Simulation of electromagnetically induced hyperthermia: a finite element gridding method

Therapeutic hyperthermia

The term hyperthermia broadly refers to either an abnormally high fever or the treatment of a disease by the induction of fever. Its effect depends on the temperature and exposure time. The increasing number of applications and clinical trials at universities, clinics, and hospitals prove the feasibility and applicability of clinical therapeutic hyperthermia. This chapter aims to outline and discuss the means by which electromagnetic energy and other techniques can provide elevation of temperature within the human body. Because of the individual characteristic of each type of treatment, different modalities of heating systems have evolved. The chapter concludes with a discussion of challenges and opportunities for further improvement in technology and routine clinical application.

Planning, optimisation and evaluation of hyperthermia treatments

BACKGROUND

Hyperthermia treatment planning using dedicated simulations of power and temperature distributions is very useful to assist in hyperthermia applications. This paper describes an advanced treatment planning software package for a wide variety of applications.

METHODS

The in-house developed C++ software package Plan2Heat runs on a Linux operating system. Modules are available to perform electric field and temperature calculations for many heating techniques. The package also contains optimisation routines, post-treatment evaluation tools and a sophisticated thermal model enabling to account for 3D vasculature based on an angiogram or generated artificially using a vessel generation algorithm. The use of the software is illustrated by a simulation of a locoregional hyperthermia treatment for a pancreatic cancer patient and a spherical tumour model heated by interstitial hyperthermia, with detailed 3D vasculature included.

RESULTS

The module-based set-up makes the software flexible and easy to use. The first example demonstrates that treatment planning can help to focus the heating to the tumour. After optimisation, the simulated absorbed power in the tumour increased with 50%. The second example demonstrates the impact of accurately modelling discrete vasculature. Blood at body core temperature entering the heated volume causes relatively cold tracks in the heated volume, where the temperature remains below 40 °C.

CONCLUSIONS

A flexible software package for hyperthermia treatment planning has been developed, which can be very useful in many hyperthermia applications. The object-oriented structure of the source code allows relatively easy extension of the software package with additional tools when necessary for future applications.

Current state of the art of regional hyperthermia treatment planning: a review

Locoregional hyperthermia, i.e. increasing the tumor temperature to 40–45 °C using an external heating device, is a very effective radio and chemosensitizer, which significantly improves clinical outcome. There is a clear thermal dose-effect relation, but the pursued optimal thermal dose of 43 °C for 1 h can often not be realized due to treatment limiting hot spots in normal tissue. Modern heating devices have a large number of independent antennas, which provides flexible power steering to optimize tumor heating and minimize hot spots, but manual selection of optimal settings is difficult. Treatment planning is a very valuable tool to improve locoregional heating. This paper reviews the developments in treatment planning software for tissue segmentation, electromagnetic field calculations, thermal modeling and optimization techniques. Over the last decade, simulation tools have become more advanced. On-line use has become possible by implementing algorithms on the graphical processing unit, which allows real-time computations. The number of applications using treatment planning is increasing rapidly and moving on from retrospective analyses towards assisting prospective clinical treatment strategies. Some clinically relevant applications will be discussed.

Simulation techniques in hyperthermia treatment planning

Abstract 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 set-ups 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.

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.

Discretizing large traceable vessels and using DE-MRI perfusion maps yields numerical temperature contours that match the MR noninvasive measurements

The success of hyperthermia treatments is dependent on thermal dose distribution. However, the three-dimensional temperature distribution remains largely unknown. Without this knowledge, the relationship between thermal dose and outcome is noisy, and therapy cannot be optimized. Accurate computations of thermal distribution can contribute to an optimized therapy. The hyperthermia modeling group in the Department of Radiotherapy, University Medical Center Utrecht devised a Discrete Vasculature [Kotte et al., Phys. Med. Biol. 41, 865-884 (1996)] model that accounts for the presence of vessel trees in the computational domain. The vessel tree geometry is tracked using magnetic resonance (MR) angiograms to a minimum diameter between 0.6 and 1 mm. However, smaller vessels (0.2-0.6 mm) are known to account for significant heat transfer. The hyperthermia group at Duke University Medical Center has proposed using perfusion maps derived from dynamic-enhanced magnetic resonance imaging to account for the tissue perfusion heterogeneity [Craciunescu et al., Int. J. Hyperthermia 17, 221-239 (2001)]. In addition, techniques for noninvasive temperature measurements have been devised to measure temperatures in vivo [Samulski et al., Int. J. Hypertherminal, 819-829 (1992)]. In this work, a patient with high-grade sarcoma has been retrospectively modeled to determine the temperature distribution achieved during a hyperthermia treatment. Available for this model were MR depicted geometry, angiograms, perfusion maps, as necessary for accurate thermal modeling, as well as MR thermometry data for validation purposes. The vasculature assembly through modifiable potential program [Van Leeuwen et al., IEEE Trans. Biomed. Eng. 45, 596-604 (1998)] was used in order to incorporate the traceable large vessels. Temperature simulations were made using different approaches to describe perfusion. The simulated cases were the bioheat equation with constant perfusion rates per tissue type, perfusion maps alone, tracked vessel tree and perfusion maps, and generated vessel tree. The results were compared with MR thermometry data for a single patient data set, concluding that a combination between large traceable vessels and perfusion map yields the best results for this particular patient. The technique has to be repeated on several patients, first with the same type of malignancy, and after that, on patients having malignancies at other different sites.

