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G C van Rhoon

Publications and source records attributed to G C van Rhoon.

46 records · Page 3Linked to original sources
PubMedDate not supplied

Stability and accuracy of power and phase measurements of a VVM system designed for online quality control of the BSD-2000 (-3D) DHT system.

Accurate control of power and phase is essential for the quality assurance of hyperthermia treatments. Hereto, an external measurement device was inserted, built around a Vector Voltmeter (VVM), in order to assess online the performance of the steering capability of the BSD-2000 and later the BSD-2000-3D system. This paper only concerns the power and phase calibration of the signal in the path between the power and phase detection probes of the BSD-system and power and phase measurement point of the VVM. The calibration is performed in the frequency range of 60-120 MHz using a network analyser with a frequency range of 0.01-500 MHz. More importantly, by repeating the calibration periodically over the last 3 years, the stability and accuracy of the power and phase measurements were determined using the VVM system. The results of the power calibration show that the VVM system, concerning its power and phase measurement, is stable in time. The variation of the power measured with the VVM system is less than 0.22 dB (5.2%) for the latest configuration of the BSD-2000-3D system. The variation of the VVM-based phase measurements of the latter configurations is 1.1 degrees or less. From the results of the power and phase measurements in the BSD 2000 system reported in previous studies using other measurement systems, it follows that the uncertainties of the power and phase measurements with the currently proposed VVM system are small enough to assess accurately the performance of the BSD system concerning its steering capability.

Calibration↗
PubMedDate not supplied

An electric field measurement system, using two-dimensional array of diodes.

In hyperthermia induced by electromagnetic applicators, one way to obtain information about the energy absorption is by measurement of electric field strength (E). This paper describes a system which can measure E-distributions, using a two-dimensional array of diodes. It was designed to be used on patient skin, during hyperthermia treatments of superficial tumours, providing additional data for applicator power control. The first prototype consists of a sheet with Schottky diode sensors with a spacing of 2.5cm, connected to high-resistance leads, printed with carbon ink. The rectified diode voltages are passed through a multiplexer unit to an AD-DA card in a PC. The sensitivity of the sensors is linearly proportional to the electric field and to the length of the extended diode connection pins. Relative E-field distributions obtained with 64 sensors are updated within 1s. Phantom measurements, performed with the sensor matrix under a Lucite Cone Applicator (LCA) and under a 2 x 2 array of Current Sheet Applicators (CSAs) were compared with infrared measurements of the temperature rise after a short power pulse. A fair agreement was found between the square of the diode voltages and the infrared distributions. Movement of a single CSA over the sensor matrix can be visualized clearly by the system. The diode matrix E-field measurement system is sufficiently fast and accurate to give valuable feedback for power steering for an array of LCAs and CSAs. The system has the potential of being a helpful tool in other fields of quality assurance as well.

Calibration↗
PubMedDate not supplied

The use of implanted closed-tip catheters for the introduction of thermometry probes during local hyperthermia treatment series.

In the past two and a half years custom-made closed-tip catheters have been used to allow extensive temperature monitoring with multi-junction thermocouple probes during local hyperthermia. Data from animal experiments indicate that the use of thermometry probes within such a catheter provides reliable temperature measurements. In order to reduce stress to the patient the catheters were fixed so that they could stay in place during the total treatment series. The catheters, outer diameter 1.22 mm, were introduced through hollow needles under sterile conditions before the first hyperthermia session, and fixed using Histo-acryl (tissue adhesive) and Tegaderm (transparent adhesive). Recently, the data concerning the first 180 catheters placed in 74 treatment areas were evaluated. In one patient, an abscess developed 10 days after removal of the (3) catheters, which required surgical intervention, but it was not clear whether these problems had developed as a result of catheter placement. There were no problems with 139 of the remaining 177 catheters (79 per cent) and in 50 treatment fields (68 per cent), respectively, for a duration of 9-55 (mean 23) days. Ten catheters (6 per cent) were lost during the treatment series after 1-22 (mean 10) days. With 28 catheters (16 per cent) infection was observed, 14-27 (mean 19) days after insertion, which necessitated catheter removal before the last treatment session in 11 cases (6 per cent). The complication rate decreased with increasing experience; from 29 per cent (25/85) in the early period to 14 per cent (13/92) in the later period. We conclude that the use of closed-tip catheters under proper control generally causes few, if any, problems. This results in the acquisition of a substantial amount of reliable temperature data at reproducible sites, and it is well tolerated by the patient.

