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Biomedical subjects

S Hyödynmaa

Publications and source records attributed to S Hyödynmaa.

14 recordsLinked to original sources
PubMed1998

Dose accuracy check of the 3D electron beam algorithm in a treatment planning system.

The accuracy of the recently implemented three-dimensional electron beam dose calculating algorithm in CADPLAN version 2.62 manufactured by Varian Dosetek was investigated. The algorithm uses a generalized Gaussian pencil beam model and the dose distributions are calculated as the sum of three weighted Gaussians. To use the calculating program in an optimum way, one needs to know the dose calculation accuracy of the algorithm as well as its limitations. This investigation includes comparisons of measured relative dose distributions with calculated dose distributions and also comparisons of measured and calculated monitor units. The geometries tested were quadratic fields, irregularly shaped fields, oblique fields, irregularly shaped phantom surfaces and internal heterogeneities and were most often irradiated with 8 and 20 MeV electrons. The results indicate that the algorithm is well suited for clinical three-dimensional dose planning. Some deviations occurred but they were most often within the limits of international criteria of acceptability.

Algorithms↗
PubMed1997

Quantification of mean energy and photon contamination for accurate dosimetry of high-energy electron beams.

The scientific background of the standard procedure for determination of the mean electron energy at the phantom surface (E0) from the half-value depth (R50) has been studied. The influence of energy, angular spread and range straggling on the shape of the depth dose distribution and the R50 and Rp ranges is described using the simple Gaussian range straggling model. The relation between the R50 and Rp ranges is derived in terms of the variance of the range straggling distribution. By describing the mean energy imparted by the electrons both as a surface integral over the incident energy fluence and as a volume integral over the associated absorbed dose distribution, the relation between E0 and different range concepts, such as R50 and the maximum dose and the surface dose related mean energy deposition ranges, Rm and R0, is analysed. In particular the influence of multiple electron scatter and phantom generated bremsstrahlung on R50 is derived. A simple analytical expression is derived for the ratio of the incident electron energy to the half-value depth. Also, an analytical expression is derived for the maximum energy deposition in monoenergetic plane-parallel electron beams in water for energies between 2 and 50 MeV. Simple linear relations describing the relative absorbed dose and mass ionization at the depth of the practical range deposited by the bremsstrahlung photons generated in the phantom are derived as a function of the incident electron energy. With these relations and a measurement of the extrapolated photon background at Rp, the treatment head generated bremsstrahlung distribution can be determined. The identification of this photon contamination allows an accurate calculation of the absorbed dose in electron beams with a high bremsstrahlung contamination by accounting for the difference in stopping power ratios between a clean electron beam and the photon contamination. The absorbed dose determined using ionization chambers in heavily photon contaminated (10%) electron beams may be too low--by as much as 1.5%--without correction.

Electrons↗
PubMed1997

Optimization of 3D conformal electron beam therapy in inhomogeneous media by concomitant fluence and energy modulation.

The possibilities of using simultaneous fluence and energy modulation techniques in electron beam therapy to shape the dose distribution and almost eliminate the influences of tissue inhomogeneities have been investigated. By using a radiobiologically based optimization algorithm the radiobiological properties of the tissues can be taken into account when trying to find the best possible dose delivery. First water phantoms with differently shaped surfaces were used to study the effect of surface irregularities. We also studied water phantoms with internal inhomogeneities consisting of air or cortical bone. It was possible to improve substantially the dose distribution by fluence modulation in these cases. In addition to the fluence modulation the most suitable single electron energy in each case was also determined. Finally, the simultaneous use of several preselected electron beam energies was also tested, each with an individually optimized fluence profile. One to six electron energies were used, resulting in a slow improvement in complication-free cure with increasing number of beam energies. To apply these techniques to a more clinically relevant situation a post-operative breast cancer patient was studied. For simplicity this patient was treated with only one anterior beam portal to clearly illustrate the effect of inhomogeneities like bone and lung on the dose distribution. It is shown that by using fluence modulation the influence of dose inhomogeneities can be significantly reduced. When two or more electron beam energies with individually optimized fluence profiles are used the dose conformality to the internal target volume is further increased, particularly for targets with complex shapes.

Algorithms↗
PubMed1996

The role of phantom and treatment head generated bremsstrahlung in high-energy electron beam dosimetry.

