Letter: Enhanced killing of hypoxic tumour cells by hyperthemia

Hyperthermia can alter tumor physiology and improve chemo- and radio-therapy efficacy

Hyperthermia has demonstrated clinical success in improving the efficacy of both chemo- and radio-therapy in solid tumors. Pre-clinical and clinical research studies have demonstrated that targeted hyperthermia can increase tumor blood flow and increase the perfused fraction of the tumor in a temperature and time dependent manner. Changes in tumor blood circulation can produce significant physiological changes including enhanced vascular permeability, increased oxygenation, decreased interstitial fluid pressure, and reestablishment of normal physiological pH conditions. These alterations in tumor physiology can positively impact both small molecule and nanomedicine chemotherapy accumulation and distribution within the tumor, as well as the fraction of the tumor susceptible to radiation therapy. Hyperthermia can trigger drug release from thermosensitive formulations and further improve the accumulation, distribution, and efficacy of chemotherapy.

Laserthermia enhances the clastogenic effects of antineoplastic agents in aerobic and chronically hypoxic HeLa cells in vitro

The interactive clastogenic effects of Nd-YAG laser induced hyperthermia (laserthermia) in combination with antineoplastic agents on normally oxygenated and chronically hypoxic HeLa cells were investigated. Exponentially growing HeLa cells were treated with bleomycin sulfate (BLM) (2-4 microg/ml), adriamycin (ADM) (2-4 microg/ml) and actinomycin D (ACT) (0.2-0.4 microg/ml) alone or in combination with laser at various powers (7-13 W) or different laser induced elevated temperatures (39.5-43.5 degrees C). HeLa cells were incubated with 3 microg/ml cytochlasin B for 36 h after treatments and the frequency of micronuclei (MN) were determined in binucleated cells. Results showed a relatively high frequency of MN formation after drug treatments in normally oxic and chronically hypoxic cells, although there was a decrease in the frequency of MN in hypoxic cells compared to oxygenated cells. Laserthermia at various powers and different induced temperatures produced a slight increase in MN formation both in oxic and hypoxic cells. When drug treatment and laserthermia was combined, a profound synergistic effect in MN formation was observed for all three drugs used in these experiments. ACT at a concentration of ten times lower than ADM and BLM produced similar effect. Also, ADM showed a marked synergistic effect with laserthermia compared to BLM at similar concentrations. This study suggests that laserthermia in combination with ADM, BLM or ACT would have a greater genotoxic effect on hypoxic cell populations. Therefore, Nd-YAG laser induced hyperthermia may be a useful modality for elimination of the radioresistant hypoxic cells.

Effect of radiation combined with hyperthermia on human prostatic carcinoma cell lines in culture

The effect of radiation combined with heat on three human prostatic carcinoma cell lines growing in vitro was investigated. Cells were exposed to different radiation doses followed by heat treatment at 43 degrees C for one hour. Heat treatment, given ten minutes after radiation, significantly enhanced the radiation response of all the cell lines studied. The combined effect of radiation and heat produced greater cytotoxicity than predicted from the additive effects of the two individual treatment modalities alone. These results indicate that a combined treatment regimen of radiation plus hyperthermia (43 degrees, 1 hr) might be an important tool in maintaining a better local control of prostatic cancer.

Whole-body hyperthermia. Rationale and potential use for cancer treatment

Whole-body hyperthermia is the controlled elevation of systemic temperature for therapeutic purposes. Historically, this treatment has been used for symptomatic control of many diseases. Recently, the potential therapeutic benefit of whole-body hyperthermia in the management of neoplastic disease has been investigated vigorously. The rationale for improved tumor control is based on heat-induced enhancement of the antineoplastic effects of radiation and chemotherapy. Although the complex biologic interaction of heat and radiation has been studied for many years, chemotherapy combined with hyperthermia has been studied less thoroughly. Despite a lack of adequate long-term laboratory and clinical investigation, use of whole-body hyperthermia with chemotherapy and radiotherapy is a logical and potentially powerful therapeutic strategy for neoplasia. Relevant issues regarding the application of whole-body hyperthermia with more traditional modes of therapy are being studied in preliminary clinical trials involving dogs and humans. Identification of optimal timing and sequencing of adjunctive therapy, proper cytotoxic drug application, methods to further minimize toxicity, and heat-sensitive tumor types will lead to expanded clinical use of whole-body hyperthermia. The historical development, clinical rationale, and application of whole-body hyperthermia for the control of disseminated or refractory neoplasia in humans and dogs is reviewed.

