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G C van Rhoon

Publications and source records attributed to G C van Rhoon.

At least 37 records · Page 2Linked to original sources
PubMed1992

Electrical impedance tomography: prospects for non-invasive control of deep hyperthermia treatments.

Electrical Impedance Tomography (EIT) is assessed as a means of monitoring deep hyperthermia treatments in both phantom and in vivo studies. EIT measurements during heating of an inhomogeneous leg phantom show good correlation with internal temperatures measured with invasive probes. Measurements whilst altering the temperature of the stomach contents in volunteers demonstrate the capability of EIT for imaging temperature change at depth in the torso. Significant problems remain to be overcome before EIT can be used in the clinic; however, the technique may be an important additional control factor as a complement to invasive monitoring.

Body Temperature↗
PubMed1991

Application of a capacitive-coupling interstitial hyperthermia system at 27 MHz: study of different applicator configurations.

A 27 MHz capacitive-coupling interstitial hyperthermia system has been developed. It uses thin flexible applicators which can easily be inserted in standard brachytherapy catheters. The system can be operated in two different configurations. In the external ground return configuration tissue is heated by currents passing from the catheters to external ground returns; in the balanced configuration, by currents passing between applicators with a phase difference of 180 degrees. The purpose of this study was to find out which configuration is preferable, in terms of temperature homogeneity and clinical usefulness. Model calculations show that, due to the high impedance associated with capacitive coupling, the applicators can be represented as current sources, in contrast to local current field electrodes which are voltage sources. SAR measurements in muscle-equivalent phantoms illustrate that homogeneous heating patterns along the catheters can be expected in both configurations in regular as well as in irregular implants. Using the external ground return configuration the power of each applicator can be controlled individually. Calculations and measurements show that cross-talk cannot be avoided completely in this configuration, but that it can be minimized by using applicators operating in phase.

Brachytherapy↗
PubMed1990

A ring capacitor applicator in hyperthermia: energy distributions in a fat-muscle layered model for different ring electrode configurations.

The energy deposition pattern within a radially layered fat-muscle phantom, diameter 135 mm, heated by a novel ring capacitor applicator has been determined experimentally as well as theoretically. Good to excellent agreement is found between measured and predicted energy distributions. For the specific absorption rate in the muscle tissue the differences are in general smaller than 6%. When the ring electrodes are placed directly on the phantom surface both measured and predicted energy distributions show the presence of superficial hot spots located within the fat layer at the site of the ring electrodes. The theoretical distributions showed that the radial component of the E-field contributes for more than 90% to the energy absorption at the hot spot in the fatty tissue in front of the ring electrodes. Introducing a small air gap (10 mm) between the phantom surface and the ring electrode results in a decrease of the energy absorption within the fatty tissue at the hot spot location by 30%. Further theoretical analysis of the energy distribution within the inhomogeneous model showed that the intensity of the hot spots at the ring electrodes can be controlled by adjustment of the applicator configuration. Independent of the size of the electrode to phantom gap the specific absorption rate values predicted in the fat-muscle model show a more favorable distribution at a frequency of 27.12 MHz than at 13.56 MHz. For a similar electrode to phantom gap the specific absorption rate within the fatty tissue is approximately two times lower at 27.12 than at 13.56 MHz. For the model calculations performed the best ratio of fat to muscle SAR (0.2) is obtained with distilled water as bolus medium in the gap.

Absorption↗
PubMed1989

A quasi-static model for the ring capacitor applicator.

The electromagnetic heat dissipation in a radially layered biological tissue inside a ring capacitor (RC) applicator has been investigated. A quasi-static model is introduced to compute the relevant electromagnetic field quantities. The method of computation employs the spatial Fourier transform of all field quantities with respect to the axial coordinate. After an iterative solution of a dual boundary value problem for the electric potential and the current density at the electrodes, an inverse Fourier transform is carried out to compute the quantities that are of interest to the deep-body system at hand. Comparison of numerical results with phantom measurements shows excellent agreement.

