The effect of shoulder variation on IMRT and SmartArc for Head and Neck Cancer

Purpose: First, to determine an average and maximum displacement of the shoulder relative to isocenter over the course of treatment. Second, to establish the dosimetric effect of shoulder displacements relative to correct isocenter alignment on the dose delivered to the target and the surrounding structures for head and neck cancer patients. Method and Materials: The frequency of shoulder shifts of various magnitudes relative to isocenter was assessed for 4 patients using image registration software. The location of the center of the right and left humeral head relative to isocenter (usually C2) was found daily from CT on rails scans, and was compared to the location of the humeral heads relative to isocenter on the initial simulation CT. Three Baseline head and neck IMRT and SmartArc plans were generated in Pinnacle based on simulation CTs. The CT datasets (external contour and boney structures) were then modified to represent shifts of the shoulder (relative to isocenter) between 3 mm and 15 mm in the SI, AP, and LR directions. The initial plans were recalculated on the image sets with shifted shoulders. Results: On average, shoulder variation was 2-5 mm in each direction, although displacements of over 1 cm in the inferior and posterior directions occurred. Shoulder shifts induced perturbations in the dose distribution, although generally only for large shifts. Most substantially, large, superior shifts resulted in coverage loss by the 95% isodose line for targets in the lower neck. Inferior shifts elevated the dose to the brachial plexus by 0.6-4.1 Gy. SmartArc plans showed similar loss of target coverage as IMRT plans. Conclusions: The position of the shoulder can have an impact on target coverage and critical structure dose. Shoulder position may need to be considered for setup of head and neck patients depending on target location.

Concept, design and implementation of a cardiovascular gene-centric 50 k SNP array for large-scale genomic association studies.

A wealth of genetic associations for cardiovascular and metabolic phenotypes in humans has been accumulating over the last decade, in particular a large number of loci derived from recent genome wide association studies (GWAS). True complex disease-associated loci often exert modest effects, so their delineation currently requires integration of diverse phenotypic data from large studies to ensure robust meta-analyses. We have designed a gene-centric 50 K single nucleotide polymorphism (SNP) array to assess potentially relevant loci across a range of cardiovascular, metabolic and inflammatory syndromes. The array utilizes a "cosmopolitan" tagging approach to capture the genetic diversity across approximately 2,000 loci in populations represented in the HapMap and SeattleSNPs projects. The array content is informed by GWAS of vascular and inflammatory disease, expression quantitative trait loci implicated in atherosclerosis, pathway based approaches and comprehensive literature searching. The custom flexibility of the array platform facilitated interrogation of loci at differing stringencies, according to a gene prioritization strategy that allows saturation of high priority loci with a greater density of markers than the existing GWAS tools, particularly in African HapMap samples. We also demonstrate that the IBC array can be used to complement GWAS, increasing coverage in high priority CVD-related loci across all major HapMap populations. DNA from over 200,000 extensively phenotyped individuals will be genotyped with this array with a significant portion of the generated data being released into the academic domain facilitating in silico replication attempts, analyses of rare variants and cross-cohort meta-analyses in diverse populations. These datasets will also facilitate more robust secondary analyses, such as explorations with alternative genetic models, epistasis and gene-environment interactions.

The role of post-translation regulation of Twist expression in the tumor progression of squamous cell carcinoma of head and neck

