Search PubMed⌕ Search

Biomedical subjects

P Wust

Publications and source records attributed to P Wust.

At least 91 records · Page 5Linked to original sources
PubMed1995

Hyperthermia--its actual role in radiation oncology. Part IV: Thermo-radiotherapy for malignant brain tumors.

BACKGROUND: High-grade malignant gliomas have a dismal prognosis. Only achievement of long-term local control improves overall survival. Conventional treatments have not been successful, but thermoradiotherapy appears to be a promising new approach to this disease. PATIENTS AND METHODS: The review addresses the various HT techniques applied for brain heating, the biological rationale and experimental studies supporting the use of heat in addition to radiotherapy for brain tumors. Normal brain damage exposed to heat is critically evaluated. Clinical trials implementing HT for tumors of the brain accumulating more than 400 patients are discussed. RESULTS: HT is a feasible and effective approach to brain tumors. Important selection criteria are tumor size (< 6 cm) and location (supratentorial, peripheral). Acute treatment toxicity is relatively low and long-term side effects are similar to those observed for RT alone. In some studies a high response rate and improvement of survival was achieved. Tumor control and survival depend on histology (anaplastic astrocytoma vs. glioblastoma multiforme), extent of surgery (complete vs. incomplete), tumor status (primary vs. recurrent), age and performance status. Satisfactory heating also correlates significantly with favourable treatment outcome. CONCLUSIONS: The encouraging results of clinical trials may be biased by favourable tumor and patient selection. Randomized clinical trials comparing RT alone versus combined RT-HT for advanced and recurrent brain tumors are justified. Part I has covered biological and technical fundamentals of clinical hyperthermia and has been published in Strahlenther. Onkol. 168 (1992), 183-190. Part II has covered clinical fundamentals and results in superficial tumors of clinical hyperthermia and has been published in Strahlenther. Onkol. 169 (1993), 633-654. Part III has covered clinical rationale and results in deep seated tumors and has been published in Strahlenther. Onkol. 171 (1995), 251-264.

Animals↗
PubMed1993

3-D computation of E fields by the volume-surface integral equation (VSIE) method in comparison with the finite-integration theory (FIT) method.

An algorithm has been developed for calculation of 3-dimensional E fields by the volume-surface integral equation (VSIE) method. Integration over surface elements is performed by elementary analytical formulas, assuming a linear interpolation of surface charges. Grid points at electrical interfaces are split off, well considering the E field behavior at these contours, specifically at sharp bends and multimedia junctions. Averaging procedures are utilized in order to avoid undefined or infinite values at critical points. The VSIE is solved by iteration using GMRES ("general minimum residuum") solver on a SUN workstation SPARC-IPX or Cray XMP, whereby convergence speed decreases considerably as the heterogeneity of the problem increases. Computation time (e.g., 20 min on a supercomputer for approximately 30,000 cells) needs to be reduced by further code development. Results for 3-D test cases (plane wave illuminating a layered cylinder) generally agree well with the finite-integration-theory (FIT) method if high E field gradients occur perpendicular to electrical boundaries. The VSIE method predicts slightly higher E fields only in critical regions. On the other hand, the FIT method at present is more efficient with respect to computation time for large domains with high cell numbers (> 100,000 cells).

Algorithms↗
PubMed1991

[Determinant factors and disturbances in controlling power distribution patterns by the hyperthermia-ring system BSD-2000. 2. Measuring techniques and analysis].

Clinical observables and phantom measurements (part 1) have suggested that the control of power deposition patterns can still be improved for the hyperthermia system BSD-2000. This is addressed to system-specific phase errors as well as inadequacies of phase selection (target point method), which might be corrected by modifications of the manufacturer. Furthermore, frequency-dependent physical effects (coupling, mode excitation) are existing, which might cause distortions and asymmetries of current distribution on antennas and consequently deteriorate the power deposition pattern (e.g. focussing capability). The application of a network analyzer system is described in order to determine electrical material constants, phase errors, coupling coefficients, reflection coefficients and current distributions on antennas. The analysis of the measurement datas suggests that ring-applicator has a variable frequency-optimum (supposed around 80 ... 95 MHz) characterized by minimal coupling and asymmetries.

