The relationship between heating time and temperature: its relevance to clinical hyperthermia

Effects of heat treatment in vitro and in vivo on human melanoma xenografts

The heat response in vitro and in vivo of five human melanoma xenografts grown in athymic nude mice was studied. The melanomas differed significantly in terms of heat sensitivity both in vitro and in vivo. At least two different mechanisms governed the overall heat response of the melanomas in vivo: the primary cell death, induced during treatment, was due to direct cytotoxic effects of the heat; the secondary cell death, induced after completion of treatment, was due to heat-induced vascular damage. The activation energies for the melanomas were not significantly different in vitro and in vivo at temperatures above the inflection point of the Arrhenius curves. Below the inflection point, on the other hand, the activation energies were higher in vitro than in vivo, probably as a consequence of differences in the physiological conditions in vitro and in vivo. The heat responsiveness of the melanomas in vivo was not related to the radioresponsiveness, whether the heat treatment was given at a low or a high temperature. All melanomas developed thermotolerance after a priming heat treatment. The thermotolerance differed significantly in magnitude among the five melanomas. It was concluded from the thermotolerance data that clinical treatment protocols probably should not prescribe more than one hyperthermic treatment per week.

Radiotherapy and hyperthermia. Analysis of clinical results and identification of prognostic variables

Site- and tumor-specific data obtained from two groups of patients with head and neck and melanoma lesions, respectively, showed that both immediate response and response duration were enhanced by the addition of heat. Two important variables, however, such as tumor volume and "isoeffect thermal dose" appeared to influence local tumor control. The volume effect was less pronounced in the lesions treated with radiotherapy plus heat than in those treated with radiotherapy alone, suggesting that the addition of heat was more damaging to the large than to the small lesions. Furthermore, a striking isoeffect thermal dose-response relationship was shown in head and neck lesions. Those data were collected and used to design a mathematical model relating the probability of local control to clinical and treatment variables. The analysis shows that, by using the same radiation parameters, the probability of local tumor control is a function of both "isoeffect thermal dose" and tumor volume.

Studies relevant to a means of quantifying the effects of hyperthermia

There is as yet no fully satisfactory method of defining 'thermal dose'. However, in order to relate different hyperthermal treatments it has been suggested that the relationship between heating time and temperature be used to derive an isoeffect parameter which might be useful in this context. The equation t2/t1 = RT1-T2, where t is the treatment time, T the temperature, R = 2 for T greater than 42.5 degrees C and R = approximately 6 for T less than 42.5 degrees C, has been shown to fit the isoeffect data from many studies both in vivo and in vitro. Whether or not this relationship is applicable when the temperature is varying has been tested using an animal model system, i.e. the response of the baby rat tail. It has shown that the relationship between time and temperature is indeed characterized by the above equation, and the phenomena of thermotolerance and step-down sensitization were clearly demonstrated. Tails were then alternated between waterbaths at different temperatures in order to simulate varying temperature. The measured effects were, in general, in good agreement with those derived from the formula. The maximum difference between the observed and predicted effect, obtained under rather extreme conditions unlikely to be experienced clinically, amounted to an under-estimate of the effective temperature by approximately 0.3 degrees C, i.e. a net small effect of thermotolerance. It is therefore suggested that the above formula for 'heat dose' provides a reasonable interim method for comparing clinical hyperthermic treatments. The formula does not, however, account for differing absolute sensitivities between tissues nor for effects of fractionating heat treatments.

Clinical application of thermal isoeffect dose

Clinically, there is strong rationale for developing a method which will provide a scientific basis for comparing the efficacy of one hyperthermia treatment with another. In order to accomplish this goal, methods must first be developed which will allow the clinician to know the three-dimensional temperature distribution in heated tissue. In this paper, examples of how this goal can be achieved are presented. Techniques for compensating for various modifiers of hyperthermia effectiveness are proposed. The limitations and advantages of these approaches are described and directions for future research are discussed.

