Hyperthermia Treatment Planning System Research Articles

Experimental validation of a thermophysical fluid model for use in a hyperthermia treatment planning system

Accurate hyperthermia treatment planning, monitoring, and evaluation of temperatures in and near fluid volumes in the body requires realistic modelling of heat transport within fluids, which is currently not implemented in available treatment planning packages. Aim of this study is to assess the accuracy of a thermophysical fluid model, developed for treatment planning near fluid volumes.A cubic phantom with inner dimensions of (7 cm)3 was filled with deionised water. The front, back, top and bottom walls of the cube consisted of PVC, the side walls of stainless steel. The left wall was kept at a constant temperature of 25 or 37 °C, the right wall at 1, 2, 5, 10, or 15 °C higher. Thermal probes mapped the temperature profile in the central vertical plane perpendicular to the cold and hot walls with a spatial resolution of 5–10 mm. The temperature distributions were compared to simulations using a finite volume-based thermophysical fluid model implementing the Boussinesq approximation to the Navier-Stokes equations, developed as an extension to our in-house developed hyperthermia treatment planning suite. The simulations were performed using three meshes at different resolutions.The fluid model predicts the temperature distribution accurately (random and systematic error <0.1 °C, at least 95% of absolute errors <0.2 °C) for hyperthermic temperature differences (<5 °C within the fluid volume). When the temperature differences reach 15 °C, the random and systematic errors increase to 0.3 °C and 0.1 °C, respectively, with absolute errors up to 1.1 °C.The thermophysical fluid model predicts temperature distributions in a convective fluid with sufficient accuracy for hyperthermia treatment planning in and near fluid regions. A mesh with a resolution of 0.25 cm combines accurate results with acceptable computation times.

SU‐F‐J‐05: The Effect of Air Pockets in the Urinary Bladder During Bladder Hyperthermia Treatment

Purpose:Hyperthermia combined with Mitomycin C is used for the treatment of non‐muscle invasive bladder cancer (NMIBC), using a phased array system of microwave antennas for bladder heating. Often some air is present in the bladder, which effectively blocks the microwave radiation, potentially preventing proper treatment of that part of the bladder. Air can be a relevant fraction of the bladder content and large air pockets are expected to have a noticeable influence on achieved temperatures.Methods:We analysed 14 NMIBC patients treated at our institute with our AMC‐4 hyperthermia device with four 70MHz antennas around the pelvis. A CT scan was made after treatment and a physician delineated the bladder on the CT scan. On the same scan, the amount of air present in the bladder was delineated. Using our in‐house developed hyperthermia treatment planning system, we simulated the treatment using the clinically applied device settings. We did this once with the air pocket delineated on the CT scan, and once with the same volume filled with bladder tissue.Results:The patients had on average 4.2ml (range 0.8–10.1ml) air in the bladder. The bladder volume was delineated by the physician, that is including air pocket and bladder wall, was on average 253ml (range 93–452ml). The average volume in which changes exceeded 0.25°C was 22ml (range 0–108 ml), with the bladder being up to 2°C cooler when an air pocket was present. Except for extreme cases, there was no evident relation between the quantity of air and the difference in temperature.Conclusion:The effect of an air pocket in the bladder during bladder hyperthermia treatment varies strongly between patients. Generally, this leads to lower temperatures in the bladder, potentially affecting treatment quality, and suggesting that care need be taken to minimise the size of air pockets during hyperthermia treatments.The KWF Dutch Cancer Society financially supported this work, grant UVA 2012‐5539.

Steering in locoregional deep hyperthermia: Evaluation of common practice with 3D-planning

