Stanford 3D hyperthermia treatment planning system. Technical review and clinical summary

Advanced Radio Frequency Applicators for Thermal Magnetic Resonance Theranostics of Brain Tumors

Thermal Magnetic Resonance (ThermalMR) is a theranostic concept that combines diagnostic magnetic resonance imaging (MRI) with targeted thermal therapy in the hyperthermia (HT) range using a radiofrequency (RF) applicator in an integrated system. ThermalMR adds a therapeutic dimension to a diagnostic MRI device. Focused, targeted RF heating of deep-seated brain tumors, accurate non-invasive temperature monitoring and high-resolution MRI are specific requirements of ThermalMR that can be addressed with novel concepts in RF applicator design. This work examines hybrid RF applicator arrays combining loop and self-grounded bow-tie (SGBT) dipole antennas for ThermalMR of brain tumors, at magnetic field strengths of 7.0 T, 9.4 T and 10.5 T. These high-density RF arrays improve the feasible transmission channel count, and provide additional degrees of freedom for RF shimming not afforded by using dipole antennas only, for superior thermal therapy and MRI diagnostics. These improvements are especially relevant for ThermalMR theranostics of deep-seated brain tumors because of the small surface area of the head. ThermalMR RF applicators with the hybrid loop+SGBT dipole design outperformed applicators using dipole-only and loop-only designs, with superior MRI performance and targeted RF heating. Array variants with a horse-shoe configuration covering an arc (270°) around the head avoiding the eyes performed better than designs with 360° coverage, with a 1.3 °C higher temperature rise inside the tumor while sparing healthy tissue. Our EMF and temperature simulations performed on a virtual patient with a clinically realistic intracranial tumor provide a technical foundation for implementation of advanced RF applicators tailored for ThermalMR theranostics of brain tumors.

Improved patient-specific hyperthermia planning based on parametrized electromagnetic and thermal models for the SIGMA-30 applicator

OBJECTIVE

To create an improved planning method for pediatric regional hyperthermia (RHT) using the SIGMA-30 applicator (SIGMA-30).

MATERIALS AND METHODS

An electromagnetic model of SIGMA-30 was generated for use with the finite-difference time-domain (FDTD) method. Applying special MATLAB-based algorithms, voxel models of a pediatric patient with pelvic rhabdomyosarcoma were created from Computed-Tomography (CT) contours for use with the FDTD method and the finite-difference (FD) method capable of using either temperature-independent or temperature-dependent perfusion models for solving the Bioheat Transfer Equation (BHTE). Patient models were parametrized regarding, first, the positioning in the applicator, second, the absorbed power range and, third, different perfusion models, resulting in the so-called Parametrized Treatment Models (PTMs). A novel dedicated optimization procedure was developed based on quantitative comparison of numerical calculations against temperature and power measurements from two RHT therapies.

RESULTS

Using measured data, a realistic absorbed power range in the patient model was estimated. Within this range, several FDTD and BHTE runs were performed and, applying the aforementioned optimization scheme, the best PTMs and perfusion models were identified for each therapy via a retrospective comparison with measurements in 14 temperature sensor positions: 5 in the tumor, 8 in rectum and one in bladder.

CONCLUSION

A novel dedicated optimization procedure for identification of suitable patient-specific electromagnetic and thermal models, which can be used for improved patient planning, was developed and evaluated by comparison with treatment-derived measurements using SIGMA-30. The optimization procedure can be extended to other hyperthermia applicators and to other patient types, including adults.

Water-soluble iron oxide nanocubes with high values of specific absorption rate for cancer cell hyperthermia treatment

Iron oxide nanocrystals (IONCs) are appealing heat mediator nanoprobes in magnetic-mediated hyperthermia for cancer treatment. Here, specific absorption rate (SAR) values are reported for cube-shaped water-soluble IONCs prepared by a one-pot synthesis approach in a size range between 13 and 40 nm. The SAR values were determined as a function of frequency and magnetic field applied, also spanning technical conditions which are considered biomedically safe for patients. Among the different sizes tested, IONCs with an average diameter of 19 ± 3 nm had significant SAR values in clinical conditions and reached SAR values up to 2452 W/g(Fe) at 520 kHz and 29 kAm(-1), which is one of the highest values so far reported for IONCs. In vitro trials carried out on KB cancer cells treated with IONCs of 19 nm have shown efficient hyperthermia performance, with cell mortality of about 50% recorded when an equilibrium temperature of 43 °C was reached after 1 h of treatment.

Benefits of superficial hyperthermia treatment planning: Five case studies

PURPOSE

To demonstrate the benefits of treatment planning in superficial hyperthermia.