Thermal isoeffect dose: addressing the problem of thermotolerance

A method of calculating a thermal isoeffect dose by converting thermal exposure into equivalent-minutes at 43 degrees C (EQ43) has been described previously by this investigator and others. Some investigators have suggested variations in the constants of this approach based on evaluations of the available in vivo data. The selection of these constants affect thermal dose calculations most at temperatures below 43 degrees C. Since treatment response appears to be most closely related to the dose in the coolest part of the tumour, the selection of appropriate constants may be quite important. The data suggest that these variations in constants are a consequence of thermotolerance. A more appropriate approach to address this problem is presented. The phenomena of thermotolerance complicates the practical application of this thermal isoeffect dose model. The dose modification caused by chronic thermotolerance which occurs during exposure at mild hyperthermic temperatures can be estimated by calculating the thermotolerance dose ratio (TTDR) between the equivalent-minute dose calculated with and without a transition temperature at 43 degrees C. The data suggest that the TTDR as a function of time can be mathematically described and used to correct for the dose modifying effect of thermotolerance. Using these results in a modification of the thermal isoeffect dose model causes a significant improvement in the fit for in vitro data below 43 degrees C.

Some problems related to the clinical use of thermal isoeffect doses

The well-known biological isoeffect relationship between treatment time and temperature has been suggested as a basis for a general biological heat dose unit which could be used to compare the effect of different heat treatment schedules. This is frequently expressed as 'equivalent heating time at 43 degrees C'. Such a conversion has in experimental studies been shown to be effective in comparing single heating schedules. However, clinical treatment has some features which may strongly influence the usefulness of an isoeffect heat dose. Firstly, the heat distribution is generally heterogeneous and fluctuates with time, which in some situations results in increased damage due to step-down heating. Secondly, in the situation where hyperthermia and radiation are given simultaneously, the time-temperature relationship may be different from that in the case of heat alone, and from the effect of heat given as an adjuvant to radiotherapy. Thirdly, most clinical treatments are given as fractionated treatments, and it is almost certain that thermotolerance may influence the biological heat effect to some extent. However, with the unknown kinetics of thermotolerance the magnitude of this phenomenon cannot be predicted. A series of experiments in a C3H mammary carcinoma were performed in order to analyse some of these problems.

1985 Douglas Lea memorial lecture. Hyperthermia in the treatment of cancer

There are sound biological reasons for using hyperthermia in the treatment of malignant disease. This review includes a discussion of this rationale and describes effects of hyperthermia either given alone or in combination with ionising radiation to cells in vitro, tumours or normal tissues. Topics discussed include thermotolerance, step-down sensitisation, fractionation, re-treatment of previously irradiated sites, thermal enhancement ratio and thermal dose. Problems of heat delivery and temperature measurement are considered and the current status of clinical studies is stated briefly.

The effect of step-down heating on mouse small intestinal mucosa

Step-down heating (SDH) was investigated in mouse small intestine by giving a primary (conditioning) treatment at or above 43 degrees C followed by a test treatment below 43 degrees C. Crypt dose-response curves following SDH were compared with those obtained using the test treatment alone; the SDH effect was characterized by a reduction in shoulder (an additive effect) and an increase in slope (thermosensitization). The thermosensitization ratio, defined as slope SDH-heated/slope single-heated, was independent of the conditioning temperature but increased to a maximum of approximately three as the duration of conditioning increased. Thermosensitization was eliminated when the conditioning treatment was itself sufficient to cause significant crypt loss and, also, when the interval between the two treatments was 0.5 h or longer. This period was less than that required for either recovery of the 'shoulder' on the crypt dose-response curve or the development of thermotolerance following the primary treatment. Thermotolerance which develops in intestine during prolonged hyperthermia (after approximately 100 min) was not affected by SDH and Arrhenius analysis indicated that the activation energy for temperatures below 43 degrees C was not significantly altered by SDH. In summary, the SDH effect on small intestine, assessed using the crypt loss endpoint, was similar to thermosensitization observed in vitro. However, the lower magnitude of the effect and its complex dependence on the primary heat treatment suggest either that crypt cells respond to SDH in a unique and characteristic manner or that the crypt assay in vivo and reproductive survival in vitro do not reflect the same endpoint.