Adenocarcinoma↗
PubMedDate not supplied

An interstitial hyperthermia system at 27 MHz.

An interstitial hyperthermia system using thin, flexible wires operating at a frequency of 27 MHz has been evaluated for possible use in combination with (iridium) brachytherapy applications employing flexible nylon afterloading catheters. This method of interstitial heating is argued to be a variation of local-current-field heating, however with the direct galvanic contact between electrode and tissue replaced by a capacitive coupling between wire and tissue. Through a special design of the wire the length of the heated area along the catheter can be chosen freely. Standard non-prepared implantation catheters can be used, so that no change from the established implantation techniques is required. Possible advantages of these 27 MHz wires over coaxial dipole antennas at higher frequencies are simplicity and low cost of the heating system, free choice of effective heating length for each wire and uniformity of heating along this effective heating length up to the tip of the wire. Flexibility of the wires is maintained, enabling application in curved (even U-shaped) catheters, e.g. for head and neck implantations. Impedance matching of the wires to the generator, necessary for an optimal transfer of power, appears possible by means of a variable air coil. Care should be taken to avoid stray capacitances of the connecting cables with respect to ground or among the different cables used. Measurements in muscle-equivalent split phantoms with infrared thermography have been performed with both single and multiple wires. The resulting SAR distributions confirm the working principle of these low-frequency wires (resistive heating) and illustrate the difference with radiative antennas at higher frequencies. The system is being applied both in experimental animal studies and in a clinical pilot study.

Body Temperature↗
PubMedDate not supplied

Radiofrequency capacitive heating of deep-seated tumours using pre-cooling of the subcutaneous tissues: results on thermometry in Dutch patients.

The capacity of a radiofrequency, 13.56 MHz, capacitive hyperthermia system using extensive pre-cooling of the subcutaneous tissue to induce locoregional deep heating has been investigated in 11 patients. Tumour location was presacral in nine--and eccentric towards the lateral side of the pelvis in two patients. For thermometry multiple catheters (mean 2.7) were inserted into the treatment volume. The mean numbers of temperature measuring points per treatment were 9.4 in tumour, 5.5 in muscle and 7.2 in subcutaneous fat. RF energy was applied after 30 min of cooling through two flexible boli perfused with saline water at 5-10 degrees C. Patient tolerance to pre-cooling was very good and after some initial discomfort the patient became rapidly accustomed to the cold water boli. For some patients better temperatures were achieved when the conventional anterior-posterior applicator set-up was replaced by a set-up with an applicator on each lateral side of the patient. As patients can tolerate temperatures within the fat tissue as high as 45.5 degrees C without complaining it appears important to monitor the temperature at the transition of fat to muscle tissue to prevent subcutaneous burns. The study shows that pre-cooling cannot avoid preferential heating at the interface from fat to muscle tissue. In this patient group the quality of the hyperthermia treatment appeared to be rather poor: 60% of the measured tumour temperatures were below 40 degrees D.

Adipose Tissue↗
PubMedDate not supplied

Some practical notes on documentation of superficial hyperthermia treatment.

A detailed and accurate documentation of the treatment setup of each individual hyperthermia session is extremely important for retrospective data analysis as well as treatment quality control. In this paper the relatively simple and cheap documentation system developed by the Hyperthermia Department of the Dr Daniel den Hoed Cancer Center, is presented.

Humans↗
PubMedDate not supplied

A 27 MHz current source interstitial hyperthermia system for small animals.