MATERIALS AND METHODS

Five patient cases are presented, in which treatment planning was applied to troubleshoot treatment-limiting hotspots, to select the optimum applicator type and orientation, to assess the risk associated with metallic implants, to assess the feasibility of heating a deeper seated tumour, and to analyse the effective SAR coverage resulting from arrays of multiple incoherent applicators. FDTD simulation tools were used to investigate treatment options, either based on segmented or simplified anatomies.

RESULTS

The background, approach and model implementation are presented per case. SAR cross-sections, profiles and isosurfaces are visualized to predict the effective SAR coverage of the target and the location of the maximum power absorption. In addition, the followed treatment strategy and the implications for the clinical treatment are given: for example, higher temperatures, relief of treatment limiting hot-spots or increased power input.

CONCLUSIONS

Treatment planning in superficial hyperthermia can be applied to improve clinical routine. Its application supports the selection of the optimum technique in non-standard cases, leading to direct benefits for the patient. In addition, treatment planning has shown to be an excellent tool for education and training for hyperthermia technicians and physicians.

Implications of clinical RF hyperthermia on protection limits in the RF range

The systemic temperature is meticulously regulated to 37-37.5 degrees C. Organ systems (skin, digestive system, muscles) have a considerable potential to regulate the perfusion for thermal regulation, physical activity, or digestion. While the regulation of the systemic temperature (37.5 degrees C) is quite strict, the tolerance and regulation potential with respect to local heat is more variable. Laboratory studies provided the relationship between thermal doses and cytotoxic effects. Tissue damage for short-term expositions (in the range of minutes) is only possible for temperatures above 50 degrees C. Radiofrequency radiation is utilized in cancer therapy, inducing local tissue temperatures in the range of 40-45 degrees C for 30-60 min. During local hyperthermia (with heated volumes <1 L) specific absorption rates (SARs) of 100-200 W kg, reactive perfusions of 20-40 mL/100 g/min, and tumor temperatures of 42-43 degrees C are achieved. Normally no side effects or damage in the normal tissue, such as muscle or skin, have been seen. During regional hyperthermia, SARs of 30-40 W kg are found in heated volumes of 10 L with temperatures of 41-42 degrees C in tumor-related measurement points. Then the reactive average perfusion is 6-9 mL/100 g/min (mean value 8 mL/100 g/min). Local temperatures even for higher SAR are regulated to values of not more than 40-42 degrees C. For these temperatures no damages in normal tissues have been found after regional hyperthermia in hundreds of patients. We conclude that the thermoregulatory potential for the whole body or large body regions is limited by the cardiac output, which can at least double the output from 5 to 10 L min. Even higher is the potential to compensate in smaller volumes. Here the perfusion in muscle can be increased from the basal value of 2-4 mL/100 g/min more than 5-10-fold.

Experimental validation of hyperthermia SAR treatment planning using MRB1+imaging

In this paper the concept of using B1+ imaging as a means to validate SAR models for radiofrequency hyperthermia is presented. As in radiofrequency hyperthermia, in common clinical MR imaging which applies RF frequencies between 64 and 128 MHz, the RF field distribution inside a patient is largely determined by the dielectric distribution of the anatomy. Modern MR imaging techniques allow measurement of the RF magnetic field component B1+ making it possible to measure at high resolution the dielectric interaction of the RF field with the patient. Given these considerations, we propose to use MR imaging to verify the validity of our dielectric patient model used for SAR models of radiofrequency hyperthermia. The aim of this study was to investigate the feasibility of this concept by performing B1+ measurements and simulations on cylindrical split phantoms consisting of materials with dielectric properties similar to human tissue types. Important topics of investigation were the accuracy and sensitivity of B1+ measurements and the validity of the electric model of the MR body coil. The measurements were performed on a clinical 1.5 T MR scanner with its quadrature body coil operating at 64 MHz. It was shown that even small B1+ variations of 2 to 5% could be measured reliably in the phantom experiments. An electrical model of the transmit coil was implemented on our FDTD-based hyperthermia treatment planning platform and the RF field distributions were calculated assuming an idealized quadrature current distribution in the coil. A quantitatively good correlation between measurements and simulations was found for phantoms consisting of water and oil, while highly conductive phantoms show considerable deviations. However, assuming linear excitation for these conductive phantoms resulted in good correspondence. As an explanation it is suggested that the coil is being detuned due to the inductive nature of the conductive phantoms, breaking up the phase difference of pi/2 between the two quadrature modes. It is concluded that B1+ imaging is an accurate and sensitive method for obtaining quantitative information about the RF field in phantoms. The electrical model of the body coil is inadequate for highly conductive phantoms. It is expected that for experiments on human bodies the inductive coupling is also significant, demonstrating the need for a full resonant FDTD model of the transmit coil. This will be pursued in the near future.

