Effect of general anaesthetics on the thermal response of normal and malignant murine tissues

Timing and sequence of hyperthermia in fractionated radiotherapy of a murine fibrosarcoma

PURPOSE

This study investigated the effect of timing and sequence of hyperthermia on fractionated radiotherapy, since it has been shown that the heat increases the size of hypoxic cell fraction which could affect the effect of subsequent radiation doses.

METHODS AND MATERIALS

Animal-tumors were early generation isotransplants of a spontaneous fibrosarcoma, FSa-II, in C3Hf/Sed mice. Tumor response was studied by tumor growth time and TCD50 (50% tumor control dose) assays. The tumor growth time is the time required for one-half of the treated tumors to reach 500 mm3 from the first treatment day. The TCD50 is a radiation dose to control one-half of the treated tumors for 120 days following treatments. One heat treatment at 43.5 degrees C for 45 min was given in a water bath in combination with fractionated doses independently (24 hr interval) or simultaneously (2 min interval). For the normal tissue study, the mouse foot was treated, and the acute foot reaction was scored daily and averaged. The late foot reaction was scored in animals used in the TCD50 assay that developed no recurrence for 120 days. The RD50(2.0) and RD50(5.0), or total radiation doses to induce an average score of 2.0 (complete epilation) and 5.0 (partial foot atrophy) in 50% of treated animals, were calculated.

RESULTS

Thermal radiosensitization was most prominent when heat was combined simultaneously with the first or last radiation dose in both the tumor growth time and TCD50 assays. However, the thermal enhancement was greatest when heat was given either with the first or last radiation dose in the TCD50 assay; whereas it was greatest when heat was administered with the last radiation dose in the tumor growth time assay. Both acute and late skin reactions were significantly potentiated by heat administered 24 hr before the first radiation dose.

CONCLUSION

A significant observation in this study was that, in both the tumor growth time and TCD50 assays, heat given independently or simultaneously did not result in any therapeutic gain compared to the radiation alone treatment.

Winner of the Lund Science Award 1992. Thermosensitization induced by step-down heating. A review on heat-induced sensitization to hyperthermia alone or hyperthermia combined with radiation

A few minute's exposure to a high temperature (sensitizing treatment, ST) may substantially increase the cytotoxic and the radiosensitizing effect of a subsequent heating at a lower temperature (test treatment, TT). This phenomenon, which is known as step-down heating (SDH) or thermosensitization, has been observed both in cultured cells in vitro and in tumours and normal tissues in vivo. The effect of SDH increases with a lowering of TT temperature, but it is rapidly lost at temperatures very close to 37 degrees C. SDH-induced thermosensitization decays within a few hours, when an interval is inserted between ST and TT. In vitro results suggest an exponential decay of the SDH effect with half times ranging from 1.5- to 3.1 h. The effect of SDH increases with increasing ST time or temperature. For single heating, the Arrhenius plot is biphasic with activation energies of 500-800 and 1200-1700 kJ/mol above and below a break point temperature in the region 42.5-43.0 degrees C, respectively. For SDH, the Arrhenius plot gradually becomes monophasic with increasing severity of ST and it approaches asymptotically to an activation energy of about 400 kJ/mol. The reduction of the activation energy depends on cell survival after the priming ST and not on the specific ST heating time or temperature. SDH strongly enhances hyperthermic radiosensitization with a 5-6-fold reduction of the radiation dose required to achieve tumour control. The thermosensitizing and the radiosensitizing effects of SDH have several features in common. Both effects become more prominent when the TT temperature is decreased and when the ST heating time or temperature increases. In addition, the decay kinetics for both effects are comparable. For heat alone, the effect of SDH in tumour and normal tissue seems to be quantitatively similar. However, the therapeutic ratio may be increased by combining SDH with radiation. Biologically, the critical subcellular targets involved in the SDH effect have not been revealed. However, the ability of SDH to inhibit the clearance of heat-induced aggregation of proteins in the nucleus is interesting. Blockage of the nuclear function by proteins is a central theory in the present molecular biological models for both cell kill by heat and heat radiosensitization. Clinically, SDH may be an advantage since even a short exposure to high temperature increases the effect of an otherwise inadequate heat treatment. The disadvantages are that SDH complicates thermal dose calculations, and may cause unacceptable damage to normal tissue.

