On verification of hyperthermia treatment planning for cervical carcinoma patients

Purpose: The aim of this study was to verify hyperthermia treatment planning calculations by means of measurements performed during hyperthermia treatments. The calculated specific absorption rate (SAR_calc) was compared with clinically measured SAR values, during 11 treatments in seven cervical carcinoma patients.

Methods: Hyperthermia treatments were performed using the 70?MHz AMC-4 waveguide system. Temperatures were measured using multisensor thermocouple probes. One invasive thermometry catheter in the cervical tumour and two non-invasive catheters in the vagina were used. For optimal tissue contact and fixation of the catheters, a gynaecological tampon was inserted, moisturized with distilled water (4 treatments), or saline (6 treatments) for better thermal contact. During one treatment no tampon was used. At the start of treatment the temperature rise (ΔT_meas) after a short power pulse was measured, which is proportional to SAR_meas. The SAR_calc along the catheter tracks was extracted from the calculated SAR distribution and compared with the ΔT_meas-profiles.

Results: The correlation between ΔT_meas and SAR_calc was on average R?=?0.56?±?0.28, but appeared highly dependent on the wetness of the tampon (preferably with saline) and the tissue contact of the catheters. Correlations were strong (R?～?0.85–0.93) when thermal contact was good, but much weaker (R?～?0.14–0.48) for cases with poor thermal contact.

Conclusion: Good correlations between measurements and calculations were found when tissue contact of the catheters was good. The main difficulties for accurate verification were of clinical nature, arising from improper use of the gynaecological tampon. Poor thermal contact between thermocouples and tissue caused measurement artefacts that were difficult to correlate with calculations.     

Reliability of temperature and SAR measurements at oesophageal tumour locations.

INTRODUCTION: For treatment of oesophageal cancer, neo-adjuvant locoregional hyperthermia (HT) has been applied in combination with chemotherapy (ChT) +/- radiotherapy (RT) at the institute. Until now, 26 patients were treated within a completed phase I study combining HT with ChT and 29 patients within an ongoing phase II study combining HT with ChT + RT. METHODS: HT was given with the 70 MHz AMC-4 waveguide system. Initially, oesophageal temperatures were measured using multi-sensor thermocouple probes (TCs) inside a nasogastric tube (NT), but the question arose whether these measurements were reliable enough to quantify the achieved tumour temperatures accurately. Presently, TCs are mounted on the outside of an inflatable balloon catheter (BC) for better intra-luminal fixation and better contact with the tumour. During 14 treatment sessions in four patients TCs inside a NT and mounted on a BC were used simultaneously. Data from these 14 treatment sessions were used to compare temperature and Specific Absorption Rate (SAR) measurements ('DeltaT-measurements') using NTs or BCs. To determine the predictive value of the local SAR for the tumour temperatures achieved during treatment, the relation between the initial DeltaT and steady state temperature (SST) was evaluated. RESULTS: There was a strong correlation between the temperature measured in the NT (Ttube) and the temperature measured with a BC (Tballoon): R = 0.88 +/- 0.13. However, Ttube was on average approximately 1 degrees C higher than Tballoon and there was a large variation between the different treatments in the relation between both measurements, rendering Ttube a probably unreliable measure for tumour temperatures. The correlation between the DeltaT measured in the NT (DeltaTtube) and with a BC (DeltaTballoon) was rather weak: R = 0.46 +/- 0.25. The correlation between the initial DeltaT and the SST was much stronger for the BC measurements, R = 0.78 +/- 0.19, than for the NT measurements, R = 0.61 +/- 0.23. Thus, DeltaTballoon has a higher predictive value for the achieved tumour temperatures than DeltaTtube. Both DeltaT and SST were generally higher for the NT measurements than for the BC measurements, suggesting an over-estimation of tumour temperatures. Averaged over all treatments in the phase I trial using a NT (20 treatments) or a BC (45 treatments), T90 was significantly higher when measured with a NT. CONCLUSION: Oesophageal temperature and SAR (DeltaT) measurements inside a NT are less reliable than BC measurements. These artefacts are due to bad thermal contact with the tumour tissue and are, therefore, not specific for thermocouple thermometry. For reliable temperature or SAR measurements inside lumina or cavities good thermal contact must be assured, e.g. by using a balloon catheter.     

