the impacts of different locomotion methods on physical characteristics. An on-site experiment was conducted in a kindergarten with 28 children (13 boys and 15 girls) of different ages (4–6 years old) who traveled up to 22 m in three different postures: upright walking (UW), stoop walking (SW), and knee and hand crawling (KHC). The results showed that: (1) The level of joint fatigue for KHC was heavier than bipedal walking (p 0.05). (2) The physical characteristics of the children in the different postures observed in this study were different (p 0.05) and gender (p > 0.05) of the children on the joints in any of the three postures. To conclude, KHC causes more damage to body joints compared to bipedal walking, as evidenced by higher exposure (mean duration, frequency), whereas UW and SW are similar in terms of the level of joint fatigue. The above findings are expected to provide a useful reference for future applications in the children’s risk assessment and in the prevention design of buildings

Fast Monte Carlo Simulation for Patient-specific CT/CBCT Imaging Dose Calculation

Recently, X-ray imaging dose from computed tomography (CT) or cone beam CT (CBCT) scans has become a serious concern. Patient-specific imaging dose calculation has been proposed for the purpose of dose management. While Monte Carlo (MC) dose calculation can be quite accurate for this purpose, it suffers from low computational efficiency. In response to this problem, we have successfully developed a MC dose calculation package, gCTD, on GPU architecture under the NVIDIA CUDA platform for fast and accurate estimation of the x-ray imaging dose received by a patient during a CT or CBCT scan. Techniques have been developed particularly for the GPU architecture to achieve high computational efficiency. Dose calculations using CBCT scanning geometry in a homogeneous water phantom and a heterogeneous Zubal head phantom have shown good agreement between gCTD and EGSnrc, indicating the accuracy of our code. In terms of improved efficiency, it is found that gCTD attains a speed-up of ~400 times in the homogeneous water phantom and ~76.6 times in the Zubal phantom compared to EGSnrc. As for absolute computation time, imaging dose calculation for the Zubal phantom can be accomplished in ~17 sec with the average relative standard deviation of 0.4%. Though our gCTD code has been developed and tested in the context of CBCT scans, with simple modification of geometry it can be used for assessing imaging dose in CT scans as well.Comment: 18 pages, 7 figures, and 1 tabl

Influence of Copper Vapor on Dielectric Properties of Hot CO2 at Different Pressures from 0.4 MPa to 0.8 MPa

The influence of copper vapor on dielectric properties of hot CO 2 at different pressures has beenanalyzed in this paper . By solving the Boltzmann equation the electron energy distribution functions andthe critical reduced electric fie ld strength (E/N) cr were obtained. The results indicate that copper markedlyreduces (E/N) cr of the CO 2 Cu gas mixtures because of copper’s large ionization cross section.Additionally, increasing the gas pressure effectively delays the pick temperature of (E/N) cr

Social isolation, rather than loneliness, is associated with cognitive decline in older adults: the China Health and Retirement Longitudinal Study

BACKGROUND: Social isolation and loneliness have each been associated with cognitive decline, but most previous research is limited to Western populations. This study examined the relationships of social isolation and loneliness on cognitive function among Chinese older adults. METHODS: This study used two waves of data (2011 and 2015) from the China Health and Retirement Longitudinal Study and analyses were restricted to those respondents aged 50 and older. Social isolation, loneliness, and cognitive function were measured at baseline. Follow-up measures on cognitive function were obtained for 7761 participants (mean age = 60.97, s.d. = 7.31; male, 50.8%). Lagged dependent variable models adjusted for confounding factors were used to evaluate the association between baseline isolation, loneliness, and cognitive function at follow-up. RESULTS: Loneliness was significantly associated with the cognitive decline at follow-up (episodic memory: β = -0.03, p 0.05). By contrast, social isolation was significantly associated with decreases in all cognitive function measures at follow-up (episodic memory: β = -0.05, p < 0.001; mental status: β = -0.03, p < 0.01) even after controlling for loneliness and all confounding variables. CONCLUSIONS: Social isolation is associated with cognitive decline in Chinese older adults, and the relationships are independent of loneliness. These findings expand our knowledge about the links between social relationships and the cognitive function in non-Western populations