Jurriaan Bakker - Academia.edu (original) (raw)

Papers by Jurriaan Bakker

Clinical trials have shown that hyperthermia (HT), i.e. an increase of tissue temperature to 39–4... more Clinical trials have shown that hyperthermia (HT), i.e. an increase of tissue temperature to 39–44 C, significantly enhance radiotherapy and chemotherapy effectiveness [1]. Driven by the developments in computational techniques and computing power, personalised hyperthermia treatment planning (HTP) has matured and has become a powerful tool for optimising treatment quality. Electromagnetic, ultrasound, and thermal simulations using realistic clinical setups are now being performed to achieve patient-specific treatment optimisation. In addition, extensive studies aimed to properly implement novel HT tools and techniques, and to assess the quality of HT, are becoming more common. In this paper, we review the simulation tools and techniques developed for clinical hyperthermia, and evaluate their current status on the path from 'model' to 'clinic'. In addition, we illustrate the major techniques employed for validation and optimisation. HTP has become an essential tool for improvement, control, and assessment of HT treatment quality. As such, it plays a pivotal role in the quest to establish HT as an efficacious addition to multi-modality treatment of cancer.

International Journal of Radiation Oncology Biology Physics, Feb 1, 2007

Purpose: To experimentally verify the feasibility of focused heating in the neck region by an arr... more Purpose: To experimentally verify the feasibility of focused heating in the neck region by an array of two rings of six electromagnetic antennas. We also measured the dynamic specific absorption rate (SAR) steering possibilities of this setup and compared these SAR patterns to simulations. Methods and Materials: Using a specially constructed laboratory prototype head-and-neck applicator, including a neck-mimicking cylindrical muscle phantom, we performed SAR measurements by electric field, Schottkydiode sheet measurements and, using the power-pulse technique, by fiberoptic thermometry and infrared thermography. Using phase steering, we also steered the SAR distribution in radial and axial directions. All measured distributions were compared with the predictions by a finite-difference time-domain-based electromagnetic simulator. Results: A central 50% iso-SAR focus of 35 ؎ 3 mm in diameter and about 100 ؎ 15 mm in length was obtained for all investigated settings. Furthermore, this SAR focus could be steered toward the desired location in the radial and axial directions with an accuracy of ϳ5 mm. The SAR distributions as measured by all three experimental methods were well predicted by the simulations. Conclusion: The results of our study have shown that focused heating in the neck is feasible and that this focus can be effectively steered in the radial and axial directions. For quality assurance measurements, we believe that the Schottky-diode sheet provides the best compromise among effort, speed, and accuracy, although a more specific and improved design is warranted.

Recent developments in simulation technology have expanded clinical use of hyperthermia treatment... more Recent developments in simulation technology have expanded clinical use of hyperthermia treatment planning (HTP). HTP involves simulation of 1) the induced power absorption and 2) the induced temperature elevation distribution. Specifically the electromagnetic (EM) simulators have evolved into reliable tools providing accurate predictions of the absorbed power distributions applied during radiofrequency (RF) hyperthermia (HT). Such simulation tools are imperative for optimizing the energy delivery by phased array applicators with over four independent antennas. Consequently, HTP is currently being utilized in the clinic before, and during, deep-regional HT. For deep-local HT this strategy has even stronger potential but, due to steep thermal gradients, also more stringent quality requirements. In this paper, we exemplify the clinical implementation process for planning guided HT using phased-array HT applicators by reporting the steps taken for the HYPERcollar, a head and neck HT ap...

Monitoring of RF hyperthermia treatment by means of Magnetic Resonance Imaging is the logical thi... more Monitoring of RF hyperthermia treatment by means of Magnetic Resonance Imaging is the logical thing to do. MRI is a versatile imaging modality capable of imaging anatomy, physiology and moreover, temperature. Thus, detailed treatment monitoring of hyperthermia becomes possible. In the last 10 to 15 years various successful MRI/hyperthermia systems have been built. In this same period, RF technology in MRI has undergone a tremendous expansion. This growth originates mainly from the RF challenges of going towards higher magnetic field strengths. Some of these developments offer very new exciting opportunities for hyperthermia and especially for hybrid MRI/hyperthermia systems. In this paper we discuss some of these prospects paving the way to next generation hybrid hyperthermia/MRI systems.

Clinical studies have established a strong benefit from adjuvant mild hyperthermia (HT) to radio-... more Clinical studies have established a strong benefit from adjuvant mild hyperthermia (HT) to radio- and chemotherapy for many tumor sites, including the head and neck (H&N). The recently developed HYPERcollar allows applying local radiofrequency HT to tumors in the entire head and neck. Treatment quality is optimized using electromagnetic and thermal simulators and, whenever placement risk is tolerable, assessed using invasively placed thermometers. To replace the current invasive procedure, we are investigating if magnetic resonance (MR) thermometry can be exploited for continuous and 3D thermal dose assessment. In this work, we used our simulation tools to design an MR compatible laboratory prototype applicator. This laboratory applicator provides the possibility for experimental assessment of the feasibility of hybrid MR-HT in the head and neck region.

Bioelectromagnetics, 2015

Among various possible health effects of mobile phone radiation, the risk of inducing cancer has ... more Among various possible health effects of mobile phone radiation, the risk of inducing cancer has the strongest interest of laymen and health organizations. Recently, the Interphone epidemiological study investigated the association between the estimated Radio Frequency (RF) dose from mobile phones and the risk of developing a brain tumor. Their dosimetric analysis included over 100 phone models but only two homogeneous head phantoms. So, the potential impact of individual morphological features on global and local RF absorption in the brain was not investigated. In this study, we performed detailed dosimetric simulations for 20 head models and quantified the variation of RF dose in different brain regions as a function of head morphology. Head models were exposed to RF fields from generic mobile phones at 835 and 1900 MHz in the "tilted" and "cheek" positions. To evaluate the local RF dose variation, we used and compared two different post-processing methods, tha...