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 (DeltaT) 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 DeltaT values and the DeltaT-ratio between the DeltaT in the tumour (DeltaToes) and near the spinal cord (DeltaTcord). 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 DeltaT-ratio was performed for this patient and the steady state temperature distribution for the obtained settings was computed. RESULTS: A higher DeltaToes was measured with the mixed settings compared to the calculated and clinical settings; DeltaTcord was higher with the mixed settings compared to the clinical settings. The DeltaT-ratio was approximately 1.5 for all three settings. These results indicate that the most effective tumour heating can be achieved with the mixed settings. DeltaT 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 DeltaT-ratio for this patient yielded a very high DeltaT-ratio ( approximately 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.     

Specific absorption rate and tissue temperature in local hyperthermia

Specific absorption rate (SAR) and tissue temperature were measured for a total of 83 treatments in 33 patients who received local hyperthermia treatment for cancer. The patients were grouped into three categories according to tumor size. Hyperthermia was induced by 13.56 MHz electromagnetic energy applied using capacitive coupling. A method is described for evaluating SAR from the tissue temperature traces at any time in the treatment when a step change is made in applied power. The method is possible only if the temperature traces are free from interference and the total power delivered to the patient is monitored. Mean values of SAR ranged from 4.6 to 89 W kg-1 depending on the treatment site. Satisfactory heating was achieved for superficial tumors, with temperatures greater than 42 degrees C being recorded in 69% of treatments. For axillary nodes only 4% of treatments exceeded 42 degrees C. For cervix tumors an idealized tumor model was used to estimate tumor temperature from the temperature and SAR measured in the adjacent normal tissue. From the model it appears necessary either to raise the systemic temperature to 40 degrees C or to increase the SAR by at least a factor of 4 to obtain a temperature of 42 degrees C in a typical tumor. Measurements of SAR and temperature are essential for feedback control of computer models which, in principle, could provide a complete distribution of temperature during a hyperthermia treatment. Furthermore, measured SAR provides a direct comparison of the power deposition from different treatment machines in a clinical environment. The data presented form a basis for comparison with the clinical use of other heating systems.     

Comparison of six microwave antennas for hyperthermia treatment of cancer: sar results for single antennas and arrays

Interstitial techniques of inserting catheters into tumors for the purpose of applying therapeutic irradiation and hyperthermia are in widespread use. Several miniature microwave antenna designs are currently used for these treatments. These include multisection, hot-tip, 2- and 3-node, dipole and helical antennas, all of which are commercially available. The antenna designs are diverse enough to have a dramatic effect on the power deposition patterns either as single antennas or when used in arrays. Aside from the dipole antenna, most of the antennas have never been evaluated experimentally or theoretically in arrays, although the array configuration is used in the vast majority of all clinical treatments. Power deposition or SAR (specific absorption rate) tests were run in muscle equivalent phantom. Single antennas were evaluated at 400 points in a plane and isoSAR contours drawn, normalized to maximum SAR. Single antennas were also compared in large and small diameter catheters to evaluate catheter dependent antenna performance. The dipole, multisection, hot-tip and helical antennas were evaluated in arrays of four antennas located at the corners of a square, spaced 2.0 cm apart. Arrays of antennas were evaluated at 441 points in three planes orthogonal to the antenna axes. Results in the single antenna studies showed that the dipole was less affected by snugness of catheter fit than the multisection, hot-tip or helical antennas. In large catheters, the latter three antennas showed more extreme tip heating performance. The 2- and 3-node antennas deposited only 20% SAR in the distal 30 mm of antenna length. In arrays, the multisection, hot-tip, and dipole antennas all yielded 80-90% SAR centrally in the central measurement plane. Comparing the three antennas, the dipole array deposited 20% more power centrally in a plane near the insertion point, and the multisection and hot-tip antenna designs deposited 10% more central power in a plane near the antenna tips. The helical antenna array deposited only 30% SAR centrally in the plane near the antenna tips and in the central plane. Only 10% SAR was measured centrally near the insertion point, as expected for tip-heating antennas. Finally, the clinical significance of the results is discussed as applied to human tumors undergoing hyperthermia treatments.     

