Cellular and Tissue Responses to Hyperthermia

Protein is the primary target for hyperthermic cytotoxicity. When cells are exposed to elevated temperatures (≥41° C), damage is inflicted in multiple sites, but the predominant molecular target appears to be protein. The heat of inactivation for cell killing and thermal damage to tissues is in the range of that necessary for protein denaturation (130 to 170 kcal/mol). Additional evidence for protein being the primary target for cell killing is the importance of heat shock proteins in protecting thermotolerant cells from thermal damage. One of the primary functions of heat shock proteins is to refold other proteins that have been damaged.⁴

Some organelles are especially important in controlling thermal response. For example, modification of membrane lipid content or use of membrane active agents, such as alcohols, can sensitize cells to heat killing, but the sensitization is probably related to destabilization of the membrane as it relates to lipid-protein interactions.⁵ The cytoskeleton of cells is particularly heat sensitive.⁶ When it is collapsed by heat, there is disruption of cytoskeletal-dependent signal transduction pathways as well as inhibition of cell motility.^7,8 Enzymes in the respiratory chain are more heat sensitive than enzymes in the glycolytic pathway.⁹ The heat sensitivity of the centriole leads to chromosomal aberrations following thermal injury.¹⁰ Finally, the DNA repair process is heat sensitive, and this may be one of the mechanisms that leads to heat-induced radiosensitization and chemosensitization.¹¹

Physiologic Response to Hyperthermia

Tumor blood flow and metabolism have important influences on the efficacy of HT, and, conversely, the physiologic consequences of HT can influence the efficacy of other treatments.

As temperatures increase, there is an increase in blood flow. The temperature threshold for this change is 41° to 41.5° C in skin.¹² Changes in vascular permeability also occur, leading to edema formation in the heated volume. At higher thermal doses, vascular stasis and hemorrhage develop. The change in normal tissue perfusion upon heating is much greater (10 times) than one sees in tumors (1.5 to 2 times).¹³ Mechanisms underlying vascular stasis may include arteriovenous shunting, thrombus formation, and leukocyte plugging.¹⁴ Importantly, physiologic changes occur in tumors at lower temperatures that are potentially beneficial (Fig. 21-2).

Figure 21-2 Overview of the beneficial physiologic and metabolic effects of HT.

Taking Advantage of the Physiologic Response to Hyperthermia

Improvement in Macromolecular and Liposomal Drug Delivery

A liposome is a small lipid vesicle (≈100 nm in diameter) that contains water or saline in the center. Drugs can be loaded into liposomes at high concentration. It has been recently shown that HT increases microvascular pore sizes preferentially in tumor microvessels, which leads to enhanced liposomal accumulation in tumor.¹⁵ The threshold for increased liposomal extravasation is 40° C, and it increases by a factor of 2 for every degree of temperature rise until vascular damage occurs.¹⁶ The effects of HT on liposomal extravasation have been studied extensively in preclinical models, but a twofold to thirteenfold enhancement in accumulation occurs in spontaneous soft tissue sarcomas of pet cats, an encouraging result that may translate to human tumors.¹⁷ HT also increases the available volume fraction (fraction of tissue not occupied by cells or stromal fibers) that may develop as cells undergo either necrosis or apoptosis following heat treatment.¹⁸ This increases the tissue space available for nanoparticles to be deposited. The increase in liposomal drug delivery achieved with HT has been shown to result in increased antitumor effect in many preclinical studies.¹⁹ HT has been used in conjunction with liposomal doxorubicin and irradiation in one small clinical series of patients with chest wall recurrences of breast cancer. Encouraging results were obtained, although drug concentrations were not measured in this series.²⁰

For relatively small agents, such as most chemotherapeutic drugs, there is no advantage gained via increased vascular permeability to macromolecules or nanoparticles unless the drugs are protein-bound.²¹ For drugs with a molecular weight of less than 1000 MW, the primary mechanism that governs drug transport is diffusion,²² which is not highly temperature dependent. However, for molecules larger than 1000 MW, the primary driving force for transport is convection, which is controlled by the pressure gradient across the vessel wall. Accordingly, HT can augment the transvascular delivery of agents such as monoclonal antibodies²³ and polymeric peptides that carry drugs or radioisotopes.²⁴ Dreher and colleagues used a novel peptide polymer that undergoes an inverse-phase transition at 41° C to “pump” polymer into tumors. The inverse-phase transition refers to a physical feature of the polymer that renders it to be water soluble below 41° C; above this temperature, the polymer comes out of solution in aggregates. The aggregation occurs primarily intravascularly but is reversible. Therefore, when the tissue is cooled down, the particles rapidly disaggregate and diffuse into the tissue down their concentration gradient²⁵ (Fig. 21-3). This same type of peptide polymer could be used for noninvasive thermometry with magnetic resonance imaging (MRI) or electron paramagnetic resonance (EPR) imaging because the interaction of the polymer with water changes when it aggregates. Proof of principle for this method was shown using EPR imaging.²⁶

Figure 21-3 Images of thermally sensitive peptide polymer (green) and thermally insensitive polymer (red) in a solid tumor before, during, and after HT. The peptides were fluorescently labeled and injected into mice with a skin-fold window chamber, which permits direct visualization of the tumor microvessels using fluorescence microscopy. Upon heating to 41° C, the temperature-sensitive liposome begins to aggregate (green dots), whereas the red nonthermally sensitive polymer remains dissolved in the blood plasma, highlighting the location of the blood vessels. When the tissue is cooled down, the aggregates disappear, leaving the now-soluble polymer in the extravascular space (green background increases with accumulation). This method could be used to literally pump drug into tumors from the vascular space. Bar = 100 μm for all images.