Hyperthermia for cancer: a practical perspective

A causal relationship between hyperpyrexia and tumor regression was first suggested in 1866, when Busch reported the cure of a histologically diagnosed sarcoma in a middle-aged woman, following a bout of erysipelas. Over the years, interest in the effect of heat on cancer has remained alive, but this interest has increased dramatically in recent years. The literature on this subject is broadly reviewed and the clinical results discussed. It is apparent from clinical studies thus far that it is a relatively simple undertaking to treat superficial neoplasms with hyperthermia. However, the major challenges in clinical thermotherapy pertain to patients with deeply situated tumors. The lack of safe and reliable methods of monitoring temperature in deep tissues is a major impediment to a thorough understanding of thermal dosimetry in clinical hyperthermia, and routine thermal dosimetry in clinical hyperthermia will have to await the development of reliable noninvasive thermometry. As responses have been reported with modest levels of hyperthermia, the need for thermometry is somewhat lessened, given that invasive monitoring is imperfect and somewhat risky when used in deeply seated tumours. The eventual place of thermotherapy in the treatment of malignant tumours in man is as yet unclear and must be rigourously and thoroughly assessed in well-designed, prospective, randomized patient trials.

Tumour growth delay, cell inactivation and vascular damage following hyperthermic treatment of a human melanoma xenograft

The effect of hyperthermia at 42.5 degrees C on a human melanoma xenograft in athymic mice was studied. The tumours were heated in vivo in a water-bath. Tumour growth delay and single-cell survival in vitro were used as endpoints. Qualitative information regarding heat-induced vascular damage was obtained from microangiographic analysis. Tumour growth delay after a given treatment was considerably longer than that expected from the cell survival measured in vitro immediately after treatment. Experiments in which removal of the tumours was delayed revealed that tumour cells were continuously dying for at least 24 hr after heat treatment. The volume of the tumour vasculature was considerably reduced after treatment, suggesting that the delayed cell death was attributed to vascular occlusion which resulted in an insufficient supply of oxygen and nutrients and an increased tumour acidity. The present work indicates that at least two mechanisms may be involved in heat-induced cell inactivation in our xenograft: firstly, direct cytotoxic effect of heat; secondly, indirect effect following heat-induced vascular damage.

Differential responses to radiation and hyperthermia of cloned cell lines derived from a single human melanoma xenograft

One uncloned and five cloned cell lines were derived from a single human melanoma xenograft. Cells from passages 7-12 were exposed to either radiation or hyperthermia (42.5 degrees C, pH = 7.4) under aerobic conditions and the colony forming ability of the cells was assayed in soft agar. The five cloned lines showed individual and characteristic responses to radiation as well as to hyperthermia. The variation in the response to radiation was mainly reflected in the size of the shoulders of the survival curves rather than in the D0-values. The variation in the response to hyperthermia was mainly reflected in the terminal slopes of the survival curves. The survival curve of cells from the uncloned line, both when exposed to radiation and hyperthermia, was positioned in the midst of those of the cloned lines. The response of the cloned lines to radiation did not correlate with the response to hyperthermia, indicating that tumor cell subpopulations which are resistant to radiation may respond well to hyperthermia.

Radiation and heat sensitivity of cells from two slowly growing human melanoma xenografts

The radiation and heat sensitivity of cells from two melanin-rich, slowly growing human melanoma xenografts (B.E. and R.A.) were studied. The volume-doubling times of the xenografts in the volume range 200-500 mm3 were found to be 22.5 47.5 days (B.E.) and 25.3-39.2 days (R.A.). The cells were suspended in culture medium during irradiation or heating, and the colony forming ability of the cells was assayed in soft agar. The X-ray survival curve parameters were found to be: Do = 1.09 +/- 0.14 Gy, Dq = 1.99 +/- 0.58 Gy (B.E.); Do = 1.23 +/- 0.08 Gy, Dq = 2.03 +/- 0.35 Gy (R.A.). The Do-values of the heat survival curves were found to be 119.0 +/- 26.6 min (42.5 degrees C), 20.4 +/- 3.9 min (43.5 degrees C) and 9.6 +/- 1.6 min (44.5 degrees C) for the B.E. melanoma and 112.9 +/- 13.3 min (42.5 degrees C), 17.9 +/- 2.0 min (43.5 degrees C) and 7.7 +/- 0.5 min (44.5 degrees C) for the R.A. melanoma. Both the radiation and the heat sensitivities of the cells are within the range of sensitivities reported for rapidly growing melanoma xenografts, suggesting that the intrinsic radiation and heat sensitivity of tumour cells are not strongly related to the rate of tumour growth prior to treatment.