Practical Limitations of Interstitial Thermometry During Deep Hyperthermia

Purpose: Intratumor thermometry during hyperthermia treatment is considered important for several reasons. The morbidity that we experienced from interstitially placed catheters in deep-seated tumors gave reason to weigh the advantages and disadvantages against each other.

Methods and Materials: The available thermometry in 215 patients treated with hyperthermia for deep-seated tumors was analyzed with the aim to evaluate practically feasible intratumor measurements. The influence of intratumor measurements on the treatment procedure was assessed.

Results: Total 120 catheters were placed interstitially in 78 patients. Over the years, the percentage of patients with interstitial thermometry decreased considerably. Forty-nine catheters could remain in place during the whole hyperthermia treatment series. The remaining catheters had to be removed for more or less severe complications, including one fatal event. In fact, the interstitial catheters caused the most severe treatment-related morbidity. During 188 of the total 859 treatments, at least one interstitial catheter was available for thermometry. Per treatment with catheter(s) in situ, the average number of intratumor measurement sites was 6.9. The value of interstitial thermometry for power steering during treatment, to both optimize intratumor temperature distribution and prevent toxicity, appeared limited. The mean volume of the tumors with interstitial thermometry was 314 cm³, SD 325. In relation to the large tumor volumes, the thermal dose parameters calculated from the available data is considered to be of limited value.

Conclusion: In view of the possible severe complications and the limited clinical value of the information achieved by interstitially placed thermometry catheters, interstitial thermometry was not found to routinely benefit the individual patient.     

Use of heating rate and specific absorption rate in the hyperthermia clinic

The specific absorption rate (SAR), which is the mass-normalized rate of energy absorption by a biological body, has been used by hyperthermia researchers to characterize energy deposition and heating patterns in tissues and in biological models. Before thermal diffusion and blood flow alter the temperature distribution, energy deposition and heating rate (HR) are related by constants. Therefore SAR is usually calculated from the initial rate of temperature rise measured during heating. SAR is an appropriate parameter for theoretical calculations or electric field measurement in tissue. However, the ultimate parameter in hyperthermia is temperature. Instead of computing the temperature rise from SAR (W/kg) and specific heat (kcal/kg. °C) which were originally obtained from temperature measurements, it is simpler and more convenient to use HR (°C/W-min) directly, especially when heterogeneous tissues are involved. The advantages of using HR instead of SAR in hyperthermia clinics are discussed.     

Use of heating rate and specific absorption rate in the hyperthermia clinic

The specific absorption rate (SAR), which is the mass-normalized rate of energy absorption by a biological body, has been used by hyperthermia researchers to characterize energy deposition and heating patterns in tissues and in biological models. Before thermal diffusion and blood flow alter the temperature distribution, energy deposition and heating rate (HR) are related by constants. Therefore SAR is usually calculated from the initial rate of temperature rise measured during heating. SAR is an appropriate parameter for theoretical calculations or electric field measurement in tissue. However, the ultimate parameter in hyperthermia is temperature. Instead of computing the temperature rise from SAR (W/kg) and specific heat (kcal/kg.degree C) which were originally obtained from temperature measurements, it is simpler and more convenient to use HR (degree C/W-min) directly, especially when heterogeneous tissues are involved. The advantages of using HR instead of SAR in hyperthermia clinics are discussed.     

Accuracy and precision of computer-simulated tissue temperatures in individual human intracranial tumours treated with interstitial hyperthermia

Accurate knowledge of tissue temperature is necessary for effective delivery of clinical hyperthermia in the treatment of malignant tumours. This report compares computer-predicted versus measured intratumoral temperatures in 11 human subjects with intracranial tumours, treated with a conceptually simple 'conductive' interstitial hyperthermia system. Interstitial hyperthermia was achieved by the use of parallel arrays of implanted, electrically heated catheters. The tissue was warmed by thermal conduction and blood convection. Simulation of intratumoral temperatures was achieved by solving a modified bioheat transfer equation on a digital computer using a finite difference method. Comparison of intratumoral temperatures from simulations and measured values differed by about +/- 0.75 degrees C. Further analysis of computed temperature distributions between catheters revealed a rapidly computable relationship between the local minimum tumour temperature and nearby catheter power and temperature that accounts for effects of varying blood flow. These findings suggest that 'on-line' prediction and control of local minimum tumour temperatures are feasible with the conductive interstitial technique.     