Humans↗
PubMed1990

[The influencing factors and interfering effects in the control of the power distributions with the BSD-20000 hyperthermia ring system. 1. The clinical observables and phantom measurements].

A new generation of annular-phased-array systems BSD-2000 has been clinically applied in a pilot study. Therapeutic intratumoral temperatures greater than 42 degrees C were obtained in 15/15 sessions with six patients. However, the control of power deposition pattern has to be improved in order to increase the therapeutic gain and to guarantee an efficient therapy. A retrospective analysis of clinical phenomena has been performed by phantom set-ups because the power deposition pattern cannot be determined during therapy. Phantom measurement techniques are outlined, specifically phantom materials and visualization of power distributions. The problem of focus balance and frequency choice is illustrated by self-developed phantoms (liquid crystal sheets, light-emitting-diode-arrays). Especially, the limitation of modeling calculations is demonstrated.

Evaluation Studies as Topic↗
PubMed1986

[Optimization of conditions for detecting density variations in focal activity in SPECT using the rotating gamma camera. Model calculations, phantom measurements and clinical applications].

The signal to noise ratio (quotient of contrast and mean noise) is a quality parameter for identifying activity gaps during single photon emission computed tomography (SPECT). Contrast and mean noise level are site-dependent on the reconstruction plane which is influenced in a complex manner by absorption conditions (strong or weak absorption), modes of rotation (full angle or partial angle imaging), interdependencies of opposing partial projections (arithmetic or geometric mean), magnitude of defect (relative to the resolution of the imaging system), geometry of the object to be measured, and scatter. In the present study the authors performed analysis and graphical representation of the SPECT imaging properties on the basis of model calculations illustrated by phantom measurements. The optimal conditions of examination or evaluation and the diagnostic criteria in SPECT of the heart, liver, brain and pelvis are discussed by comparative calculation of signal to noise ratios in defect identification.

Brain↗
PubMedDate not supplied

Three-dimensional monitoring of small temperature changes for therapeutic hyperthermia using MR.

Radiofrequency hyperthermia of deep-seated pelvic tumors requires noninvasive monitoring of temperature distributions in patients. Methods of MR thermography were reported to be a promising tool in solving this problem. However, to be truly useful for monitoring hyperthermia treatments, MR thermography should be able to cover the entire pelvis in acquisition times no longer than for a breath-hold (< or = 15 seconds) and to resolve small temperature differences (< 1 degrees C). Three methods exploiting the temperature dependence of spin-lattice relaxation time (T1), of self-diffusion coefficient (D), and of chemical shift of proton resonance frequency (PRF) were applied in phantom experiments; the pulse sequences were the T1-weighted gradient echo, the pulsed diffusion gradient spin echo made faster through the keyhole technique, and the gradient echo with the phase reconstruction, respectively. The high planar resolution was compromised, and instead, coarse and more isotropic voxels were used. Experiments were performed in two consecutive steps, thus imitating a possible scenario for monitoring hyperthermia. In the first step, calibration curves were recorded, which were then used in the second step to obtain maps of temperature changes. The results show a clear superiority of the PRF method, followed by the D and the T1 methods. The uncertainty of temperature changes predicted both from calibration curves and from maps was less than 1 degrees C only with the PRF and the D-based methods.

Calibration↗
PubMedDate not supplied

A pilot study of a new therapeutic approach in the treatment of locally advanced stages of rectal cancer: neoadjuvant radiation, chemotherapy and regional hyperthermia.