A temperature-based feedback control system for electromagnetic phased-array hyperthermia: theory and simulation

A hybrid proportional-integral-in-time and cost-minimizing-in-space feedback control system for electromagnetic, deep regional hyperthermia is proposed. The unique features of this controller are that (1) it uses temperature, not specific absorption rate, as the criterion for selecting the relative phases and amplitudes with which to drive the electromagnetic phased-array used for hyperthermia and (2) it requires on-line computations that are all deterministic in duration. The former feature, in addition to optimizing the treatment directly on the basis of a clinically relevant quantity, also allows the controller to sense and react to time- and temperature-dependent changes in local blood perfusion rates and other factors that can significantly impact the temperature distribution quality of the delivered treatment. The latter feature makes it feasible to implement the scheme on-line in a real-time feedback control loop. This is in sharp contrast to other temperature optimization techniques proposed in the literature that generally involve an iterative approximation that cannot be guaranteed to terminate in a fixed amount of computational time. An example of its application is presented to illustrate the properties and demonstrate the capability of the controller to sense and compensate for local, time-dependent changes in blood perfusion rates.

Experimental and numerical investigation of feed-point parameters in a 3-D hyperthermia applicator using different FDTD models of feed networks

Experimental and numerical methods were used to determine the coupling of energy in a multichannel three-dimensional hyperthermia applicator (SIGMA-Eye), consisting of 12 short dipole antenna pairs with stubs for impedance matching. The relationship between the amplitudes and phases of the forward waves from the amplifiers, to the resulting amplitudes and phases at the antenna feed-points was determined in terms of interaction matrices. Three measuring methods were used: 1) a differential probe soldered directly at the antenna feed-points; 2) an E-field sensor placed near the feed-points; and 3) measurements were made at the outputs of the amplifier. The measured data were compared with finite-difference time-domain (FDTD) calculations made with three different models. The first model assumes that single antennas are fed independently. The second model simulates antenna pairs connected to the transmission lines. The measured data correlate best with the latter FDTD model, resulting in an improvement of more than 20% and 20 degrees (average difference in amplitudes and phases) when compared with the two simpler FDTD models.

Regional hyperthermia applicator design using FDTD modelling

Recently published results confirm the positive effect of regional hyperthermia combined with external radiotherapy on pelvic tumours. Several studies have been published on the improvement of RF annular array applicator systems with dipoles and a closed water bolus. This study investigates the performance of a next-generation applicator system for regional hyperthermia with a multi-ring annular array of antennas and an open water bolus. A cavity slot antenna is introduced to enhance the directivity and reduce mutual coupling between the antennas. Several design parameters, i.e. dimensions, number of antennas and operating frequency, have been evaluated using several patient models. Performance indices have been defined to evaluate the effect of parameter variation on the specific absorption rate (SAR) distribution. The performance of the new applicator type is compared with the Coaxial TEM. Operating frequency appears to be the main parameter with a positive influence on the performance. A SAR increase in tumour of 1.7 relative to the Coaxial TEM system can be obtained with a three-ring, six-antenna per ring cavity slot applicator operating at 150 MHz.

Quasistatic zooming of FDTD E-field computations: the impact of down-scaling techniques

Due to current computer limitations, regional hyperthermia treatment planning (HTP) is practically limited to a resolution of 1 cm, whereas a millimetre resolution is desired. Using the centimetre resolution E-field distribution, computed with, for example, the finite-difference time-domain (FDTD) method and the millimetre resolution patient anatomy it is possible to obtain a millimetre resolution SAR distribution in a volume of interest (VOI) by means of quasistatic zooming. To compute the required low-resolution E-field distribution, a low-resolution dielectric geometry is needed which is constructed by down-scaling the millimetre resolution dielectric geometry. In this study we have investigated which down-scaling technique results in a dielectric geometry that yields the best low-resolution E-field distribution as input for quasistatic zooming. A segmented 2 mm resolution CT data set of a patient has been down-scaled to 1 cm resolution using three different techniques: 'winner-takes-all', 'volumetric averaging' and 'anisotropic volumetric averaging'. The E-field distributions computed for those low-resolution dielectric geometries have been used as input for quasistatic zooming. The resulting zoomed-resolution SAR distributions were compared with a reference: the 2 mm resolution SAR distribution computed with the FDTD method. The E-field distribution for both a simple phantom and the complex partial patient geometry down-scaled using 'anisotropic volumetric averaging' resulted in zoomed-resolution SAR distributions that best approximate the corresponding high-resolution SAR distribution (correlation 97, 96% and absolute averaged difference 6, 14% respectively).