Accuracy and precision of computer-simulated tissue temperatures in individual human intracranial tumours treated with interstitial hyperthermia

Accurate knowledge of tissue temperature is necessary for effective delivery of clinical hyperthermia in the treatment of malignant tumours. This report compares computer-predicted versus measured intratumoral temperatures in 11 human subjects with intracranial tumours, treated with a conceptually simple ‘conductive’ interstitial hyperthermia system. Interstitial hyperthermia was achieved by the use of parallel arrays of implanted, electrically heated catheters. The tissue was warmed by thermal conduction and blood convection. Simulation of intratumoral temperatures was achieved by solving a modified bioheat transfer equation on a digital computer using a finite difference method. Comparison of intratumoral temperatures from simulations and measured values differed by about ±0?75°C. Further analysis of computed temperature distributions between catheters revealed a rapidly computable relationship between the local minimum tumour temperature and nearby catheter power and temperature that accounts for effects of varying blood flow. These findings suggest that ‘on-line’ prediction and control of local minimum tumour temperatures are feasible with the conductive interstitial technique.     

Intra-luminal thermometry: is tissue type assignment a necessity for thermal analysis?

INTRODUCTION: Tissue type assignment, i.e. differentiation tumour from normal tissue, is a normal procedure for interstitial thermometry. In our department, thermometry in patients with a tumour in the lower pelvis is usually restricted to the intra-luminal tracks. It is unknown whether discrimination between normal and tumour tissue is relevant for deep regional hyperthermia thermal dosimetry using only intra-luminal tumour contact and tumour adjacent thermometry. This study has analysed the acquired temperature data in order to answer this question. PATIENTS AND METHODS: Seventy-five patients with locally advanced cervical carcinoma were selected randomly. Patients were treated with a two or three modality combination, i.e. radiotherapy +hyperthermia or radiotherapy + hyperthermia + chemotherapy from October 1997 to September 2003. The first 100 hyperthermia treatments fulfilling the only selection criterion: no displacement of the thermometry catheter along the insertion length during the treatment, were included in the study, resulting in 43 patients with one-to-five treatments/patient (median 2). Using RHyThM (Rotterdam Hyperthermia Thermal Modulator), for each single treatment tissue type, was defined on the basis of information given by a CT scan in radiotherapy position. A step change in the slope of the profile of the first temperature map was identified to verify the insertion length of the catheter. RESULTS: The average T50 (median temperature) in bladder tumour indicative, vagina tumour contact and rectum tumour indicative was 40.9 +/- 0.9 degrees C, 39.7 +/- 0.9 degrees C and 40.6 +/- 0.8 degrees C, respectively. The average normal tissue T50 in bladder, vagina and rectum was 40.8 +/- 0.9 degrees C, 40.1 +/- 0.9 degrees C and 40.7 +/- 0.8 degrees C, respectively. The differences between bladder tumour indicative T50 and bladder normal tissue T50 and also between vagina tumour contact T50 and vagina normal tissue T50 were significant ( p = 0.0001). No statistical difference was found between rectum tumour indicative t50 and rectum normal tissue T50. CONCLUSION: At present the cause of the temperature difference is not known. However, as the difference between tumour (indicative/contact) and normal tissue is very small and considering also the inaccuracy in the tissue type assignment it can be stated that this study does not provide sufficient evidence to conclude that the statistical difference has clinical relevance. Therefore, it was concluded that at this time there is no need to differentiate between normal and tumour tissue in intra-luminal thermometry.     

CT fluoroscopy-guided closed-tip catheter placement before regional hyperthermia treatment of soft tissue sarcomas: 5-Year experience in 35 consecutive patients