Influence of prior heat treatment on the effects of heat alone or combined with X-rays on mouse stromal tissue

The tumour bed effect assay was used to study the sensitivity of mouse stromal tissue to heat applied alone or combined with irradiation. Prior heat treatment, 30 min at 43 degrees C, of the tumour bed led to thermotolerance. After priming, thermotolerance developed fully within 24 h and it had disappeared completely after about 10 days. The kinetics of development and decay of thermotolerance in this slowly dividing tissue is similar to that which we had observed previously in skin. When decay rates of several normal tissues with different proliferation characteristics are compared, it is obvious that there is not a clear relationship between proliferation rate of the presumed target cells in the tissue and thermotolerance decay rate.

Factors of importance for the development of the step-down heating effect in a C3H mammary carcinoma in vivo

The effect of step-down heating (SDH) was investigated in a C3H mammary carcinoma inoculated into the feet of CDF1 mice. The SDH effect was evaluated by comparing slopes of time versus growth delay curves of SDH-heated with the curve for single-heated controls. The effect was quantified by a ratio: 'step-down ratio' (SDR), defined as slope (SDH-heated)/slope (single-heated). Step-down heating resulted in thermosensitization in contrast to step-up heating which did not affect the heat sensitivity. The kinetics of the step-down heating effect was investigated by inserting an interval between a 44.5 degrees C/10 min sensitizing treatment (ST) and a 42.0 degrees C test treatment (TT). The effect of SDH was maximal with no interval between ST and TT (SDR = 2.3), decayed within 2 h and turned into thermotolerance. This thermotolerance was maximal after 12 h and decayed within 120 h. The effect of varying the TT temperature was investigated in the range 39.0-44.5 degrees C (ST = 44.5 degrees C/10 min). Below 42.5 degrees C the SDR value increased exponentially, and even a 39 degrees C TT produced a significant heat damage. An Arrhenius analysis was made showing a straight line in the whole temperature range with an activation energy of 526 kJ/mol and an increased activation entropy. These data show that thermosensitization can be induced by SDH in C3H mammary carcinomas in vivo. The effect seems to decay within 2 h, and by decreasing the heat activation energies the effect of low temperature heating is increased.

Arrhenius analysis of the heat response in vivo and in vitro of human melanoma xenografts

The response to heat treatment in vivo (40.5-44.0 degrees C) and in vitro (40.5-45.5 degrees C) of five human melanoma xenografts was studied. Specific growth delay was used as a measure of response after treatment in vivo. Colony-forming ability was assayed in soft agar after treatment in vitro. Dose-response curves were established and subjected to Arrhenius analysis. The Arrhenius curves were found to have an inflection point at 42.0-43.0 degrees C in vivo and 41.5-42.5 degrees C in vitro. The activation energies were in the ranges 426-771 kJ/mol in vivo and 676-739 kJ/mol in vitro above the inflection point and 774-1661 kJ/mol in vivo and 1118-2190 kJ/mol in vitro below the inflection point. Above the inflection point the activation energies in vivo and in vitro were not significantly different for any of the melanomas, and furthermore were similar to those reported for rodent tumours, normal tissues and cells in culture in the same temperature range. Below the inflection point on the other hand the activation energies were lower in vivo than in vitro. This difference was probably a consequence of differences in the physiological conditions in vivo and in vitro. The activation energies in vitro in this temperature range were comparable to those reported for normal tissues and cells in culture.

Retrospective analysis of the response of tumours in patients treated with a combination of radiotherapy and hyperthermia