Physics in medicine and biology, Jan 7, 2011

To avoid potentially adverse health effects of electromagnetic fields (EMF), the International Co... more To avoid potentially adverse health effects of electromagnetic fields (EMF), the International Commission on Non-Ionizing Radiation Protection (ICNIRP) has defined EMF reference levels. Restrictions on induced whole-body-averaged specific absorption rate (SAR(wb)) are provided to keep the whole-body temperature increase (T(body, incr)) under 1 °C during 30 min. Additional restrictions on the peak 10 g spatial-averaged SAR (SAR(10g)) are provided to prevent excessive localized tissue heating. The objective of this study is to assess the localized peak temperature increase (T(incr, max)) in children upon exposure at the reference levels. Finite-difference time-domain modeling was used to calculate T(incr, max) in six children and two adults exposed to orthogonal plane-wave configurations. We performed a sensitivity study and Monte Carlo analysis to assess the uncertainty of the results. Considering the uncertainties in the model parameters, we found that a peak temperature increase as...

International journal of hyperthermia : the official journal of European Society for Hyperthermic Oncology, North American Hyperthermia Group, 2007

The purpose of this work was to define all features, and show the potential, of the novel HYPERco... more The purpose of this work was to define all features, and show the potential, of the novel HYPERcollar applicator system for hyperthermia treatments in the head and neck region. The HYPERcollar applicator consists of (1) an antenna ring, (2) a waterbolus system and (3) a positioning system. The specific absorption rate (SAR) profile of this applicator was investigated by performing infra-red measurements in a cylindrical phantom. Mandatory patient-specific treatment planning was performed as an object lesson to a patient with a laryngeal tumour and an artificial lymph node metastasis. Comfort tests with healthy volunteers have revealed that the applicator provides sufficient comfort to maintain in treatment position for an hour: the standard hyperthermia treatment duration in our centre. By phantom measurements, we established that a central focus in the neck can be obtained, with 50% iso-SAR lengths of 3.5 cm in transversal directions (x/y) and 9-11 cm in the axial direction (z). Us...

International Symposium on Electromagnetic Compatibility - EMC EUROPE, 2012

At the Erasmus Medical Center, we apply hyperthermia treatments (HT) of cancer in the pelvic area... more At the Erasmus Medical Center, we apply hyperthermia treatments (HT) of cancer in the pelvic area using an electromagnetic applicator installed in a Faraday shielded treatment room. Consequently, medical staff and accompanying persons are exposed to electromagnetic fields (EMF) during treatment. In the past, procedures were defined based on measurements defining compliant areas where the fields are expected to be below the exposure guidelines. Today, advanced electromagnetic models enable more precise dose assessment in the human anatomy and better evaluation of the procedures. The objective of this study was to investigate theoretically if the exposure of medical staff and accompanying persons is compliant to the guidelines when adhering to the procedures. Therefore, the whole-body averaged (SARwb) and spatial-averaged (SAR10g) specific absorption rate was assessed numerically in whole-body models of the patient, medical staff and an accompanying person. We found that predicted fields can exceed the exposure guidelines ten fold. However, the basic restrictions on SARwb and SAR10g are not exceeded for workers, but SARwb can exceed the guideline by a factor of two for accompanying persons that are in close vicinity of the applicator. In conclusion, adhering to the procedures defined in the early nineties is effective in keeping the exposure in medical staff and accompanying persons compliant to the exposure guidelines.

Health Physics, 2014

This study analyzes the exposure of pregnant women and their fetuses in three different gestation... more This study analyzes the exposure of pregnant women and their fetuses in three different gestational stages to electromagnetic radiation in the radio frequency range in the near- and the far-field using numerical modeling. For far-field exposure, the power density at which the basic restriction for the whole body SAR is reached is calculated for both the mother and the fetus at whole body resonance and at frequencies between 450 MHz and 2,450 MHz. The near-field exposure is assessed at 450 MHz, 900 MHz, and 2,450 MHz using half wavelength dipoles as generic sources located at different locations around the abdomen of the mother. For the investigated cases, the exposure of the mother is always below or on the order of magnitude of the basic restriction for exposure at the reference level. When applying the reference levels for the general public, the fetus is sufficiently shielded by the mother. However, the basic restrictions for general public exposure can be exceeded in the fetus when the mother is exposed at reference levels for occupational conditions. For plane wave exposure at occupational levels, the whole body SAR in the fetus can exceed the basic restrictions for the general population by at least 1.8 dB, and in the near-field of professional devices, the 10 g SAR can be non-compliant with the product standard for the general public by > 3.5 dB.

International Journal of Hyperthermia, 2007

Purpose: Definition of all features and the potential of the novel HYPERcollar applicator system ... more Purpose: Definition of all features and the potential of the novel HYPERcollar applicator system for hyperthermia treatments in the head and neck (H&N) region. Methods and Materials: The HYPERcollar applicator consists of 1) an antenna ring, 2) a waterbolus system and 3) a positioning system. The specific absorption rate (SAR) profile of this applicator is investigated by performing infra-red