Purpose: This study was designed to assess technical success and complications in patients with high-risk soft tissue sarcomas undergoing CT fluoroscopy-guided closed-tip catheter placement before treatment with combined chemotherapy and regional hyperthermia. Materials and methods: This retrospective study comprised all patients referred for insertion of closed-tip catheters for the introduction of thermometry probes before regional hyperthermia treatment at a single university centre from 2010 to 2015. Catheter placements were performed under local anaesthesia and intermittent CT fluoroscopy guidance. Technical success, complication rate, duration of catheter insertion and dose–length product (DLP) were analysed. Technical success was defined as intratumoural catheter placement suitable for subsequent thermometry. Results: A total of 35 procedures were performed on 35 patients (22 men, 13 women). In 34 out of 35 interventions catheters were inserted successfully; in one patient catheter placement was not feasible. No intra-interventional complications occurred. In six patients post-interventional complications were observed – two major (one abscess formation and one severe catheter dislocation) and four minor complications. Technical failure was observed in 11.4% of patients, especially catheter kinking. A total of 55 catheters were placed, with a mean number of 1.7?±?0.7 per patient. Mean total DLP was 723.2?±?355.9 mGy*cm. Conclusion: CT fluoroscopy-guided closed-tip catheter placement into high-risk soft tissue sarcomas was characterised by high technical success and relatively low complication rate. While major complications were rarely observed, catheter-kinking preventing successful thermometry represented the most frequent technical failure.     

The distribution of power and heat produced by interstitial microwave antenna arrays: I. Comparative phantom and canine studies

To adequately plan and administer localized hyperthermia with interstitial microwave antennas, the thermal distribution patterns generated by such antennas must be characterized. This study evaluated the performance of single node 915 MHz antennas operating either alone or as a 2 cm square array of four parallel antennas using both muscle-equivalent phantoms and canine thigh muscle. Two types of measurements were compared. Specific absorption rate (SAR), where temperature increases resulting from short duration microwave pulses were used to define power distribution, and temperature gradients during simulated hyperthermia treatments. SAR measurements in phantoms were comparable to those obtained in non-perfused canine muscle demonstrating the usefulness of the phantom for these measurements. For a single antenna there was a rapid decrease in power radially which resulted in a steep thermal gradient at distances within 0.5 cm. However, the power generated by a four-antenna array was highest along its central axis and declined to approximately 50% near the antennas at the array periphery. Along the central axis of the array power decreased most rapidly distal to the antenna nodes. The distribution of temperature measured during simulated hyperthermia treatments in phantoms paralleled the SAR distribution and was comparable to the temperature gradient observed in perfused canine muscle, suggesting that phantoms could be used to predict temperature distributions in resting muscle tissue.     

Absorbed power deposition for various insertion depths for 915 MHz interstitial dipole antenna arrays: experiment versus theory

Dipole antennas are commonly used in interstitial clinical hyperthermia treatments because of their compatibility with brachytherapy techniques and their good power deposition patterns when used in arrays. For accurate treatment planning, however, there must be a comprehensive knowledge base to predict the power deposition patterns when insertion depth is a non-resonant length. This is especially true for insertion depths that result in significant power deposition outside of the antenna junction plane and presumably outside of the tumor volume. A computer controlled measurement system was used with a muscle equivalent phantom to make measurements of specific absorption rate (SAR) or absorbed power per unit mass of tissue at 598 points in a plane. The diagonal plane was the measurement plane of choice because it characterized the SAR profiles at the array center as well as areas in the proximity of the antennas. Dartmouth dipole antennas were used (0.9 mm O.D.) in brachytherapy catheters with inner catheters (2.2 mm O.D./1.2 mm I.D.). The resonant half-wavelength of this dipole antenna/catheter combination is 7.8 cm. A choke modification of the dipole was also investigated. Four antennas were used in a boxlike configuration with 2.0 cm separation. Insertion depths of 5.9, 7.8, 9.8, 12.7, 15.6 and 17.6 cm were used. The hA subsection (junction to tip) was held constant at 3.9 cm. Plots were made of the experimental SAR data normalized to the maximum SAR measured in the plane. Theoretical plots were calculated in the same plane for each of the insertion depths. SAR comparisons were also made longitudinally along the central axis of the array and through the antenna junctions in the diagonal plane for resonant half-wavelength insertion depth. Experimental results verified theoretical predictions of the existence of a secondary hot-spot in the center of the array, but outside of the antenna junction plane and approximately a quarter-wavelength from the insertion point. This secondary hot-spot appears for all insertion depths greater than 10 cm. At longer insertion depths approaching a full wavelength, however, this secondary peak is not dominant. Choke antennas demonstrated a solution to the problem of shifting SAR patterns with varying insertion depths by restricting the active length of the antenna.     