Data reproduced from Dreher MR, Liu W, Michelich CR, et al: Thermal cycling enhances the accumulation of a temperature-sensitive biopolymer in solid tumors, Cancer Res 67(9):4418-4424, 2007.

Effects of HT on Tumor Metabolism and Oxygenation.

Enzymes for aerobic metabolism are more heat sensitive than those involved in anaerobic metabolism.⁹ Decreases in adenosine triphosphate (ATP) concentration after HT are inversely related to the temperatures achieved during HT in preclinical models and canine sarcomas^27,28 and are related to the pathologic complete response (pCR) rate in human sarcomas.²⁷ Decreases in ATP and increases in lactate concentration occur following tumor blood flow reduction after HT.²⁹ In human patients with soft tissue sarcomas who were treated preoperatively with HT and radiation therapy, reduction in the ratio of ATP to inorganic phosphate (Pi) (measured using phosphorus-31 [³¹P] magnetic resonance spectroscopy) was significantly correlated with a higher probability for pCR.²⁷ These results are consistent with the theory that tumor respiration can decrease after HT treatment.

A shift toward anaerobic metabolism would decrease oxygen consumption rates, which could lead to improvement in tumor oxygenation. Some of the benefits of HT may therefore result from improvements in oxygenation.³⁰ Several preclinical studies have shown that mild temperature heating (between 40° and 42° C for an hour) can lead to improvements in the oxygen partial pressure (pO₂) in the tumor up to 24 hours after heating.³¹ However, in some models, the extent of reoxygenation is quite heterogeneous between and even within individual tumors.³² It has been reported that HT improves tumor oxygenation in canine and human soft tissue sarcomas, with reoxygenation being associated with a greater probability for pCR in human tumors.^33,34 In a canine study, median temperatures of less than 44° C improved oxygenation, whereas median temperatures higher than 44° C for 60 minutes led to decreased oxygenation.³³ In patients with locally advanced breast cancer who were treated in a phase II study with preoperative HT plus radiotherapy and taxol, there was a significantly increased likelihood for achieving pCR if the tumors reoxygenated 24 hours after the first HT treatment. The probability for achieving a pCR was greater when the median temperatures were below 41.5° C.³⁵ Improvement in oxygenation does not occur in all tumors, however, and currently there is no method for predicting which tumors will show this effect.³⁶ In summary, mild temperature elevations during HT will benefit some patients if HT induces reoxygenation.³¹ To take full advantage of this effect, it will be necessary to use methods to enhance the reoxygenation effect, such that it occurs in the majority of patients. Song and Griffin have suggested that the addition of hyperoxic gas breathing after HT but during radiotherapy may achieve this goal.^37,38

pH Modification

It is well established that acidification can enhance sensitivity to HT. Changes in intracellular pH are responsible for increased thermal sensitivity.^39,40 The most widely studied method to achieve acidification has been induction of hyperglycemia. The logic is that excess glucose load will push tumors toward glycolysis and lactic acid production.^41,42 However, hyperglycemia alone is insufficient to reliably achieve adequate acidification.^43,44 The addition of agents that can selectively acidify tumor intracellular pH, such as glucose combined with the respiratory inhibitor meta-iodobenzylguanadine (MIBG), have the potential to further enhance hyperthermic cytotoxicity selectively in tumor tissues^45,46 because the acidification appears to be selective for tumors. Quercetin, a naturally occurring flavinoid, inhibits thermotolerance induction, particularly at a low pH, and can accordingly increase the thermal sensitivity of cells.^47–49 An alternative approach is to block the extrusion of hydrogen ions from cells, which is normally accomplished via membrane-bound pumps. Use of such agents, in combination with acidification of the extracellular space, can lead to enhanced hyperthermic cell killing both in vitro and in vivo.⁵⁰

Rationale for Combining Hyperthermia with Radiotherapy

When irradiation is combined with HT, complementary effects occur. Cells in S phase are radioresistant but are sensitive to HT. Hypoxic cells are three times more resistant to radiation compared with aerobic cells, but there is no difference in thermal sensitivity between aerobic and hypoxic cells. As discussed above, HT can lead to reoxygenation, which will further improve radiotherapy response.31,33,34 Finally, HT inhibits the repair of both sublethal and potentially lethal damage by inactivating crucial DNA repair pathways.^62–64

Factors to Consider when Combining Hyperthermia with Radiotherapy

The interaction between radiation and HT is described by the thermal enhancement ratio (TER), which is defined as the ratio of doses of radiation to achieve an isoeffect for radiotherapy alone compared with radiotherapy plus HT. TERs for local tumor control have been estimated for a number of human tumors using historical control data for radiation alone.⁶⁵ In most tumor types examined, these ratios were higher than 1. Assessment of normal tissue TER has not been attempted except in a few cases. For those examples, TER values for normal tissue damage have been less than those for tumor tissue damage in the same patient population, suggesting that there is potential for therapeutic gain with the use of radiotherapy combined with HT compared with radiotherapy alone.⁶⁵ Prospective randomized trials in dogs with spontaneous tumors have also shown evidence for improved local tumor control with radiotherapy plus HT compared with radiotherapy alone,^58,66,67,68 with no clinically observable increase in the frequency of clinically relevant late normal tissue complications. In one canine trial, enhancement of late radiation damage (as assessed histologically) was reported, and the duration of acute radiation complications was prolonged.⁶⁸ There is clinical evidence, however, that excessively high intratumoral temperatures (i.e., >45° C for 60 min) can lead to damage to surrounding normal tissues, an effect that is often caused by rapid tumor regression.^66,69 Such damage is not easily repaired and can lead to chronic tissue consequences, such as fibrosis, fistula formation, and bone necrosis.