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.

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

BACKGROUND

A hyperthermia planning system has been developed for generating patient and applicator models as well as calculating and visualizing E-field and temperature distributions. Significant dependencies on models and algorithms have been found.

METHODS

Computerized tomography (CT) data sets are first transformed into so called 'labelled CT-volume'-data sets of equal resolution, which are used for segmentation. The first type of patient model obtained subsequently is based on regions with specified electrical properties representing tissues or organs (so called 'region-based model'). The second patient model renders a direct transformation of Hounsfield Units (HU) to electrical constants (so called 'HU-based model'). The FDTD-method (finite difference time domain) is then applied on a cubic lattice employing either an auxiliary 'sub-cubic lattice' (for HU-based segmentation) or a tetrahedron grid (for region-based segmentation) to assign the electrical properties, both representing the anatomy of the patient. E-field distributions are corrected by a post-processing procedure with respect to the geometry of interfaces defined by the tetrahedron grid. For comparison, the VSIE method (volume surface integral equation) is performed on the same tetrahedron grid. The applicator model assumes eight half-wavelength dipole antennas fed with constant voltages with water as background medium.

RESULTS

For both numerical methods (FDTD, VSIE) the resulting antenna input impedances as well as the current distributions along the antennas were quite similar and almost insensitive to the particular geometry model (region-based, HU-based). In contrast to that, the power deposition patterns in the interior of the patient depended strongly on those models. Major differences can be related to different labels of the tissue type bone in the HU-based model in comparison to the definition via regions. Conversely, comparable results were obtained using the VSIE method and the FDTD method on the region-based patient model with a posteriori correction at the tetrahedron grid points. SAR (specific absorption rate) elevations up to a factor of 10 were predicted when employing region-based models. Those peaks might correspond to specific toxicity of electromagnetic radiation clinically known as hot spot phenomena or musculo-skeletal syndromes. Conversely, HU-based models generated quite homogeneous power deposition patterns with fluctuations of at most factor 2.

CONCLUSION

The methods employing region-based geometry models such as the VSIE method and FDTD method in conjunction with a posteriori correction at tissue interfaces result in comparable E-field distributions for regional hyperthermia. Due to its shorter calculation time, the FDTD method is currently used in the clinic. Predictions derived from HU-based models without prior corrections of tissue specifications are not always supported by clinical experience.

Electromagnetic thermal therapy power optimization for multiple source applicators

The optimization of power deposition for electromagnetic (EM) thermal therapy is investigated. Several goal or objective functions are examined using a generalized mathematical formulation. These include maximization of: (1) target power absorption, (2) the ratio of target to non-target power absorption, (3) target power absorption weighted by the ratio of target to non-target power absorption, and (4) target power absorption subject to the constraint that the non-target high power volume ('hot spot' volume) is below a chosen level. The merit of these functions was retrospectively tested using an anatomic data base containing 38 cancer patients that were clinically heated with EM phased arrays. CT and/or MRI image data were used to define relevant anatomic geometries and tissue properties for finite element numerical models. Power optimization is achieved by variation of seven available control parameters (four amplitudes and three phases) for these clinical array devices. The results indicate that site dependent improvements in target power absorption can be achieved using these goal functions relative to a configuration that utilizes equal phase and amplitude for the sources. The relative merit among these various functions favours an optimization strategy that maximizes the target power absorption weighted by the ratio of target power to non-target power absorption.

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).

Combined external beam irradiation and external regional hyperthermia for locally advanced adenocarcinoma of the prostate

PURPOSE

To determine the safety and efficacy of combined external beam irradiation and external regional hyperthermia in the treatment of adenocarcinoma of the prostate.

METHODS AND MATERIALS

From 1987 to 1994, 30 patients received combined external beam irradiation and external regional hyperthermia for locally advanced prostate cancer. The results of the 21 patients with newly diagnosed (n = 18) or locally recurrent (n = 3) adenocarcinoma are reported herein. No patient had evidence of distant metastases. Total radiotherapy doses of 65-70 Gy to the prostate were planned using a four-field box technique. Hyperthermia treatments were delivered using an annular phased array microwave device. The treatment goal was to achieve temperatures > or = 42 degrees C in all measured points within the prostate.

RESULTS

Of the newly diagnosed patients, 16 out of 18 (89%) had T3 or T4 tumors, 11 out of 18 (61%) had Gleason scores of 7-9, and the mean pretreatment Prostate Specific Antigen (PSA) was 69 ng/ml. The median follow-up of all 21 patients was 36 months. None of the patients achieved the treatment goal of all intratumoral temperatures > or = 42 degrees C. The mean CEM 43 T90 was 2.34 min. The disease-free survival at 36 months is 25%; 12 out of 18 (67%) of the patients have relapsed. The only significant predictor of relapse was pretreatment PSA. There were no complications > Grade 3.

CONCLUSIONS

In spite of the inability to achieve high tumor temperatures, the relapse-free survival rate in this population of patients with very advanced localized prostate cancer treated with radiation therapy plus hyperthermia compares favorably with most series using radiation therapy alone. Further studies aimed at improving the ability to deliver hyperthermia to the prostate are warranted.