Thermotolerance and radiation sensitizing effects of long duration, mild temperature hyperthermia

Hyperthermia combined with radiation therapy for superficial breast cancer and chest wall recurrence: a review of the randomised data

Hyperthermia has long been used in combination with radiation for the treatment of superficial malignancies, in part due to its radiosensitising capabilities. Patients who suffer superficial recurrences of breast cancer, be it in their chest wall following mastectomy, or in their breast after breast conservation, typically have poor clinical outcomes. They often develop distant metastatic disease, but one must not overlook the problems associated with an uncontrolled local failure. Morbidity is enormous, and can significantly impair quality of life. There is no accepted standard of care in treating superficial recurrences of breast cancer, particularly in patients that have previously been irradiated. There is a substantial literature regarding the combined use of hyperthermia and radiotherapy for these superficial recurrences. Most of it is retrospective in nature, but there are several larger phase III randomised trials that show an improved rate of clinical complete response in patients treated with both modalities. In this review article, we will highlight the important prospective data that has been published regarding the combined use of hyperthermia and radiation.

Long duration mild temperature hyperthermia and brachytherapy

Combining long duration mild temperature hyperthermia (LDMH) and low dose-rate (LDR) brachytherapy to enhance therapeutic killing of cancer cells was proposed many years ago. The cellular and tumour research that supports this hypothesis is presented in this review. Research describing LDMH interaction with pulsed brachytherapy and high dose-rate brachytherapy using clinically relevant parameters are compared with LDMH/LDR brachytherapy. The mechanism by which LDMH sensitizes LDR has been established as the inhibition of sublethal damage repair. The molecular mechanisms have been shown to involve DNA repair enzymes, but the exact nature of these processes is still under investigation. The relative differences between LDMH interactions with human and rodent cells are presented to help in the understanding of possible roles of LDMH in clinical application. The role of LDMH in modifying tumour blood flow and its possible role in LDR sensitization of tumours is also presented. The positive aspects of LDMH-brachytherapy for clinical application are sixfold; (1) the thermal goals (temperature, time and volume) are achievable with currently available technology, (2) the hyperthermia by itself has no detectable toxic effects, (3) thermotolerance appears to play a minor if any role in radiation sensitization, (4) TER of around 2 can be expected, (5) hypoxic fraction may be decreased due to blood flow modification and (6) simultaneous chemotherapy may also be sensitized. Combined LDMH and brachytherapy is a cancer therapy that has established biological rationale and sufficient technical and clinical advancements to be appropriately applied. This modality is ripe for clinical testing.

Thermal radiosensitization of human tumour cell lines with different sensitivities to 41.1 degrees C

While much work on radiosensitization by hyperthermia in the 43 degrees C and higher temperature range has been done, relatively little work has been done at temperatures in the 41-42 degrees C range. In this moderate hyperthermia range there are dramatic differences in the resistance of mammalian cells to hyperthermia. Therefore, thermal radiosensitization was measured in two human colon adenocarcinoma cell lines, one that expresses chronic thermotolerance and proliferates at 41.1 degrees C, NSY 42129 (NSY) cells and one that is slowly killed at 41.1 degrees C, HCT15 cells. Heat-resistant NSY cells were found to be more radioresistant than heat-sensitive HCT15 cells. Hyperthermia at 41.1 degrees C enhanced the radiation sensitivity in NSY cells, but no significant induction of heat-induced radiosensitization was observed in HCT15 cells. The radiation sensitivity induced by 41.1 degrees C in NSY cells appeared to be related to both intrinsic heat-induced radiosensitization (HIR) and cell-cycle redistribution at 41.1 degrees C. Incidentally, cells incubated at 41.1 degrees C for between 8-16 h displayed an identical radiosensitivity to those heated for 24 h. This result implies that modest hyperthermia for 2 h or more can have a radiosensitizing effect in heat-resistant cells.

Spatial temperature control with a 27 MHz current source interstitial hyperthermia system

PURPOSE

This article gives an overview of the properties of a 27 MHz current source interstitial hyperthermia system, affecting temperature uniformity.

