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.     

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.     

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 CDFl mice and the skin of normal CDFl feet is presented. Water-bath heating was used, and SDH was obtained by giving a 44.7°C/10 min treatment followed by heating at 42.2°C for variable times. Single heating at 42.2°C and step-up heating (SUH), i.e. 42.2°C followed by 44.7°C/10 min, were used as controls. The endpoint was the heating time at 42.2°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°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.     

Effect of heating order on radiation response of mouse tumor and skin.

The response of C3H mouse mammary adenocarcinomas and skin to irradiation either immediately before, or during heating at 43°C was evaluated. The therapeutic gain factor (TGF) was 1.3 for irradiation during heating, but only 0.9 for irradiation prior to heating. X-irradiation also was done 2 hr before, during or 2 hr after heating at 42.5°C. Heating times were 15, 30 and 60 min. There was no TGF for irradiation following 60 min. heating. When irradiation preceded 1 hr of heating, the TGF was 1.2. Therapeutic gains also were achieved for 30 min. of heating regardless of sequence, although the largest TGF of 1.3 was obtained for simultaneous treatment. TGFs of 1.3 were obtained for 15 min. heating 2 hr prior to or during irradiation. The greater TGFs for shorter heating times resulted from the small effect of heat on skin response, but significant effect on tumor control.     

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)     

Interaction of misonidazole, hyperthermia, and irradiation in a C3H mammary carcinoma and its surrounding skin in vivo.

The interaction of irradiation, Misonidazole (MISO), and hyperthermia was studied in a C3H mouse mammary carcinoma and its surrounding skin in vivo. MISO (0.5-1.0 mg/g) was injected 30 min before irradiation. Hyperthermia (41.5 degrees-43.5 degrees C for 60 min) was given either simultaneously, 0.5 hr, or 4 hr after X rays. The results were evaluated as the radiation dose to achieve tumor control (TCD50) or moist desquamation of the skin (DD50) in half of the treated animals. A therapeutic gain was found when the enhancement in tumors were greater than that found in skin. The combination of simultaneous heat and irradiation caused great enhancement in radiation response, but with no therapeutic gain. A slightly lower enhancement of the damage in both tissues was found with a 30 min interval between irradiation and hyperthermia, whereas heat 4 hr after X rays gave a small, but significant therapeutic gain. MISO significantly enhanced the response in tumors but not in skin. Combined trimodality treatment with MISO, irradiation, and hyperthermia resulted in enhancement ratios up to 15, dependent on temperature, radiation-heat interval, and to a lesser extent the MISO dose. The enhancement was for all schedules most pronounced in the tumors, resulting in an improved therapeutic effect. The combination of MISO and hyperthermia may be a valuable addition to radiotherapy, especially if heat and irradiation can be applied with close interval and with one of the modalities given selectively to the tumor.     

Simultaneous and sequential hyperthermia and radiation treatment of an experimental tumor and its surrounding normal tissue in vivo

In order to optimize the therapeutic effect of a combined hyperthermia-radiation treatment, the influence of sequence and interval between the two modalities on local tumor control and normal tissue damage was studied during variation of heating time and temperature. A C3H mouse mammary carcinoma transplanted into the feet of C3D2F1 mice was used as a model. Local hyperthermia was given to unanesthetized mice by immersion of the tumor-bearing foot into a water bath. Radiation was given either before, during or after heating. After simultaneous treatment an increasing thermal enhancement ratio (TER) was observed with increasing temperatures and/or increasing heating time with TER values ranging from 1.2 at 41° C to about 5 at 43.5° c after a one hour heating. In all simultaneous treatment schedules, the TER values were almost similar in tumor and normal tissue; no improvement in therapeutic gain was observed. Different results were obtained by using an interval between the two modalities. Hyperthermic treatment (42.5° C/60 min) given with intervals up to 24 hours before radiation showed no definitive improvement in therapeutic ratio. However, if radiation was given before the heating, the normal tissue completely recovered from thermal sensitization within 4 hours, whereas a marked thermal enhancement was persistent in the tumor for more than 24 hours. Thus, an increased therapeutic ratio could be obtained if radiation was given before heat and an interval of 4 hours or more was allowed. This improved therapeutic ratio was dependent on the temperature and ranged from about 1.1 at 41.5° C to 2.1 at 43.5° C given for one hour. These data indicate that if both tumor and normal tissue are heated, the optimal tumor effect may not be a hyperthermic radiosensitization, but rather a direct heat killing of radioresistant tumor cells. This special heat sensitivity of radioresistant tumor cells may be explained by the characteristic environmental conditions (e.g. chronic hypoxia and acidity) influencing such cells.     

Skin responses to step-up and step-down heating in C3H mice

The effects of step-up (42 leads to 44 degrees C sequence) and step-down (44 leads to 42 degrees C sequence) heating were studied on the skin of C3H/He mouse feet. Skin damage in mouse feet exposed to 44 degrees C alone became proportionally more severe with increasing exposure time, while it was slight or not observed at 42 degrees C for up to 2 hr. Preheating to 42 degrees C for 60 or 120 min (step-up) had little effect on the damage from 30-60 min exposure at 44 degrees C, but increased the damage seen with 90 min at 44 degrees C. However, skin damage was markedly enhanced by exposure to step-down heating. On the other hand, thermal resistance was induced in the mouse skin by 44 degrees C fractionated treatments. The induced thermal resistance reached a maximum with a 1 day interval, and then disappeared after a 3 day interval. In another fractionation schedule of step-down heating, the enhanced skin damage which was observed when here was no interval, recovered rapidly and attained an additive response within a 1 day interval. No thermal resistance was observed.     