The synergistic effects of hyperthermia (temperatures > or = 41 degrees C) when combined with radiotherapy or cytotoxic drugs, as well as a modulation of tumour-related immunological phenomena have been demonstrated preclinically. Local or regional hyperthermia in combination with radiation or chemotherapy has been studied in patients during recent years, and has convincingly demonstrated that hyperthermia is feasible and tolerated by patients. Furthermore, there is strong evidence that hyperthermia may provide an improvement in local control as compared with radiotherapy or chemotherapy alone. Systems based on radiowave irradiation allow sufficiently tolerable and effective regional hyperthermic therapy in patients with rectal carcinomas. Used as part of curative pre-operative and postoperative multimodal therapeutic strategies in high-risk patients with locally advanced rectal carcinomas, hyperthermia may result in improved local control and a higher rate of sphincter-sparing procedures. 20 patients with non-resectable, locally advanced primary or recurring rectal carcinoma T3/4 entered a phase I/II study of pre-operative radiochemothermotherapy with folinic acid and 5-fluorouracil, radiation (45 Gy HD), as well as regional hyperthermia once a week followed by chemotherapy after surgery. The regimen proved to be sufficiently tolerable. Acute grade III or IV toxicities did not occur after hyperthermia. Tumour resections were performed on 14 of the 20 patients, with 13 being complete. In 9 of the carcinomas, downstaging compared with the pretherapeutic stage was achieved. In 3 of 6 patients with persistent non-resectable tumours, local control has now been maintained for more than 12 months. One patient progressed locally during neoadjuvant combination therapy. These results prompted the initiation of a prospective randomised study to evaluate the relative importance of regional hyperthermia in this setting.

Adult↗
PubMedDate not supplied

Feasibility and analysis of thermal parameters for the whole-body-hyperthermia system IRATHERM-2000.

The infrared system IRATHERM-2000, with water-filtered infrared A wavelength underwent 20 treatments of whole body hyperthermia in conjunction with chemotherapy. In all the sessions, the aimed systemic temperature (41.8 degrees C, maximum 42.0 degrees C) could be achieved and maintained for 60 min. Due to increasing clinical experience, the unnegligible local toxicity, exhibited as heat-induced superficial lesions, and neurotoxicity, could be reduced during the course of the study. Data from three other series accomplished at the von Ardenne Clinic, totalling 120 heat sessions, were available and included for a comparative analysis. Analysis of the toxicity shows that a correlation exists between thermal side-effects and heat-up periods (until steady-state), maximum temperatures, and superficial thermal doses. The time needed to reach the plateau seems to correlate with fluid loss, which, thus, indirectly influences toxicity, and most importantly the initial power level. The typical heat-up time in such a standard set-up amounts to 100-150 min, for a temperature rise from 37.5 to 42.0 degrees C. Evaluation of the energy balance reveals a highly patient-specific range for the reactive evaporation in the IRATHERM system, resulting in a power (heat) loss of up to 1400 W via sweat production of approximately 2 l/h. In order to counterbalance this effect, an accordingly high infrared power, ranging from 1200-1500 W, needs to be delivered, resulting in a significant thermal skin exposition. Concepts used to reduce the heat loss by reactive evaporation include prevention of convection by appropriate sealing of the heating chamber and increasing the humidity by a nebulizer. For the more trained user, the heat-up time can be considerably shortened, particularly, in the introductory phase of the heating process, by employing higher, but still tolerable, patient-specific power levels. However, such a strategy requires, due to higher risks, close monitoring of skin temperatures together with a considerable amount of clinical experience. The results of the IRATHERM pilot study were compared, not only with previous groups where the IRATHERM was applied, but also with results of various other investigators where the Enthermics Radiant Heat Device was employed. In the authors' opinion, improved understanding of the mechanisms and crucial parameters underlying whole body hyperthermia, will enable a controllable and tolerable therapy through proficient contribution to equipment and methods.

Body Temperature↗
PubMedDate not supplied

Electromagnetic phased arrays for regional hyperthermia: optimal frequency and antenna arrangement.

This paper investigates the effects of the three-dimensional arrangement of antennae and frequency on temperature distributions that can be achieved in regional hyperthermia using an electromagnetic phased array. It compares the results of power-based and temperature-based optimization. Thus, one is able to explain the discrepancies between previous studies favouring more antenna rings on the one hand and more antennae per ring on the other hand. The sensitivity of the results is analysed with respect to changes in amplitudes and phases, as well as patient position. This analysis can be used for different purposes. First, it provides additional criteria for selecting the optimal frequency. Secondly, it can be used for specifying the required phase and amplitude accuracy for a real phased array system. Furthermore, it may serve as a basis for technological developments in order to reduce both types of sensitivities described above.