Physics in Medicine and Biology, 2009

The objective of this theoretical study is to design an ultrasound (US) cylindrical phased array ... more The objective of this theoretical study is to design an ultrasound (US) cylindrical phased array that can be used for hyperthermia (40-44 degrees C) treatment of tumours in the intact breast. Simultaneously, we characterize the influence of acoustic and thermal heterogeneities on the specific absorption rate (SAR) and temperature patterns to determine the necessity of using heterogeneous models for a US applicator design and treatment planning. Cylindrical configurations of monopole transducers are studied on their ability to generate interference patterns that can be steered electronically to the location of the target region. Hereto, design parameters such as frequency, number of transducers per ring, ring distance and number of rings are optimized to obtain a small primary focus, while suppressing secondary foci. The models account for local heterogeneities in both acoustic (wave velocity and absorption) and thermal (blood perfusion rate, heat capacity and conductivity) tissue properties. We used breast models with a central tumour (30x20x38 mm3) and an artificial thorax tumour (sphere with a radius of 25 mm) to test the design. Simulations predict that a US cylindrical phased array, consisting of six rings with 32 transducers per ring, a radius of 75 mm and 66 mm distance between the first and sixth transducer ring, operating at a frequency of 100 kHz, can be used to obtain 44 degrees C in the centre of tumours located anywhere in the intact breast. The dimensions of the volumes enclosed by the 41 degrees C iso-temperature are 19x19x21 mm3 and 21x21x32 mm3 for the central and the thorax tumours, respectively. It is demonstrated that acoustic and thermal heterogeneities do not disturb the SAR and temperature patterns.

Physics in Medicine and Biology, 2010

To apply high-quality hyperthermia treatment to tumours at deep locations in the head and neck (H... more To apply high-quality hyperthermia treatment to tumours at deep locations in the head and neck (H&N), we have designed and built a site-specific phased-array applicator. Earlier, we demonstrated its features in parameter studies, validated those by phantom measurements and clinically introduced the system. In this paper we will critically review our first clinical experiences and demonstrate the pivotal role of hyperthermia treatment planning (HTP). Three representative patient cases (thyroid, oropharynx and nasal cavity) are selected and discussed. Treatment planning, the treatment, interstitially measured temperatures and their interrelation are analysed from a physics point of view. Treatments lasting 1 h were feasible and well tolerated and no acute treatment-related toxicity has been observed. Maximum temperatures measured are in the range of those obtained during deep hyperthermia treatments in the pelvic region but mean temperatures are still to be improved. Further, we found that simulated power absorption correlated well with measured temperatures illustrating the validity of our treatment approach of using energy profile optimizations to arrive at higher temperatures. This is the first data proving that focussed heating of tumours in the H&N is feasible. Further, HTP proved a valuable tool in treatment optimization. Items to improve are (1) the transfer of HTP settings into the clinic and (2) the registration of the thermal dose, i.e. dosimetry.

Physics in Medicine and Biology, 2013

Accumulating evidence shows that hyperthermia improves head-and-neck cancer treatment. Over the l... more Accumulating evidence shows that hyperthermia improves head-and-neck cancer treatment. Over the last decade, we introduced a radiofrequency applicator, named HYPERcollar, which enables local heating also of deep locations in this region. Based on clinical experience, we redesigned the HYPERcollar for improved comfort, reproducibility and operator handling. In the current study, we analyze the redesign from an electromagnetic point of view. We show that a higher number of antennas and their repositioning allow for a substantially improved treatment quality. Combined with the much better reproducibility of the water bolus, this will substantially minimize the risk of underexposure. All improvements combined enable a reduction of hotspot prominence (hot-spot to target SAR quotient) by 32% at an average of 981 W, which drastically reduces the probability for system power to become a treatment limiting source. Moreover, the power deposited in the target selectively can be increased by more than twofold. Hence, we expect that the HYPERcollar redesign currently under construction allows us to double the clinically applied power to the target while reducing the hot-spots, resulting in higher temperatures and, consequently, better clinical outcome.

Physics in Medicine and Biology, 2014

Clinical studies have established a strong benefit from adjuvant mild hyperthermia (HT) to radio-... more Clinical studies have established a strong benefit from adjuvant mild hyperthermia (HT) to radio-and chemotherapy for many tumor sites, including the head and neck (H&N). The recently developed HYPERcollar allows the application of local radiofrequency HT to tumors in the entire H&N. Treatment quality is optimized using electromagnetic and thermal simulators and, whenever placement risk is tolerable, assessed using invasively placed thermometers. To replace the current invasive procedure, we are investigating whether magnetic resonance (MR) thermometry can be exploited for continuous and 3D thermal dose assessment. In this work, we used our simulation tools to design an MR compatible laboratory prototype applicator. By simulations and measurements, we showed that the redesigned patch antennas are well matched to 50 (S11<−10 dB). Simulations also show that, using 300 W input power, a maximum specific absorption rate (SAR) of 100 W kg −1 and a temperature increase of 4.5 • C in 6 min is feasible at the center of a cylindrical fat/muscle phantom. Temperature measurements using the MR scanner confirmed the focused heating capabilities and MR compatibility of the setup. We conclude that the laboratory applicator provides the possibility for experimental assessment of the feasibility of hybrid MR-HT in the H&N region. This versatile design allows rigorous analysis of MR

Physics in Medicine and Biology, 2011

To avoid potentially adverse health effects of electromagnetic fields (EMF), the International Co... more To avoid potentially adverse health effects of electromagnetic fields (EMF), the International Commission on Non-Ionizing Radiation Protection (ICNIRP) has defined EMF reference levels from the basic restrictions on the induced whole-body-averaged specific absorption rate (SAR wb ) and the peak 10 g spatial-averaged SAR (SAR 10g ). The objective of this study is to assess if the SAR in children remains below the basic restrictions upon exposure at the reference levels. Finite difference time domain (FDTD) modeling was used to calculate the SAR in six children and two adults when exposed to all 12 orthogonal plane wave configurations. A sensitivity study showed an expanded uncertainty of 53% (SAR wb ) and 58% (SAR 10g ) due to variations in simulation settings and tissue properties. In this study, we found that the basic restriction on the SAR wb is occasionally exceeded for children, up to a maximum of 45% in small children. The maximum SAR 10g values, usually found at body protrusions, remain under the limit for all scenarios studied. Our results are in good agreement with the literature, suggesting that the recommended ICNIRP reference levels may need fine tuning.