Electric Conductivity↗
PubMed1988

Interstitial hyperthermia using 27 MHz wire antennas and interstitial photodynamic therapy in a rat rhabdomyosarcoma: phantom and animal studies.

This paper deals with the interaction of interstitial hyperthermia (HT) and interstitial photodynamic therapy (PDT). Its main focus, however, is on a newly developed heating system; phantom studies as well as temperature-response data obtained from the in vivo experiments are presented. Heat was delivered by thin, flexible wire antennas operating at a frequency of 27 MHz. Measurements in muscle-equivalent phantom with infrared thermography were performed. Uniform heating over the inserted length of the antenna was obtained and impedance matching appears possible by simple variable air coils, thereby minimizing the reflected power to less than 20%. Light was obtained from an Argon-Dye laser system tuned to a wavelength of 625 nm at a dose rate of 75-100 mW per fiber to a total incident dose of 900 J from four linear light applicators. An experimental murine tumor (Rhabdomyosarcoma, type R-1) was transplanted in WAG/Rij rats and, after reaching an average diameter of 2 cm, the active component of haematoporphyrin derivative (HPD), Photofrin II, was injected intravenously. The tumors were subsequently implanted with four flexible catheters, through which either light or heat could be applied. Dose-response relationships for PDT alone, HT alone and PDT followed by HT were established with cure as endpoint. The animal experiments showed that with the use of low-frequency wires a good localized heat distribution in the tumors can be obtained. Moreover, this study showed that PDT and HT, in the proper sequence and only when optimal temperatures are reached, result in an augmented cytotoxicity on the tumor cells in vivo; i.e. a cure rate of 41% was obtained.

Animals↗
PubMed1988

Interaction of interstitial photodynamic therapy and interstitial hyperthermia in a rat rhabdomyosarcoma--a pilot study.

Photodynamic therapy (PDT) involves the activation of photosensitizing drugs by light of appropriate wavelength. The photosensitive agent Hematoporphyrin Derivative (HPD) appears to be preferentially retained in malignant tumors; irradiation of HPD-containing tissue by light of appropriate wavelength (625 nm) and dose leads to (tumor) tissue destruction. The aim of this study is to achieve maximum tumor control probability with minimum normal tissue photosensitivity. In previous work from our laboratory it has been demonstrated that PDT has its fundamental effects on the tumor and normal tissue microcirculation. As it is well established that hyperthermia (HT) has its major effects in less well vascularized areas of the tumor, the combined modality of HT and PDT might prove to be advantageous. Moreover, suppression of sublethal damage repair by HT has been observed. To overcome the problem of poor light penetration into tissues and the high rate of recurrences following PDT with external irradiation, the combined effects of interstitial PDT with interstitial hyperthermia in a new line of animal experiments were studied in our laboratory. An experimental murine tumor (Rhabdomyosarcoma, type R-1) was transplanted in WAG/Rij rats and, after reaching an average diameter of 2 cm, the active component of HPD, that is Photofrin II, was injected intravenously in different dose schedules (5 mg/kg, 10 mg/kg). After 24 or 48 hrs the tumors were implanted with four flexible catheters, through which either light or heat could be applied. Light was obtained from an Argon-Dye laser system tuned to a wavelength of 625 nm at a dose rate of 75-100 mW per fiber to a dose level of 900 Joule from four linear light applicators. Heat (44 degrees C/30') was delivered by four 27 MHz radiofrequency antennas. Dose response relationships for PDT alone, HT alone and PDT combined with HT were established with cure as endpoint. This study showed that these two modalities, in the proper sequence and spacing, result in an augmented cytotoxicity on the tumor cells in vivo. With the combined modality treatment a cure rate of 41% (90 days) was obtained. As the implantation of flexible catheters is a well-known technique in radiation therapy practice, the potentiating effects of interstitial HT combined with interstitial PDT in solid tumors is very promising and clinical studies are warranted.

Animals↗
PubMed1988

Low dose reirradiation in combination with hyperthermia: a palliative treatment for patients with breast cancer recurring in previously irradiated areas.