Two cases of fatal pelvic necrosis have been reported as a late complication in patients with locally advanced cervical cancer treated with thermoradiotherapy,⁷⁰ and there was speculation as to whether the complication was the result of HT. More recently, however, this rare complication has been reported following chemoradiotherapy treatment of cervical cancer in two small series.^71,72 Thermal doses achieved in the HT series were several degrees lower than the radiotherapy doses reported to be associated with enhancement of normal tissue complications. This finding strongly suggests that these two cases were not the result of HT but more likely resulted from the radiotherapy treatment; An additional complication may have been the heavy smoking history of both patients.⁷⁰

In summary, most available data from preclinical and clinical studies indicate that therapeutic gain is achievable for the combination of HT with radiotherapy. There is very little evidence to suggest that HT enhances the incidence or severity of late normal tissue complications from radiotherapy, particularly when excessively high intratumoral temperatures are avoided.

Rationale for Using Hyperthermia with Chemotherapy

Many chemotherapeutic agents have been shown to exhibit synergism with HT, including cisplatin and related compounds, melphalan, cyclophosphamide, nitrogen mustards, anthracyclines, nitrosoureas, bleomycin, mitomycin C, and hypoxic cell sensitizers.²¹ The mechanisms that underlie the synergy may include (1) increased cellular uptake of drug, (2) increased oxygen radical production, (3) increased DNA damage and inhibition of repair, and (4) reversal of drug resistance mechanisms.^73,74–76 Hypoxia and pH are also important in the thermochemotherapeutic response.^77–81 There are some classes of drugs, such as etoposide and vinca alkaloids, that do not synergistically interact with HT.²¹

Rationale for Using Hyperthermia with Targeted Agents

Very little is known about whether HT affects the cytotoxicity of newer targeted agents, such as those that block epidermal growth factor receptor (EGFR), mitogen-activated protein (MAP) kinase, mammalian target of rapamycin (mTOR), or other signal transduction pathways. Identification of targets can be accomplished by examining changes in gene expression brought on by HT treatment. Such studies have been conducted in vitro and in vivo. Genomic analysis of cells from U937, a human myelomonocytic tumor, exhibited differential expression in approximately 1000 genes, following a noncytotoxic treatment of 41° C for 30 minutes.⁸² A higher thermal exposure of 42° C for 90 minutes led to more significant change in proapoptotic signaling pathways,⁸³ in addition to the expected heat shock response in both cases. Genome-wide analysis has also been done on one rat tumor line in vivo, following local heating (43° C for 60 min).^84,85 The effects of HT were rather pleiotropic, involving over 1200 genes and a host of cellular responses, including apoptosis regulation, cell cycle control, MAP kinase- and calcium-regulated cell signaling, and genes involved in angiogenesis and metabolism regulation. There was also substantial down-regulation of a number of genes involved in immune function at 3 hours after treatment, which recovered to baseline by 24 hours. These results suggested that HT does not adversely affect immune function. The fact that HT appears to augment proapoptotic signaling pathways suggests that it could work additively with targeted agents that promote apoptosis. To date, such strategies have not been examined, however.

It has been reported recently that HT can induce autophagy via activation of nuclear factor kappa B (NFκB) as a protective mechanism against cell death.⁸⁶ Therefore it would make sense to consider combining agents that inhibit NFKB activation with HT as a means of enhancing thermal cell killing. Alternatively, the combination of HT at 43° C with increased oxidative stress can increase cell killing via inhibition of the Akt pathway and induction of irreversible autophagy.⁸⁷ Pajonk and coworkers⁸⁸ reported that inhibition of proteasome activity in combination with HT at 44° C for 60 minutes was particularly effective in augmenting apoptosis, and, in addition, caused substantial radiosensitization.

It is important to note that in most of the studies cited above, a single thermal exposure was used and the severity of the exposure ranged from sublethal to exposures that would be expected to kill logs of cells (i.e., 41° C for 30 min vs. 44° C for 60 min). It is not legitimate to make comparisons between studies with such divergent treatment conditions, let alone the variations in cell types and other methodologic differences. Nevertheless, it appears that there is an emerging rationale for combining HT with the targeting of various signal transduction pathways and other cellular adaptation processes as a means of enhancing cell killing.

There has been some effort, however, related to the idea of using HER2-targeted thermosensitive liposomes to enhance specificity of drug delivery to HER2-positive tumors.⁸⁹ Alternatively, magnetic nanoparticles have been coated with HER2 antibodies to enhance delivery to tumor cells, where they can induce HT when placed in a high-powered magnetic field.^90,91 Targeted nanorods have also been investigated for therapeutic potential with HT.⁹²

Factors to Consider when Combining Hyperthermia with Chemotherapy

Electromagnetic Heating Devices

Electromagnetic heating devices can be separated into two categories: superficial applicators with effective penetration into tissue of less than 4 cm, and deep heating devices that have effective penetration of more than 4 cm. Superficial devices include waveguides (Fig. 21-5) and microstrip or patch antennas (Fig. 21-6) operating at microwave frequencies of 433, 915, or 2450 MHz.^{111,112,113,114} Microwave energy is usually coupled into tissue through a temperature-controlled de-ionized water bolus to maintain the skin temperature below 44° C.