Theoretical and experimental investigations of a newly developed intracavitary applicator system for the radiothermotherapy of gynaecological tumours

This paper describes a new type of applicator system for the intracavitary radiothermotherapy of gynaecological tumours. Hyperthermia and radiation can be delivered by one and the same applicator. The hyperthermia is delivered by a modified capacitive heating technique (27–12 MHz) combined with cooling systems, the radiation by the numerically controlled oscillation of a gamma source on the inside of the main applicator, the RF gamma applicator (HDR afterloading technique). Isotherms and isodoses can be adapted to the anatomical-pathological situation. Interactions between the applicator system and the surrounding tissue were investigated in a theoretical model (FEM). Complex two-dimensional SAR calculations as well as three-dimensional temperature calculations were carried out. The RF gamma applicator was also examined by thermography; the thermography proved the theoretical modelling to be correct. The applicators were also tested in animal experiments.     

Interstitial microwave-induced hyperthermia and iridium brachytherapy for the treatment of obstructing biliary carcinomas.

In a phase I clinical study, 10 patients with obstructive biliary carcinomas were treated with single-antenna interstitial microwave hyperthermia and iridium-192 brachytherapy. For each patient a standard biliary drainage catheter was implanted percutaneously through the obstructed common bile duct. This catheter accommodated a single microwave antenna which operated at 915 MHz, and one or two fibreoptic thermometry probes for temperature measurement. Under fluoroscopic guidance the microwave antenna and temperature probes were positioned in the CT-determined tumour mass. The 60-min heat treatment achieved a central tumour temperature of 45-55 degrees C while keeping temperatures at the proximal and distal margins at 43 degrees C. Immediately following the hyperthermia treatment the microwave antenna and temperature probes were removed, and a single strand of iridium-192 double-strength seeds was inserted to irradiate the tumour length. A dose of 5500-7900 cGy calculated at 0.5 cm radially from the catheter was administered over 5-7 days. Upon removal of the iridium a second hyperthermia treatment was performed. A total of 18 hyperthermia treatments were administered to the 10 patients. In two cases the second hyperthermia treatment after brachytherapy was not possible due to a kink in the catheter, or bile precipitation in the catheter. All patients tolerated the procedure well, and there were no acute complications. To evaluate the volumetric heating potential of this hyperthermia method, specific absorption rate (SAR) values were measured at 182 planar points in muscle phantom. Insulated and non-insulated antenna performance was tested at 915 MHz in a biliary catheter filled with air, saline, or bile to mimic clinical treatments. The insulated antenna exhibited the best performance. Differences between antenna performance in saline and bile were also noted. In summary, this technique may have potential for tumours which obstruct biliary drainage and are accessible to percutaneous decompression using standard diagnostic radiological procedures.     

Reliability of temperature and SAR measurements at oesophageal tumour locations

Introduction: For treatment of oesophageal cancer, neo-adjuvant locoregional hyperthermia (HT) has been applied in combination with chemotherapy (ChT)?±?radiotherapy (RT) at the institute. Until now, 26 patients were treated within a completed phase I study combining HT with ChT and 29 patients within an ongoing phase II study combining HT with ChT?+?RT.

Methods: HT was given with the 70?MHz AMC-4 waveguide system. Initially, oesophageal temperatures were measured using multi-sensor thermocouple probes (TCs) inside a nasogastric tube (NT), but the question arose whether these measurements were reliable enough to quantify the achieved tumour temperatures accurately. Presently, TCs are mounted on the outside of an inflatable balloon catheter (BC) for better intra-luminal fixation and better contact with the tumour. During 14 treatment sessions in four patients TCs inside a NT and mounted on a BC were used simultaneously. Data from these 14 treatment sessions were used to compare temperature and Specific Absorption Rate (SAR) measurements (‘ΔT-measurements’) using NTs or BCs. To determine the predictive value of the local SAR for the tumour temperatures achieved during treatment, the relation between the initial ΔT and steady state temperature (SST) was evaluated.