International Journal of Radiation Oncology*Biology*Physics, 2007

Purpose: To experimentally verify the feasibility of focused heating in the neck region by an arr... more Purpose: To experimentally verify the feasibility of focused heating in the neck region by an array of two rings of six electromagnetic antennas. We also measured the dynamic specific absorption rate (SAR) steering possibilities of this setup and compared these SAR patterns to simulations. Methods and Materials: Using a specially constructed laboratory prototype head-and-neck applicator, including a neck-mimicking cylindrical muscle phantom, we performed SAR measurements by electric field, Schottkydiode sheet measurements and, using the power-pulse technique, by fiberoptic thermometry and infrared thermography. Using phase steering, we also steered the SAR distribution in radial and axial directions. All measured distributions were compared with the predictions by a finite-difference time-domain-based electromagnetic simulator. Results: A central 50% iso-SAR focus of 35 ؎ 3 mm in diameter and about 100 ؎ 15 mm in length was obtained for all investigated settings. Furthermore, this SAR focus could be steered toward the desired location in the radial and axial directions with an accuracy of ϳ5 mm. The SAR distributions as measured by all three experimental methods were well predicted by the simulations. Conclusion: The results of our study have shown that focused heating in the neck is feasible and that this focus can be effectively steered in the radial and axial directions. For quality assurance measurements, we believe that the Schottky-diode sheet provides the best compromise among effort, speed, and accuracy, although a more specific and improved design is warranted.

International Journal of Hyperthermia, 2014

Magnetic resonance thermometry (MRT) offers non-invasive temperature imaging and can greatly cont... more Magnetic resonance thermometry (MRT) offers non-invasive temperature imaging and can greatly contribute to the effectiveness of head and neck hyperthermia. We therefore wish to redesign the HYPERcollar head and neck hyperthermia applicator for simultaneous radio frequency (RF) heating and magnetic resonance thermometry. In this work we tested the feasibility of this goal through an exploratory experiment, in which we used a minimally modified applicator prototype to heat a neck model phantom and used an MR scanner to measure its temperature distribution. We identified several distorting factors of our current applicator design and experimental methods to be addressed during development of a fully MR compatible applicator. To allow MR imaging of the electromagnetically shielded inside of the applicator, only the lower half of the HYPERcollar prototype was used. Two of its antennas radiated a microwave signal (150 W, 434 MHz) for 11 min into the phantom, creating a high gradient temperature profile (DT max ¼ 5.35 C). Thermal distributions were measured sequentially, using drift corrected proton resonance frequency shift-based MRT. Measurement accuracy was assessed using optical probe thermometry and found to be about 0.4 C (0.1-0.7 C). Thermal distribution size and shape were verified by thermal simulations and found to have a good correlation (r 2 ¼ 0.76).

International Journal of Hyperthermia, 2014

Purpose: Magnetic resonance thermometry (MRT) is an attractive means to non-invasively monitor in... more Purpose: Magnetic resonance thermometry (MRT) is an attractive means to non-invasively monitor in vivo temperature during head and neck hyperthermia treatments because it can provide multi-dimensional temperature information with high spatial resolution over large regions of interest. However, validation of MRT measurements in a head and neck clinical setup is crucial to ensure the temperature maps are accurate. Here we demonstrate a unique approach for temperature probe sensor localisation in head and neck hyperthermia test phantoms. Methods: We characterise the proton resonance frequency shift temperature coefficient and validate MRT measurements in an oil-gel phantom by applying a combination of MR imaging and 3D spline fitting for accurate probe localisation. We also investigate how uncertainties in both the probe localisation and the proton resonance frequency shift (PRFS) thermal coefficient affect the registration of fibre-optic reference temperature probe and MRT readings. Results: The method provides a two-fold advantage of sensor localisation and PRFS thermal coefficient calibration. We provide experimental data for two distinct head and neck phantoms showing the significance of this method as it mitigates temperature probe localisation errors and thereby increases accuracy of MRT validation results. Conclusions: The techniques presented here may be used to simplify calibration experiments that use an interstitial heating device, or any heating method that provides rapid and spatially localised heat distributions. Overall, the experimental verification of the data registration and PRFS thermal coefficient calibration technique provides a useful benchmarking method to maximise MRT accuracy in any similar context.

International Journal of Hyperthermia, 2013

Background and purpose: In Rotterdam, patient-specific hyperthermia (HT) treatment planning (HTP)... more Background and purpose: In Rotterdam, patient-specific hyperthermia (HT) treatment planning (HTP) is applied for all deep head and neck (H&N) HT treatments. In this paper we introduce VEDO (the Visualisation Tool for Electromagnetic Dosimetry and Optimisation), the software tool required, and demonstrate its value for HTP-guided online complaint-adaptive (CA) steering based on specific absorption rate (SAR) optimisation during a H&N HT treatment. Materials and methods: VEDO integrates CA steering, visualisation of the SAR patterns and mean tumour SAR (SAR target ) optimisation in a single screen. The pre-calculated electromagnetic fields are loaded into VEDO. During treatment, VEDO shows the SAR pattern, overlaid on the patients' CT-scan, corresponding to the actually applied power settings and it can (re-)optimise the SAR pattern to minimise SAR at regions where the patient senses discomfort while maintaining a high SAR target . Results: The potential of the quantitative SAR steering approach using VEDO is demonstrated by analysis of the first treatment in which VEDO was used for two patients using the HYPERcollar. These cases show that VEDO allows response to power-related complaints of the patient and to quantify the change in absolute SAR: increasing either SAR target from 96 to 178 W/kg (case 1); or show that the first SAR distribution was already optimum (case 2). Conclusion: This analysis shows that VEDO facilitates a quantitative treatment strategy allowing standardised application of HT by technicians of different HT centres, which will potentially lead to improved treatment quality and the possibility of tracking the effectiveness of different treatment strategies.