Squamous cell carcinoma of head and neck (SCCHN) is the tenth most common cancer in the world. Unfortunately, the survival of patients with SCCHN has not improved in the last 40 years. Therefore new targets for therapy are needed, and to this end we are studying signaling pathways activated by IL-6 which we have found stimulates cell migration and soft agar growth in SCCHN. Our data show that IL-6 increases TWIST expression in a transcription-independent mechanism in many SCCHN cell lines. Further investigation reveals TWIST can be phosphorylated upon IL-6 treatment. By computation prediction (http://scansite.mit.edu/motifscan_seq.phtml ), we found that TWIST has a putative phosphorylation site for casein kinase 2 (CK2) suggesting that this kinase could serve as a link between IL-6 stimulation and Twist stability. To test this hypothesis, we used a CK2 inhibitor and shRNA to CK2 and found that these interventions inhibited IL-6 stimulation of TWIST stability. In addition, mutation of the putative CK2 phosphorylation site (S18/S20A) in TWIST decreased the amount of phospho-ATP incorporated by TWIST in an in vitro kinase assay, and altered TWIST stability. In Boyd chamber migration assay and wound-healing assay, the CK2 inhibitor, DMAT, was found to decrease the motility of IL-6 stimulated SCCHN cells and over expression of either a wild-type or the hyperphosphorylated mimicking mutant S18/20D –Twist rather than the hypo-phosphorylated mimicking mutant S18/20A-Twist can promote SCCHN cell motility.To our knowledge, this is the first report to identify the importance of IL-6 stimulated CK2 phosphorylation of TWIST in SCCHN. As CK2 inhibitors are currently under phase I clinical trials, our findings indicate that CK2 may be a viable therapeutic target in SCCHN. Therefore, further pre-clinical studies of this inhibitor are underway.

Polymorphisms of the DNA repair gene MGMT and risk and progression of head and neck cancer.

Methylating agents are involved in carcinogenesis, and the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) removes methyl group from O(6)-methylguanine. Genetic variation in DNA repair genes has been shown to contribute to susceptibility to squamous cell carcinoma of the head and neck (SCCHN). We hypothesize that MGMT polymorphisms are associated with risk of SCCHN. In a hospital-based case-control study of 721 patients with SCCHN and 1234 cancer-free controls frequency-matched by age, sex and ethnicity, we genotyped four MGMT polymorphisms, two in exon 3, 16195C>T and 16286C>T and two in the promoter region, 45996G>T and 46346C>A. We found that none of these polymorphisms alone had a significant effect on risk of SCCHN. However, when these four polymorphisms were evaluated together by the number of putative risk genotypes (i.e. 16195CC, 16286CC, 45996GT+TT, and 46346CA+AA), a statistically significantly increased risk of SCCHN was associated with the combined genotypes with three to four risk genotypes, compared with those with zero to two risk genotypes (adjusted odds ratio (OR)=1.27; 95% confidence interval (CI)=1.05-1.53). This increased risk was also more pronounced among young subjects (OR=1.81; 95% CI=1.11-2.96), men (OR=1.24; 95% CI=1.00-1.55), ever smokers (OR=1.25; 95%=1.01-1.56), ever drinkers (OR=1.29; 95% CI=1.04-1.60), patients with oropharyngeal cancer (OR=1.45; 95% CI=1.12-1.87), and oropharyngeal cancer with regional lymph node metastasis (OR=1.52; 95% CI=1.16-1.89). In conclusion, our results suggest that any one of MGMT variants may not have a substantial effect on SCCHN risk, but a joint effect of several MGMT variants may contribute to risk and progression of SCCHN, particularly for oropharyngeal cancer, in non-Hispanic whites.

Characterization of the cytochrome P-450 drug metabolism system of LS174T human colon tumor cell line

The human colon tumor cell line, LS174T, has been shown to have four major components of the drug metabolizing system; cytochrome b$\sb5$ reductase, cytochrome b$\sb5$, cytochrome P450 reductase and cytochrome P450, by activity measurements, spectral studies and antibody cross-reactivity. Cytochrome P450IA1 is induced by benzanthracene in these cells as shown by activity with the specific substrate, ethoxyresorufin, cross-reactivity with rabbit antibodies to rat IA1, and by a hybridizing band on a Northern blot to a rat IA1 probe.^ Further, this system has proven responsive to various inducers and conditions of growth. The enzyme activities were found stable over limited cell passages with control values of 0.03 and 0.13 $\mu$mol/min/mg protein for NADPH and NADH cytochrome c (cyt c) reducing activity, 0.05 nmol cyt b$\sb5$ per milligram and 0.013 nmol cytochrome P450 per milligram of microsomal protein. Phenobarbital/hydrocortisone treatment showed a consistent, but not always significant increase in the NADPH and NADH cyt c reducing activity and benzanthracene treatment an increase in the NADH cyt c reducing activity. Delta-aminolevulinic acid (0.5mM) caused a significant decrease in the specific activity of all enzyme contents and activities tested.^ Finally, the cytochrome b$\sb5$ to cytochrome P450, by the coordinate induction of the cytochrome b$\sb5$ pathway by P450 inducers, by the high ratio of NADH to NADPH ethoxycoumarin deethylase activity in uninduced cell microsomes, and by the increase in NADH and NADPH ethoxycoumarin deethylase activity when the microsomes were treated with potassium cyanide, a desaturase inhibitor. ^