Figure 21-5 Example of a microwave waveguide applicator heating system for superficial HT treatments. Typical systems operate at 915 MHz or 430 MHz for heating a depth of 2 to 4 cm. Energy is usually coupled into tissue through a thin layer of deionized water that is temperature controlled to maintain the skin surface below 44° C. A single waveguide applicator provides no control of power distribution in the x-y plane, aside from what can be achieved by physically moving the applicator.

Figure 21-6 Two examples of thin, flexible microstrip antenna array applicators currently under development for superficial HT. A temperature-controlled, deionized water bolus less than 1 cm thick is used to couple microwaves into tissue and maintain the skin surface below 44° C. These conformal array applicators are driven with eight or more microwave power sources to provide adjustment of the power distribution across the tissue surface.

In order to heat at depths of more than 4 cm, radiofrequencies between 5 and 200 MHz are used. There are three basic techniques for electromagnetic deep heating: magnetic induction, capacitively coupled electric fields, and radiated fields from multiple antenna phased arrays.

Radiofrequency Phased Array

The third option for heating with electromagnetic fields is the radiofrequency phased array.119,120,121,122 These devices consist of an array of radiofrequency antennas arranged in a geometric pattern surrounding the target body region.¹²³ Antennas are driven with multiple independently controllable power amplifiers but use a common radiofrequency source, allowing the radiofrequency fields from all antennas to add together to form a focus in the center of the array. The focus can usually be shifted off center and into the tumor by variation of the relative phase of the signals driving each amplifier. For an array with multiple antennas in phase, one can achieve better and deeper power deposition into tissue than is possible with operation of the antennas independently (i.e., driving the antennas without phase addition). By varying the phase and amplitude of multiple antennas, the phased array technique has more flexibility for controlling power deposition than the magnetic induction and capacitive techniques, which depend heavily on tissue properties and have few power control variables. Figure 21-7 shows one example of a 12-antenna annular phased array applicator mounted on the patient table of an MRI system. This MR-compatible 100-MHz radiofrequency phased array applicator fits inside the bore of a 1.5-T magnet, allowing MR imaging of a temperature rise inside the body during HT treatment.

Figure 21-7 Example of a 12-antenna radiofrequency phased array applicator for deep regional HT treatments. This array is driven at 100 MHz and has appropriate filtering to enable use inside a magnetic resonance (MR) imaging system for monitoring deep tissue temperatures with proton resonance frequency shift-based MR thermal imaging. The large focus of heating inside the phased array applicator may be guided into the tumor by adjusting phase and amplitude of the 12 antennas, in response to real-time feedback from the MR thermal images.

Invasive Thermometry

Invasive thermometry, the current standard, involves physically placing thermometry probes into the tumor within implanted needles or catheters to read subsurface temperatures.129,130,131 Although the accuracy of invasive thermometers is sufficiently precise (typically, ±0.2° C) to resolve important differences in thermal dose (as described above), this type of thermometry has many disadvantages. Disadvantages include discomfort to the patient and risk of hemorrhage and/or infection, the expense of physician time required for catheter placement, and the necessity for imaging to verify placement of thermometers. In addition, the sparse nature of the data obtained makes it difficult to spatially control power deposition or to alter treatment to improve temperature distributions.^132,133 The most common method for doing invasive thermometry is to insert blind-ended catheters into a tumor, using either ultrasound or computed tomography (CT) guidance.¹³⁰ Alternatively, for deep-seated tumors, thermometers may be placed inside orifices that are surrounded by tumor, such as the rectum, urethra, or cervix.^134,135 Thermometers are then placed inside these catheters during treatment. The probes can be multipoint sensors that remain fixed in place, or the sensors can be moved cyclically during treatment to record temperatures at many points along the catheter. There are guidelines published for how these measurements should be taken for all HT devices.^{130,136–138} In spite of its sparse nature, invasive thermometry can provide valuable information about the quality of a treatment. A number of clinical reports have correlated temperature-related parameters to clinical response.^{66,139,140,141–144}

Noninvasive Thermometry

Noninvasive thermometry is emerging in answer to the critical need for volumetric thermometry to visualize complete three-dimensional temperature distributions in real time during deep HT treatments. In previous treatments using the radiofrequency phased array applicators monitored only by invasive probes, there were not a sufficient number of thermal monitoring points to allow adequate characterization of the heating pattern; such monitoring is necessary for effective three-dimensional steering of the heat focus into the tumor tissue. Parts of the tumor are often unheated, whereas too much heat is generated in surrounding normal tissues. Similarly, multiple-element array superficial HT applicators require more temperature measurements than are possible with a limited number of invasive probes to monitor and control all independently powered heat sources. In general, insertion of multiple interstitial probes is clinically impractical, and for superficial tumors (<1.5 cm in depth); probes are often placed in fixed positions on the skin or mapped through catheters lying on the skin to characterize the distribution of heating across a large area of disease as a prediction of subsurface temperatures.