Quasistatic zooming for regional hyperthermia treatment planning

Due to current computer limitations, specific absorption rate (SAR) distributions in regional hyperthermia treatment planning (HTP) are limited to centimetre resolution. However, since patient anatomy is highly structured on a millimetre scale, millimetre-resolution SAR modelling is required. A method called quasistatic zooming has been developed to obtain a high-resolution SAR distribution within a volume of interest (VOI): using the low-resolution E-field distribution and the high-resolution patient anatomy, the high-resolution SAR distribution is computed within a small zoom volume Q (small compared with the wavelength in water (lambda(w))). Repeating this procedure yields the zoomed-resolution SAR distribution in an arbitrary VOI. To validate this method for a VOI that is not small compared with lambda(w), high-resolution finite-difference time-domain (FDTD) modelling is needed. Since this is impractical for a clinical applicator, a computer model of a small applicator has been created. A partial patient anatomy is inserted into the applicator and both high- and low-resolution SAR distributions are computed for this geometry. For the same geometry, zoomed-resolution SAR distributions are computed with different sizes of Q. To compare the low- and zoomed-resolution SAR distributions with the high-resolution one, the correlation and averaged absolute difference are computed. These numbers are improved considerably using zooming (correlation 58% to 92%; averaged absolute difference 43% to 20%). These results appear to be independent of the size of Q, up to 0.3 lambda(w). Quasistatic zooming is a valuable tool in high-resolution regional HTP.

High-resolution SAR modelling for regional hyperthermia: testing quasistatic zooming at 10 MHz

Present-day regional hyperthermia treatment planning systems are limited to centimetre resolution. To obtain CT-resolution SAR distributions, a method called quasistatic zooming has been developed: using the centimetre-resolution E-field distribution and the CT-resolution tomogram, the CT-resolution SAR distribution is obtained. For a low frequency of 10 MHz this method has been validated sucessfully using CT-resolution SAR computations. It appears that these CT-resolution SAR distributions are completely different from centimetre-resolution SAR distributions, indicating the necessity for high-resolution SAR modelling. Using the presented zooming technique, reliable CT-resolution SAR modelling is now possible with relatively short computation times. So far, the zooming method has only been validated for low frequencies, but clinically relevant frequencies appear to be possible.

Clinical evaluation and verification of the hyperthermia treatment planning system hyperplan

PURPOSE

A prototype of the hyperthermia treatment planning system (HTPS) HyperPlan for the SIGMA-60 applicator (BSD Medical Corp., Salt Lake City, Utah, USA) has been evaluated with respect to clinical practicability and correctness.

MATERIALS AND METHODS

HyperPlan modules extract tissue boundaries from computed tomography (CT) images to generate regular and tetrahedral grids as patient models, to calculate electric field (E-field) distributions, and to visualize three-dimensional data sets. The finite difference time-domain (FDTD) method is applied to calculate the specific absorption rate (SAR) inside the patient. Temperature distributions are calculated by a finite-element code and can be optimized. HyperPlan was tested on 6 patients with pelvic tumors. For verification, measured SAR values were compared with calculated SAR values. Furthermore, intracorporeal E-field scans were performed and compared with calculated profiles.

RESULTS

The HTPS can be applied under clinical conditions. Measured absolute SAR (in W/kg), as well as relative E-field scans, correlated well with calculated values (+/-20%) using the contour-based FDTD method. Values calculated by applying the FDTD method directly on the voxel (CT) grid, were less well correlated with measured data.

CONCLUSION

The HyperPlan system proved to be clinically feasible, and the results were quantitatively and qualitatively verified for the contour-based FDTD method.

Optimization of pelvic heating rate distributions with electromagnetic phased arrays

Deep heating of pelvic tumours with electromagnetic phased arrays has recently been reported to improve local tumour control when combined with radiotherapy in a randomized clinical trial despite the fact that rather modest elevations in tumour temperatures were achieved. It is reasonable to surmise that improvements in temperature elevation could lead to even better tumour response rates, motivating studies which attempt to explore the parameter space associated with heating rate delivery in the pelvis. Computational models which are based on detailed three-dimensional patient anatomy are readily available and lend themselves to this type of investigation. In this paper, volume average SAR is optimized in a predefined target volume subject to a maximum allowable volume average SAR outside this zone. Variables under study include the position of the target zone, the number and distribution of radiators and the applicator operating frequency. The results show a clear preference for increasing frequency beyond 100 MHz, which is typically applied clinically, especially as the number of antennae increases. Increasing both the number of antennae per circumferential distance around the patient, as well as the number of independently functioning antenna bands along the patient length, is important in this regard, although improvements were found to be more significant with increasing circumferential antenna density. However, there is considerable site specific variation and cases occur where lower numbers of antennae spread out over multiple longitudinal bands are more advantageous. The results presented here have been normalized relative to an optimized set of antenna array amplitudes and phases operating at 100 MHz which is a common clinical configuration. The intent is to provide some indications of avenues for improving the heating rate distributions achievable with current technology.

