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Biomedical subjects

T Holmes

Publications and source records attributed to T Holmes.

At least 19 recordsLinked to original sources
PubMed2003

Evolving neurovascular relationships in the RCS rat with age.

PURPOSE: To examine the course of development of vascular disorders in the Royal College of Surgeons (RCS) rat and how these may lead to retinal ganglion cell loss. METHODS: Whole-mount retinae from RCS rats were first stained for neurofilament protein and then for NADPH-diaphorase staining. A separate group of RCS rats was injected with Type II Peroxidase and the retinae were subsequently processed for peroxidase histochemistry. RESULTS: The first changes in the deep vascular plexus occur as the photoreceptor layer is lost and it comes into close proximity to the retinal pigment epithelial (RPE) cell layer. RPE cells migrate onto retinal vessels, and at such locations vascular complex develop. These are first found ventral to the optic nerve head and then gradually progress over most of the retina. The inner retinal vessels that supply the complexes cross the optic nerve fiber layer and appear to be under tension. They ligate axons, which leads to retinal ganglion cell loss. CONCLUSIONS: These observations show vascular changes can have secondary repercussions for neurons distant from the primary lesion.

Aging↗
PubMed2001

Optimized intensity-modulated arc therapy for prostate cancer treatment.

We recently implemented intensity-modulated arc therapy (IMAT) at our institution. In this study, we evaluate the dosimetric merits of the application of this technique to the treatment of prostate cancer. Each IMAT treatment plan incorporated bilateral overlapping arcs. The dose from each beam segment was computed using the three-dimensional dose model of a clinical treatment planning system (Render Plan 3.5, Precision Therapy). The weights assigned to the individual arc segments were optimized using a gradient search method. For 12 patients, comparisons were made between the IMAT treatment plans and corresponding plans using fixed cone-beam intensity-modulated radiotherapy (IMRT) from a commercial inverse planning system (CORVUS, NOMOS Corp.). We found that the optimized IMAT treatments produced similar dose distributions to the IMRT deliveries. Compared with the IMRT treatments, the IMAT treatments produced slightly less target dose homogeneity with consistently greater sparing of the rectum in regions of lower dose. The trade-off between target dose conformity and rectum sparing can be adjusted in both optimization procedures. Because the total beam-on time for IMAT delivery is 1 to 2 minutes with approximately 5-6 minutes of patient setup time, the delivery efficiency of the IMAT treatment was significantly better than the multiple-beam IMRT treatment.

Humans↗
PubMed2001

Analysing collimator structure effects in head-scatter calculations for IMRT class fields using scatter raytracing.

The frequent blocking of the irradiated volume in intensity modulated radiation therapy (IMRT) makes the head-scatter fraction of the incident photon fluence more significant than that in conventional therapy with open fields. On the other hand. certain collimator configurations block scatter photons directed to a given observation point while allowing primary photons to be transmitted. The 'anomalous blocking' makes the primary field a poor indicator of the scatter fluence. Since large MU-to-cGy ratios in IMRT can magnify head-scatter uncertainties, it becomes necessary to accurately model both the effective scatter source and the collimator structure that limits the scatter reaching the irradiated volume. First we obtain a dual-source model, using a Taylor series expansion to derive the effective scatter source distribution from the data measured for the Elekta SL20 linac equipped with a multi-leaf collimator (MLC). Then, using a raytracing algorithm, we calculate the transmission of scatter rays from the effective scatter source plane to points in the patient plane. The method can account for the anomalous blocking of scatter by the MLC leaves and the backup diaphragms. For a variety of collimator settings tested, the calculations agree with measurements to an accuracy of 0.002psi10 x 10, where psi10 x 10 is the total (primary + scatter) photon fluence of an open 10 x 10 cm2 field for the same MU delivered. Although the significance of collimator structure in IMRT depends strongly on fields shapes employed for the delivery, potential cumulative errors on the order of a few per cent can be avoided in fluence calculations if the proposed method is used.

Algorithms↗
PubMed2000

PTSD and utilization of medical treatment services among male Vietnam veterans.