To address the shortcomings of invasive thermometry, a number of noninvasive techniques are under investigation. Infrared thermography has been available for years for monitoring surface temperature distributions.¹⁴⁵ Although spatial resolution and accuracy are excellent, the method requires open access to visualize the surface, so it is not compatible with most heating technologies. A new approach for monitoring two-dimensional surface temperature distributions, even when the surface is buried under a water bolus-coupled applicator, involves use of high-density arrays of fiberoptic sensors mounted in a large surface-conforming thermal monitoring sheet (TMS) configuration.^146,147 Other noninvasive thermometry approaches under development include electrical impedance tomography (EIT), which is sometimes called applied potential tomography,^148,149 ultrasonic temperature imaging using changes in backscattered energy,^150,151 electron paramagnetic resonance (EPR) imaging,²⁶ active microwave imaging or tomography,¹⁵² microwave radiometry,^153–159 and real-time multislice MR thermal imaging (MRTI).

Although there are clinical applications for each of these technologies, MRTI is being studied intensively for volumetric characterization and control of deep tissue hyperthermia. Although several MR tissue parameters are sensitive to changes in temperature, attention has focused in recent years on investigation of the change in water proton resonance frequency shift (PRFS),160,161,162 even in tissues with low water content such as breast tissue.^163,164 Noninvasive MR thermometry has been shown to assist in the delivery of HT treatments and can offer better characterization of the treatment efficacy by allowing calculation of the required thermal metrics from volumetric data. Figure 21-8 shows an example of noninvasive thermometry with MRTI during heat treatment of a lower leg sarcoma with a four-antenna, 140-MHz mini-annular phased array applicator. The method in human patients has yielded a temperature resolution of better than 1° C compared with calibrated invasive sensors.¹⁶⁵ Moreover, MRI offers opportunities for validation of treatment planning systems,^166–168 dynamic control of treatment delivery,^{169,170–172} and post-treatment assessment of tissue damage.¹⁷³ The use of MRTI has been increasing rapidly in recent years for clinical applications in both moderate- and high-temperature thermal therapies such as radiofrequency, laser, and high-intensity focused ultrasound.^135,174,175 The International Journal of Hyperthermia summarized the status of MR thermometry in a special issue,^{162,176,177,178} and additional background information can be found in review articles by Rieke and Butts¹⁷⁹ and MacFall and Soher.¹⁶¹

Figure 21-8 Real-time display during MR monitoring of heat treatment. Left panel shows anatomic image of leg in 24-cm diameter, phased array applicator. The central image shows a typical differential temperature image display at about 20 minutes into heat treatment, after the phase of the four 140-MHz twin-dipole heating antennas had been adjusted to move the heat focus into the tumor at upper right of leg. The proton resonance frequency shift-calculated temperature rise is displayed on the same plot as invasive fiberoptic sensors in the tumor. The dramatic refocusing of power deposition at 18 minutes into treatment is seen to shift heating away from normal tissues in upper left of image and into the tumor at upper right, as indicated qualitatively by the brightness of the temperature image and quantitatively in the calculated temperature rise for tumor in the plot at right.

In a recent paper by Craciunescu and colleagues,¹⁶⁵ important steps toward rapid volumetric real-time imaging-guided HT treatments were presented. With update times of the order of minutes, the real-time treatment control claim made here is in agreement with an accepted definition of real time given for thermal therapies by Rieke and Butts,¹⁷⁹ as “an update time that is small compared to significant changes in temperatures during treatment.” This update time is different in HT (minutes or more) versus ablation therapies (less than a second). This whole process lends credibility to the idea that future HT systems will use image feedback to control and focus heating in tumors.

Hyperthermia Alone

HT alone has been studied in the treatment of superficial tumors. Although an occasional response is noted, the duration is typically quite short.^180,181 No long-term tumor control has been described with the use of HT as a sole treatment modality. HT alone remains widely practiced in certain alternative and complementary medicine clinics. Its use, however, is not supported by any peer-reviewed published scientific data.

Hyperthermia with Radiotherapy

A large number of reports attest to the efficacy of HT in combination with radiotherapy. The majority of publications have been from phase II trials involving patients with superficial malignant disease which is more amenable to heating) as a component of more generalized disease, such as local recurrence of breast carcinoma on the chest wall. With the addition of HT to irradiation, clinical response rates have approximately doubled, from 25% to 35% with radiation alone to 50% to 70% with radiotherapy plus HT.¹⁸² In addition to phase II studies, several phase III trials have now been conducted worldwide with approximately two-thirds published in the English-language literature. These trials generally have been positive, lending additional validity to the information obtained from phase II studies.

Hyperthermia with Chemotherapy

Published data on the efficacy of locoregional HT in conjunction with chemotherapy is much less frequent. Both animal and human data have been reviewed.¹⁸² Particularly encouraging phase II results were reported for sarcomas,¹⁸³ which led to a recently completed phase III trial (see section on phase III trials). A small phase III study from Japan compared the preoperative combination of bleomycin and cisplatin with and without HT for esophageal cancer.¹⁸⁴ The pathologic response rate was higher in the combined treatment arm compared with the CT-alone arm. A phase III trial, recently reported by Issels and associates,¹⁸⁵ compared neoadjuvant thermochemotherapy with chemotherapy followed by surgical resection and radiotherapy for locally advanced soft tissue sarcomas (see section on phase III trials, below, for details).