Computational techniques for fast hyperthermia temperature optimization

Hyperthermia temperature optimization involves arriving at a temperature distribution which minimizes a stated goal function, the goal function having a biological basis in maximizing tumor cell kill while not exceeding normal tissue toxicity. This involves the computationally intensive process of multiple evaluations of the temperature goal function, requiring repeated evaluations of the power deposition and its corresponding temperature distribution. Two computational schemes are proposed to expedite the temperature optimization process: (1) temperature distribution evaluation by superpositioning precomputed distributions, and (2) using representative tissue groups (rather than every point in the domain) to evaluate the goal function. The application of these schemes is illustrated with a typical optimization problem, as applied to symmetric and asymmetric, heterogeneous models. Application of these schemes reduced the optimization time on a DEC Alpha 1000 4/266 (Alpha is a registered trademark of Digital Equipment Corporation.) from several h to min, with little difference in results. The computational schemes, though demonstrated in the context of electromagnetic hyperthermia, are generally applicable to other forms of nonionizing radiation employed in hyperthermia therapy.

Rationale for using invasive thermometry for regional hyperthermia of pelvic tumors

PURPOSE

Invasive thermometry for regional hyperthermia is time-consuming, uncomfortable, and risky for the patient. We tried to estimate the benefit/cost ratio of invasive thermometry in regional hyperthermia using the radiofrequency system BSD-2000.

METHODS AND MATERIALS

We evaluated 182 patients with locally advanced pelvic tumors that underwent regional hyperthermia. In every patient a tumor-related temperature measurement point was obtained either by invasive or minimally invasive catheter measurement tracks. In the earlier period for every patient an intratumoral measurement point was decided as obligatory and intratumoral catheters were implanted intraoperatively, CT guided, or under fluoroscopy. In the later period, invasive thermometry often was avoided, if a measurement point in or near the tumor was reached by an endoluminally inserted catheter (rectal, vaginal, cervical, urethral, or vesical). For every patient side effects and complications referred to thermometry were evaluated and compared with the potential benefit of the invasively achieved temperature data. The suitability of endolumimally registered temperatures is analyzed to estimate local feasibility (specific absorption rate achieved) and local effectiveness (thermal parameters correlated with response).

RESULTS

In 74 of 182 patients invasive thermometry was performed, at most CT-guided for soft tissue sarcomas and rectal recurrences. In 14 of 74 (19%) side effects such as local inflammation, pain, or abscess formation occurred that enforced removal of the catheter. However, local problems were strongly correlated with the dwell time of the catheter and nearly never occurred for dwell times less than 5 days. Fortunately, no fatal complications (e.g., bleeding or perforation) occurred during or after implantation which could be attributed to the invasive thermometry procedure. Endoluminal tumor-related temperature rises per time unit (to estimate power density) were correlated with intratumoral rises at the same patients (where both measurements were available). For a subgroup of patients pooled in two Phase II studies with rectal (n = 37) and cervical (n = 18) carcinomas thermal parameters derived from endoluminal measurements were correlated with response or local control, resp.

CONCLUSIONS

If a tumor-related endoluminal temperature measurement point is available, additional invasive thermometry gives no further information to improve the power deposition pattern. For primary rectal and cervical cancer, and probably as well for prostate, bladder and anal cancer, endoluminal measurements are suitable to estimate local feasibility and effectiveness. Therefore, invasive thermometry is dispensable in the majority of patients. In some selected cases, temperature measurement in the tumor center is required to estimate the maximum temperature. In those cases, dwell time of catheters should be minimized--and it should be considered to perform invasive thermometry at the beginning (one or two heat treatments).