Analyses of pp60$\sp{{\rm c}-src}$ and epidermal growth factor receptor protein tyrosine kinases in a human colon adenocarcinoma cell line following induction of goblet cell-like characteristics by tumor necrosis factor-alpha

Regulation of colonic epithelial cell proliferation and differentiation remains poorly understood due to the inability to design a model system which recapitulates these processes. Currently, properties of "differentiation" are studied in colon adenocarcinoma cell lines which can be induced to express some, but not all of the phenotypes of normal cells. In this thesis, the DiFi human colon adenocarcinoma cell line is utilized as an in vitro model system in which to study mucin production. In response to treatment with tumor necrosis factor-alpha, DiFi cells acquire some properties of mucin-producing goblet cells including altered morphology, increased reactivity to wheat germ agglutinin, and increased mucin production as determined by RNA expression as well as reactivity with the MUC-1 antibodies, HMFG-1 and SM-3. Thus, TNF-treated DiFi cells represent one of the few in vitro systems in which mucin expression can be induced.^ DiFi cells express an activated pp60$\sp{{\rm c}-src},$ as do most colon adenocarcinomas and derived cell lines, as well as an amplified epidermal growth factor (EGF) receptor. To assess potential changes in these enzymes during induction of differentiation characteristics, potential changes in the levels and activities of these enzymes were examined. For pp60$\sp{{\rm c}-src},$ no changes were observed in protein levels, specific activity of the kinase, cellular localization, or phosphorylation pattern as determined by Staphylococcus aureus V8 protease partial proteolytic mapping after induction of goblet cell-like phenotypic changes. These results suggest that pp60$\sp{{\rm c}-src}$ is regulated differentially in goblet cells than in absorptive cells, as down-modulation of pp60$\sp{{\rm c}-src}$ kinase occurs in the latter. Therefore, effects on pp60$\sp{{\rm c}-src}$ may be critical in colon regulation, and may be important in generating the various colonic epithelial cell types.^ In contrast to pp60$\sp{{\rm c}-src},$ EGF receptor tyrosine kinase activity decreased ($<$5-fold) after TNF treatment and at the time in which morphologic changes were observed. Similar decreases in tyrosine phosphorylation of EGF receptor were observed as assessed by immunoblotting with an anti-phosphotyrosine antibody. In addition, ($\sp{125}$I) -EGF cell surface binding was reduced approximately 3-fold following TNF treatment with a concomitant reduction in receptor affinity ($<$2-fold). These results suggest that modulation of EGF receptor may be important in goblet cell differentiation. In contrast, other published studies have demonstrated that increases in EGF receptor mRNA and in ($\sp{125}$I) -EGF binding accompany differentiation toward the absorptive cell phenotype. Therefore, differential regulation of both EGF receptor and pp60$\sp{{\rm c}-src}$ occur along the goblet cell and absorptive cell differentiation pathways. Thus, my results suggest that TNF-treated DiFi cells represent a unique system in which to study distinct patterns of regulation of pp60$\sp{{\rm c}-src}$ and EGF receptor in colonic cells, and to determine if increased MUC-1 expression is an early event in goblet cell differentiation. ^

Establishing occupational and environmental health design requirements for Lunar and Mars settlements

In a study of Lunar and Mars settlement concepts, an analysis was made of fundamental design assumptions in five technical areas against a model list of occupational and environmental health concerns. The technical areas included the proposed science projects to be supported, habitat and construction issues, closed ecosystem issues, the "MMM" issues--mining, material-processing, and manufacturing, and the human elements of physiology, behavior and mission approach. Four major lessons were learned. First it is possible to relate public health concerns to complex technological development in a proactive design mode, which has the potential for long-term cost savings. Second, it became very apparent that prior to committing any nation or international group to spending the billions to start and complete a lunar settlement, over the next century, that a significantly different approach must be taken from those previously proposed, to solve the closed ecosystem and "MMM" problems. Third, it also appears that the health concerns and technology issues to be addressed for human exploration into space are fundamentally those to be solved for human habitation of the earth (as a closed ecosystem) in the 21st century. Finally, it is proposed that ecosystem design modeling must develop new tools, based on probabilistic models as a step up from closed circuit models. ^