Electromagnetic Fields↗
PubMedDate not supplied

Temperature data and specific absorption rates in pelvic tumours: predictive factors and correlations.

The system BSD 2000 has been in clinical use for regional hyperthermia for more than 10 years. Several technical details of this hyperthermia system, as well as the results of clinical studies employing this system have been investigated. The intention of this paper is to investigate the correlation between technical efficiency or feasibility of hyperthermia with the BSD 2000, in terms of power densities and temperatures depending upon parameters such as tumour histology, tumour location, patient age, patient sex, and patient cross section. The possible conclusions of predictive factors derived from the above correlations were closely scrutinized. Data acquired from 772 treatment sessions of 190 patients with pelvic tumours, mainly sarcomas and carcinomas of the rectum, cervix, prostate and anus, have been evaluated. For every session, index temperatures T90 (temperature attained at 90% of tumour related measurement points), cumulative minutes for T90 > Tref, tumour related power density (SAR: specific absorption rate, in W/kg) and the effective perfusion Weff (in ml/100 g min) were calculated. Temperatures were measured either invasively or endoluminally. The statistics software SPSS was employed subsequently for univariate, as well as multivariate analyses. The results exhibit that index temperatures mainly depend on the power density SAR and the hyperthermia induced effective perfusion. The total power P (in 100 W) and, complementarily, the relative power density absolute value(SAR) (= SAR/P) seem to have lesser influence. Clear differences between the tumour entities were established regarding their index temperatures and temperature distributions. SAR, Weff and P were correlated with several anatomical, biological and clinical factors. Sessions rendering low index temperatures and SAR values also revealed decreased individual tolerance to the treatment. This clearly displays that power-induced side effects define the limits of the efficiency of regional hyperthermia. Equivalent relationships and correlations are derived from intratumoural and endoluminal thermometry. Individual limitations of regional hyperthermia caused by anatomical, biological and clinical factors are liable to be difficult to overcome with the rather restricted potentials of the BSD 2000 system to control the SAR distribution.

Analysis of Variance↗
PubMedDate not supplied

Performance and use of current sheet antennae for RF-hyperthermia of a phantom monitored by 3 tesla MR-thermography.

Several MR-compatible current sheet antennae (CSA) of different height (h) (16 cm (l) x 8 cm (w) x 1-5 cm (h)) were built for simulated RF (96 MHz) hyperthermia of a medium-sized (12l) tissue-equivalent phantom inside a 3 tesla whole body tomograph. Prior to use, efficiencies of the CSA were determined by network analysis and by calorimetry. Depending on the height h of the CSA and on the thickness d(bolus) of the water bolus used for RF-coupling of the CSA to the lossy medium, their efficiency varied between 20-70% and the CSA with h = 3 cm was selected for simulated RF hyperthermia. During heating, spatial temperature distributions (20-42 degrees C) of five slices (voxel size 2 x 2 x 10mm(3)) were recorded intermittently within 4 s/slice by measuring the temperature dependent shift of the (1)H resonance frequency (125.32 MHz). A phased array consisting of two identical CSA produced distinctly different spatial temperature distributions at 0 and 180 degrees phase difference between both RF channels feeding the antennae. Within a one-dimensional heat diffusion model, the specific absorption rate (SAR) of the electromagnetic wave generated by a single antenna was deduced from the experimental data resulting in a penetration depth (1/e(2)) of approximately 4 cm.

Biophysical Phenomena↗
PubMedDate not supplied

Anaesthesiological experiences with whole body hyperthermia.