METHODS AND MATERIALS

Applicators can be inserted in standard flexible afterloading catheters. Maximum temperatures are measured with seven-point constantan-manganin thermocouple probes inside each applicator. Temperature can be controlled automatically using a simple control algorithm. Three-dimensional power absorption and thermal models for inhomogeneous tissues are available to optimize applicator geometry and phase configuration. Properties of the interstitial heating system have been verified both in phantom experiments and in in vivo treatments of rhabdomyosarcomas implanted in the flank of a rat.

RESULTS

An experiment with four electrodes in one catheter proves that longitudinal control of the specific absorption rate (SAR) is feasible. Local cooling applied by cold water circulation through a catheter perpendicular to the afterloading catheter could be compensated by independent control of electrode power. Furthermore, comparison of two different phase configurations using four dual electrode applicators shows that the SAR distribution can be manipulated significantly, utilizing the phase of the electrodes. Finally, the temperature can be controlled safely and model calculations are in fair agreement with the measurements.

CONCLUSIONS

The features of the 27 MHz current source interstitial hyperthermia system enable spatial temperature control at approximately 1.5 cm.

Direct-coupled interstitial ultrasound applicators for simultaneous thermobrachytherapy: a feasibility study

This study presents the design and performance evaluation of interstitial ultrasound applicators designed specifically for thermal therapy with simultaneous brachytherapy. The applicator consists of a multielement array of piezoceramic tubular radiators, each with separate power control, surrounded by thin layers of electrically-insulating and biocompatible coatings (< 2.6 mm OD). A catheter which is compatible with remote afterloaders and standard brachytherapy technology forms the inner lumen. These 'direct-coupled interstitial ultrasound applicators' (DCIUA's) are placed within the tumour or target region, with the coated transducer surface forming the outer wall of the implant catheter. Thermocouple sensors embedded in the coating over each transducer can be used for continuous monitoring of the tissue/applicator interface temperatures for feedback control of power to each transducer segment. Theoretical acoustic power deposition and corresponding temperature distributions from thermal simulations have demonstrated that the radius of effective heating is highly dependent upon the acoustic efficiency of the piezoceramic transducers, with effective heating extending > 1 - 1.5 cm radially for typical DCIUA applicators that are 60-65% efficient. This exceeds the effective heating radius of both thermal conduction and RF heating technologies. Measurements with prototype multielement ultrasound applicators have demonstrated acoustic efficiencies between 60 and 65% and beam distributions which are fairly uniform and collimated to the transducer axial length. Thermal dosimetry measurements within in vivo tissues have demonstrated controllable therapeutic temperature rises at 1 - 1.5 cm radial depth from the applicators, which were in agreement with the simulations. This study demonstrates that direct-coupled ultrasound applicators, designed without an active cooling mechanism in order to accommodate the insertion of radiation sources, are practicable for simultaneous thermobrachytherapy and promises to give more adjustable heating patterns than current alternative techniques.

Enhanced cytotoxicity in combination of low dose-rate irradiation with hyperthermia in vitro

We have pursued an in vitro investigation using a rodent cell line to characterize the interaction of simultaneous low dose-rate irradiation (LDRI) and hyperthermia and to determine the role of LDRI in the development of thermotolerance in a fractionated hyperthermia schedule. Yoshida sarcoma cells growing in vitro were used in this study. Cell survivals were estimated by the Courteney soft agar clonogenic assay. A treatment device for LDRI treatment, which held eight 137Cs sources and agar plus containing the cells, gave an irradiation dose of 51.8 cGy/h to the cells. In the experiment of simultaneous LDRI and hyperthermia, the LDRI cytotoxicities were enhanced by hyperthermia over a non-lethal temperature range. High synergistic effects were observed in a lower temperature range (40, 41 and 41.5 degrees C) in contrast to higher temperature (42, 43 and 44 degrees C). In the experiment of alternate LDRI and hyperthermia, thermotolerance induced by initial heating was well developed during the LDR exposure but less expressed in comparison with the cells which had no LDRI exposure. The treatment of LDRI for 8 h may have affected the thermotolerance development in our experimental condition.