Enhanced tumor growth in experimental whole body hyperthermia

The purpose of this study was to investigate the interaction of heat and chemotherapy in experimental whole body hyperthermia (WBH). A vascular technique of extracorporeal perfusion was employed to rapidly elevate body temperature in rabbits carrying the transplantable VX-2 carcinoma. Hyperthermia (greater than 41 degrees C) was achieved in a mean time of 28 minutes. Tumor-bearing rabbits receiving WBH alone exhibited poor mean survival (14 days) relative to untreated tumor-bearing animals (32 days) and the group receiving IV Cis-platinum alone (50 days) (NS, p less than .05 respectively). WBH with Cis-platinum was intermediate in terms of mean survival (30 days, NS) between the WBH alone and Cis-platinum alone groups. In this experimental model we have determined WBH to be a detrimental form of cancer therapy. Its action appears to enhance tumor proliferation, resulting in rapid animal demise. In light of these findings a reevaluation of clinical WBH may be warranted.

Tumor control and therapeutic gain with different schedules of combined radiotherapy and local external hyperthermia in human cancer

Tumor control and therapeutic gain have been evaluated in a series of studies on patients with multiple lesions employing different protocols of combined radiotherapy (RT) and local external hyperthermia (HT). Tumor response has been evaluated during a follow-up ranging 6 to 18 months. Therapeutic enhancement factor (TEF) was defined as the ratio of thermal enhancement (TE) of tumors to TE of skin, where TE was clinically evaluated as the ratio of percent response (i.e., complete tumor clearance and moist desquamation, respectively) after combined modality to percent response after RT alone. Local tumor control was constantly better in lesions treated with any combined modalities in comparison with RT alone. The use of high RT dose per fraction appeared to increase tumor control only in the combined modalities groups, the immediate (so called "simultaneous") schedule (HT at 42.5 degrees C/45 min, applied immediately after each RT fraction, twice a week) being more effective than the delayed (so called "sequential") treatment (HT at 42.5 degrees C/45 min, delivered 4 h after each RT fraction, twice a week). The combination of high RT dose per fraction with high temperature HT (45 degrees C for 30 min) achieved the best tumor control. No increased radiation skin reaction was observed when a conventional fraction size of RT was used (3 daily fractions of 1.5-2 Gy, 4 h interval between fractions) in association with HT (42.5 degrees C/45 min, every other day, immediately after the second daily RT fraction). A remarkable enhancement of skin reaction was observed, however, when using high RT doses per fraction in association with 42.5 degrees C HT, especially with the immediate treatment schedule. No enhancement of skin reaction was obtained after high RT doses per fractions and 45 degrees C HT because an active skin cooling by means of circulating cold water was used in these cases. Consequently, a good TEF (1.58) was obtained when conventional RT doses per fraction were used in association with 42.5 degrees C HT. TEF values of 1.40 and 1.15 were observed when high RT doses per fraction were employed in association with the delayed and immediate 42.5 degrees C HT, respectively. HT at 45 degrees C can be safely employed only when tumors can be heated selectively or at least preferentially in comparison with normal tissue; in the lesions treated with such a schedule a TEF of 2.10 was obtained.

Effect of hyperthermia on human melanoma cells heated either as solid tumors in athymic nude mice or in vitro

Human melanoma cells were exposed to clinically acceptable hyperthermia (42.5 degrees C) either as solid tumors in athymic nude mice or suspended in culture medium. Single cell survival was in both cases assayed in vitro in soft agar. The response to heat varied considerably among the five melanomas studied. The D0-values ranged from 21 to 590 min when the cells were heated in vitro. The response to heat following treatment in vivo was for a given melanoma larger than that following treatment in vitro. However, cells which were resistant to heat treatment in vitro, were also resistant to treatment in vivo, and those which were sensitive in vitro were also sensitive in vivo.

