Biomedical subjects

R A Siwek

Publications and source records attributed to R A Siwek.

5 recordsLinked to original sources

PubMed1991

Shielding effects of Selectron applicator and pellets on isodose distributions.

The Selectron is a mobile remote afterloading device which was developed specifically to reduce the radiation exposure to hospital personnel during intracavitary brachytherapy. Applicators for these devices contain source trains of 48 spherical pellets per channel, composed of both active and inactive pellets which have an external diameter of 2.5 mm. The configuration of pellets is chosen to optimize the dose distribution in accordance with the patient's geometry. A new dose calculation model has been proposed based on the separation of radiation into primary and secondary components. This model takes into account the attenuation of the primary radiation by the pellets. Isodose distributions around a Selectron standard applicator containing a typical patient loading of active pellets were measured and compared with those calculated by program SEEDS of the Atomic Energy of Canada Ltd. (AECL) Theraplan computer planning system. The shielding effects of the stainless steel applicator tubes, the ovoids, screws, inactive and active pellets will be considered. The effects of scatter and attenuation by the pellets will be illustrated by measured isodose distributions around a single applicator tube.

Brachytherapy↗

PubMed1991

A control method for a nonlinear multivariable system: application to interstitial laser hyperthermia.

An original adaptive control method is presented for controlling a nonlinear multivariable system. The method, which could be described as a modified quasi-linear approach, involves dividing the source excitation into a series of pulsing rounds and is implemented as a control algorithm on a computer. The theory underlying the method is developed with reference to an application involving temperature control in interstitial laser hyperthermia. In this application, the method is both successful and necessary to achieve optimally uniform elevated temperatures in a ground beef phantom. Apart from variable and parameter definitions, the method is otherwise general and might be useful for controlling a nonlinear system in which no prior exact characterization of the system is possible. Simulations were conducted to assess the effectiveness of the method in systems for which the unit excitation response changes by factors ranging from zero to three over the total period of excitation. In each case the method has proven stable.

Algorithms↗

PubMed1987

Body composition of fasting obese patients measured by in vivo neutron activation analysis and isotopic dilution.

The masses of fat, water, protein and minerals in ten obese patients (seven women and three men) have been measured, by in vivo neutron activation analysis and isotopic dilution, before and after four weeks of starvation. Mass of skeletal muscle was estimated from measured total body potassium (TBK) and nitrogen (TBN). Before starvation the patients weighed, on average, 56.5 kg more than predicted from sex, age and height. The composition of their excess tissue was consistent with that of adipose tissue. During starvation the patients lost, on average, 12.1 kg of tissue containing more water and potassium than adipose tissue. Regression analysis of the measured changes in potassium and protein suggested that approximately 230 mmol of potassium was lost independently of protein, the remainder being lost with 'non-muscle' fat-free tissue. Because some potassium was lost without protein, the change in muscle mass calculated from the changes in TBK and TBN overestimated the true mass of muscle lost. After starvation the patients still had 44.5 kg of excess tissue. Their composition was consistent with an excess of adipose tissue and a deficit of potassium.

Adolescent↗

PubMed1985

Thermic effect of feeding carbohydrate, fat, protein and mixed meal in lean and obese subjects.

The thermic effect of 1.67 MJ (400 kcal) of carbohydrate (glucose), fat, protein and mixed meal were examined in 11 lean and 11 obese subjects by indirect calorimetry. The changes in metabolic rate in response over 90 min period (30-120 min after the meal) to the different meals were compared with that seen after a similar volume of low calorie drink. The thermic effects of glucose and protein were not significantly different between lean and obese subjects. Obese subjects showed very little increase in metabolic rate following ingestion of fat (-0.9 +/- 2.0%, mean +/- SEM) and this was significantly different from that seen in lean subjects (14.4 +/- 3.4%). The thermogenic response to mixed meal was also significantly lower in obese subjects when expressed as percentage change (12.9 +/- 2.3% compared to 25.0 +/- 4.8%). There was no evidence for delay in gastric emptying times for glucose and fatty meal in the six obese subjects in whom these were measured. We conclude that obese subjects show a reduced thermogenic response to fat.

Adult↗

PubMedDate not supplied

Measurements in high-intensity beams from medical linear accelerators.

There is a potential to produce very high doses and dose rates on dual mode (electron and x-ray capability) medical electron accelerators [C. J. Karzmark, Br. J. Radiol. 40, 697 (1967)]. Measurements were taken on two accelerators to determine the size of the doses which were possible. The experiments demonstrate the limitations of many of the commonly used dosimeters when they are used outside of the dose and dose rate ranges normally encountered in a radiotherapy department. Dose rates from 110 to 165 cGy/pulse were measured using ionization chamber, ferrous sulphate and thermoluminescent dosimetry, and water calorimetry.

Calorimetry↗