Clinical trials have shown that hyperthermia (HT), i.e. an increase of tissue temperature to 39–4... more Clinical trials have shown that hyperthermia (HT), i.e. an increase of tissue temperature to 39–44 C, significantly enhance radiotherapy and chemotherapy effectiveness [1]. Driven by the developments in computational techniques and computing power, personalised hyperthermia treatment planning (HTP) has matured and has become a powerful tool for optimising treatment quality. Electromagnetic, ultrasound, and thermal simulations using realistic clinical setups are now being performed to achieve patient-specific treatment optimisation. In addition, extensive studies aimed to properly implement novel HT tools and techniques, and to assess the quality of HT, are becoming more common. In this paper, we review the simulation tools and techniques developed for clinical hyperthermia, and evaluate their current status on the path from 'model' to 'clinic'. In addition, we illustrate the major techniques employed for validation and optimisation. HTP has become an essential tool for improvement, control, and assessment of HT treatment quality. As such, it plays a pivotal role in the quest to establish HT as an efficacious addition to multi-modality treatment of cancer.

International Journal of Radiation Oncology Biology Physics, Feb 1, 2007

Purpose: To experimentally verify the feasibility of focused heating in the neck region by an arr... more Purpose: To experimentally verify the feasibility of focused heating in the neck region by an array of two rings of six electromagnetic antennas. We also measured the dynamic specific absorption rate (SAR) steering possibilities of this setup and compared these SAR patterns to simulations. Methods and Materials: Using a specially constructed laboratory prototype head-and-neck applicator, including a neck-mimicking cylindrical muscle phantom, we performed SAR measurements by electric field, Schottkydiode sheet measurements and, using the power-pulse technique, by fiberoptic thermometry and infrared thermography. Using phase steering, we also steered the SAR distribution in radial and axial directions. All measured distributions were compared with the predictions by a finite-difference time-domain-based electromagnetic simulator. Results: A central 50% iso-SAR focus of 35 ؎ 3 mm in diameter and about 100 ؎ 15 mm in length was obtained for all investigated settings. Furthermore, this SAR focus could be steered toward the desired location in the radial and axial directions with an accuracy of ϳ5 mm. The SAR distributions as measured by all three experimental methods were well predicted by the simulations. Conclusion: The results of our study have shown that focused heating in the neck is feasible and that this focus can be effectively steered in the radial and axial directions. For quality assurance measurements, we believe that the Schottky-diode sheet provides the best compromise among effort, speed, and accuracy, although a more specific and improved design is warranted.

Recent developments in simulation technology have expanded clinical use of hyperthermia treatment... more Recent developments in simulation technology have expanded clinical use of hyperthermia treatment planning (HTP). HTP involves simulation of 1) the induced power absorption and 2) the induced temperature elevation distribution. Specifically the electromagnetic (EM) simulators have evolved into reliable tools providing accurate predictions of the absorbed power distributions applied during radiofrequency (RF) hyperthermia (HT). Such simulation tools are imperative for optimizing the energy delivery by phased array applicators with over four independent antennas. Consequently, HTP is currently being utilized in the clinic before, and during, deep-regional HT. For deep-local HT this strategy has even stronger potential but, due to steep thermal gradients, also more stringent quality requirements. In this paper, we exemplify the clinical implementation process for planning guided HT using phased-array HT applicators by reporting the steps taken for the HYPERcollar, a head and neck HT ap...

Monitoring of RF hyperthermia treatment by means of Magnetic Resonance Imaging is the logical thi... more Monitoring of RF hyperthermia treatment by means of Magnetic Resonance Imaging is the logical thing to do. MRI is a versatile imaging modality capable of imaging anatomy, physiology and moreover, temperature. Thus, detailed treatment monitoring of hyperthermia becomes possible. In the last 10 to 15 years various successful MRI/hyperthermia systems have been built. In this same period, RF technology in MRI has undergone a tremendous expansion. This growth originates mainly from the RF challenges of going towards higher magnetic field strengths. Some of these developments offer very new exciting opportunities for hyperthermia and especially for hybrid MRI/hyperthermia systems. In this paper we discuss some of these prospects paving the way to next generation hybrid hyperthermia/MRI systems.

Clinical studies have established a strong benefit from adjuvant mild hyperthermia (HT) to radio-... more Clinical studies have established a strong benefit from adjuvant mild hyperthermia (HT) to radio- and chemotherapy for many tumor sites, including the head and neck (H&N). The recently developed HYPERcollar allows applying local radiofrequency HT to tumors in the entire head and neck. Treatment quality is optimized using electromagnetic and thermal simulators and, whenever placement risk is tolerable, assessed using invasively placed thermometers. To replace the current invasive procedure, we are investigating if magnetic resonance (MR) thermometry can be exploited for continuous and 3D thermal dose assessment. In this work, we used our simulation tools to design an MR compatible laboratory prototype applicator. This laboratory applicator provides the possibility for experimental assessment of the feasibility of hybrid MR-HT in the head and neck region.

Bioelectromagnetics, 2015

Among various possible health effects of mobile phone radiation, the risk of inducing cancer has ... more Among various possible health effects of mobile phone radiation, the risk of inducing cancer has the strongest interest of laymen and health organizations. Recently, the Interphone epidemiological study investigated the association between the estimated Radio Frequency (RF) dose from mobile phones and the risk of developing a brain tumor. Their dosimetric analysis included over 100 phone models but only two homogeneous head phantoms. So, the potential impact of individual morphological features on global and local RF absorption in the brain was not investigated. In this study, we performed detailed dosimetric simulations for 20 head models and quantified the variation of RF dose in different brain regions as a function of head morphology. Head models were exposed to RF fields from generic mobile phones at 835 and 1900 MHz in the "tilted" and "cheek" positions. To evaluate the local RF dose variation, we used and compared two different post-processing methods, tha...