Effect of hyperthermia and misonidazole on the radiosensitivity of a transplantable murine tumor: influence of factors modifying the fraction of hypoxic cells

Hypoxia has been demonstrated to play an important role in the effect of hyperthermia on tumors. We have studied the influence of different factors modifying the oxygenation status of a transplantable murine mammary adenocarcinoma (tumor volume and pentobarbital sodium anesthesia). The effect of hyperthermia alone on the tumor is not significantly influenced by the change in oxygenation status during the growth of the tumor. Also, the large increase of the acutely hypoxic cell fraction, as a result of anesthesia, does not change the effect of hyperthermia alone. In the combined irradiation-heat treatment there is a clear influence of the chronically hypoxic cell fraction on the response to hyperthermia: an increase in tumor size, resulting in a larger hypoxic cell fraction, leads to an increase in thermal enhancement ratio. However, the increased acutely hypoxic cell fraction, resulting from anesthesia, did not lead to an increase in thermal enhancement ratio; in fact the enhancement ratio apparently decreased. In spite of the fact that hyperthermia was applied immediately after irradiation no potentiation of radiation effects was found. The thermal enhancement of the radiation response was never larger than the enhancement as a result of misonidazole. All thermal enhancement could be explained by effects of heat on the chronically hypoxic cell fraction. Misonidazole had no effect on the response of tumors to heat alone, but greatly enhanced the effect of heat combined with irradiation. Anesthesia of the animals did not influence these effects of misonidazole.

Effect of fractionated hyperthermia on hypoxic cells in vitro

The lethal response of asynchronous exponentially growing mouse lung (L1A2) cells heated to 42 degrees C under hypoxic conditions was demonstrated in vitro. Acutely hypoxic cells (i.e. heated immediately after 30 min of N2 plus CO2 gassing) and aerobic cells treated under the same extracellular pH were equally sensitive to a single hyperthermic treatment, and incubation under hypoxia for up to 24 hours prior to treatment did not influence cell survival. Similarly, under controlled pH conditions (pH within 7.0 to 7.4) recovery from hyperthermic damage demonstrated by two-dose hyperthermic fractionation (each of 1.5 hours at 42 degrees C) was identical in hypoxic and aerobic cells, and the highest recovery was found at a 10-hour interval. Preheating for 1.5 hours at 42 degrees C induced thermal resistance to a second treatment at 42 degrees C (thermotolerance). At the 10-hour interval the degree of thermotolerance was not influenced by incubation under hypoxic conditions (thermotolerance ratio, TTR equals 4.7 in both aerobic and hypoxic cells). The data indicate that hypoxic conditions do not influence the heat response in L1A2 cells to either a single or a two-dose fractionated hyperthermic treatment in which hypoxia or aerobic conditions were maintained in the interval between the heat treatment.

Spinal anaesthesia for paediatric radiotherapy

Painful radiotherapeutic procedures on lower limbs are susceptible to spinal anaesthesia. This method is shown to be suitable in children when neuroleptanalgesia provides amnesia, an adequate period of patient immobility, and stable cardiovascular and respiratory conditions. The technique is useful in overcoming the problems of anaesthesia in the radiotherapy environment.

Influence of hyperthermia on the oxygen enhancement ratio for x-rays, measured in vivo

The skin of mouse tail has been used to study the effect of hyperthermia on the oxygen enhancement ratio (OER). Heating was by immersion of a portion of the tail in hot water. Radiation was given either immediately before or after hyperthermia. The average skin reaction between 15 and 50 days after treatment was taken as the end-point. The OER in the absence of hyperthermia was 1.77, suggesting significant hypoxia of the skin. When hyperthermia was given after irradiation the measured value for the OER was not significantly different, but with prior hyperthermia the OER was increased to an average value of 2.3. This increase in OER is probably due to a transient increase in blood circulation following hyperthermia and causing improved tissue oxygenation during irradiation. As a consequence we would expect a greater thermal enhancement ratio for heat given before irradiation than afterwards, and this has frequently been observed with other normal tissues. There was no evidence that heat reduces OER, as has been reported by some authors on the basis of experiments performed on cells in vitro.