Purpose: In Rotterdam, fifteen years of clinical experience with deep hyperthermia has sublimated in empirical treatment guidelines. In this paper, a hyperthermia treatment planning system (HTPS) is employed to investigate the effect of these guidelines on global power distribution, their effectiveness and the rationale behind each guideline.Materials and methods: Four guidelines were investigated. The first two prescribe steering actions for balancing intraluminal temperatures and alleviating complaints of deep-seated pain or pressure. The third guideline handles superficial complaints of pain or heat sensation. The last guideline states that frequency should be increased from 77 MHz upwards in case of multiple, opposite, painful regions uncontrollable by the previous steering actions.For all steering actions it is assumed that input power is increased until complaints occur. Sigma Hyperplan was used to calculate specific absorption rate (SAR) distributions for five patient models with locally advanced cervical cancer. Absorbed power ratios of different regions of interest were evaluated to illustrate steering efficacy and complaint reduction.Results and conclusions: Phase steering is effective in shifting the central power distribution to the periphery, and is an appropriate method to balance temperatures or to handle deep-seated complaints. Reduction of amplitude is the proper action to alleviate superficial complaints of heat or pressure. Compression of the SAR distribution, mainly in the lateral direction, is predicted with increasing frequency. Hence, for complaints in the lower back or on the sides, a frequency increase should be considered.We conclude that the results of the HTPS are in close agreement with the empirical steering guidelines.

Improvement of absorbing structures used in regional hyperthermia

Local pain is a major limiting factor in regional hyperthermia treatment with radiative applicators. Absorbing structures, consisting of agar bound saline water, have been used successfully to reduce peripheral hot spots. However, both clinical experience and simulation results indicate a SAR elevation in the tissue under the edges of the absorber block. This paper investigates the effect of modification of shape, position and spatial composition of the absorber blocks on the central attenuating effect and the SAR elevating effect at the edges. A selection from a set of five options is made based on simulations with a phantom and a single ring dipole applicator. The simulations have been performed with the FDTD core of the regional hyperthermia treatment planning system. It is shown that tapering of the absorber edge and introduction of a water layer between the absorber and the skin can reduce the edge effect in the superficial fat layer by ∼50% with respect to a rectangular absorber. A further reduction of 15% can be obtained by an absorber with an appropriate gradient of its conductivity in the direction of the dominant -field. The modified absorbers produce a central attenuating effect comparable to the rectangular type. The use of a water layer type and a sigma gradient type absorber is also analysed in a patient anatomy, both in the dipole ring applicator, operating at 70 MHz, as well as in a three ring Cavity Slot (CS) applicator, operating at 150 MHz. The mutual influence of phase-amplitude steering and the application of absorbers is investigated in the CS applicator. It appears that absorbers have a significant influence on the interference pattern in the patient model, possibly causing substantial reduction of the SAR value in the tumour and limiting the possibility of ad hoc application of absorbers. Re-optimization can only partly cancel this effect. Local SAR reduction by phase-amplitude control alone can match or improve the effect obtained with modified absorbers.

Clinical use of the hyperthermia treatment planning system HyperPlan to predict effectiveness and toxicity

Purpose: The main aim is to prove the clinical practicability of the hyperthermia treatment planning system HyperPlan on a β-test level. Data and observations obtained from clinical hyperthermia are compared with the numeric methods FE (finite element) and FDTD (finite difference time domain), respectively. Methods and Materials: The planning system HyperPlan is built on top of the modular, object-oriented platform for visualization and model generation AMIRA. This system already contains powerful algorithms for image processing, geometric modeling, and three-dimensional graphics display. A number of hyperthermia-specific modules are provided, enabling the creation of three-dimensional tetrahedral patient models suitable for treatment planning. Two numeric methods, FE and FDTD, are implemented in HyperPlan for solving Maxwell’s equations. Both methods base their calculations on segmented (contour based) CT or MR image data. A tetrahedral grid is generated from the segmented tissue boundaries, consisting of approximately 80,000 tetrahedrons per patient. The FE method necessitates, primarily, this tetrahedral grid for the calculation of the E-field. The FDTD method, on the other hand, calculates the E-field on a cubical grid, but also requires a tetrahedral grid for correction at electrical interfaces. In both methods, temperature distributions are calculated on the tetrahedral grid by solving the bioheat transfer equation with the FE method. Segmentation, grid generation, E-field, and temperature calculation can be carried out in clinical practice at an acceptable time expenditure of about 1–2 days. Results: All 30 patients we analyzed with cervical, rectal, and prostate carcinoma exhibit a good correlation between the model calculations and the attained clinical data regarding acute toxicity (hot spots), prediction of easy-to-heat or difficult-to-heat patients, and the dependency on various other individual parameters. We could show sufficient agreement between the calculations and measurements for power density (specific absorption rate) within the range of assessed precision. Tumor temperatures can only be estimated, because of the rather variable perfusion conditions. The results of the FE and FDTD methods are comparable, although slight differences exist resulting from the differences in the underlying models. There are also statistically provable differences among the tumor entities regarding the attained specific absorption rate, temperatures, and volume loads in normal tissue. However, gross fluctuations exist from patient to patient. Conclusion: The hyperthermia planning system HyperPlan could be validated for a number of the 30 patients. Further improvements in the implemented models, FE and FDTD, are required. Even at its present state of development, hyperthermia planning for regional hyperthermia delivers valuable information, not only for clinical practice, but also for further technologic improvements.