Physics in medicine and biology, Jan 7, 2011

To avoid potentially adverse health effects of electromagnetic fields (EMF), the International Co... more To avoid potentially adverse health effects of electromagnetic fields (EMF), the International Commission on Non-Ionizing Radiation Protection (ICNIRP) has defined EMF reference levels. Restrictions on induced whole-body-averaged specific absorption rate (SAR(wb)) are provided to keep the whole-body temperature increase (T(body, incr)) under 1 °C during 30 min. Additional restrictions on the peak 10 g spatial-averaged SAR (SAR(10g)) are provided to prevent excessive localized tissue heating. The objective of this study is to assess the localized peak temperature increase (T(incr, max)) in children upon exposure at the reference levels. Finite-difference time-domain modeling was used to calculate T(incr, max) in six children and two adults exposed to orthogonal plane-wave configurations. We performed a sensitivity study and Monte Carlo analysis to assess the uncertainty of the results. Considering the uncertainties in the model parameters, we found that a peak temperature increase as...

International journal of hyperthermia : the official journal of European Society for Hyperthermic Oncology, North American Hyperthermia Group, 2007

The purpose of this work was to define all features, and show the potential, of the novel HYPERco... more The purpose of this work was to define all features, and show the potential, of the novel HYPERcollar applicator system for hyperthermia treatments in the head and neck region. The HYPERcollar applicator consists of (1) an antenna ring, (2) a waterbolus system and (3) a positioning system. The specific absorption rate (SAR) profile of this applicator was investigated by performing infra-red measurements in a cylindrical phantom. Mandatory patient-specific treatment planning was performed as an object lesson to a patient with a laryngeal tumour and an artificial lymph node metastasis. Comfort tests with healthy volunteers have revealed that the applicator provides sufficient comfort to maintain in treatment position for an hour: the standard hyperthermia treatment duration in our centre. By phantom measurements, we established that a central focus in the neck can be obtained, with 50% iso-SAR lengths of 3.5 cm in transversal directions (x/y) and 9-11 cm in the axial direction (z). Us...

International Symposium on Electromagnetic Compatibility - EMC EUROPE, 2012

At the Erasmus Medical Center, we apply hyperthermia treatments (HT) of cancer in the pelvic area... more At the Erasmus Medical Center, we apply hyperthermia treatments (HT) of cancer in the pelvic area using an electromagnetic applicator installed in a Faraday shielded treatment room. Consequently, medical staff and accompanying persons are exposed to electromagnetic fields (EMF) during treatment. In the past, procedures were defined based on measurements defining compliant areas where the fields are expected to be below the exposure guidelines. Today, advanced electromagnetic models enable more precise dose assessment in the human anatomy and better evaluation of the procedures. The objective of this study was to investigate theoretically if the exposure of medical staff and accompanying persons is compliant to the guidelines when adhering to the procedures. Therefore, the whole-body averaged (SARwb) and spatial-averaged (SAR10g) specific absorption rate was assessed numerically in whole-body models of the patient, medical staff and an accompanying person. We found that predicted fields can exceed the exposure guidelines ten fold. However, the basic restrictions on SARwb and SAR10g are not exceeded for workers, but SARwb can exceed the guideline by a factor of two for accompanying persons that are in close vicinity of the applicator. In conclusion, adhering to the procedures defined in the early nineties is effective in keeping the exposure in medical staff and accompanying persons compliant to the exposure guidelines.

Health Physics, 2014

This study analyzes the exposure of pregnant women and their fetuses in three different gestation... more This study analyzes the exposure of pregnant women and their fetuses in three different gestational stages to electromagnetic radiation in the radio frequency range in the near- and the far-field using numerical modeling. For far-field exposure, the power density at which the basic restriction for the whole body SAR is reached is calculated for both the mother and the fetus at whole body resonance and at frequencies between 450 MHz and 2,450 MHz. The near-field exposure is assessed at 450 MHz, 900 MHz, and 2,450 MHz using half wavelength dipoles as generic sources located at different locations around the abdomen of the mother. For the investigated cases, the exposure of the mother is always below or on the order of magnitude of the basic restriction for exposure at the reference level. When applying the reference levels for the general public, the fetus is sufficiently shielded by the mother. However, the basic restrictions for general public exposure can be exceeded in the fetus when the mother is exposed at reference levels for occupational conditions. For plane wave exposure at occupational levels, the whole body SAR in the fetus can exceed the basic restrictions for the general population by at least 1.8 dB, and in the near-field of professional devices, the 10 g SAR can be non-compliant with the product standard for the general public by &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; 3.5 dB.

International Journal of Hyperthermia, 2007

Purpose: Definition of all features and the potential of the novel HYPERcollar applicator system ... more Purpose: Definition of all features and the potential of the novel HYPERcollar applicator system for hyperthermia treatments in the head and neck (H&N) region. Methods and Materials: The HYPERcollar applicator consists of 1) an antenna ring, 2) a waterbolus system and 3) a positioning system. The specific absorption rate (SAR) profile of this applicator is investigated by performing infra-red