Significance of additive heat effect in the therapeutic gain factor in combined hyperthermia and radiotherapy: murine tumor response and foot reaction

The thermal enhancement ratio (TER) and therapeutic gain factor (TGF) were evaluated for combined hyperthermia and radiation treatments of a murine fibrosarcoma, FSa-II. The TER is the ratio of the radiation dose that induces a given reaction without hyperthermia to that with hyperthermia. The TGF is defined as the ratio of TER for tumor response to TER for normal tissue response. Tumors in the subcutaneous tissue of the right foot were irradiated with graded radiation doses when they reached an average diameter of 6 mm (110 mm3). Hyperthermia was given by immersing animal feet in a constant temperature water bath 10 min before or after irradiation. The tumor growth time to reach 500 mm3 was obtained for each tumor and the median tumor growth time was calculated for each treatment group. For the normal tissue study, the non-tumor bearing murine foot was treated, as was the tumor, and the foot reaction was scored after treatment, according to our numerical score system for radiation damage, until the 35th post-treatment day and averaged. Using the fraction of animals showing a given average foot reaction score in a treatment group, the RD50, or the radiation dose to induce the given foot reaction or greater, was calculated. A single heating at 45.5 degrees C for 10 min and a step-down heating (first heat at 45.5 degrees C for 10 min immediately followed by the second heat at 41.5 degrees C for 60 min) prolonged the tumor growth time, indicating that hyperthermia per se resulted in some cell killing. The prolongation was greater following step-down heating than following single heating. These heat treatments alone induced no noticeable heat damage on the foot, but decreased the threshold dose observed on the radiation dose response curves for the foot reaction. Accordingly, TER and TGF were evaluated with or without normalizing this thermal effect. TER's for both tumor and foot responses without normalization were greater than the TER's after normalization and decreased with increasing radiation dose (between 1.9 and 7.1 or greater for tumor and between 1.3 and 4.3 or greater for foot reaction), whereas the normalized TER's were relatively constant (between 1.6 and 1.7 for tumor and between 0.7 and 1.5 for foot reaction). TGF's without normalization were greater than those obtained after normalization. The former was large at small doses and decreased with increasing radiation dose (between 1.5 and 4.0 or greater), whereas the latter was within 0.8 and 1.3 and relatively independent of radiation dose.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison between the effect of step-down heating in a tumour and a normal tissue in vivo

A comparison between the effect of step-down heating (SDH) obtained in a C3H mammary carcinoma grown in the feet of CDF1 mice and the skin of normal CDF1 feet is presented. Water-bath heating was used, and SDH was obtained by giving a 44.7 degrees C/10 min treatment followed by heating at 42.2 degrees C for variable times. Single heating at 42.2 degrees C and step-up heating (SUH), i.e. 42.2 degrees C followed by 44.7 degrees C/10 min, were used as controls. The endpoint was the heating time at 42.2 degrees C to obtain either a definite tumour growth time (TGT50) or a specific skin score level (RD50) in 50% of the animals. The effect of SDH and SUH was quantified by the step-down ratio (SDR), calculated as the ratio of the heating times at 42.2 degrees C to obtain the specific endpoint. In both assays the effect of SDH was seen as a significant left shift of the SDH dose-response curve compared to the curve for single heating and SUH. For the comparison of the tumour and the normal tissue response, damage levels with comparable heating times for single heating were used. The therapeutic effect was then investigated by calculating the therapeutic gain factor (TGF), where TGF = SDR(tumour)/SDR(normal tissue). Neither SUH nor SDH gave a TGF significantly different from 1. The results suggest that SDH may be used clinically to shorten the heating time without decreasing the therapeutic effect.