One hundred and twelve patients with various carcinomas were treated on 112 fields with radiotherapy and hyperthermia, using non-invasive techniques. Radiotherapy dose ranged from 13-70 Gy (except for one patient receiving hyperthermia alone) with a mean of 28.6 Gy. The combined treatment was primarily aimed at giving palliation; 79 per cent of the patients had received previous irradiation on the same area. Hyperthermia was given twice weekly following radiotherapy. From the temperature data collected, 12 different parameters expressing the hyperthermia 'dose' were derived. The various parameters for both treatment modalities, i.e. radiotherapy and hyperthermia, and some of the tumour parameters were statistically evaluated with respect to their influence on tumour response. The overall response rate was 87 per cent including 33 per cent complete response. The complete response rate increased with increasing radiotherapy total dose, i.e. from 23 per cent (14-25 Gy) and 38 per cent (28-36 Gy) to 60 per cent (greater than 38 Gy). A positive correlation between the tumour temperature parameter representative of the coldest spot in the tumour, and the level of response was found. Achievement of complete response appeared also to be determined to a considerable extent by radiotherapy total dose as well as tumour volume. The correlation between response level and the minimum hyperthermia dose parameters persisted, however, after correction for the influence of tumour volume and radiotherapy total dose. These results support the opinion that higher tumour response rates can be achieved by increasing the hyperthermia treatment level at the coldest spot in the tumour.

Estimation of therapeutic gain in clinical trials involving hyperthermia and radiotherapy

It is clear from discussions in this paper that phase III testing of hyperthermia in human patients must proceed in a cautious and stepwise fashion. Because of the risks of increasing late effects, either due to direct thermal damage or thermo-radiosensitization of normal tissues, it is not prudent to proceed with such testing in sites where there is a risk of excessive normal tissue heating. The correlations between temperature and prognosis in heated tumours implies that sites and techniques should be chosen where the chance of achieving relatively uniform heating are maximized. Methods of quality assurance are of equal importance and need to be carefully designed. Even then, retrospective analyses with temperature variations used as prognostic covariates are essential. Other factors, such as tumour volume and radiotherapy dose should be carefully controlled in experimental and control groups. Finally, protocol compliance is a real problem which will cause problems in interpretation of results, especially in studies designed to look at hyperthermic time-dose fractionation.

An assessment of local hyperthermia in clinical practice

A total of 116 small superficial tumours have been treated by radiation alone, hyperthermia alone, or radiation and hyperthermia combined in a Phase I/II study. Most tumours were metastases or local recurrences of adenocarcinoma of breast but other histologies were involved including melanoma. Hyperthermia was delivered predominantly by microwaves, but radiofrequency and ultrasound methods were also used. Rigorous thermal dosimetry, based on measurements from invasive multipoint thermocouple arrays, has shown that 58 per cent of hyperthermal treatments reached a minimum dose within tumour equivalent to 20 min at 43 degrees C (minEq43); 24 per cent reached at least 60 minEq43. Minima of 20 minEq43 were achieved successfully on every intended occasion in a quarter of the 75 tumours heated, and on one/two occasions in 39; unfortunately, this minimum threshold was not reached at any point monitored at any hyperthermia session in 17(23 per cent) tumours. Tumours that received radiation and effective hyperthermia were more likely to disappear completely (CR rate 86 per cent) than those that were irradiated but inadequately heated (CR rate 35 per cent) (P less than 0.001) or were treated by the same doses of radiation alone (CR rate 35 per cent) (P less than 0.05). This improvement with hyperthermia became more apparent with suboptimal radiation doses. A small but measurable growth delay was imposed by heat alone with a poor complete response rate (11 per cent). The real-time use of a thermal dose unit in clinical practice facilitates hyperthermal treatment comparisons and provides an important parameter for checking the technical performance of a heat delivery system. The results of this study emphasizes the need for improvements in intratumour temperature distribution, in order to establish minimum threshold temperatures to enhance tumour response rates.

A clinical microwave hyperthermia system with multipoint real-time thermal dosimetry

A clinical hyperthermia system using a 915 MHz microwave generator and incorporating multipoint thermocouple thermometry is described. Temperatures can be monitored simultaneously at 16 points and measurements displayed on a visual display unit and a plotter. The power output of the generator is adjusted under computer control to maintain a constant predetermined temperature in a chosen control channel. A clinically useful feature of the system is the ability to determine the effective cumulative thermal dose delivered to the tissue at points monitored in real time. The basis for the thermal dose calculation is discussed in detail. The calculated dose parameter is displayed for each of the 16 channels during the treatment and updated every 30 s. This real-time display of a thermal dose parameter has made possible a more homogeneous heating of tumours.