The response of six mouse tumours to combined heat and X rays: implications for therapy

The response of six types of mouse tumour to single doses of X rays alone or to X rays in combination with moderate hyperthermia (42.5 degrees C/60 min) has been assessed using delay in tumour regrowth. Thermal sensitization was observed in five of the six tumours. The degree of sensitization varied with the size of the X-ray dose, being larger at higher doses. The degree of sensitization also depended on the sequence and separation of the heat and irradiation. The thermal sensitization has been measured in terms of the X-ray doses to produce the same level of tumour damage with or without heat, i.e. thermal enhancement ratios. These TER values, measured for X-ray doses in excess of 20 Gy, are not greater in any of the tumours than in a range of normal tissues, if the X rays and heat are given in close succession. Separation of the heat and X rays reduces the TER values slightly, but some effect is still apparent at 3--24 hours. In normal tissues the effect of heat is totally lost within four hours. Comparison of these tumour data with published normal tissue data indicates a therapeutic advantage if the heat and X rays are separated by more than one hour. This therapeutic gain is most reliably achieved and heat given after irradiation.

The effect of vascular occlusion on the thermal sensitization of a mouse tumour

The effect of occluding the blood supply to a mouse tumour (with a metal clamp) has been studied for both irradiation and heating. Local heat was applied by immersion in a water bath for one hour at 42.8 degrees C or for 15 minutes at 44.8 degrees C. Occlusion of the blood supply during heating has a profound cytotoxic effect on the tumour, even in the absence of irradiation. Most tumours treated with 42.8 degrees C for one hour under clamped conditions were locally controlled whether they were irradiated or not. Tumours heated with their blood supply unobstructed showed a lesser sensitivity to heat, seen as an increased sensitivity to X rays with a thermal enhancement ratio of 1.8--2.6. With the shorter period of more intense heat (44.8 degrees C for 15 min), the effect of increasing the clamping time before heating was studied. The proportion of tumours locally controlled increased from 33% if the clamp was applied immediately before heating to 83% if the clamp was present for 60 minutes before heating commenced. No cures were observed for heat applied immediately before clamping, or immediately after release of the clamp. Accumulation of metabolic products or pH changes are implicated as the factors which alter the thermal sensitivity of these tumour cells.

Enhancement of thermal killing by polyamines. II. Uptake and metabolism of exogenous polyamines in hyperthermic Chinese hamster cells

The uptake and metabolism of the polyamines spermine, spermidine, cadaverine and putrescine, previously shown to potentiate heat sensitivity, were studied in cultured Chinese hamster cells. Heat (42 degrees C) causes enhanced uptake of exogenously supplied polyamines into the acid-soluble fraction of the cells. Putrescine is taken up exceptionally fast at 37 degrees C, about 10 times faster than its homologue, cadaverine. This uptake is slower at 42 degrees C. The polyamines taken up were metabolized to some extent and the metabolites were similar at 37 degrees C and 42 degrees C except in the case of putrescine. These results suggest that potentiation of heat-sensitivity is probably mediated by the polyamines as such and not by their metabolites. Polyamines slightly protect the cells against the inhibitory effect of heat on RNA and protein synthesis. It is suggested that exogenous polyamines interact with nucleic acids inside the cell, and this interaction may underlie their synergism with heat. The exact nature of this interaction and the way it leads to enhanced thermal sensitivity are still obscure.

Local tumor hyperthermia in combination with radiation therapy. 1. Malignant cutaneous lesions

There is increasing evidence that the use of hyperthermia alone or in conjunction with other modalities may improve the therapeutic effectiveness of treatment of cancer. The present clinical studies were carried out to evaluate the response of normal and tumor tissues in patients with various cutaneous malignant lesions to repeated courses of hyperthermia alone or in conjunction with radiation therapy. Thirty-six patients with malignant cutaneous lesions (mycosis fungoides, Kaposi sarcoma, malignant melanoma, lymphoma cutis, and other metastatic skin lesions) have been studied. The heating methods used were: 1) temperature regulated water bath immersion; and 2) radiofrequency inductive heating. The normal tissue effects of the combined treatments of radiation and hyperthermia do not appear to be greater than those treated with radiation alone. The initial tumor regression rates were faster in patients treated with radiation plus hyperthermia than in radiation alone, particularly in patients with Kaposi sarcoma and lymphoma cutis. Among ten locally recurrent patients, seven showed significant prolonged benefits achieved by the combined treatments as compared with the radiation therapy alone. Fractionated hyperthermia alone caused significant tumor regression in four out of five patients. Possible mechanisms leading to the improved results from the combined treatments are discussed.