Expression and regulation of interleukins in human head and neck squamous cell carcinoma

Patients with head and neck squamous cell carcinoma (HNSCC) demonstrate abnormal cell-mediated immunity which is most pronounced at the primary tumor site. Therefore, we tested whether this aberrant immunity could be due to tumor-derived cytokines. We investigated the presence of cytokine mRNA and protein in 8 HNSCC-derived cell lines; RT-PCR results indicated mRNA's for IL-1$\alpha$ and TGF-$\alpha$ (8/8), TGF-$\beta$ (7/8), IL-1$\beta$ (7/8), IL-4 and IL-6 (4/8). IL-2, IFN-$\gamma,$ and TNF-$\alpha$ mRNA was not detected. Supernatants from 6 of these cell lines were analyzed by ELISA and IL-1$\alpha,$ IL-1$\beta,$ and IL-6 were markedly increased compared to HPV-16 immortalized human oral keratinocytes. IL-1$\alpha$ was found in the highest concentration $>$IL-6 $>$ IL-1$\beta.$^ To approach the mechanisms of cytokine regulation, 4 cell lines were compared for HPV DNA presence, p53 status, and cytokine expression. An association between HPV DNA and cytokine expression was not found. However, cell lines secreting the most IL-6 had mutant p53 and/or HPV 16 E6/E7 expression. Further regulatory investigations revealed that exogenous IL-1$\alpha$ and/or IL-1$\beta$ minimally stimulated the proliferation of 2/3 cell lines, as well as strongly induced IL-6 production in 3/3; this effect was completely abrogated by IL-1Ra. IL-1Ra also inhibited the secretion of IL-1$\alpha$ and IL-1$\beta$ in 2/3 cell lines. These data suggest an IL-1 autocrine loop in certain HNSCC cell lines. Because IL-2 induces IL-1 and is used in therapy of HNSCC, the expression of IL-2 receptor was also investigated; IL-2 $\alpha$ and $\beta$ subunits were detected in 3/3 cell lines and $\gamma$ subunits was detected in one. Exogenous IL-2 inhibited the proliferation, but stimulated the secretion of IL-1$\alpha$ in 2/3, and IL-1$\beta$ and IL-6 in 1/3 cell lines.^ To determine if our cell line findings were applicable to patients, immunohistochemistry was performed on biopsies from 12 invasive tumors. Unexpectedly, universal intracellular production of IL-1$\alpha,$ IL-1$\beta,$ and IL-6 protein was detected. Therefore, the aberrant elaboration of biologically active IL-1 and IL-6 may contribute to altered immune status in HNSCC patients. ^

Design and evaluation of the parent education component of a comprehensive violence prevention project in middle schools

Low parental monitoring is related to youth risk behaviors such as delinquency and aggression. The purpose of this dissertation was to describe the development and evaluation of a parent education intervention to increase parental monitoring in Hispanic parents of middle school children.^ The first study described the process of intervention mapping as used to develop Padres Trabajando por la Paz, a newsletter intervention for parents. Using theory, empirical literature, and information from the target population, performance objectives and determinants for monitoring were defined. Learning objectives were specified and a staged social-cognitive approach was used to develop methods and strategies delivered through newsletters.^ The second study examined the outcomes of a randomized trial of the newsletter intervention. Outcome measures consisted of a general measure of monitoring, parent and child reports of monitoring behaviors targeted by the intervention, and psychosocial determinants of monitoring (self-efficacy, norms, outcome expectancies, knowledge, and beliefs). Seventy-seven parents completed the randomized trial, half of which received four newsletters over an eight-week period. Results revealed a significant interaction effect for baseline and treatment for parent's reports of norms for monitoring (p =.009). Parents in the experimental condition who scored low at baseline reported increased norms for monitoring at follow-up. A significant interaction effect for child reports of parental monitoring behaviors (p =.04) reflected an small increase across baseline levels in the experimental condition and decreases for the control condition at higher baseline scores. Both groups of parents reported increased levels of monitoring at follow-up. No other outcome measures varied significantly by condition.^ The third study examined the relationship between the psychosocial determinants of parental monitoring and parental monitoring behaviors in the study population. Weak evidence for a relationship between outcome expectancies and parental monitoring behaviors suggests further research in the area utilizing stronger empirical models such as longitudinal design and structural equation modeling.^ The low-cost, minimal newsletter intervention showed promise for changing norms among Hispanic parents for parental monitoring. In light of the importance of parental monitoring as a protective factor for youth health risk behaviors, more research needs to be done to develop and evaluate interventions to increase parental monitoring. ^