Thermotolerance kinetics and growth rate changes in the R1H tumour heated at 43 degrees C

R1H rhabdomyosarcomas implanted into the foot of the right hind leg of female WAG/Rij rats were exposed to fractionated hyperthermia at 43 degrees C and the kinetics of thermotolerance and heat-induced growth rate changes were studied. Tumours of anaesthetized animals were exposed to heat by immersing the leg up to the thigh in a water bath. Tumour growth delay (TGD) and tumour volume doubling time were calculated from individual growth curves. After single heating, TGD increased with increasing heating time, the increase being linear for heating times exceeding 60 min. Thermotolerance was induced by a priming heat treatment at 43 degrees C for 60 min and the kinetics of development and decay was studied for fractionation intervals ranging from 4 to 144 h. After 4 h the thermal sensitivity of the tumours was enhanced by about 30 per cent, probably due to the sensitizing effect of heat-induced physiological alterations in the tumour tissue such as suboptimal environmental conditions caused by depressed blood flow. For longer time intervals thermotolerance developed and reached a maximum at 24 h where the thermotolerance ratio was 4.5 +/- 1.5. From 24 to 144 h thermotolerance decayed exponentially with a half-time of 28 +/- 8 h. Heat also affected the growth rate of the treated tumours. After single heat treatments at 43 degrees C for 15-60 min the tumours grew faster than untreated control tumours. This change was statistically significant. After prolonged single heating, growth rate was found to be reduced. Tumour volume doubling time was not detectably changed after fractionated heat treatments.

Effect of step-down heating on hyperthermic radiosensitization in an experimental tumor and a normal tissue in vivo

The effect of step-down heating (SDH) on the radiosensitization induced by simultaneous hyperthermia and radiation was investigated in a C3H mammary carcinoma inoculated into the feet of CDF1 mice and the skin of normal CDF1 feet. SDH consisted of a sensitizing treatment (ST) of 44.5 degrees C/10 min followed by a test treatment (TT) of 41.5 degrees C for 30, 60 or 120 min. Simultaneous administration of radiation and hyperthermia was achieved by delivering radiation in the middle of the TT. The endpoint selected was the radiation dose needed to achieve either tumor control or moist desquamation in 50% of the animals. The results were evaluated by the thermal enhancement ratio (TER), defined as dose of radiation needed to achieve endpoint in relation to dose of combined radiation and hyperthermia needed to achieve the endpoint. SDH of tumors increased the TER significantly compared with step-up heating (SUH). The ratios between TCD50 values for corresponding SDH and SUH increased with TT heating time and at 120 min a 2.5-fold increase in the radiosensitizing effect was achieved. It has previously been shown that SDH alone causes thermosensitization in tumors by decreasing the activation energy. However, the effect was too small to explain the increased radiosensitization observed with SDH. In the normal tissue studies SDH combined with radiation treatment gave a lower TER compared to the SDH tumor results, suggesting a possible therapeutic gain.

Modification of the thermal sensitivity of the murine foot and tumour by prior hypoxic treatment

The response of Furth's murine mastocytoma to heat and the effect of prior heating and hypoxic treatment by clamping on the thermal sensitivity of the murine foot and the tumour were investigated. Two different-sized tumours, with average diameters of 3-4 mm and 7-8 mm were used. The tumour response to heat was evaluated by tumour growth (TG) time. Thermotolerance developed in both the foot and tumour tissues by a prior heating for 5 or 10 min at 45 degrees C. Hypoxic treatment resulted in dual effects on tumours. A 10 or 20 min hypoxic pretreatment increased thermal sensitivity of the tumour, while more than a 30 min hypoxic pretreatment induced thermotolerance. In contrast, a 20 min hypoxic treatment induced thermotolerance in the normal tissues. These results indicate that an appropriate period of prior hypoxic treatment may lead to a differential response between normal and tumour tissues.

Site dependent response of tumours to combined heat and radiation

In previous experiments, large differences in thermal sensitisation were observed for tumours grown on the tails of the chest of mice. The present work reports the results of experiments to compare the response of tumours in four different sites to the radiosensitising effects of both heat and misonidazole. Factors influencing tumour response, e.g., tumour growth rate, blood flow, temperature uniformity, temperature increase during heating and drug availability, were also studied. Tumour response and most of the parameters measured varied according to the site of tumour implantation. Growth rate, blood flow and natural tumour temperature are all likely to be important. However, there appears to be no simple relationship by which tumour response could be predicted, although heat dose, the product of temperature elevation above the natural level and treatment time, may be the most relevant parameter. Clearly the choice of implant site does influence response to treatment. Tumours grown on the extremities may be poor models for human tumours, because of their low natural temperatures.