Physics in Medicine and Biology, 2009

The objective of this theoretical study is to design an ultrasound (US) cylindrical phased array ... more The objective of this theoretical study is to design an ultrasound (US) cylindrical phased array that can be used for hyperthermia (40-44 degrees C) treatment of tumours in the intact breast. Simultaneously, we characterize the influence of acoustic and thermal heterogeneities on the specific absorption rate (SAR) and temperature patterns to determine the necessity of using heterogeneous models for a US applicator design and treatment planning. Cylindrical configurations of monopole transducers are studied on their ability to generate interference patterns that can be steered electronically to the location of the target region. Hereto, design parameters such as frequency, number of transducers per ring, ring distance and number of rings are optimized to obtain a small primary focus, while suppressing secondary foci. The models account for local heterogeneities in both acoustic (wave velocity and absorption) and thermal (blood perfusion rate, heat capacity and conductivity) tissue properties. We used breast models with a central tumour (30x20x38 mm3) and an artificial thorax tumour (sphere with a radius of 25 mm) to test the design. Simulations predict that a US cylindrical phased array, consisting of six rings with 32 transducers per ring, a radius of 75 mm and 66 mm distance between the first and sixth transducer ring, operating at a frequency of 100 kHz, can be used to obtain 44 degrees C in the centre of tumours located anywhere in the intact breast. The dimensions of the volumes enclosed by the 41 degrees C iso-temperature are 19x19x21 mm3 and 21x21x32 mm3 for the central and the thorax tumours, respectively. It is demonstrated that acoustic and thermal heterogeneities do not disturb the SAR and temperature patterns.

Physics in Medicine and Biology, 2010

To apply high-quality hyperthermia treatment to tumours at deep locations in the head and neck (H... more To apply high-quality hyperthermia treatment to tumours at deep locations in the head and neck (H&N), we have designed and built a site-specific phased-array applicator. Earlier, we demonstrated its features in parameter studies, validated those by phantom measurements and clinically introduced the system. In this paper we will critically review our first clinical experiences and demonstrate the pivotal role of hyperthermia treatment planning (HTP). Three representative patient cases (thyroid, oropharynx and nasal cavity) are selected and discussed. Treatment planning, the treatment, interstitially measured temperatures and their interrelation are analysed from a physics point of view. Treatments lasting 1 h were feasible and well tolerated and no acute treatment-related toxicity has been observed. Maximum temperatures measured are in the range of those obtained during deep hyperthermia treatments in the pelvic region but mean temperatures are still to be improved. Further, we found that simulated power absorption correlated well with measured temperatures illustrating the validity of our treatment approach of using energy profile optimizations to arrive at higher temperatures. This is the first data proving that focussed heating of tumours in the H&N is feasible. Further, HTP proved a valuable tool in treatment optimization. Items to improve are (1) the transfer of HTP settings into the clinic and (2) the registration of the thermal dose, i.e. dosimetry.

Physics in Medicine and Biology, 2013

Accumulating evidence shows that hyperthermia improves head-and-neck cancer treatment. Over the l... more Accumulating evidence shows that hyperthermia improves head-and-neck cancer treatment. Over the last decade, we introduced a radiofrequency applicator, named HYPERcollar, which enables local heating also of deep locations in this region. Based on clinical experience, we redesigned the HYPERcollar for improved comfort, reproducibility and operator handling. In the current study, we analyze the redesign from an electromagnetic point of view. We show that a higher number of antennas and their repositioning allow for a substantially improved treatment quality. Combined with the much better reproducibility of the water bolus, this will substantially minimize the risk of underexposure. All improvements combined enable a reduction of hotspot prominence (hot-spot to target SAR quotient) by 32% at an average of 981 W, which drastically reduces the probability for system power to become a treatment limiting source. Moreover, the power deposited in the target selectively can be increased by more than twofold. Hence, we expect that the HYPERcollar redesign currently under construction allows us to double the clinically applied power to the target while reducing the hot-spots, resulting in higher temperatures and, consequently, better clinical outcome.

Physics in Medicine and Biology, 2014

Clinical studies have established a strong benefit from adjuvant mild hyperthermia (HT) to radio-... more Clinical studies have established a strong benefit from adjuvant mild hyperthermia (HT) to radio-and chemotherapy for many tumor sites, including the head and neck (H&N). The recently developed HYPERcollar allows the application of local radiofrequency HT to tumors in the entire H&N. Treatment quality is optimized using electromagnetic and thermal simulators and, whenever placement risk is tolerable, assessed using invasively placed thermometers. To replace the current invasive procedure, we are investigating whether magnetic resonance (MR) thermometry can be exploited for continuous and 3D thermal dose assessment. In this work, we used our simulation tools to design an MR compatible laboratory prototype applicator. By simulations and measurements, we showed that the redesigned patch antennas are well matched to 50 (S11<−10 dB). Simulations also show that, using 300 W input power, a maximum specific absorption rate (SAR) of 100 W kg −1 and a temperature increase of 4.5 • C in 6 min is feasible at the center of a cylindrical fat/muscle phantom. Temperature measurements using the MR scanner confirmed the focused heating capabilities and MR compatibility of the setup. We conclude that the laboratory applicator provides the possibility for experimental assessment of the feasibility of hybrid MR-HT in the H&N region. This versatile design allows rigorous analysis of MR

Physics in Medicine and Biology, 2011

To avoid potentially adverse health effects of electromagnetic fields (EMF), the International Co... more To avoid potentially adverse health effects of electromagnetic fields (EMF), the International Commission on Non-Ionizing Radiation Protection (ICNIRP) has defined EMF reference levels from the basic restrictions on the induced whole-body-averaged specific absorption rate (SAR wb ) and the peak 10 g spatial-averaged SAR (SAR 10g ). The objective of this study is to assess if the SAR in children remains below the basic restrictions upon exposure at the reference levels. Finite difference time domain (FDTD) modeling was used to calculate the SAR in six children and two adults when exposed to all 12 orthogonal plane wave configurations. A sensitivity study showed an expanded uncertainty of 53% (SAR wb ) and 58% (SAR 10g ) due to variations in simulation settings and tissue properties. In this study, we found that the basic restriction on the SAR wb is occasionally exceeded for children, up to a maximum of 45% in small children. The maximum SAR 10g values, usually found at body protrusions, remain under the limit for all scenarios studied. Our results are in good agreement with the literature, suggesting that the recommended ICNIRP reference levels may need fine tuning.