Investigation of thermotolerance in mouse testis

The effect of a two-fraction heat treatment on mouse testis has been assessed by measuring testis weight loss at 1 week after treatment. The rate of repair of 'sublethal' heat damage following the first treatment was dependent on the severity of the treatment. Using a primary treatment of 41.5 degrees C for 30 min, the weight loss following a test treatment of 41.5 degrees C for 30 min returned to that of the test treatment alone within an interval of 16-24 h. Using a milder primary treatment of 40.0 degrees C for 30 min, repair of sublethal heat damage appeared to be complete by 1-2 h. When a single test treatment was used, there was no evidence of heat-induced thermal resistance (thermotolerance) following primary treatments of 40.0 or 41.5 degrees C for 30 min, for periods up to 24 h between treatments. A small degree of thermotolerance could, however, be demonstrated following the most severe primary treatment used if full dose: effect curves were obtained. Thermotolerance, manifest as a decrease in slope, was maximal at approximately 4 h after the primary treatment. The results are discussed with reference to other normal tissue data.

The influence of pre-treatment temperature on the thermal sensitivity of a mouse tumour

The response of tumours to hyperthermia was tested by giving graded heat treatments and assessing local control at 90 days. Mice were divided into three groups which were pre-treated for 3 days in ambient temperatures of 4, 21 or 35 degrees C. This enabled the mean tumour resting temperature to be varied by up to 11 degrees C, before subsequent heat treatment. For the heat treatments, the tumours were clamped in order to eliminate blood flow, resulting in uniform temperature distributions and hence more uniform thermal sensitivity. TCD50 values were used to construct Arrhenius plots. For all three pre-treatment temperatures, these plots demonstrated a factor of 1.6 increase in heating time per degree Celsius reduction in heating temperature. However, tumours kept in a 4 degrees C environment before treatment were more thermally sensitive than those kept in 21 degrees C conditions, while those in a 35 degrees C environment were more resistant. Pretreatment at 4 degrees C was equivalent to an increase of either 0.5 degree C in heating temperature or 28 per cent in heating time, compared with pre-treatment at 21 degrees C. Pre-treatment at 35 degrees C was equivalent to a reduction of either 0.6 degree C in heating temperature or 25 per cent in heating time. These data indicate that the pre-treatment tumour temperature is an important parameter, but the effect of heat treatment is more closely related to absolute heating temperature rather than to the increase in temperature above the normal resting level.

Tumour response to heat and radiation: prognostic variables in the treatment of neck node metastases from head and neck cancer

A total of 38 patients with 81 multiple neck node metastases from squamous cell carcinoma of head and neck were treated with radiotherapy alone or with radiotherapy plus hyperthermia. Irradiation was delivered following a three fractions per day schedule of 2 + 1.5 + 1.5 Gy/day, with 4 h intervals between fractions, up to a total dose of 60 Gy. Heat was applied by means of a 500 MHz apparatus. Temperature data were converted to equivalent minutes at 42.5 degrees (Eq 42.5). Initial complete response rates and local control distribution were compared for subgroups of tumour volume and thermal dose. The data indicated that the volume effect was less pronounced in the combined modality than in the radiation alone arm, suggesting that the addition of heat was more damaging to the large than to the small lesions. A striking thermal dose-response relationship was shown, although complete response rates increased only after a certain thermal dose was accumulated, clearly indicating the presence of a threshold dose.

Clinically derived dose effect relationship for hyperthermia given in combination with low dose radiotherapy

A group of 36 patients, treated on 44 fields with fractionated treatments of local high-frequency induced hyperthermia and radiotherapy to the low total dose of 14-25 Gy, was retrospectively evaluated for differences between responders (i.e., complete and partial responders) and non-responders. A response rate of 86% was achieved, comprising 14% (6/44) complete and 73% (32/44) partial response. There was an indication that the probability of complete response is greater in smaller tumours. The tumour temperatures achieved were higher in the responders than in the non-responders. Logistic analysis was performed on the mean temperature achieved. This temperature parameter yielded a significant positive dose-effect relationship with regard to therapeutic effect, from which a 50% effective dose of 38.8 degrees C was calculated. The data indicate that to obtain the maximum therapeutic effect, i.e., greater than 95% response rate, tumour temperatures above 42.5 degrees C need to be achieved.