Minimax optimization-based inverse treatment planning for interstitial thermal therapy

The following work represents the development and evaluation of a minimax optimization-based inverse treatment planning approach for interstitial thermal therapy of cancer and benign disease. The goal is to determine a priori optimal applicator placements and power level settings to maintain the minimum tumour temperature, Tmin, and maximum normal tissue temperature, Tmax within a prescribed therapeutic temperature range. The temperature distribution is approximated by a finite element method (FEM) solution of a bioheat transfer equation on a nonuniform finite element mesh. Lower and upper therapeutic temperature thresholds are specified in the tumour and surrounding normal tissues. A constrained minimax optimization problem is formulated to determine optimal applicator positions and power level settings that minimize the maximum (rather than average) temperature errors in the target tumour region and surrounding normal tissues. The optimization problem is formulated for two general classes of interstitial heating applicators, those with and without a surface cooling mechanism. The viability and sensitivity of this approach is investigated in the two-dimensional setting for various tumour shapes and blood perfusion levels using surface-cooled and direct-coupled interstitial ultrasound applicator power deposition models. These preliminary results indicate the utility of this approach for meeting a prescribed Tmin/Tmax-based clinical objective criterion, and its potential for generating optimal treatment plans that can withstand variations or uncertainty in blood perfusion levels.

ESHO quality assurance guidelines for regional hyperthermia

The Technical Committee and the Clinical Committee of the ESHO evaluated the experience of the institutes which are active in clinical regional hyperthermia using radiative equipment. Based on this evaluation, QA guidelines have been formulated. The focus of these guidelines lies on what must be done not on how it should be done. Subjects covered are: treatment planning, treatment, treatment documentation, requirements and characterization of equipment, safety aspects, hyperthermia staff requirements and instrumentation for quality assurance.

Research and development of hyperthermia machines for present and future clinical needs

This article describes the clinical problems encountered with the use of hyperthermia equipment and the requisites in the development of more advanced systems. A summary of the trends in the development of hyperthermia equipment is presented. In addition, a description from the physical point of view is included for the design of new applicators for deep heating.

Molecular chaperones in the etiology and therapy of cancer

Molecular chaperones are ubiquitous, well-conserved proteins that account for 2-5 % of all cellular proteins in most cells. The present review summarizes our current knowledge about their involvement in the etiology and therapy of cancer with special emphasis on the expression of chaperones in malignant cells, their role in folding of (proto)oncogene products, cell cycle regulation, cell differentiation and apoptosis, development of metastasis, and their participation in the recognition of malignant cells. We also overview the importance of chaperones in hyperthermia, drug resistance, and recent approaches in chaperone-immunotherapy.

Simulation studies promote technological development of radiofrequency phased array hyperthermia

A treatment planning program package for radiofrequency hyperthermia has been developed. It consists of software modules for processing three-dimensional computerized tomography (CT) data sets, manual segmentation, generation of tetrahedral grids, numerical calculation and optimisation of three-dimensional E field distributions using a volume surface integral equation algorithm as well as temperature distributions using an adaptive multilevel finite-elements code, and graphical tools for simultaneous representation of CT data and simulation results. Heat treatments are limited by hot spots in healthy tissues caused by E field maxima at electrical interfaces (bone/muscle). In order to reduce or avoid hot spots suitable objective functions are derived from power deposition patterns and temperature distributions, and are utilised to optimise antenna parameters (phases, amplitudes). The simulation and optimisation tools have been applied to estimate the improvements that could be reached by upgrades of the clinically used SIGMA-60 applicator (consisting of a single ring of four antenna pairs). The investigated upgrades are increased number of antennas and channels (triple-ring of 3 x 8 antennas and variation of antenna inclination. Significant improvement of index temperatures (1-2 degrees C) is achieved by upgrading the single ring to a triple ring with free phase selection for every antenna or antenna pair. Antenna amplitudes and inclinations proved as less important parameters.

A phase I/II study to evaluate radiation therapy and hyperthermia for deep-seated tumours: a report of RTOG 89-08