Local tumor hyperthermia in combination with radiation therapy. 1. Malignant cutaneous lesions

There is increasing evidence that the use of hyperthermia alone or in conjunction with other modalities may improve the therapeutic effectiveness of treatment of cancer. The present clinical studies were carried out to evaluate the response of normal and tumor tissues in patients with various cutaneous malignant lesions to repeated courses of hyperthermia alone or in conjunction with radiation therapy. Thirty-six patients with malignant cutaneous lesions (mycosis fungoides, Kaposi sarcoma, malignant melanoma, lymphoma cutis, and other metastatic skin lesions) have been studied. The heating methods used were: 1) temperature regulated water bath immersion; and 2) radiofrequency inductive heating. The normal tissue effects of the combined treatments of radiation and hyperthermia do not appear to be greater than those treated with radiation alone. The initial tumor regression rates were faster in patients treated with radiation plus hyperthermia than in radiation alone, particularly in patients with Kaposi sarcoma and lymphoma cutis. Among ten locally recurrent patients, seven showed significant prolonged benefits achieved by the combined treatments as compared with the radiation therapy alone. Fractionated hyperthermia alone caused significant tumor regression in four out of five patients. Possible mechanisms leading to the improved results from the combined treatments are discussed.

Effect of hyperthermia on malignant cells in vivo. A review and a hypothesis

The relevant literature is reviewed in an attempt to clarify the mechanism of heat-dependent tumor cell destruction in vivo. Malignant cells in vivo appear to be selectively destroyed by hyperthermia in the range of 41-43 degrees C. Heat evidently affects nuclear function, expressed by an inhibited RNA, DNA and protein synthesis and characteristic arrest or delay of cells in certain locations of the cell cycle. However, as these effects appear to be reversible and are observed in normal cells as well as malignant cells, they probably do not explain the hyperthermic induced selective in vivo destruction of malignant cells. Heat-induced cytoplasmic damage appears to be of more importance. Increased lysosomal activation is observed, and is further intensified by a relatively increased anaerobic glycolysis which develops selectively in tumor cells. A hypothesis is proposed and discussed which explains the marked and selective in vivo tumor cell destruction as a consequence of the enhancing effect on the cytoplasmic damage of certain environmental factors (e.g. increased acidity, hypoxia and insufficient nutrition.

Hyperthermic tumour-cell devitalization in vivo

A review of the morphologic, biochemical and clinical effects of hyperthermia on malignant cells indicates the presence of two principally different heat-induced alterations. (1) A 'destructive' lysosomal dependent cytoplasmic reaction dominates the tumour-cell devitalization in vivo, probably influenced by the characteristic tumour cell environment. (2) 'Repressive' nuclear abnormalities may be observed, but seem to be secondary in the in vivo reaction. However, under certain conditions (combined treatment modalities) this nuclear effect may be of importance.

Interaction of hyperthermia and the hypoxic cell sensitizer Ro-07-0582 on the EMT6 mouse tumour

The combination of hyperthermia and the hypoxic cell radiosensitizer Ro-07-0582 has been investigated on the EMT6 tumour implanted into the legs of BALB/c mice. Treatments at a drug dose of 1 mg/g over a range of waterbath temperatures from 37 to 45 degrees C are described. The surviving clonogenic fraction following treatment was assayed in vitro. Measurements of intra-tumour temperature have been made, and shown to be better correlated with the cytocidal effect on the tumour than the waterbath temperature. No significant effect of Ro-07-0582 was observed at 37 degrees C. However, marked cytotoxicity due to the drug was seen at intra-tumour temperatures above 42-5 degrees C for 1 h. These were in addition to the cytocidal effect of the hyperthermia. The results are discussed in relation to the distribution of temperatures and hypoxic cell populations throughout the tumour.

Hyperthermic potentiation: biological aspects and applications to radiation therapy

Experimental studies have provided evidence that hyperthermia may be an effective agent, either alone or in combination with ionizing radiation, in the treatment of cancer. Results have shown that temperatures in the range of 42 degrees to 45 degrees C: 1) are cytotoxic, with cell lethality showing little or no dependence on levels of oxygenation; 2) inhibit the recovery of cells from sub-lethal and potentially lethal radiation damage while enhancing the levels of lethal damage; and 3) may be combined with x-irradiation in a manner to improve therapeutic ratios. The observed interaction between hyperthermia and x-rays may in part be due to differences in the Age Response Functions and reassortment of cycling cells to these two agents. Hyperthermia may also greatly change repopulation and re-oxygenation parameters in irradiated tumor and normal tissue volumes. An overall consideration of these and other factors is essential in the design of optimal schedules of combined hyperthermia and x-irradiation treatments in the management of malignant disease.