Mechanism of N-(4-hydroxyphenyl)retinamide (4HPR)-induced apoptosis in head and neck squamous cell carcinoma cells

4HPR is a synthetic retinoid that has shown chemopreventive and therapeutic efficacy against premalignant and malignant lesions including oral leukoplakia, ovarian and breast cancer and neuroblastoma in clinical trials. 4HPR induces growth inhibition and apoptosis in various cancer cells including head and neck squamous cell carcinoma (HNSCC) cells. 4HPR induces apoptosis by several mechanisms including increasing reactive oxygen species (ROS), or inducing mitochondrial permeability transition (MPT). 4HPR has also been shown to modulate the level of different proteins by transcriptional activation or posttranslational modification in various cellular contexts. However, the mechanism of its action is not fully elucidated. In this study, we explored the mechanism of 4HPR-induced apoptosis in HNSCC cells. ^ First, we identified proteins modulated by 4HPR by using proteomics approaches including: Powerblot western array and 2-dimensional polyacrylamide gel electrophoresis. We found that 4HPR modulated the levels of several proteins including c-Jun. Further analysis has shown that 4HPR induced activation of Activator Protein 1 (AP-1) components, c-Jun and ATF-2. We also found that 4HPR increased the level of Heat shock protein (Hsp) 70 and phosphorylation of Hsp27. ^ Second, we found that 4HPR induced prolonged activation of JNK, p38/MAPK and extracellular signal-regulated kinase (ERK). We also demonstrated that the activation of these kinases is required for 4HPR-induced apoptosis. JNK inhibitor SP600125 and siRNA against JNK1 and JNK2 suppressed, while overexpression of JNK1 enhanced 4HPR-induced apoptosis. p38/MAPK inhibitor PD169316 and MEK1/2 inhibitor PD98059 also suppressed 4HPR-induced apoptosis. We also demonstrated that activation of JNK, p38/MAPK and ERK is triggered by ROS generation induced by 4HPR. We also found that translation inhibitor, cycloheximide, suppressed 4HPR-induced apoptosis through inhibition of 4HPR-induced events (e.g. ROS generation, cytochrome c release, JNK activation and suppression of Akt). We also demonstrated that MPT is involved in 4HPR-induced apoptosis. ^ Third, we demonstrated the presence of NADPH oxidase in HNSCC 2B cells. We also found that 4HPR increased the level of the p67phox, a subunit of NADPH oxidase which participates in ROS production and apoptosis induced by 4HPR. ^ The novel insight into the mechanism by which 4HPR induces apoptosis can be used to improve design of future clinical studies with this synthetic retinoid in combination with specific MAPK modulators. ^