International Journal of Radiation Oncology*Biology*Physics, 2007

Purpose: To experimentally verify the feasibility of focused heating in the neck region by an arr... more Purpose: To experimentally verify the feasibility of focused heating in the neck region by an array of two rings of six electromagnetic antennas. We also measured the dynamic specific absorption rate (SAR) steering possibilities of this setup and compared these SAR patterns to simulations. Methods and Materials: Using a specially constructed laboratory prototype head-and-neck applicator, including a neck-mimicking cylindrical muscle phantom, we performed SAR measurements by electric field, Schottkydiode sheet measurements and, using the power-pulse technique, by fiberoptic thermometry and infrared thermography. Using phase steering, we also steered the SAR distribution in radial and axial directions. All measured distributions were compared with the predictions by a finite-difference time-domain-based electromagnetic simulator. Results: A central 50% iso-SAR focus of 35 ؎ 3 mm in diameter and about 100 ؎ 15 mm in length was obtained for all investigated settings. Furthermore, this SAR focus could be steered toward the desired location in the radial and axial directions with an accuracy of ϳ5 mm. The SAR distributions as measured by all three experimental methods were well predicted by the simulations. Conclusion: The results of our study have shown that focused heating in the neck is feasible and that this focus can be effectively steered in the radial and axial directions. For quality assurance measurements, we believe that the Schottky-diode sheet provides the best compromise among effort, speed, and accuracy, although a more specific and improved design is warranted.

International Journal of Hyperthermia, 2014

Magnetic resonance thermometry (MRT) offers non-invasive temperature imaging and can greatly cont... more Magnetic resonance thermometry (MRT) offers non-invasive temperature imaging and can greatly contribute to the effectiveness of head and neck hyperthermia. We therefore wish to redesign the HYPERcollar head and neck hyperthermia applicator for simultaneous radio frequency (RF) heating and magnetic resonance thermometry. In this work we tested the feasibility of this goal through an exploratory experiment, in which we used a minimally modified applicator prototype to heat a neck model phantom and used an MR scanner to measure its temperature distribution. We identified several distorting factors of our current applicator design and experimental methods to be addressed during development of a fully MR compatible applicator. To allow MR imaging of the electromagnetically shielded inside of the applicator, only the lower half of the HYPERcollar prototype was used. Two of its antennas radiated a microwave signal (150 W, 434 MHz) for 11 min into the phantom, creating a high gradient temperature profile (DT max ¼ 5.35 C). Thermal distributions were measured sequentially, using drift corrected proton resonance frequency shift-based MRT. Measurement accuracy was assessed using optical probe thermometry and found to be about 0.4 C (0.1-0.7 C). Thermal distribution size and shape were verified by thermal simulations and found to have a good correlation (r 2 ¼ 0.76).

International Journal of Hyperthermia, 2014

Purpose: Magnetic resonance thermometry (MRT) is an attractive means to non-invasively monitor in... more Purpose: Magnetic resonance thermometry (MRT) is an attractive means to non-invasively monitor in vivo temperature during head and neck hyperthermia treatments because it can provide multi-dimensional temperature information with high spatial resolution over large regions of interest. However, validation of MRT measurements in a head and neck clinical setup is crucial to ensure the temperature maps are accurate. Here we demonstrate a unique approach for temperature probe sensor localisation in head and neck hyperthermia test phantoms. Methods: We characterise the proton resonance frequency shift temperature coefficient and validate MRT measurements in an oil-gel phantom by applying a combination of MR imaging and 3D spline fitting for accurate probe localisation. We also investigate how uncertainties in both the probe localisation and the proton resonance frequency shift (PRFS) thermal coefficient affect the registration of fibre-optic reference temperature probe and MRT readings. Results: The method provides a two-fold advantage of sensor localisation and PRFS thermal coefficient calibration. We provide experimental data for two distinct head and neck phantoms showing the significance of this method as it mitigates temperature probe localisation errors and thereby increases accuracy of MRT validation results. Conclusions: The techniques presented here may be used to simplify calibration experiments that use an interstitial heating device, or any heating method that provides rapid and spatially localised heat distributions. Overall, the experimental verification of the data registration and PRFS thermal coefficient calibration technique provides a useful benchmarking method to maximise MRT accuracy in any similar context.

International Journal of Hyperthermia, 2013

Background and purpose: In Rotterdam, patient-specific hyperthermia (HT) treatment planning (HTP)... more Background and purpose: In Rotterdam, patient-specific hyperthermia (HT) treatment planning (HTP) is applied for all deep head and neck (H&N) HT treatments. In this paper we introduce VEDO (the Visualisation Tool for Electromagnetic Dosimetry and Optimisation), the software tool required, and demonstrate its value for HTP-guided online complaint-adaptive (CA) steering based on specific absorption rate (SAR) optimisation during a H&N HT treatment. Materials and methods: VEDO integrates CA steering, visualisation of the SAR patterns and mean tumour SAR (SAR target ) optimisation in a single screen. The pre-calculated electromagnetic fields are loaded into VEDO. During treatment, VEDO shows the SAR pattern, overlaid on the patients' CT-scan, corresponding to the actually applied power settings and it can (re-)optimise the SAR pattern to minimise SAR at regions where the patient senses discomfort while maintaining a high SAR target . Results: The potential of the quantitative SAR steering approach using VEDO is demonstrated by analysis of the first treatment in which VEDO was used for two patients using the HYPERcollar. These cases show that VEDO allows response to power-related complaints of the patient and to quantify the change in absolute SAR: increasing either SAR target from 96 to 178 W/kg (case 1); or show that the first SAR distribution was already optimum (case 2). Conclusion: This analysis shows that VEDO facilitates a quantitative treatment strategy allowing standardised application of HT by technicians of different HT centres, which will potentially lead to improved treatment quality and the possibility of tracking the effectiveness of different treatment strategies.