Tri-modality Therapies with Hyperthermia, Radiotherapy, and Chemotherapy

The combination of HT, chemotherapy, and irradiation has been evaluated preclinically and has been shown to yield superior antitumor effects.¹⁸⁶ Clinical studies have reported encouraging results with this approach as well. For example, pilot clinical studies have evaluated the combination of the hypoxic cell sensitizer etanidazole, irradiation, and HT for superficial tumors.¹⁸⁷ A small randomized phase III trial involving patients with esophageal cancer showed a higher pathologic complete response (pCR) rate when HT was added to chemoradiotherapy compared with chemoradiotherapy alone.¹⁸⁸ More recently, the combination of HT plus cisplatin plus radiotherapy has been evaluated in patients with locally advanced cervical cancer. Twelve patients were treated in this series, with local control being achieved in 10 of them. The two patients with local failure presented with local recurrence following hysterectomy.¹⁸⁹ In a follow-up report, Westermann and associates¹⁹⁰ combined results of this trial (with longer follow-up) with results from several other institutions that had conducted similar studies. A total of 68 patients were included in the analysis. Of these, 61 achieved a complete response. At a median follow-up of 538 days, 74% of patients were disease free. This is a favorable outcome, compared with historical controls.

The combination of 5-FU, HT, and radiation therapy yielded favorable responses in patients with locally advanced colorectal cancer in a phase II study.¹⁹¹ This formed the basis for a randomized phase III trial of 5-FU plus irradiation with or without HT by investigators in Germany.

Normal Tissue Damage from HT

HT does not significantly increase the early or late toxicity of radiation.58,65,67,68 The most common toxicities of HT are superficial or subcutaneous tissue burns. They are usually first- or second-degree burns and are small in volume. Such burns occur in approximately 5% of patients treated in the Duke experience; characteristically, they do not exceed 3 to 4 cm in maximum diameter. Third-degree burns are infrequently observed (<1% incidence). Complications from thermometry catheters are infrequent if the catheters are removed following each HT treatment.¹⁹² When catheters are left in place throughout the course of HT (e.g., for several weeks), the frequency of complications rises significantly, particularly of secondary infections.¹⁹³

There are potential medical contraindications to deep regional HT related to the physiologic stress. Guidelines for patient selection for deep HT have been developed.^138,194

Breast Cancer Trials

Chest wall recurrences of breast cancer are difficult to control with conventional approaches, and many patients develop such recurrences despite prior adjuvant radiotherapy or systemic therapy. Because breast tumors are superficial, they have been amenable to HT trials. Five separate phase III trials were combined for analysis in an international collaborative study by Vernon and colleagues.²⁰⁵ Patients were randomized to either radiotherapy alone or radiotherapy with HT. Treatment was prescribed according to ESHO and RTOG guidelines. HT techniques differed somewhat between institutions but are well documented, as is information concerning temperature distributions and thermal dosimetry.¹⁴⁴ Overall, the five trials demonstrated a significant improvement in the complete response rate for patients receiving HT plus radiotherapy (59%) compared with radiotherapy alone (41%), with an odds ratio of 2.3 (95% confidence interval, 1.4 to 3.8) (see Table 21-2). The greatest effect was observed in patients with recurrent lesions in previously irradiated areas where further irradiation was of necessity limited to low doses. No survival advantage was seen.

Trials of Other Superficial Malignant Tumors

A study was conducted by the RTOG in the 1980s comparing radiotherapy alone with radiotherapy plus HT in superficial measurable tumors (RTOG 8104).^197,198 Of the 307 patients included in the study, approximately half had head and neck tumors, one-third had breast carcinomas (with chest wall recurrences), and the remainder had a variety of superficial malignant tumors. Patients treated with radiotherapy plus HT had a complete response rate of 32% compared with 30% for those receiving radiotherapy alone. Subgroup analysis revealed significant improvements in the duration of local control only in patients with tumors less than 3 cm and in those with breast and/or chest wall recurrences. It was postulated that the better outcome in smaller tumors was a consequence of better heating. This trial was plagued by highly variable heating techniques and crude thermal dosimetry. These problems led to the development of subsequent RTOG guidelines for performing HT.¹³⁰

Head and Neck Cancer Trials

There are two randomized series demonstrating an advantage to HT combined with radiotherapy (see Table 21-2). The first study by Datta and associates²⁰⁶ randomized 65 patients to radiation alone versus radiotherapy and HT. Radiotherapy doses consisted of 50 Gy in 5 weeks to the primary site and regional lymphatics followed by 10 to 15 Gy given to sites of gross disease in daily fractions of 2 Gy. HT was given twice a week, with 72 hours between each session. The HT plus radiotherapy arm showed significant improvement in response in patients with stage III and IV disease. For example, patients with stage III disease receiving combined treatment had a 58% complete response rate compared with 20% in the radiotherapy-alone group. There was no benefit for patients with stage I and II disease, with more than 90% of these patients achieving a complete response with either treatment. This trial from India evaluated both the primary site as well as neck nodes, despite the recognized difficulties of effectively delivering heat to most primary head and neck tumors.

A second trial from Italy by Valdagni and colleagues207,208 restricted treatment and evaluation to metastatic cervical lymph nodes. This study randomized 41 patients with advanced locoregional squamous cell carcinoma of the head and neck to treatment with either radiotherapy alone or radiotherapy combined with HT. Long-term follow-up allowed for analysis of response, duration of local control, and survival times. The 5-year actuarial local control rate in the neck with radiotherapy alone versus radiotherapy plus HT was 24% versus 69%, and the 5-year overall survival rate was 0% versus 53%; all of these differences were statistically significant. There was no clear enhancement of toxicity or any clear relationship between thermal dose received and outcome. Nonetheless, this well-executed, well-described phase III trial, despite the relatively small number of patients, is an important component of the evidence suggesting the value of HT.