Ninety-seven patients with breast cancer recurring in a previously irradiated area (mean dose 44 Gy) were reirradiated in combination with hyperthermia and had evaluable tumor responses. In the reirradiation series, radiotherapy was given twice weekly in most patients, with a fraction size varying from 200 to 400 cGy, the total dose varying from 8 to 32 Gy. Hyperthermia was given following the radiotherapy fractions. The combined treatment resulted in 35% complete and 55% partial responses. Duration of response was median 4 months for partial response and 26 months for complete response, respectively. The median survival time for all patients was 12 months. Acute skin reaction was mild, with more than moderate erythema in only 14/97 patients. Thermal burns occurred in 44/97 patients, generally at sites where pain sensation was decreased, and therefore they did not cause much inconvenience. In the 19 patients who survived more than 2 years, no late radiation damage was observed. When patients who received a "high dose" (greater than 29 Gy and hyperthermia) were compared with those who received a "low dose" (less than 29 Gy and hyperthermia), a higher complete response rate was observed in the high dose group (58% vs. 24%), whereas no difference in acute toxicity was found. We conclude that reirradiation with 8 x 4 Gy in combination with hyperthermia twice weekly is a safe, effective and well tolerated method for palliative treatment of patients with breast cancer recurring in previously irradiated areas.

Adult↗
PubMed1985

Clinically derived dose effect relationship for hyperthermia given in combination with low dose radiotherapy.

A group of 36 patients, treated on 44 fields with fractionated treatments of local high-frequency induced hyperthermia and radiotherapy to the low total dose of 14-25 Gy, was retrospectively evaluated for differences between responders (i.e., complete and partial responders) and non-responders. A response rate of 86% was achieved, comprising 14% (6/44) complete and 73% (32/44) partial response. There was an indication that the probability of complete response is greater in smaller tumours. The tumour temperatures achieved were higher in the responders than in the non-responders. Logistic analysis was performed on the mean temperature achieved. This temperature parameter yielded a significant positive dose-effect relationship with regard to therapeutic effect, from which a 50% effective dose of 38.8 degrees C was calculated. The data indicate that to obtain the maximum therapeutic effect, i.e., greater than 95% response rate, tumour temperatures above 42.5 degrees C need to be achieved.

Adult↗
PubMed1984

Cardiovascular changes during whole body hyperthermia treatment of advanced malignancy.

Cardiovascular studies were carried out on patients subjected to whole body hyperthermia treatment for advanced malignancy in order to assess the magnitude of the changes occurring and the degree of strain imposed on the system. The subjects, who were anaesthetised with a nitrous oxide/oxygen and relaxant sequence, were heated in a modified Siemens hyperthermia cabin and maintained at a body temperature of 41.8 degrees C for 2 h. The results of 30 treatments are presented. Large increases in cardiac output and heart rate were accompanied by large decreases in peripheral resistance in both the systemic and pulmonary vascular beds. The pulmonary arterial pressure rose whereas that in the systemic circulation fell. This caused right ventricular work to increase proportionately more than left ventricular work. Care should be exercised when subjecting patients with limited right ventricular function to this treatment.

Cardiac Output↗
PubMed1984

Human tumour pH changes following hyperthermia and radiation therapy.

Measurement of human tumour pH was performed during treatment with whole-body hyperthermia (WBHT) at 41.8 degrees C, and prior to and following a series of combined local hyperthermia (LHT) and radiotherapy treatments. During WBHT no changes were seen in the 11 tumour measurements performed during heating, 'plateau' and cooling phases of treatment. Tumour pH rose significantly in 24 tumours in which paired determinations were performed prior to and following local therapy (mean rise, 0.23 pH units), while the 11 subcutaneous controls, measured in a non-treated area, remained unchanged. The lack of change in tumour pH during WBHT is probably related to the moderate treatment temperature (42 degrees C or less), whereas the rise seen following local therapy is probably the result of changes in tissue oxygenation and blood flow following therapy. These changes may have implications to the treatment of malignant disease.

Combined Modality Therapy↗
PubMed1983

Cerebral temperature and epidural pressure during whole body hyperthermia in dogs.

