J. Van de Kamer

High-resolution temperature-based optimization for hyperthermia treatment planning

In regional hyperthermia, optimization techniques are valuable in order to obtain amplitude/phase settings for the applicators to achieve maximal tumour heating without toxicity to normal tissue. We implemented a temperature-based optimization technique and maximized tumour temperature with constraints on normal tissue temperature to prevent hot spots. E-field distributions are the primary input for the optimization method. Due to computer limitations we are restricted to a resolution of 1 x 1 x 1 cm3 for E-field calculations, too low for reliable treatment planning. A major problem is the fact that hot spots at low-resolution (LR) do not always correspond to hot spots at high-resolution (HR), and vice versa. Thus, HR temperature-based optimization is necessary for adequate treatment planning and satisfactory results cannot be obtained with LR strategies. To obtain HR power density (PD) distributions from LR E-field calculations, a quasi-static zooming technique has been developed earlier at the UMC Utrecht. However, quasi-static zooming does not preserve phase information and therefore it does not provide the HR E-field information required for direct HR optimization. We combined quasi-static zooming with the optimization method to obtain a millimetre resolution temperature-based optimization strategy. First we performed a LR (1 cm) optimization and used the obtained settings to calculate the HR (2 mm) PD and corresponding HR temperature distribution. Next, we performed a HR optimization using an estimation of the new HR temperature distribution based on previous calculations. This estimation is based on the assumption that the HR and LR temperature distributions, though strongly different, respond in a similar way to amplitude/phase steering. To verify the newly obtained settings, we calculate the corresponding HR temperature distribution. This method was applied to several clinical situations and found to work very well. Deviations of this estimation method for the AMC-4 system were typically smaller than 0.2 degrees C in the volume of interest, which is accurate enough for treatment planning purposes.

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.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.

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.

Treatment planning for capacitive regional hyperthermia

Capacitively coupled hyperthermia devices are widely in use, mainly in Asian countries. In this paper, a comprehensive treatment planning system, including a Specific Absorption Rate (SAR) and thermal model for capacitively coupled hyperthermia, is described and demonstrated using a heterogeneous patient model. In order to accurately model a hyperthermia treatment, simulation at high resolution is mandatory. Using the quasi-static approximation, the electromagnetic problem can be solved at high resolution with acceptable computational effort. The validity of the quasi-static approximation is demonstrated by comparing the Maxwell solution of a phantom problem to the quasi-static approximation. Modelling of capacitive hyperthermia of the prostate reveals the difficulty of heating deep-seated tumours in the pelvic area. Comparison of the SAR distribution in the heterogeneous patient model and a patient shaped agar phantom shows a shielding effect of the pelvic bone and the influence of the fat-muscle distribution. It is shown that evaluation of capacitive hyperthermia with agar phantoms leads to overly optimistic conclusions. Therapeutic relevant tumour temperatures can only be obtained by permitting temperature extrema in normal tissue. This concurs with clinical practice, where treatment-limiting hot spots restrict the tumour temperature. It is demonstrated that the use of very cold overlay bolus bags has only a very superficial effect. The presented model can be used for individual treatment planning and optimization, for the evaluation of capacitive applicator modifications and comparison with other devices.

Prospective treatment planning to improve locoregional hyperthermia for oesophageal cancer

Background: In the Academic Medical Center (AMC) Amsterdam, locoregional hyperthermia for oesophageal tumours is applied using the 70 MHz AMC-4 phased array system. Due to the occurrence of treatment-limiting hot spots in normal tissue and systemic stress at high power, the thermal dose achieved in the tumour can be sub-optimal. The large number of degrees of freedom of the heating device, i.e. the amplitudes and phases of the antennae, makes it difficult to avoid treatment-limiting hot spots by intuitive amplitude/phase steering. Aim: Prospective hyperthermia treatment planning combined with high resolution temperature-based optimization was applied to improve hyperthermia treatment of patients with oesophageal cancer. Methods: All hyperthermia treatments were performed with ‘standard’ clinical settings. Temperatures were measured systemically, at the location of the tumour and near the spinal cord, which is an organ at risk. For 16 patients numerically optimized settings were obtained from treatment planning with temperature-based optimization. Steady state tumour temperatures were maximized, subject to constraints to normal tissue temperatures. At the start of 48 hyperthermia treatments in these 16 patients temperature rise (ΔT) measurements were performed by applying a short power pulse with the numerically optimized amplitude/phase settings, with the clinical settings and with mixed settings, i.e. numerically optimized amplitudes combined with clinical phases. The heating efficiency of the three settings was determined by the measured ΔT values and the ΔT-ratio between the ΔT in the tumour (ΔToes) and near the spinal cord (ΔTcord). For a single patient the steady state temperature distribution was computed retrospectively for all three settings, since the temperature distributions may be quite different. To illustrate that the choice of the optimization strategy is decisive for the obtained settings, a numerical optimization on ΔT-ratio was performed for this patient and the steady state temperature distribution for the obtained settings was computed. Results: A higher ΔToes was measured with the mixed settings compared to the calculated and clinical settings; ΔTcord was higher with the mixed settings compared to the clinical settings. The ΔT-ratio was ∼1.5 for all three settings. These results indicate that the most effective tumour heating can be achieved with the mixed settings. ΔT is proportional to the Specific Absorption Rate (SAR) and a higher SAR results in a higher steady state temperature, which implies that mixed settings are likely to provide the most effective heating at steady state as well. The steady state temperature distributions for the clinical and mixed settings, computed for the single patient, showed some locations where temperatures exceeded the normal tissue constraints used in the optimization. This demonstrates that the numerical optimization did not prescribe the mixed settings, because it had to comply with the constraints set to the normal tissue temperatures. However, the predicted hot spots are not necessarily clinically relevant. Numerical optimization on ΔT-ratio for this patient yielded a very high ΔT-ratio (∼380), albeit at the cost of excessive heating of normal tissue and lower steady state tumour temperatures compared to the conventional optimization. Conclusion: Treatment planning can be valuable to improve hyperthermia treatments. A thorough discussion on clinically relevant objectives and constraints is essential.