This study investigated the effect of posttraumatic stress disorder (PTSD) on help-seeking for physical problems. Merging two large data sets resulted in a sample of 1773 male Vietnam veterans from white, black, Hispanic, Native Hawaiian, and Japanese American ethnic groups. Predictors of utilization included PTSD, other axis I disorders, and substance abuse. In analyses that adjusted only for age, PTSD was related to greater utilization of recent and lifetime VA medical services, and with recent inpatient care from all sources. Further analysis showed that the increased utilization associated with PTSD was not merely due to the high comorbidity between PTSD and other axis I disorders. The uniqueness of the association between PTSD and medical utilization is discussed in terms of somatization and physical illness.

Adult↗
PubMed2000

Predictors for people's response to a tornado warning: Arkansas, 1 March 1997.

On 1 March 1997, powerful tornadoes touched down in Arkansas (USA) on a Saturday afternoon. Twenty-six fatalities and 400 non-fatal injuries were reported. We performed a population-based cross-sectional study to determine factors associated with appropriate responses to tornado warnings. Of 146 survey participants, 140 (96 per cent) knew the difference between 'tornado watch' and 'tornado warning' and were aware of when the warning was announced. Of those 140 participants, 64 (45.7 per cent) responded to the warning by seeking shelter, and 58 (90.6 per cent) of those 64 acted within five minutes of hearing the warning. Four factors were positively associated with those seeking shelter: having graduated from high school (OR = 4.2, 95 per cent CI = 1.1-15.5); having a basement in one's house (OR = 3.8, 95 per cent exact CI = 1.1-17.1); hearing a siren (OR = 4.4, 95 per cent CI = 1.3-18.9); and having prepared a household plan of response when tornadoes occur (OR = 2.6, 95 per cent CI = 1.1-6.3). On the basis of these findings, we recommend: first, that people who live in tornadoprone areas have a personal plan of action to help them respond immediately to warnings; second, public-health education officials in areas with frequent tornadic activity should do more to educate the public about what they can do to protect themselves from a tornado; and third, that emergency-management officials planning protection measures for vulnerable communities should consider that most people have limited time (our study documented five minutes) in which to respond to a tornado warning. Thus, shelters in tornado-prone areas should be quickly accessible by residents.

Arkansas↗
PubMed2000

A systematic evaluation of air cavity dose perturbation in megavoltage x-ray beams.

The EGS4 Monte Carlo radiation transport code was used to systematically study the dose perturbation near planar and cylindrical air cavities in a water medium irradiated by megavoltage x-ray beams. The variables of the problem included x-ray energy, cavity shape and dimension, and depth of the cavity in water. The Monte Carlo code was initially validated against published measurements and its results were found to agree within 2% with the published measurements. The study results indicate that the dose perturbation is strongly dependent on x-ray energy, field size, depth, and size of cavity in water. For example, the Monte Carlo calculations show dose reductions of 42% and 18% at 0.05 and 2 mm, respectively, beyond the air-water interface distal to the radiation source for a 3 cm thick air slab irradiated by a single 5x5 cm2 15 MV beam. The dose reductions are smaller for a parallel-opposed pair of 5x5 cm2 15 MV x-ray beams, being 21% and 11% for the same depths. The combined set of Monte Carlo calculations showed that the dose reduction near an air cavity is greater for: (a) Smaller x-ray field size, (b) higher x-ray energy, (c) larger air-cavity size, and (d) smaller depth in water where the air cavity is situated. A potential clinical application of these results to the treatment of prostate cancer is discussed.

Air↗
PubMed1999

Dosimetric verification of a commercial inverse treatment planning system.

A commercial three-dimensional (3D) inverse treatment planning system, Corvus (Nomos Corporation, Sewickley, PA), was recently made available. This paper reports our preliminary results and experience with commissioning this system for clinical implementation. This system uses a simulated annealing inverse planning algorithm to calculate intensity-modulated fields. The intensity-modulated fields are divided into beam profiles that can be delivered by means of a sequence of leaf settings by a multileaf collimator (MLC). The treatments are delivered using a computer-controlled MLC. To test the dose calculation algorithm used by the Corvus software, the dose distributions for single rectangularly shaped fields were compared with water phantom scan data. The dose distributions predicted to be delivered by multiple fields were measured using an ion chamber that could be positioned in a rotatable cylindrical water phantom. Integrated charge collected by the ion chamber was used to check the absolute dose of single- and multifield intensity modulated treatments at various spatial points. The measured and predicted doses were found to agree to within 4% at all measurement points. Another set of measurements used a cubic polystyrene phantom with radiographic film to record the radiation dose distribution. The films were calibrated and scanned to yield two-dimensional isodose distributions. Finally, a beam imaging system (BIS) was used to measure the intensity-modulated x-ray beam patterns in the beam's-eye view. The BIS-measured images were then compared with a theoretical calculation based on the MLC leaf sequence files to verify that the treatment would be executed accurately and without machine faults. Excellent correlation (correlation coefficients > or = 0.96) was found for all cases. Treatment plans generated using intensity-modulated beams appear to be suitable for treatment of irregularly shaped tumours adjacent to critical structures. The results indicated that the system has potential for clinical radiation treatment planning and delivery and may in the future reduce treatment complexity.