OBJECTIVE: In order to investigate the safety of whole body hyperthermia (WBH) within the context of systemic Cancer Multistep Therapy (sCMT) in patients with disseminated malignancies, cardiopulmonary changes and various organ functions were examined. METHODS AND PROCEDURES: Fifty-seven sCMT treatments were performed in 22 patients. WBH with a plateau phase of 1 h at 41.8 degrees C was induced by an IRATHERM 2000 device. Cardiopulmonary parameters were measured at 37, 40, 41.8 and 39 degrees C by use of a pulmonary artery catheter, femoral oxymetry and a radial artery catheter. Organ functions of the liver, kidney, cardiovascular and central nervous system were evaluated before and after treatment. RESULTS: Compared with the initial values, significant alterations were found of most cardiopulmonary parameters in the sense of hypercirculation at 41.8 degrees C. With the exception of extra vascular lung water index, all parameters showed a clear tendency towards the pre-treatment levels at 39 degrees C. In eight out of 57 sCMT treatments, reversible organ dysfunctions were observed. Comparison of radial and femoral arterial blood pressure showed significantly different values at 40 and 41.8 degrees C. CONCLUSIONS: WBH induces cardiovascular stress, but by careful selection of patients and appropriate anaesthesiological monitoring it can be performed safely using general anaesthesia. This enables further evaluation of WBH in multimodal treatment concepts.

Adult↗
PubMedDate not supplied

Influence of patient models and numerical methods on predicted power deposition patterns.

BACKGROUND: A hyperthermia planning system has been developed for generating patient and applicator models as well as calculating and visualizing E-field and temperature distributions. Significant dependencies on models and algorithms have been found. METHODS: Computerized tomography (CT) data sets are first transformed into so called 'labelled CT-volume'-data sets of equal resolution, which are used for segmentation. The first type of patient model obtained subsequently is based on regions with specified electrical properties representing tissues or organs (so called 'region-based model'). The second patient model renders a direct transformation of Hounsfield Units (HU) to electrical constants (so called 'HU-based model'). The FDTD-method (finite difference time domain) is then applied on a cubic lattice employing either an auxiliary 'sub-cubic lattice' (for HU-based segmentation) or a tetrahedron grid (for region-based segmentation) to assign the electrical properties, both representing the anatomy of the patient. E-field distributions are corrected by a post-processing procedure with respect to the geometry of interfaces defined by the tetrahedron grid. For comparison, the VSIE method (volume surface integral equation) is performed on the same tetrahedron grid. The applicator model assumes eight half-wavelength dipole antennas fed with constant voltages with water as background medium. RESULTS: For both numerical methods (FDTD, VSIE) the resulting antenna input impedances as well as the current distributions along the antennas were quite similar and almost insensitive to the particular geometry model (region-based, HU-based). In contrast to that, the power deposition patterns in the interior of the patient depended strongly on those models. Major differences can be related to different labels of the tissue type bone in the HU-based model in comparison to the definition via regions. Conversely, comparable results were obtained using the VSIE method and the FDTD method on the region-based patient model with a posteriori correction at the tetrahedron grid points. SAR (specific absorption rate) elevations up to a factor of 10 were predicted when employing region-based models. Those peaks might correspond to specific toxicity of electromagnetic radiation clinically known as hot spot phenomena or musculo-skeletal syndromes. Conversely, HU-based models generated quite homogeneous power deposition patterns with fluctuations of at most factor 2. CONCLUSION: The methods employing region-based geometry models such as the VSIE method and FDTD method in conjunction with a posteriori correction at tissue interfaces result in comparable E-field distributions for regional hyperthermia. Due to its shorter calculation time, the FDTD method is currently used in the clinic. Predictions derived from HU-based models without prior corrections of tissue specifications are not always supported by clinical experience.

Humans↗
PubMedDate not supplied

Strategies for optimized application of annular-phased-array systems in clinical hyperthermia.

A theoretical framework is presented for optimized heating of deep-seated tumours by phase and amplitude steering. The optimization problem for a specific tumour and perfusion case results in a functional dependency between power-level and maximum obtainable therapeutic efficiency. Different optimization criteria and strategies are outlined, which cause an increase of power or thermal dose in the tumour. Three tumour models (central pelvic tumour, eccentric abdominal tumour with or without necrosis) are analysed in detail. The simulation studies predict that appreciable parts of these tumours (50-100%) can be heated efficiently (42.5-43 degrees C) within the range of available and clinically tolerated power levels (1-5 kW/m), if tumour perfusion is less than 20-25 ml/100 g min. Some improvements are obtained by increasing the number of independent channels (from four to eight) and by the application of time-dependent (complementary) power-deposition patterns.

Abdominal Neoplasms↗