An analysis of the RightCare policy at Ben Taub General Hospital, Houston, Texas

Introduction. This study is a two-part evaluation of the RightCare policy, a policy implemented to reduce crowding at the Emergency Center (EC) at Ben Taub General Hospital in Houston, Harris County, Texas. This research includes an evaluation of the policy's impact on specific hospital measures, along with a description of the policy's demise from the point of view of hospital staff. Objective. The purpose of this study is two-fold: (1) To determine whether RightCare policy affected the level of crowding in the Emergency Center and (2) to identify the conditions that may have led to the policy's demise. Methods. For the policy impact portion of this research, hospital measures were collected from existing databases. Analysis included a pre-post comparative design in which the 12 months preceding the policy's implementation were compared with the 12 months following the policy's implementation. For the policy perception portion, employees were surveyed using an on-line questionnaire. Results. The results of the study are mixed. Some measures improved, including time spent on ambulance diversion and the proportion of those who left without being seen, while others did not, such as return visits and total length of stay. Employees generally supported the policy, but expressed concerns over insufficient training and funding. Conclusion. The RightCare policy was a good initial attempt to improve crowded conditions in the EC. The study showed that a clearer policy design, improved training, adequate staffing levels, and better communication would improve operational outcomes in the future.^

Validity and reliability of using MVP 4 Function Walk4Life digital pedometers to assess physical activity levels among preschool-aged Head Start children

The Surgeon General recommends preschoolers 3-5 years old accumulate 60 minutes of moderate-to-vigorous physical activity (MVPA) per day. However, there is limited data measuring physical activity (PA) and MVPA amongst this population. The purpose of this cross-sectional study is to determine the validity, reliability, and feasibility of using MVP 4 Function Walk4Life digital pedometers (MVP-4) in measuring MVPA among preschoolers using the newly modified direct observational technique, System for Observing Fitness Instruction Time-Preschool Version (SOFIT-P) as the gold standard. An ethnically diverse population of 3-5 year old underserved children were recruited from two Harris County Department of Education (HCDE) Head Start centers. For 2 days at baseline and 2 days at post-test, 75 children enrolled wore MVP-4 pedometers for approximately 6-hours per observation day and were observed using SOFIT-P during predominantly active times. Statistical analyses used Pearson "r" correlation coefficients to determine mean minutes of PA and MVPA, convergent and criterion validity, and reliability. Significance was set at p = <0.05. Feasibility was determined through process evaluation information collected during this study via observations from data collectors and teacher input. Results show mean minutes of PA and MVPA ranged between 30-42 and 11-14 minutes, respectively. Convergent validity comparing BMI percentiles with MVP-4 PA outcomes show no significance at pre-test; however, each measurement at post-test showed significance for MVPA (p = 0.0247, p = 0.0056), respectively. Criterion validity comparing percent MVPA time between SOFIT-P and MVP-4 pedometers was determined; however, results deemed insufficient due to inconsistency in observation times while using the newly developed SOFIT-P. Reliability measures show no significance at pre-test, yet show significant results for all PA outcomes at post-test (p = 0.001, p = 0.001, p = 0.0010, p = 0.003), respectively. Finally, MVP-4 pedometers lacked feasibility due to logistical barriers in design. Researchers feel the significant results at post-test are secondary to increased familiarity and more accurate placement of pedometers across time. Researchers suggest manufacturers of MVP-4 pedometers further modify the instrument for ease of use with this population, following which future studies ought to determine validity using objective measures or all-day direct observation techniques.^

Factors that affect non-adherence to first line antiretroviral medications among HIV infected children and adolescents in Botswana

The aims of the study were to determine the prevalence of and factors that affect non-adherence to first line antiretroviral (ARV) medications among HIV infected children and adolescents in Botswana. The study used secondary data from Botswana-Baylor Children's Clinical Center of Excellence for the period of June 2008 to February 10th, 2010. The study design was cross-sectional and case-comparison between non-adherent and adherent participants was used to examine the effects of socio-demographic and medication factors on non-adherence to ARV medications. A case was defined as non-adherent child with adherence level < 95% based on pill count and measurement of liquid formulations. The comparison group consisted of children with adherence levels ≥95%.^ A total of 842 participants met the eligibility criteria for determination of the prevalence of non-adherence and 338 participants (169 cases and 169 individuals) were used in the analysis to estimate the effects of factors on non-adherence. ^ Univariate and multivariable logistic regression were used to estimate the association between non-adherence (outcome) and socio-demographic and medication factors (exposures). The prevalence of non-adherence for participants on first line ARV medications was 20.0% (169/842).^ Increase in age (OR (95% CI): 1.10 (1.04–1.17) p = 0.001) was associated with nonadherence, while increase in number of caregivers (OR (95% CI): 0.72 (0.56–0.93) p = 0.01) and increase in number of monthly visits (OR (95% CI): 0.92 (0.86–0.99) p = 0.02), were associated with good adherence in both the unadjusted and the adjusted models. For the categorical variables, having more than two caregivers (OR (95% CI): 0.66 (0.28–0.84), p = 0.002) was associated with good adherence even in the adjusted model. ^ Conclusion. The prevalence of non-adherence to antiretroviral medicines among the study population was estimated to be 20.0%. In previous studies, adherence levels of ≥ 95% have been associated with better clinical outcomes and suppression of virus to prevent development of resistance. Older age, fewer numbers of caregivers and fewer monthly visits were associated with non-adherence. Strategies to improve and sustain adherence especially among older children are needed. The role of caregivers and social support should be investigated further.^