Algorithms↗
PubMed1999

Marijuana use among American Indian adolescents: a growth curve analysis from ages 14 through 20 years.

OBJECTIVE: To describe the developmental course of marijuana use among a group of American Indian adolescents, aged 14 through 20 years. METHOD: A group of 1,766 American Indian adolescents from 3 culture groups provided repeated measures of 30-day marijuana use twice a year across a 3-year period. Linking 5 age cohorts, hierarchical linear modeling was used to model a curvilinear trajectory of marijuana use. Gender and community differences were examined as well. RESULTS: Support was found for a "maturational" model of marijuana use across time: Use increased in middle adolescence, peaked in later adolescence, and began to decrease in early adulthood. Both gender and community differences in trajectories were significant as well. CONCLUSIONS: Marijuana use among American Indian adolescents follows a clear developmental trajectory. Growth curve analysis can provide an additional tool for studying the effects of interventions that may not be apparent in a traditional evaluation design.

Adolescent↗
PubMed1994

A comparison of three inverse treatment planning algorithms.

Three published inverse treatment planning algorithms for physical optimization of external beam radiotherapy are compared. All three algorithms attempt to minimize a quadratic objective function of the dose distribution. It is shown that the algorithms are based on the common framework of Newton's method of multi-dimensional function minimization. The approximations used within this framework to obtain the different algorithms are described. The use of these algorithms requires that the number of weights of elemental dose distributions be equal to the number of sample points taken in the dose volume. The primary factor in determining how the algorithms are implemented is the dose computation model. Two of the algorithms use pencil beam dose models and therefore directly optimize individual pencil beam weights, whereas the third algorithm is implemented to optimize groups of pencil beams, each group converging upon a common point. All dose computation models assume that the irradiated medium is homogeneous. It is shown that the two different implementations produce similar results for the simple optimization problem of conforming dose to a convex target shape. Complex optimization problems consisting of non-convex target shapes and dose limiting structures are shown to require a pencil beam optimization method.

Algorithms↗
PubMed1994

A filtered backprojection dose calculation method for inverse treatment planning.

An efficient method of calculating dose distributions in homogeneous media for megavoltage photons is described. The method is similar to filtered backprojection image reconstruction and is based on the analogy between external beam radiotherapy and SPECT image reconstruction. The filtered backprojection dose calculation significantly reduces the computation time for a large number of x-ray beams compared to a conventional convolution dosimetry method. A factor of 20 reduction in computation time is demonstrated for a 2D implementation of the model. The method has proved useful for speeding up an inverse treatment planning algorithm for conformal radiotherapy, and has the potential to be implemented in the reconstruction hardware of a radiotherapy CT simulator. Results of computer simulations based on the model are presented.

Humans↗
PubMed1994

Relations of exercise to body image and sexual desirability among a sample of university students.

College students (N = 212) completed a survey designed to elicit information about their exercise habits, perceived sexual desirability, and body image. A number of body-image items were associated with both perceived physical attractiveness and perceived sexual desirability. Total body image was related to gender, exercise level, perceived physical attractiveness, perceived sexual desirability, and perception of self as a sexual partner.

Adolescent↗
PubMed1993

Spontaneous assembly of a self-complementary oligopeptide to form a stable macroscopic membrane.