The purpose of this paper is to evaluate the safety and efficacy of deep hyperthermia in conjunction with radiation therapy. This study employed 'second generation' electromagnetic devices which were felt to be better able to confine heating and spare normal tissue than the devices evaluated in a previous study (RTOG 84-01). Sixty six patients at six institutions were enrolled on a prospective Phase I/II study. Eligible deep seated tumours were treated with a combination of external hyperthermia and radiation therapy. Radiation consisted of 1.7-2 Gy per fraction, 4-5 fractions per week, to > 20 Gy (previously irradiated lesions) or > 50 Gy (no previous radiation). Deep hyperthermia was delivered with electromagnetic devices: BSD 2000 for 92% of cases, Thermotron for 5% of cases, other low frequency electromagnetic for 4% of cases. Hyperthermia was delivered < or = twice weekly. Overall complete and partial response rates were 34% and 16% respectively. Response was not correlated with maximum tumour temperature or disease site. There was, however, a strong association with radiation dose: 54% CR with > or = 45 Gy versus 7% with < 45 Gy (p < 0.0001). The achieved temperatures were less than ideal. Although the average maximum tumor temperature was 41.9 degrees C (range 35.7 degrees C-46.7 degrees C), the minimum tumour temperatures were low. The average minimum tumour temperature was 38.5 degrees C and was never > 41.8 degrees C. Treatment was well tolerated with no fatalities. There were four acute grade 3 or 4 toxicities (6% of patients). Patient discomfort resulted in interruption or discontinuation of sessions in 30% of the sessions. In 12 cases (18% of patients) the planned course of hyperthermia was discontinued due to acute discomfort. The devices used in this study were better tolerated than the devices used in the previous Phase I/II deep hyperthermia trial (RTOG 84-01) with less patient discomfort and no problems with severe systemic cardiovascular stress. In the previous study 68% of the hyperthermia courses were prematurely terminated primarily due to patient discomfort and toxicity; in the present study 18% were prematurely terminated. However, as indicated by the low minimum tumour temperature, fundamental problems with achieving acceptable temperature distributions remain.

Hyperthermia treatment planning and temperature distribution reconstruction: a case study

While a great deal of effort has been applied toward solving the technical problems associated with modelling clinical hyperthermia treatments, much of that effort has focused on only estimating the power deposition. Little effort has been applied toward using the modelled power depositions (either electromagnetic (EM) or ultrasonic) as inputs to estimate the hyperthermia induced three-dimensional temperature distributions. This paper presents a case report of a patient treated with hyperthermia at the Duke University Medical Center where numerical modelling of the EM power deposition was used to prospectively plan the treatment. Additionally, the modelled power was used as input to retrospectively reconstruct the transient three-dimensional temperature distribution. The modelled power deposition indicated the existence of an undesirable region of high power in the normal tissue. Based upon this result, amplitudes and phases for driving the hyperthermia applicator were determined that eliminated the region of high power and subsequent measurements confirmed this. The steady-state and transient three-dimensional temperature distributions were reconstructed for four out of the seven treatments. The reconstructed steady-state temperatures agreed with the measured temperatures; root-mean-square error ranged from 0.45 to 1.21 degrees C. The transient three-dimensional tumour temperature was estimated assuming that the perfusion was constant throughout the treatment. Using the computed three-dimensional transient temperature distribution, the hyperthermia thermal dose was computed. The equivalent minutes at 43 degrees C achieved by 50% (T50Eq43) of the tumour volume was computed from the measured data and the three-dimensional reconstructed distribution yielding T50Eq43 = 40.6 and 19.8 min respectively.

Quality control of the SIGMA applicator using a lamp phantom: a four-centre comparison

An elliptical phantom with a fat-equivalent ring and lamp matrix was developed for observing the power distribution in ring applicators used for regional hyperthermia. This phantom was used on four European BSD-2000-type therapy systems under routine conditions to test the quality of the SIGMA-60 applicator (systems in Berlin, Essen, Munich and Rotterdam). Frequency-dependent focusing imbalances were observed in all systems. At the time of the quality control test two of the systems displayed considerable errors in their settings. The system setups and possible ways of correcting errors are described in detail. Small maladjustments are caused by coupling effects between antennas and their surroundings and by interactions between the coaxial cables which supply the power. Serious distortions can be caused by phase errors and defects in cables or plugs; the latter can result in significant long-term restrictions on the ability to control the SAR (specific absorption rate) distribution in a way the user may not notice. The measurements gained from these four systems confirm the need for a practical and universal procedure for quality control in regional hyperthermia.

Verification of a hyperthermia model method using MR thermometry

Simulation of hyperthermia induced power and temperature distributions is becoming generally accepted and finding its way into clinical hyperthermia treatments. Such simulations provide a means for understanding the complete three-dimensional temperature distribution. However, the results of the simulation studies should be regarded with caution since modelling errors will result in differences between the actual and simulated temperature distribution. This study uses a diffusion weighted magnetic resonance (MR) based technique to measure hyperthermia induced temperature distributions in a three-dimensional space in a non-perfused phantom. The measured data are used to verify the accuracy of numerical simulations of the same three-dimensional temperature distributions. The simulation algorithm is a finite element based method that first computes the electromagnetic induced power deposition then the temperature distribution. Two non-perfused phantom studies were performed and qualitatively the MR and simulated distributions agreed for steady-state. However, due to the long MR sampling time (approximately 4 min), poor agreement between the simulations and MR measurements were obtained for thermal transients. Good agreement between the simulations and fibreoptic thermometry measurements were obtained. The fiberoptic measurements differed from the simulations by 0.11 +/- 0.59 degrees C and -0.17 +/- 0.29 degrees C (mean +/- standard deviation for the two studies).