Design of mixed-longitudinal studies

Cross-sectional designs, longitudinal designs in which a single cohort is followed over time, and mixed-longitudinal designs in which several cohorts are followed for a shorter period are compared by their precision, potential for bias due to age, time and cohort effects, and feasibility. Mixed longitudinal studies have two advantages over longitudinal studies: isolation of time and age effects and shorter completion time. Though the advantages of mixed-longitudinal studies are clear, choosing an optimal design is difficult, especially given the number of possible combinations of the number of cohorts and number of overlapping intervals between cohorts. The purpose of this paper is to determine the optimal design for detecting differences in group growth rates.^ The type of mixed-longitudinal study appropriate for modeling both individual and group growth rates is called a "multiple-longitudinal" design. A multiple-longitudinal study typically requires uniform or simultaneous entry of subjects, who are each observed till the end of the study.^ While recommendations for designing pure-longitudinal studies have been made by Schlesselman (1973b), Lefant (1990) and Helms (1991), design recommendations for multiple-longitudinal studies have never been published. It is shown that by using power analyses to determine the minimum number of occasions per cohort and minimum number of overlapping occasions between cohorts, in conjunction with a cost model, an optimal multiple-longitudinal design can be determined. An example of systolic blood pressure values for cohorts of males and cohorts of females, ages 8 to 18 years, is given. ^

Statistical issues on studies of mixed longitudinal design

Mixed longitudinal designs are important study designs for many areas of medical research. Mixed longitudinal studies have several advantages over cross-sectional or pure longitudinal studies, including shorter study completion time and ability to separate time and age effects, thus are an attractive choice. Statistical methodology used in general longitudinal studies has been rapidly developing within the last few decades. Common approaches for statistical modeling in studies with mixed longitudinal designs have been the linear mixed-effects model incorporating an age or time effect. The general linear mixed-effects model is considered an appropriate choice to analyze repeated measurements data in longitudinal studies. However, common use of linear mixed-effects model on mixed longitudinal studies often incorporates age as the only random-effect but fails to take into consideration the cohort effect in conducting statistical inferences on age-related trajectories of outcome measurements. We believe special attention should be paid to cohort effects when analyzing data in mixed longitudinal designs with multiple overlapping cohorts. Thus, this has become an important statistical issue to address. ^ This research aims to address statistical issues related to mixed longitudinal studies. The proposed study examined the existing statistical analysis methods for the mixed longitudinal designs and developed an alternative analytic method to incorporate effects from multiple overlapping cohorts as well as from different aged subjects. The proposed study used simulation to evaluate the performance of the proposed analytic method by comparing it with the commonly-used model. Finally, the study applied the proposed analytic method to the data collected by an existing study Project HeartBeat!, which had been evaluated using traditional analytic techniques. Project HeartBeat! is a longitudinal study of cardiovascular disease (CVD) risk factors in childhood and adolescence using a mixed longitudinal design. The proposed model was used to evaluate four blood lipids adjusting for age, gender, race/ethnicity, and endocrine hormones. The result of this dissertation suggest the proposed analytic model could be a more flexible and reliable choice than the traditional model in terms of fitting data to provide more accurate estimates in mixed longitudinal studies. Conceptually, the proposed model described in this study has useful features, including consideration of effects from multiple overlapping cohorts, and is an attractive approach for analyzing data in mixed longitudinal design studies.^