Results: There was a strong correlation between the temperature measured in the NT (T_tube) and the temperature measured with a BC (T_balloon): R?=?0.88?±?0.13. However, T_tube was on average ～1°C higher than T_balloon and there was a large variation between the different treatments in the relation between both measurements, rendering T_tube a probably unreliable measure for tumour temperatures. The correlation between the ΔT measured in the NT (ΔT_tube) and with a BC (ΔT_balloon) was rather weak: R?=?0.46?±?0.25. The correlation between the initial ΔT and the SST was much stronger for the BC measurements, R?=?0.78?±?0.19, than for the NT measurements, R?=?0.61?±?0.23. Thus, ΔT_balloon has a higher predictive value for the achieved tumour temperatures than ΔT_tube. Both ΔT and SST were generally higher for the NT measurements than for the BC measurements, suggesting an over-estimation of tumour temperatures. Averaged over all treatments in the phase I trial using a NT (20 treatments) or a BC (45 treatments), T₉₀ was significantly higher when measured with a NT.

Conclusion: Oesophageal temperature and SAR (ΔT) measurements inside a NT are less reliable than BC measurements. These artefacts are due to bad thermal contact with the tumour tissue and are, therefore, not specific for thermocouple thermometry. For reliable temperature or SAR measurements inside lumina or cavities good thermal contact must be assured, e.g. by using a balloon catheter.     

Clinical thermometry, using the 27 MHz multi-electrode current-source interstitial hyperthermia system in brain tumours.

BACKGROUND AND PURPOSE: In interstitial hyperthermia, temperature measurements are mainly performed inside heating applicators, and therefore, give the maximum temperatures of a rather heterogeneous temperature distribution. The problem of how to estimate lesion temperatures using the multi-electrode current-source interstitial hyperthermia (MECS-IHT) system in the brain was studied. MATERIALS AND METHODS: Temperatures were measured within the electrodes and in an extra catheter at the edge of a 4 x 4 x 4.5 cm(3) glioblastoma multiforme resection cavity. From the temperature decays during a power-off period, information was obtained about local maximum and minimum tissue temperatures. The significance of these data was examined through model calculations. RESULTS: Maximum tissue temperatures could be estimated roughly by switching off all electrodes for about 5 s. Model calculations showed that the minimum tissue temperatures near a certain afterloading catheter correspond well with the temperature of the applicator inside, about 1 min after this applicator was switched off. CONCLUSIONS: Although the electrode temperatures read during heating are not suitable to assess the temperature distribution, it is feasible to heat the brain adequately using the MECS-IHT system with extra sensors outside the electrodes and/or application of decay methods.     

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).     

Interstitial microwave hyperthermia and brachytherapy for malignancies of the vulva and vagina. I: Design and testing of a modified intracavitary obturator.

A vaginal obturator was fabricated to be used in combination with implanted catheters to provide microwave hyperthermia and brachytherapy to the vulva and vaginal wall. This site is difficult to heat or irradiate solely with interstitial techniques. The obturator was modified to provide grooves for the mounting of interstitial catheters into the outer wall and was matched with a template for circumferential implants. Power deposition tests were done using arrays of three microwave antenna designs: dipole (hA = hB = 3.9 cm), helical (3.9 cm coil, shorted), and modified dipole (1.0 cm helix on dipole tip) to test the performance of the obturator. The obturator and four non-obturator catheters were positioned in muscle-equivalent phantom. Two obturator catheters along with two free-standing catheters formed the obturator array. Four freestanding catheters formed the non-obturator array. Power deposition or specific absorption rate (SAR) measurements were made along the central axis, bisect, and diagonal transect of each array. SAR results showed that antennas in the obturator wall radiated as dipole theory predicts, although with less power density when compared to antennas in the same catheters spaced 1.8 cm from the obturator. This could be compensated for by increasing the power to the antennas in the obturator by 42%. Adjacent pairs of antennas were placed 90 degrees out of phase for 0.25 sec and rotated around the array. Phase rotation demonstrated that the central array SAR peaks could be lowered from 100% to 50% SAR, with dipole antennas thus resulting in lowered peak temperatures and the ability to heat larger volumes by improving the distribution of power. With helical antennas, there was 50% SAR at the array center when operated coherently without phase rotation. Three patients were treated with the obturator and a custom-made template using dipole antennas, and temperatures were measured in five obturator catheters. Therapeutic heating was measured in the catheters on the obturator between antennas in contact with the vaginal mucosa.