Theoretical comparison of intraluminal heating techniques

Introduction: This study compared simulated temperature distributions of intraluminal heating devices, concerning penetration and homogeneity. A hot water balloon, a 434-MHz monopole and a 915-MHz dipole antenna, both with incorporated cooling, and a 27-MHz applicator were investigated. Methods: The hot water balloon had an inlet temperature of 45°C and a flow rate of 7.85 ml s−1. The cooling water and air had a temperature of 41°C and 37°C and a flow rate of 5.89 ml s−1 and 1.8 l s−1, respectively. A 27-MHz applicator consisting of one or two electrode(s) was modelled to demonstrate axial steering for inhomogeneous tissue properties. Calculated power distributions were scaled to a total power of 10 W in tissue before the corresponding temperature distributions were calculated. Results: The hot water balloon and the 27-MHz device showed a thermal penetration depth of ∼4 and ∼10 mm, respectively. The penetration depths of the 434- and 915-MHz applicators were comparable: ∼10 and ∼16 mm with water and air cooling, respectively. With the 27-MHz applicator, spatial steering was applied to minimize temperature gradients along the applicator. The 434- and 915-MHz antennas have no steering possibilities. The temperature distribution of the hot water balloon is not affected by inhomogeneous dielectric properties, only slightly by inhomogeneous perfusion. Conclusion: A hot water balloon is useful for heating tumours with a limited infiltration in tissue, while a 27-MHz device has the best potential to realize a homogeneous temperature distribution in larger tumours.

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.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 eŒect 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£9£4 cm 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 eŒect on the central (i.e. tumour) region is less than 20% . Larger absorbers have a more global eŒect and cause more attenuation in the central region. The attenuating eŒect depends strongly on the thickness of the fat layer between muscle and absorber. More than 2 cm fat limits the eŒective use of absorbers. Absorbers can induce a signi® cant increase of SAR in muscle and fat near their edges. This eŒect also depends on absorber size and salinity and the thickness of the fat layer. The eŒect of an absorber was also evaluated with a patient anatomy, yielding results in agreement with the phantom experiments.

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.

Finite Element-Based Biomechanical Modeling of the Bladder for Image Guided Radiotherapy

A realistic biomechanical bladder model is constructed that gives insight into pelvic organ motion as a result of bladder filling changes. We use finite element (FE) modeling to simulate bladder wall deformation caused by urine inflow. First, all pelvic structures were defined from MRI including bladder wall, small bowel, prostate, rectum, pelvic bone and the rest of the body. These were translated to FE meshes. Using appropriate material properties for all organs, displacements of these organs as a response to changing bladder pressure were computed. After fitting only the volume, the computed bladder shape has a good agreement with real bladder shape (overlap from 0.84 to 0.92). In conclusion, a FE bladder model can successfully predict the bladder shape change given a known bladder volume change. This model can potentially be used to improve image-guided radiotherapy for bladder cancer patients.

EP-1869: Metabolic response between primary tumor and lymph nodes in NSCLC patients during treatment course

University Medical Center Freiburg and Ortenau Klinikum Offenburg, Dept. of Radiation Oncology and Dept. of Neurology, Freiburg and Offenburg, Germany University Medical Center Freiburg, Dept. of Nuclear Medicine, Freiburg, Germany University Medical Center Freiburg and Ortenau Klinikum Offenburg, Dept. of Radiation Oncology, Freiburg and Offenburg, Germany University Medical Center Freiburg and Ortenau Klinikum Offenburg, Dept. of Nuclear Medicine and Dept. of Radiology, Freiburg and Offenburg, Germany University Medical Center Freiburg and German Cancer Consortium DKTK, Dept. of Radiation Oncology, Freiburg, Germany