Time-temperature relationships for hyperthermal radiosensitisation in mouse intestine: influence of thermotolerance

Thermal enhancement of radiation injury to the crypt compartment of mouse small intestinal mucosa has been measured as a function of heating time for temperatures in the range 41.0-44.0 degrees C. All the hyperthermal treatments used were themselves subthreshold for gross tissue injury. With this limitation, thermoradiosensitisation increased linearly with duration of hyperthermia for temperatures in the range 42.3-44.0 degrees C. Using temperatures below 42.0 degrees C, there was a saturation in effect for treatments longer than approximately 40-90 min, possibly due to the development of thermotolerance. The thermoradiosensitisation isoeffect curve relating heating time with temperature was biphasic with the transition occurring between 41.8 and 42.0 degrees C. For temperatures above the transition, a 1 degree C change was equivalent to a factor of 2.6 in heating time; below the transition, a 1 degree C change was equivalent to a factor of 5.4. Time-temperature relationships for thermoradiosensitisation in other rodent tissues are reviewed and compared with the general relationships for direct thermal injury, previously derived from experimental studies. The results are discussed with relevance to the interpretation of in vivo thermal enhancement of radiation injury.

The relationship between heating time and temperature for rat tail necrosis with and without occlusion of the blood supply

The relationship between time of heating and temperature has been investigated for necrosis resulting in the loss of distal vertebrae in the rat tail. The study was made in both normal conditions and with the blood supply to the tail occluded. In normal conditions there was a transition in the isoeffect relationship close to 42.5 degrees C. Above this temperature a 1 degree C change was equivalent to a change in heating time by a factor of 1.95 +/- 0.01; below 42.5 degrees C the factor increased to 8.1 +/- 0.3. When the tail blood supply was occluded by a clamp the factor was 1.86 +/- 0.01 at temperatures above 42 degrees C and the tissue was considerably more sensitive to hyperthermia. The factor decreased to 1.3 +/- 0.01 at lower temperatures so that the difference in sensitivity between normal and clamped tissue markedly increased with increasing heating time. The results are interpreted in terms of decreased pH resulting from occlusion of the blood supply which renders the tissue more sensitive. The transition in the isoeffect relationship for normal tails is thought to result from the induction of thermal tolerance and is eliminated when the blood supply is occluded. The result is clearly relevant to the heat treatment of regions of tumours with poor blood supply.

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.

The sensitivity to hyperthermia of human granulocyte/macrophage progenitor cells (CFU-GM) derived from blood or marrow of normal subjects and patients with chronic granulocytic leukaemia

To compare the relative heat sensitivities of human normal and neoplastic cells of the same tissue type, a study was carried out of the relative sensitivities to heat of granulocyte/macrophage progenitor cells (CFU-GM) derived from the peripheral blood and bone marrow of normal subjects and patients with chronic granulocytic leukaemia (CGL). Nucleated haemopoietic cells were incubated at temperatures in the range 41.5 degrees C to 44.0 degrees C for various periods before culture in agar. The results of these experiments showed that CFU-GM from normal blood were consistently less sensitive to damage by heat than normal marrow CFU-GM. There was no comparable difference in the relative heat sensitivities of CFU-GM from blood and marrow of patients with CGL and no significant difference between the heat sensitivities of CFU-GM derived from marrow from normal individuals and patients with CGL. The observed difference in heat sensitivity of CFU-GM from normal blood and marrow accords with other data suggesting that the two progenitor cell compartments are distinct: the blood CFU-GM may represent a more primitive population of committed progenitor cells. In CGL, CFU-GM in the blood may much more closely resemble those in the marrow. The data provide no support for the hypothesis that malignant cells differ intrinsically from their normal counterparts in respect of sensitivity to damage by hyperthermia.