Whole body hyperthermia was performed on 13 anaesthetized and intubated dogs. Cerebral temperature and epidural pressure were measured and related to rectal and oesophageal temperatures. The brain temperature was significantly higher than that of the rectum at normotemperature. The temperature gradients lessened during heating. At maximum rectal temperature (plateau) small differences between the left and right hand sides of the brain were seen, the temperatures being 0.4 degrees C and 0.3 degrees C higher than the rectal temperature, respectively. During the 2-h plateau period the difference between brain and oesophageal temperature became progressively smaller. The mean epidural pressure decreased slightly during the course of the treatment, but remained within the normal range of 1.2 +/- 0.4 KPa (8.7 +/- 2.5 mm Hg). From this investigation may be concluded that (1) the cerebral temperature is reflected better by the oesophageal than by the rectal temperature and (2) the epidural pressure does not increase during whole body hyperthermia for 2 h at a rectal temperature of up to 42.5 degrees C.

Animals↗
PubMed1983

Whole-body hyperthermia in cancer therapy: a report of a phase I-II study.

Twenty-seven patients were treated with whole-body hyperthermia alone or in combination with either chemotherapy or radiotherapy. Whole-body hyperthermia was performed in the Pomp-Siemens cabin with hot air and a warm water mattress, the patient being covered with plastic film to avoid cooling by perspiration. The heat treatment lasted for 2 hr at 41.8-42.0 degrees C. Toxicity, such as liver damage and respiratory problems, was considerable. There were two fatalities. Hyperthermia gave an improved therapeutic effect in 6 of the patients. Considerable pain relief was observed in 8 of 10 patients. Whole-body hyperthermia at 42 degrees C can be effective but the potential toxicity limits its use to those patients with severe complaints for whom no other palliative treatment is available.

Adult↗
PubMedDate not supplied

Accuracy of electrical field measurement using the flexible Schottky diode sheet at 433 MHz.

In this study, the accuracy of Schottky diode sensors mounted as a two-dimensional array on a flexible 125 micro m thick polyester foil has been studied. The diodes are placed at a distance of 2.5 x 2.5 cm, resulting in a measuring area of 20 x 20 cm. The diodes are placed across the gap between both arms (3 x 5 mm) of a dipole, total length 12 mm. High resistance (1 M Omega/m) carbon transmission lines printed on the sheet are used to connect each electrical (E) field sensor to the read-out electronics and a data-acquisition system. It is demonstrated that the flexible Schottky diode sheet can quantitatively measure E-field distributions at 433 MHz with an overall accuracy of approximately 6% (1 SD). The largest contribution to the inaccuracy is related to the phantom heterogeneity. The absolute sensitivity of this electrical field sensor is 0.71 V/m per V/m of the applied external electromagnetic field. The DC-voltage signal of the diodes shows a more or less square root relation to the RF-power applied to the applicator over a 15-fold range. An important feature of the system is that it provides the ability to perform on-line monitoring of the E-field, i.e. the SAR distribution of 433 MHz applicators. Further, it enables the introduction of fast and easy quality control protocols for superficial hyperthermia applicators.

Electricity↗
PubMedDate not supplied

Theoretical investigation of measurement procedures for the quality assurance of superficial hyperthermia applicators.

This work presents the results obtained from simple numerical models concerning the measurement uncertainty with thermographic techniques used for the evaluation of superficial hyperthermia applicators. Based upon the calculations performed, it is shown that, when using a thermographic technique to measure the SAR distribution of an applicator, heating times from 60-120s and measuring times of 10s are acceptable for an accurate assessment of the half-width at half power (HWHP) of an applicator (error less than 2%) with an expected HWHP larger than 2.5 cm. Only when the HWHP is expected to be less than 2.5 cm does the heating time need to be adapted to obtain an accuracy of 2% or better. For the assessment of the maximum SAR, the situation is worse. Even with a careful experimental design, it is difficult to measure the maximum SAR with an error less than 7%.

Biophysical Phenomena↗