Noninvasive prediction of SAR distributions with an electro-optical E field sensor

An integrated electro-optical (eo) E field sensor is developed on the basis of a Ti:LiNbO3 Mach-Zehnder interferometer. A measuring device based on the lock-in principle is introduced to register the E field in phase and amplitude using this E field probe. Segmented electrodes are used to minimize influences from the dielectric surroundings on the base point capacitance of the receiving dipole. The operating point is stabilized against drift phenomena resulting from optical damage and pyroelectric effect. Sensitivity, dynamic range, harmonic distortions and mechanical properties of a prototype of this electro-optical E field sensor are evaluated. A phantom setup in the SIGMA-60 applicator was developed to test this electro-optical sensor for hyperthermia applications. Power deposition patterns of various standard adjustments of the SIGMA ring are visualized in an elliptical lamp phantom. Simultaneously, E field in phase and amplitude is determined on a closed curve in 10 degrees steps around the phantom in a substitute bolus. The numbers are stored and utilized as boundary conditions in a two-dimensional finite elements code which calculates the SAR distribution on an appropriate triangular grid inside the closed curve. An excellent qualitative agreement is obtained between visualized and calculated SAR patterns. This novel measurement method is therefore suitable for noninvasive monitoring of SAR patterns during clinical application of regional radiofrequency hyperthermia.

Clinical, physiological and anatomical determinants for radiofrequency hyperthermia

Temperature/time curves and corresponding CT scans of > 200 regional heat treatments with the hyperthermia system BSD-2000 in 43 patients have been analysed. In vivo variables and treatment parameters such as local specific absorption rate SAR, local relative SAR parallel SAR parallel, total power P, local cooling coefficients wb, and local steady-state temperature elevations delta Tss (above systemic temperature) have been determined. For determination of wb the well-known and accepted steady-state approach has been used, which was slightly modified for the purposes of this study. Specifically, comparison of cooling coefficients at the beginning and end of heat treatments were performed in tumours and normal tissues. Other variables are anatomical descriptors from CT scans, score of side effects plim, and various clinical factors. A variance analysis of the dependent variables, specifically delta Tss and parallel SAR parallel, is performed with respect to factors which were estimated as predictive. The intratumoral steady-state temperature elevations are determined by the perfusion-related cooling coefficients and local SAR to almost the same extent. Increase of cooling coefficients in tumours during the heat treatment characterizing the thermoregulatory potential have a slight but less important influence with respect to the achieved temperature elevations. SAR is influenced by several anatomical factors which determine the relative SAR distribution and clinical factors which limit the total power P. However, options for controlling present RHT systems in order to optimize the relative SAR distribution or to avoid hot spot phenomena appear limited. Three-dimensional modelling calculations show that the spatial arrangement of electrical interfaces emerging from bone and fat structures limits SAR control in available RHT technology and is mainly responsible for local power-dependent discomfort (Wust et al. 1994b). Some conclusions are drawn, about how technological development of hyperthermia technology can contribute towards overcoming this problem.

Application of new technology in clinical hyperthermia

Two areas of technical progress related to hyperthermic oncology are presented: (1) numerical modelling of absorbed power and temperature distributions; and (2) non-invasive thermometry using magnetic resonance imaging. The results represent achievements made during the past 5 years at Duke University Medical Center's Departments of Radiation Oncology and Radiology. They represent examples of progress in the technology of hyperthermia that have potential for greatly improving the delivery, monitoring and assessment of clinical hyperthermia.

Development and testing of SAR-visualizing phantoms for quality control in RF hyperthermia

A new prototype of an elliptical standard phantom with fat-equivalent walls and a lamp matrix for SAR (specific absorption rate) visualization has been developed. This paper outlines the manufacture of solid components based upon either polyester resin or epoxy resin, as well as the adjustment of their electrical conditions (epsilon r, sigma) by admixtures of carbon and/or aluminium powder. Visualizing sensors (LED = light-emitting diodes, miniature lamps) are evaluated with respect to their transformation of electric field strength into light. Standard SAR patterns of the hyperthermia system BSD-2000 have been semiquantitatively assessed by the visualizing technique (power stepping method) and quantitatively by E field sensor scans. Extracted iso-SAR distributions are in good agreement with E field sensor scans performed with a lamp sensor coupled to a fibre or using a dipole probe with high resistive leads. The requirement for periodic quality control of SAR patterns of RF (radio frequency) hyperthermia systems is demonstrated. Comparisons between techniques are given, specifically with respect to the LED phantom of Schneider and van Dijk.