A trial of interstitial HT was carried out primarily in head and neck patients by the RTOG.¹⁹⁶ This trial studied 173 patients with persistent or recurrent tumors after prior radiotherapy and/or surgery that were amenable to interstitial radiotherapy. The lesion site was the head and neck in approximately 45% of patients and the pelvis in approximately 40%, with miscellaneous sites accounting for the rest. The patients were randomized to receive interstitial radiotherapy alone with or without interstitial HT. Overall, there was no difference in complete response rates or 2-year survival rates. There were major quality assurance issues with this trial. Only one patient in the entire group was considered to have had adequate HT. This trial, as well as the previously mentioned RTOG trial, provided impetus for the subsequent development of HT guidelines by the RTOG.¹³⁷

Esophageal Cancer Trials

Two randomized studies demonstrated an advantage to the addition of HT to chemoradiotherapy or chemotherapy alone in the neoadjuvant treatment of esophageal cancer. In the first study by Sugimachi and colleagues, 53 patients with squamous cell carcinoma of the thoracic esophagus were randomized to preoperative HT, radiotherapy, and chemotherapy, compared with chemoradiotherapy alone.¹⁸⁸ The chemotherapy (bleomycin) and HT were given concurrently 1 hour prior to the irradiation in a 3-week regimen with a total radiotherapy dose of approximately 32 Gy. Clinical complete responses and pathologic responses were significantly improved in the tri-modality arm, with a pCR of 26% combined in the tri-modality group versus 8% in the chemoradiotherapy group (see Table 21-2).

In a follow-up study, an additional 40 patients were treated with chemotherapy alone (bleomycin and cisplatin) or chemotherapy combined with HT.¹⁸⁴ No radiotherapy was given in this trial. Again, an improvement in histopathologic response was noted favoring the HT group (19% vs. 41%).

Malignant Melanoma Trials

A major multicenter trial was conducted in patients with metastatic melanoma by Overgaard and colleagues.⁵⁷ Seventy patients with 134 metastatic or recurrent malignant melanoma lesions were randomized to receive radiotherapy with or without HT. Overall, there was a significant benefit for the addition of HT, with a 2-year local control rate of 46% in the combined group compared with 28% for those receiving radiotherapy alone (see Table 21-2). Quality assurance issues were problematic in this trial, with only 14% of treatments achieving the protocol objective of 43° C for 60 minutes. Despite this, positive benefits were seen.

Glioblastoma Multiforme Trials

A University of California San Francisco study by Sneed and coworkers²⁰⁹ evaluated interstitial HT combined with a brachytherapy boost for selected patients with glioblastoma multiforme. One hundred and twelve patients were entered into this trial, which randomized patients whose tumor was implantable following external beam radiation therapy and chemotherapy to receive brachytherapy with or without HT. Seventy-nine were randomized; 33 patients were dropped from the protocol due to disease progression. Both time to tumor progression and survival times were significantly improved for the HT patients compared with those treated with brachytherapy alone (see Table 21-2). Two-year survival rates in the two groups were 31% and 15%, respectively. Toxicity appeared to be slightly greater in the HT patients, with seven grade 3 toxicities reported compared with one in the brachytherapy-alone group. Thermal dose data showed that good heating was achieved in most patients. No good correlation was seen between thermal dose and response.²⁰⁹

Pelvic Tumor (Cervical, Bladder, and Colorectal) Trials

The Dutch hyperthermia group conducted a study (van der Zee and colleagues)²¹⁰ in which 358 patients with previously untreated locally advanced pelvic tumors were randomized to receive radiotherapy alone or radiotherapy plus HT (see Table 21-2). HT treatments were given once weekly for a total of five treatments. Generally, thermal goals were not achieved. Detailed thermal dose analyses have not been published. There were approximately equal numbers of patients with bladder, colorectal, and cervical carcinomas. Complete response rates were 39% and 55% after radiotherapy alone and radiotherapy plus HT, respectively (p <.001). Duration of local control was also significantly improved with radiotherapy plus HT. Patients with cervical carcinoma benefited the most. Three-year survival rates were 27% and 51% in the radiotherapy and radiotherapy plus HT groups, respectively (p = .003). This study has been criticized for suboptimal therapy in the control arm, namely, radiotherapy alone as opposed to the combination of radiotherapy and chemotherapy. At the time the trial was initiated in 1990, the role of chemotherapy in cervical carcinoma was not established. Subsequently, a number of studies published from 1999 to 2000 have demonstrated a survival advantage for concurrent cisplatin-based chemotherapy in cervical carcinoma.^211–215 Additional phase III trials are needed to establish whether there is an advantage to using thermochemoradiotherapy compared with chemoradiotherapy for locally advanced cervical cancer.

Soft Tissue Sarcoma Trials

The European Organization for Research and Treatment of Cancer (EORTC) recently reported on results of a phase III trial by Issels and associates that compared neoadjuvant thermochemotherapy with chemotherapy (etoposide, ifosfamide, and doxorubicin)¹⁸⁵ (see Table 21-2). A total of 341 patients were enrolled. Patients received four cycles of neoadjuvant treatment before receiving definitive surgery and/or radiotherapy for the primary tumor. The hazard ratio for local progression-free survival, the primary outcome variable, was 0.58 for patients receiving thermochemotherapy (p = .003). At the 2-year follow-up point, the combined arm had a 15% higher rate of progression-free survival. Disease-free survival analysis showed a hazard ratio for the combined arm of 0.7 (p = .011). For those patients who completed the combined-arm treatment, the hazard ratio for overall survival was 0.66 (p = .038).