Temperature distribution is an important factor in thermo-radiotherapy and it is greatly dependent on the applied heating technique. Consistency of the heating method is therefore important in translating in vivo experimental data to the clinical situation. To further evaluate the combination of interstitial hyperthermia and interstitial radiotherapy, an experimental interstitial hyperthermia system has been developed for small (500-2000 mm3) tumours growing in the flank of a rat. The system used reproduces the properties of our clinical current source interstitial hyperthermia system. The heating system consists of four applicators, each with independent tuning and power control. The applicators are situated inside plastic afterloading catheters and are capacitively coupled with the surrounding tissue. The tumour is heated through dissipation of a 27 MHz current flowing to an external ground plane. An effective RF-filter allows reliable thermocouple temperature measurements when the power is switched on. The tumour temperature is easily controlled by means of a continuous temperature read-out and a clear temperature display. A minimum temperature up to 46 degrees C can be reached within 4-10 min and maintained (+/-0.5 degrees C) throughout the treatment period. Modelling calculations performed for this heating system indicate that the applicator temperatures should be kept equal in order to minimize the difference between maximum and minimum temperature. Significantly higher applicator currents are needed at larger distances from the ground plane. In addition, the homogeneity of the temperature distribution is improved when either the tumour is isolated or when the environmental temperature is increased. The calculations also show that temperature distribution is strongly dependent on effective heat conductivity. A description of the system and its performance is presented.

Animals↗
PubMedDate not supplied

Intraspinal temperatures during regional deep hyperthermia induced by electromagnetic radiation in the Macaca mulatta.

A capacitive ring system operating at 27 MHz was tested for temperature distribution in the abdominal cavity of the living Rhesus monkey. Catheters were introduced through the abdomen and their location checked after the experiment. During two of these experiments a catheter was also introduced into the intraspinal canal. Temperature distribution was rather homogeneous within the abdominal cavity. Core temperature remained 3-5 degrees C below intra-abdominal temperatures. The intraspinal temperature increase was of the same magnitude compared with intra-abdominal temperature increase. Similar findings in other species have been reported by others. It is concluded that during non-invasive induction of regional hyperthermia by electromagnetic radiation, where a high level of energy is directed close to the spine, the intraspinal temperatures may be increased as high as the temperatures within the surrounding tissues. The central nervous system may be relatively sensitive to hyperthermia and hyperthermia may sensitize the spinal cord to radiation. Therefore, when hyperthermia is targeted at areas near the spine, either alone or in combination with radiotherapy applied to the tolerance limit of the spinal cord, these findings will have to be taken into consideration.

Abdomen↗
PubMedDate not supplied

A 433 MHz Lucite cone waveguide applicator for superficial hyperthermia.

The effective field size (EFS, SAR > or = 50% of the maximum SAR at 1 cm depth) of a conventional 433 MHz water filled waveguide applicator (32 cm2, aperture area 100 cm2) has been increased by: (1) replacement of the two diverging brass side walls which are parallel to the direction of the electric field by Lucite walls; and (2) Placement of a heterogeneous permittivity in the centre of the aperture. SAR distributions were measured at several depths in layered fat-muscle phantoms. With Lucite side walls the SAR distribution becomes wider in the H-plane of the aperture, resulting in a circular SAR distribution. In this situation the EFS is 67 cm2. Additional insertion of a PVC cone with a top angle of 15 degrees at the centre of the aperture increases the EFS to 91 +/- 6 cm2 for a waterbolus of 18 x 18 x 1 cm3. The experiments also demonstrated that the resulting EFS is affected by the waterbolus size and shape. Calorimetric measurements showed that the efficiency of the improved applicator is comparable to the efficiency of the conventional water filled waveguide applicator, 50 and 56% respectively. The modifications reported provide a simple and inexpensive means to increase the EFS and can be easily implemented in water filled waveguide applicators.

Breast Neoplasms↗