Determination and validation of the actual 3D temperature distribution during interstitial hyperthermia of prostate carcinoma

To determine the thermal dose of a hyperthermia treatment, knowledge of the three-dimensional (3D) temperature distribution is mandatory. The aim of this paper is to validate an interstitial hyperthermia treatment planning system with which the full 3D temperature distribution can be obtained in individual patients. Within a phase I study, 12 patients with prostate cancer were treated with interstitial hyperthermia using our multi electrode current source interstitial hyperthermia treatment (MECS IHT) system. The temperature distribution was measured from within the heating devices and by additional thermometry. The perfusion level was estimated and the heating implant reconstructed. The steady-state temperature distribution was calculated using our interstitial hyperthermia treatment planning system. The simulated temperature distribution was validated by individually comparing the measured and simulated thermo-sensors, both for the thermometry integrated with the heating applicators and the additional thermometry. The entire procedure was also performed on a no-flow agar–agar phantom. It was shown that the calculated temperature distribution of an individual patient during MECS interstitial hyperthermia is very heterogeneous. The validation indicates that the calculated temperature elevations match the measurements within approximately 1 °C. Possible improvements are more precise reconstruction, incorporation of discrete vasculature and using a temperature-dependent, heterogeneous perfusion distribution. Further technical improvements of the MECS-IHT system may also result in better temperature calculations.

Development of a regional hyperthermia treatment planning system.

A flexible and fast regional hyperthermia treatment planning system for the Coaxial TEM System has been devised and is presented. Using Hounsfield Unit based thresholding and manually outlining of the tumour, a 40 cm CT data set (slice thickness 5 mm) is segmented and down scaled to a resolution of 1 cm, requiring only 30 min. The SAR model is based on the finite-difference time-domain (FDTD) method. The number of time steps to achieve numerical stability has been determined and was found to be 7000. Various optimizations of the SAR model have been applied, resulting in a relatively short computation time of 3.7 h (memory requirements 121 MB) on a Pentium III, 450 MHz standard personal computer, running GNU/Linux. The model has been validated using absolute value(Ez) measurements in a standard phantom inserted in the Coaxial TEM Applicator under different conditions and a good agreement was found. Hyperthermia treatment planning in combination with the homemade visualization tools have provided much insight in the regional hyperthermia treatment with the Coaxial TEM Applicator.

The use of absorbing structures during regional hyperthermia treatment.

Local pain is the main factor that limits regional hyperthermia treatment. Using the SAR model of the regional hyperthermia treatment planning system, the capability of absorbing blocks to reduce peripheral hot spots was investigated. The effect of rectangular absorbers of various size and salinity on an elliptical phantom in the Coaxial TEM was evaluated. The computed results were compared with SAR values measured in the phantom. Absorbers of 9 x 9 x 4 cm3 and a salinity of 18 gram l(-1) provide a SAR reduction in the muscle equivalent material, centrally under the absorber of at least 50% at a depth of up to 3 cm. The effect on the central (i.e. tumour) region is less than 20%. Larger absorbers have a more global effect and cause more attenuation in the central region. The attenuating effect depends strongly on the thickness of the fat layer between muscle and absorber. More than 2 cm fat limits the effective use of absorbers. Absorbers can induce a significant increase of SAR in muscle and fat near their edges. This effect also depends on absorber size and salinity and the thickness of the fat layer. The effect of an absorber was also evaluated with a patient anatomy, yielding results in agreement with the phantom experiments.