An analytical expression has been derived for the phantom generated bremsstrahlung photons in plane-parallel monoenergetic electron beams normally incident on material of any atomic number (Be, H2O, Al, Cu and U). The expression is suitable for the energy range from 1 to 50 MeV and it is solely based on known scattering power and radiative and collision stopping power data for the material at the incident electron energy. The depth dose distribution due to the bremsstrahlung generated by the electrons in the phantom is derived by convolving the bremsstrahlung energy fluence produced in the phantom with a simple analytical energy deposition kernel. The kernel accounts for both electrons and photons set in motion by the bremsstrahlung photons. The energy loss by the primary electrons, the build-up of the electron fluence and the generation, attenuation and absorption of bremsstrahlung photons are all taken into account in the analytical formula. The longitudinal energy deposition kernel is derived analytically and it is consistent with both the classical biexponential relation describing the photon depth dose distribution and the exponential attenuation of the primary photons. For comparison Monte Carlo calculated energy deposition distributions using ITS3 code were used. Good agreement was found between the results with the analytical expression and the Monte Carlo calculation. For tissue equivalent materials, the maximum total energy deposition differs by less than 0.2% from Monte Carlo calculated dose distributions. The result can be used to estimate the depth dependence of phantom generated bremsstrahlung in different materials in therapeutic electron beams and the bremsstrahlung production in different electron absorbers such as scattering foils, transmission monitors and photon and electron collimators. By subtracting the phantom generated bremsstrahlung from the total bremsstrahlung background the photon contamination generated in the treatment head can be determined to allow accurate dosimetry of heavily photon contaminated electron beams.

Electrons↗
PubMed1996

Optimization of conformal electron beam therapy using energy- and fluence-modulated beams.

Fluence modulation of multiple electron beams of various energies has been used to optimize the delivered dose distribution during electron beam radiation therapy. By maximizing the probability of achieving tumor control without causing severe complications electron beam fluence profiles have been optimized for superficial target volumes. It is possible to use several equiportal fluence-modulated electron beams to modify the energy deposition with depth in a controlled manner making it possible to use the technique as an alternative to bolus. The technique was tested in two representative phantom geometries and in three clinical patient geometries using a set of five and two different energies. The local maxima in dose for the plans with five energies were typically lower than with the conventional or advanced bolus techniques. The principles for how the technique could be carried out in the future with a fourth generation radiotherapy accelerator are also indicated.

Electrons↗
PubMed1995

An improved energy-range relationship for high-energy electron beams based on multiple accurate experimental and Monte Carlo data sets.

A theoretically based analytical energy-range relationship has been developed and calibrated against well established experimental and Monte Carlo calculated energy-range data. Only published experimental data with a clear statement of accuracy and method of evaluation have been used. Besides published experimental range data for different uniform media, new accurate experimental data on the practical range of high-energy electron beams in water for the energy range 10-50 MeV from accurately calibrated racetrack microtrons have been used. Largely due to the simultaneous pooling of accurate experimental and Monte Carlo data for different materials, the fit has resulted in an increased accuracy of the resultant energy-range relationship, particularly at high energies. Up to date Monte Carlo data from the latest versions of the codes ITS3 and EGS4 for absorbers of atomic numbers between four and 92 (Be, C, H2O, PMMA, Al, Cu, Ag, Pb and U) and incident electron energies between 1 and 100 MeV have been used as a complement where experimental data are sparse or missing. The standard deviation of the experimental data relative to the new relation is slightly larger than that of the Monte Carlo data. This is partly due to the fact that theoretically based stopping and scattering cross-sections are used both to account for the material dependence of the analytical energy-range formula and to calculate ranges with the Monte Carlo programs. For water the deviation from the traditional energy-range relation of ICRU Report 35 is only 0.5% at 20 MeV but as high as -2.2% at 50 MeV. An improved method for divergence and ionization correction in high-energy electron beams has also been developed to enable use of a wider range of experimental results.

Electrons↗
PubMed1994

Comparison of performance standards for radiation beam uniformity characteristics of a linear accelerator.

The recent international standards published by the International Electrotechnical Commission (IEC) on the performance of medical electron accelerators describe suggested test procedures for the performance of radiotherapy accelerators. The recommendations of the Nordic Association of Clinical Physics for testing of the radiation beam characteristics include test conditions, methods and suggested tolerances that are different from those of the IEC. In this work the two publications were compared for acceptance testing of a Philips SL25 linear accelerator. It is important to gain experience on the practical use of these standards.

Europe↗
PubMed1989

Testing of autonomic cardiovascular regulation--methodological considerations.

The different analyses of the results on autonomic nervous function tests were evaluated in 43 male and 32 female diabetic patients and in 24 male and 24 female control subjects, aged 47-67 years, all without any known heart disease. The Valsalva ratio of the first effort did not differ from the mean Valsalva ratio of three efforts. During deep breathing, heart rate variation and max/min R-R interval ratio determined from the first three breathing cycles did not differ from the respective variables calculated from six consecutive breathing cycles. Diastolic blood pressure response to isometric handgrip was greater during the third minute than during the first and the first two minutes. In conclusion, the tests for the evaluation of autonomic nervous function can be simplified without losing their diagnostic value.