A 16-residue peptide [(Ala-Glu-Ala-Glu-Ala-Lys-Ala-Lys)2] has a characteristic beta-sheet circular dichroism spectrum in water. Upon the addition of salt, the peptide spontaneously assembles to form a macroscopic membrane. The membrane does not dissolve in heat or in acidic or alkaline solutions, nor does it dissolve upon addition of guanidine hydrochloride, SDS/urea, or a variety of proteolytic enzymes. Scanning EM reveals a network of interwoven filaments approximately 10-20 nm in diameter. An important component of the stability is probably due to formation of complementary ionic bonds between glutamic and lysine side chains. This phenomenon may be a model for studying the insoluble peptides found in certain neurological disorders. It may also have implications for biomaterials and origin-of-life research.

Amino Acid Sequence↗
PubMed1991

A unified approach to the optimization of brachytherapy and external beam dosimetry.

Semi-automated optimization of dose distributions is possible using techniques borrowed from imaging science. The ideal distribution of dose is first deconvolved by a convolution kernel yielding an ideal weighting distribution in the patient. The weighting distribution describes the total energy released per unit mass of the irradiated medium. For internal and external radiation sources, this is directly related to the amount and distribution of radioactivity and energy fluence in the medium, respectively. For external sources, the exponential Radon transform is used to obtain ideal fluence projections incident on the patient. In both instances negative values are produced, which when set to zero result in perturbed dose distributions. This may necessitate iterative techniques to reduce the 'residual dose' produced by the zeroing process. Application of the approach is presented for the optimization of 2-dimensional dose distributions in external beam therapy, radioimmunotherapy, and brachytherapy sources.

Brachytherapy↗
PubMed1988

A low-cost method for simultaneous video-recording of ambulant subject and electroencephalograph: the Quarrier's system.

A system is described by means of which the ambulant subject may be video-filmed over several hours with the electroencephalograph superimposed on the same tape, allowing correlations to be observed in the active state. Our first subjects have been girls with Rett syndrome. Requirements are: one portable video camera/recorder, a conventional e.e.g. machine, the 'Medilog 9000' system for prolonged ambulatory recording (Oxford Medical), a specialized video mixer (videogram ForA FVG600, commercially available) a video timer, a conventional video cassette recorder and a television monitor. The cost is small for a department already equipped to carry out ordinary ambulatory e.e.g. monitoring. The method is safe, well tolerated, simple to use and adaptable. To the best of the authors' knowledge, they are the first to use this system and they believe that it should find wider application when it is desirable to correlate the behaviour of a subject with physiological parameters, on a restricted budget.

Ambulatory Care↗
PubMed1988

Diet-induced changes in trough theophylline concentrations in an elderly asthmatic patient.

A 66-year old obese man with asthma was given a hypocaloric (1100 Kcal) and low protein (35 g) diet for nine days. While receiving theophylline (Theodur), 200 mg bid, his morning trough theophylline concentrations rose from 3.40 micrograms/ml to 12.7 micrograms/ml by day 9 of this diet. Following discontinuation, his theophylline concentration fell to 5.95 micrograms/ml by day 6 on home diet. The patient lost 3.67 kg during the nine-day study. Thus, a brief exposure to a hypocaloric, low protein diet in this elderly patient with asthma caused a dramatic rise in trough theophylline concentrations.

Aged↗
PubMed1986

The responsiveness of rabbit bronchial rings to antigen, AGEPC and histamine.

Rings of intrapulmonary bronchi isolated from rabbits producing anti-horseradish peroxidase IgE antibodies contracted when exposed to antigen. The contractile response had a lag period of about 1 min, reached a peak at 6 min and then subsided. Bronchi from rabbits with detectable levels of specific IgG (in addition to IgE) antibodies did not differ in response from those with undetectable specific IgG levels. Histamine also contracted rabbit intrapulmonary bronchi with an EC50 of 10 micro (SD 1.29). The response to antigen was completely inhibited with chlorpheniramine (30 microM). In contrast to intrapulmonary bronchi, responsiveness of mainstem bronchi to antigen was observed only occasionally, whereas histamine was equipotent on both mainstem and intrapulmonary bronchi. Thus, the amount of antigen-induced mediator release may be less in the mainstem bronchi. Acetyl glyceryl ether phosphorylcholine, in concentrations up to 10 microM, did not contract either mainstem or intrapulmonary bronchi. This study indicates that histamine is the major mediator of (and acetyl glyceryl ether phosphorylcholine does not significantly participate in) antigen-induced contraction of isolated bronchi from IgE-producing rabbits. The results provide a likely mechanism for the increase in pulmonary resistance observed in IgE anaphylaxis in this species.

Animals↗