Development of a regional hyperthermia treatment planning system

A flexible and fast regional hyperthermia treatment planning system for the Coaxial TEM System has been devised and is presented. Using Hounsfield Unit based thresholding and manually outlining of the tumour, a 40cm CT data set (slice thickness 5mm) is segmented and down scaled to a resolution of 1cm, requiring only 30min. The SAR model is based on the finite-difference time-domain (FDTD) method. The number of time steps to achieve numerical stability has been determined and was found to be 7000. Various optimizations of the SAR model have been applied, resulting in a relatively short computation time of 3.7h (memory requirements 121MB) on a Pentium III, 450MHz standard personal computer, running GNU/Linux. The model has been validated using |Ez| measurements in a standard phantom inserted in the Coaxial TEM Applicator under different conditions and a good agreement was found. Hyperthermia treatment planning in combination with the homemade visualization tools have provided much insight in the regional hyperthermia treatment with the Coaxial TEM Applicator.

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.

Numerical analysis of capacitively coupled electrodes for interstitial hyperthermia

Multi electrode current source interstitial hyperthermia (MECS-IHT) employs individually controlled, 27 MHz radiofrequency electrodes inserted into plastic brachytherapy catheters. In order to get a firm understanding of the physical behaviour of the electrodes and to verify the current source approximation in our hyperthermia treatment planning system we have investigated (1) the electrical properties of the electrode-catheter-tissue system, and (2) the impact of inhomogeneity of the electrical properties of the tissue in the vicinity of the electrodes. The results validate the use of the ideal current source approximation in the treatment planning SAR model. The models predict the presence of a significant heat source inside the electrode wall when lossy catheter materials are used, producing a conductive heating component in addition to the SAR in the tissue. For a given catheter spacing this conductive component will produce a more heterogeneous temperature distribution. Thus, low-loss catheter materials like polyethylene and Teflon are recommended. The SAR is highly localized near the catheter. Calculations concerning a fat-muscle interface show that the SAR is higher in the fatty tissue than in the muscle tissue; 3D SAR control by individually controlled electrode segments is essential in such a situation.

Treatment planning for hyperthermia with ultrasound phased arrays

Treatment planning for ultrasound phased arrays suggests a strategy for hyperthermia therapy which satisfies therapeutic conditions at the target and spares other sensitive anatomical structures. To predict both desirable and harmful interactions between ultrasound and important structures such as the tumor, bones, and air pockets, a hyperthermia treatment planning system has been developed for ultrasound phased arrays. This collection of treatment planning routines consists of geometric and thermal optimization procedures specific to ultrasound phased arrays, where geometric treatment planning, combined with thermal treatment planning and three-dimensional visualization, provides essential information for the optimization of individual patient treatments. A patient image data set for cancer of the prostate, a difficult target situated in the midst of multiple pelvic bone obstructions, illustrates the geometric treatment planning algorithm and other tools for treatment analysis. The results indicate that the analysis of complex three-dimensional relationships between the applicator, anatomical structures, and incident fields provides an important means of predicting treatment limiting conditions, thereby allowing the hyperthermia applicator to electronically adapt to individual patients and specific sites.

A fast and simple algorithm for the calculation of convective heat transfer by large vessels in three-dimensional inhomogeneous tissues

A fast and simple algorithm has been presented for the calculation of time-dependent temperature distributions in inhomogeneous vascularized tissue. Three-dimensional anatomical data of tissues and vessel structures are decomposed into elementary cubic nodes by a special digitizing routine with vessels represented by connected strings of vessel nodes. Vessel cross sections may be irregular shaped and/or tapered. Conductive and convective heat transfer was calculated through use of the heat balance technique on each cubic node resulting in an explicit finite difference computational scheme. Employing a three time level scheme, the Fourier stability criterion is circumvented allowing arbitrary time steps to be defined in the algorithm. Time steps as large as 100 times the Fourier restricted one still result in stable and convergent calculations of the stationary temperature distribution. Vessels with different flows and diameters are incorporated by performing a vessel specific second discretization step in time. Using the new algorithm as a mathematical tool the thermal equilibration length of vessel segments have been established under a broad range of geometrical and flow conditions. Validation followed from comparing transient and stationary temperature distributions derived by the proposed algorithm to those from an accurate cylindrical numerical model. Predicted values for the thermal equilibration lengths are compared to an analytical expression and phantom experiments. The algorithm is incorporated in a thermal model being the main part of our hyperthermia treatment planning system.