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

New experimental model for single liver lobe hyperthermia in small animals using non-directional microwaves

PURPOSE

Our aim was to develop a new experimental model for in vivo hyperthermia using non-directional microwaves, applicable to small experimental animals. We present an affordable approach for targeted microwave heat delivery to an isolated liver lobe in rat, which allows rapid, precise and stable tissue temperature control.

MATERIALS AND METHODS

A new experimental model is proposed. We used a commercial available magnetron generating 2450 MHz, with 4.4V and 14A in the filament and 4500V anodic voltage. Modifications were required in order to adjust tissue heating such as to prevent overheating and to allow for fine adjustments according to real-time target temperature. The heating is controlled using a virtual instrument application implemented in LabView® and responds to 0.1° C variations in the target. Ten healthy adult male Wistar rats, weighing 250-270 g were used in this study. The middle liver lobe was the target for controlled heating, while the rest of the living animal was protected.

RESULTS

In vivo microwave delivery using our experimental setting is safe for the animals. Target tissue temperature rises from 30°C to 40°C with 3.375°C / second (R2 = 0.9551), while the increment is lower it the next two intervals (40-42°C and 42-44°C) with 0.291°C/ s (R2 = 0.9337) and 0.136°C/ s (R2 = 0.7894) respectively, when testing in sequences. After reaching the desired temperature, controlled microwave delivery insures a very stable temperature during the experiments.

CONCLUSIONS

We have developed an inexpensive and easy to manufacture system for targeted hyperthermia using non-directional microwave radiation. This system allows for fine and stable temperature adjustments within the target tissue and is ideal for experimental models testing below or above threshold hyperthermia.

The response of human and rodent cells to hyperthermia

Inherent cellular radiosensitivity in vitro has been shown to be a good predictor of human tumor response in vivo. In contrast, the importance of the intrinsic thermosensitivity of normal and neoplastic human cells as a factor in the responsiveness of human tumors to adjuvant hyperthermia has never been analyzed systematically. A comparison of thermal sensitivity and thermo-radiosensitization in four rodent and eight human-derived cell lines was made in vitro. Arrhenius plots indicated that the rodent cells were more sensitive to heat killing than the human, and the break-point was 0.5 degrees C higher for the human than rodent cells. The relationship between thermal sensitivity and the interaction of heat with X rays at low doses was documented by thermal enhancement ratios (TER's). Cells received either a 1 hr exposure to 43 degrees C or a 20 minute treatment at 45 degrees C before exposure to 300 kVp X rays. Thermal enhancement ratios ranged from 1.0 to 2.7 for human cells heated at 43 degrees C and from 2.1 to 5.3 for heat exposures at 45 degrees C. Thermal enhancement ratios for rodent cells were generally 2 to 3 times higher than for human cells, because of the fact that the greater thermosensitivity of rodent cells results in a greater enhancement of radiation damage. Intrinsic thermosensitivity of human cells has relevance to the concept of thermal dose; intrinsic thermo-radiosensitization of a range of different tumor cells is useful in documenting the interactive effects of radiation combined with heat.

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.

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.

Thermal radiosensitization and thermotolerance in cultured cells from a murine mammary carcinoma

Cultured murine mammary carcinoma cells M8013 could be made thermotolerant by a priming heat treatment, 30 min at 43 degrees C, applied 5 h prior to subsequent heat treatment. The sensitivity of non-tolerant and thermotolerant cells to either radiation or heat combined with radiation was investigated. Analysis of survival curves with respect to D0 and N showed that thermotolerance had no influence on the radiation sensitivity of the cells. Thermal enhancement of radiation effects (in combined heat/irradiation treatments) was however reduced as a result of thermotolerance. When thermal enhancement ratios were (D0) plotted as a function of the cell killing effects of heat treatment alone, thermotolerance did not seem to have any influence. This latter observation suggests that thermotolerance modifies the effectiveness of the heat treatment for heat-induced cell lethality and radiosensitization equally. Comparison of our in vitro results with several in vivo data on normal tissues suggest that the reduction in 'effective' treatment temperature which has been observed in the in vivo studies as a result of thermotolerance may be explained by equal modification of the effects of heat by thermotolerance both for its direct effects and the radiosensitization.

Acute and late damage in mouse tail following hyperthermia and X-irradiation

Tissue injury following X-irradiation of adult mouse tail was assessed by: (a) the severity of the acute skin reaction, which appeared on day 13 and peaked at approximately 20 days after treatment; (b) the occurrence of necrosis by three months; and (c) the incidence of "late' necrosis which developed between 6.5 and 11 months after treatment, in tails whose gross appearance was normal at 3 months. Hyperthermia (44.0 or 44.5 degrees C for 30 min, by hot-water immersion) given 9 days after radiation, i.e. close to the time of appearance of the acute skin reaction, resulted in an increased thermal response and, in addition, an increase in tissue response for all three waves of radiation injury studied. The times of expression of injury remained unaltered and the degrees of thermal enhancement of both early and "late' necrosis were similar to that measured for the acute skin reaction following radiation. All values were significantly lower than the thermal enhancement ratio (TER), measured as a result of a primary interaction between heat and radiation when the two modalities are given in close association. The observed increase in "late' injury could be accounted for by thermal potentiation of the acute radiation response; there was no evidence for a significant exacerbation by a "late' or progressing thermal injury.