Aged↗
PubMed1985

Estimation of parameters affecting the uptake of 99mTc-methylenediphosphonate in rat femur with model simulation.

The uptake of 99mTc-methylenediphosphonate (MDP) in different parts of rat femur was simulated using a local three-space model for tracer transfer. The model consisted of bone blood, bone ECF-space and space for tracer deposition. The measured 99mTc-MDP concentration in the systemic blood and the local bone blood flow measured by 131I-macroaggregated albumin microspheres were used as input parameters. The measured blood flow values were 6.3, 3.1 and 15.3 ml/100 g/min for proximal, middle and distal femur, respectively. The model parameters that gave the best fit to measured 99mTc-MDP uptake curves in computer simulation showed that bone blood flow, volume of ECF-space, permeability surface area product and accretion constant from ECF-space to space for tracer deposition were highest in distal and lowest in middle femur. The values corresponded to peak extraction fractions of 0.38, 0.62, and 0.31 for proximal, middle and distal femur, respectively. We conclude that the simulation gives acceptable model parameters, and indicates applicability of a similar model into clinical quantitative bone scintigraphy.

Animals↗
PubMed1985

Exercise capacity in subjects with high oxygen affinity.

Exercise capacity and hemodynamic parameters were measured in ten patients with a previously unpublished variant of hemoglobin (Hb-Linköping) and in ten age- and sex-matched controls. Bicycle ergometer test was almost maximal and the indices of working capacity and cardiac tolerance were similar in patients and controls. The hemoglobin dissociation curve was shifted to the left at rest and after exercise the shift to right was half of the corresponding shift in controls. Lactate accumulation during exercise was similar in patients and controls, also the elimination rate seems identical, but requires further studies. Hydrogen ion production in the patients during exercise was more marked than in the controls. If this is a sign of altered energy metabolism remains to be ascertained in future studies. In acute short-term exercise the patients with abnormally high hemoglobin oxygen affinity seem to have similar tolerance and cardiovascular load. Aberrant metabolism and adaptation to relative hypoxia in spite of erythrocytosis may have long-term effects that require long-term follow-up of these patients.

Adult↗
PubMed1985

Quantitative radioisotope measurement of duodenogastric reflux in patients with ulcer or gastrectomized for ulcer.

In this work the duodenogastric reflux was quantified as the amount of radioactivity entering the stomach after an i.v. administration of 99mTc-HIDA in ulcer patients and in patients who had undergone BI gastrectomy. The results were compared with visual evidence of gastric activity in the gamma camera images and biochemical determination of gastric bile reflux. The method is useful in quantifying the reflux if the activity is above the background activity. It allows the determination of an upper limit for the reflux when the reflux is evident visually. Only two or three images are needed for the quantitation. No correlation was found between biochemical measurement of fasting bile reflux in the stomach and radioisotopic quantification.

Adult↗
PubMed1983

Increased bone marrow blood flow in polycythemia vera.

Bone marrow blood flow was measured in polycythemia vera, in compensatory and in relative polycythemia with a 133Xe washout method. In the treated polycythemia vera bone marrow blood flow was significantly increased compared with the age-matched controls. The fraction of blood flow entering the bone and flowing through the hematopoietic marrow was markedly increased in both the untreated and the treated polycythemia vera. Although the number of observations in compensatory and relative polycythemia was small, the results suggest that bone marrow blood flow is not markedly increased in these diseases. The results also suggest that in older patients the simple 133Xe method may support the diagnosis of polycythemia vera.

Adult↗
PubMed1982

Quantitative sacro-iliac scintigraphy. I. Methodological aspects.

Methodological aspects of quantitative sacroiliac scintigraphy (QSS) using [99mmTc]methylene diphosphonate (MDP) were studied. To improve the diagnostic value of QSS it is important to understand the errors involved in the calculated indices. The accumulated radioactivity in the sacro-iliac joint, compared with that in the sacral bone (SI ratio), decreased linearly with age, by about 24% from the age of 15 years to 71 years. The post-injection time of scintigraphy was not critical, if the scintigraphy was carried out after 2.5 hours. Although the variances of the SI ratios in our control material were relatively high, the repeatability of the SI ratio measurement was 3.9%. The influence of measuring geometry on the QSS and the acceptability of the sacrum as the reference area were tested.

Adolescent↗