Bladder Cancer Trials

Colombo and colleagues²¹⁶ conducted a randomized phase III trial for recurrent superficial bladder cancer that used intravesicular mitomycin C with and without bladder HT, by means of an intravesicularly placed microwave antenna to heat the bladder contents and wall (see Table 21-2). Eighty-six patients were enrolled and 75 completed therapy and were evaluable. Treatment of the bladder commenced after surgical removal of the tumor. The hazard ratio for chemotherapy alone versus thermochemotherapy was 4.8 (p = .0002). The heated group had a higher rate of complications, particularly related to thermal damage to the posterior bladder wall. Most of this damage resolved quickly, with only one patient experiencing a prolonged several-month recovery period. These results are encouraging, considering the fact that the large majority of these patients had failed mitomycin C therapy previously.

Thermal Dose Prescription

Even if noninvasive thermometry were available, until recently there has been no information on what target temperature ranges are needed to optimize this therapy. Such information is the second key for providing a quantitative assessment of HT treatment adequacy (i.e., it establishes a method for writing an HT prescription). In spite of extensive phase II data suggesting that higher temperatures yield improved likelihood for effective therapy, until recently there have been no successful prospective trials showing that escalation of thermal dose yields improved antitumor effect. However, recently, two clinical trials have been reported in which thermal dose was controlled during application of HT.^140,204 These trials have set the stage for establishing realistic goals for how to administer this treatment.

These two trials were designed to perform a “test” HT treatment first, to determine whether the tumor was heatable, as defined by preestablished criteria. If the tumor was “heatable,” then the patient was randomized to receive either a low cumulative thermal dose or a high thermal dose that was at least 10-fold higher than the low dose. The inclusion of the “test” treatment was key in both studies because about 12% of all eligible patients were excluded from randomization because their tumors could not be heated. The first trial was in patients with superficial tumors and the second was in pet dogs with spontaneous soft tissue sarcomas. Both trials demonstrated that cumulative thermal doses of more than 10 CEM43°CT₉₀ yielded improved tumor responses and durations of local control^140,204 (Fig. 21-9). To put this into perspective, this cumulative thermal dose is equivalent to achieving a temperature of 40° C for 1 hour, once or twice a week, at the 10th percentile of the temperature distribution. Importantly, this thermal goal was reached in more than 90% of patients who randomized to the high thermal dose group.

Figure 21-9 A, Clinical trial design. B, Duration of local control in patients with superficial tumors treated with a low or high cumulative thermal dose combined with radiation therapy. The difference in the complete response rate was significant, as was the duration of local control after administration of a higher cumulative thermal dose. CEM43°C, cumulative equivalent minutes at 43° C; HT, hyperthermia; T90, interval of heating.

From Jones E, Oleson JR, Dewhirst MW, et al: A randomized trial of hyperthermia and radiation for superficial tumors, J Clin Oncol 23:3079-3085, 2005.

Trimodality Therapies

Important observations have been made about HT that provide a strong rationale for its continued development. First, although the addition of chemotherapy to radiation therapy has improved the treatment outcome for many diseases, there is still a long way to go. The addition of HT to chemoradiotherapy protocols may be what is needed to push the therapies further toward the goal of reaching 100% local control. Because HT provides a synergistic interaction with many chemotherapeutic drugs as well as radiation therapy, this general approach is compelling.

Augmentation of Drug Delivery

Drug delivery to tumors remains a major challenge due to physiologic barriers such as high interstitial fluid pressure and heterogeneous perfusion. As noted above, HT has been shown to augment macromolecule and nanoparticle drug delivery by increasing blood flow, available volume fraction, and tumor vascular permeability. Furthermore, local HT can be used as a trigger for temperature-sensitive drug delivery systems such as liposomes, polymers, and hydrogels.217,218,219,220 In this manner, site-specific bioavailability can be achieved. For example, a temperature-sensitive liposome containing doxorubicin (ThermoDox) has been developed to release rapidly at clinically achievable temperatures of 40° to 42° C. This formulation has shown dramatic improvement in tumor drug concentration and antitumor efficacy in preclinical studies^221,222,223 and is currently being investigated in phase II/III clinical trials for chest wall recurrences of breast cancer and radiofrequency ablation of liver metastases, respectively. As thermal technology and liposomal formulations advance, this method has the potential for precise control of drug delivery independent of tumor phenotype and drug composition. Newer applications of this liposome have contained both drug and MR contrast agents, and this tandem has been used to monitor drug delivery to heated tumors in real time, as the MR contrast agents change the MR signal when they are released from the liposomes^224,225,226 (Fig. 21-10).

Figure 21-10 Example of monitoring drug delivery in real time, using liposomes containing an MR contrast agent (MnSO₄) and doxorubicin. Calibration studies showed that the change in T1 relaxation time, induced by the Mn⁺⁺, is linearly proportional to drug concentration, in vivo. HT is delivered to the tumor using a catheter containing circulating hot water at the center of the tumor (shows as a dark spot near the center of each tumor). A, When HT is administered, prior to and during drug administration, drug deposition is highest at the periphery of the tumor. B, When drug is administered first, followed by HT, the drug accumulates around the catheter, yielding a centralized drug deposition pattern. C, When drug dosing is split in half and given before HT and after HT has started, a more uniform drug deposition pattern is seen. Interestingly, the peripheral drug delivery pattern was most efficacious. LTSL, low-temperature-sensitive liposome.

Data from Ponce AM, Viglianti BL, Yu D, et al: Magnetic resonance imaging of temperature-sensitive liposome release: drug dose painting and antitumor effects, J Natl Cancer Inst 99(1):53-63, 2007.