Effects of step-up and step-down heating on a transplantable murine tumor

The effects of step-up (42----44 degrees sequence) and step-down (44----42 degrees sequence) heating were studied on a transplantable mammary adenocarcinoma of C3H/He mouse. Tumor-bearing legs were immersed in a water bath and the response to hyperthermia was evaluated in terms of the delay in tumor growth. Tumor growth was delayed greatly with increase in the duration of treatment with 44 degrees hyperthermia, whereas with 42 degrees hyperthermia of up to 180 min, tumor growth was delayed only slightly. The effects of step-up heating were similar to those of 44 degrees hyperthermia alone and the response was enhanced by a factor of 0.9-1.1 with the 60-min treatment at 42 degrees followed by treatment at 44 degrees. Thermal resistance developed when the preheating time at 42 degrees was longer than 30 min. On the other hand, the tumor response was markedly enhanced by step-down heating by a factor of 1.8-2.4 with the treatment at 44 degrees followed by 60-min treatment at 42 degrees. Since the enhancement factor for skin damage found previously was similar to that for the tumor, therapeutic gain cannot be expected by the use of these combined heat treatments.     

An investigation into the combined effects of X-irradiation and 3MHz ultrasound-induced hyperthermia on pig skin

The thermal enhancement of radiation-induced damage in pig skin has been investigated. Heating at 43 degrees C for 60 min was produced by a scanned 3MHz ultrasound transducer, immediately after single doses of X rays. The ED50 values for the dermal reactions of dusky/mauve erythema and necrosis after irradiation alone were 18.6 +/- 0.5 Gy and 20.5 +/- 0.4 Gy, respectively. The reduction in the ED50 values to 15.3 +/- 0.4 Gy and 17.7 +/- 0.5 Gy after irradiation plus heating was significant and suggested a thermal enhancement ratio (TER) of between 1.15 and 1.22. These TER values were within the range obtained in both pig and rat skin using other 'dry' heating methods. This would suggest that the non-thermal effects of ultrasound do not influence the thermal enhancement of x-irradiation damage.     

Hyperthermia as an adjuvant to radiotherapy in the treatment of malignant melanoma

One hundred and fifteen cutaneous or lymph node metastases from malignant melanoma were treated with three fractions of irradiation alone in 8 days (62 tumours) or followed by heat either immediately (simultaneous treatment, 26 tumours) or after an interval of 3-4 h (sequential therapy, 27 tumours). In addition, three tumours were treated unsuccessfully with heat alone. The total doses of radiation varied between 15 and 30 Gy, allowing a dose-response analysis. For irradiation alone the isoeffective dose to obtain 50 per cent complete response (TCD50) was 26.3 Gy. Addition of heat reduced the TCD50 significantly (p less than 0.05) with a thermal enhancement ratio (TER) of 1.43 for simultaneous treatment and 1.24 for sequential therapy. Also the persistent local control at 18 months was improved by hyperthermia (56 per cent versus 86 per cent, p less than 0.05). However, simultaneous treatment also enhanced the acute skin response to the same extent as the tumour (TER 1.42 for severe erythema). This schedule thus gave no therapeutic gain. In contrast, no normal tissue enhancement was found after sequential treatment (TER 1.02). Such a treatment schedule resulted in a significantly improved therapeutic ratio of 1.22. This effect was especially prominent in larger tumours (if sufficiently heated) and an analysis corrected for volume showed a TER of 1.51. A special analysis was performed in patients with multiple lesions. 15 pairs of tumours were given the same radiation dose, with or without hyperthermia. Out of these, 11 showed a better response, three showed the same response, and only in one pair was the best response in the tumour obtained by radiation alone.     

Thermotolerance of tumors MA737 and LA795 of mice

The effects of splitted heating of tumor, alone or in combination with local radiation, were studied on mouse mammary cancer MA737 in JB 2 mice and lung cancer LA795 in 739 mice. Hyperthermia was done by immersing the tumor-bearing foot into a water bath at 44 +/- 0.2 degrees C. When combined with radiation, 8 Mev beta irradiation from a linear accelerator was given. The response was assessed by tumor growth retardation calculated according to a special computer program. A priming dose of 5 to 6-minute exposure to 44 degrees C was followed, after various time intervals (1, 4, 5, 24 hr), by a longer exposure. Thermotolerance was induced in these two animal tumor models. Tolerance to heat, however, did not affect the combined effect of hyperthermia and radiation. Electron microscopic examination of the treated tumors confirmed the synergistic killing effect of the combined treatment.     

The effect of hyperthermia on the early and late appearing mouse foot reactions and on the radiation carcinogenesis: effect on the early and late appearing reactions

The effect of hyperthermia on radiation-induced early- and late-appearing foot reactions was studied in C3Hf/Sed mice derived from our defined flora mouse colony. The animal foot was irradiated with 137Cs gamma-rays under hypoxic, air, or hyperbaric oxygen (O2 30 psi) conditions. Hyperthermia of 43.5 degrees C for 45 min was given locally in a water bath where a constant temperature +/- 0.1 degrees C was maintained. Treatment intervals between the 2 treatments were 20 min and 2 days. For the early-appearing reactions scores taken between the 14th and 35th post-irradiation days were averaged. Late-appearing reactions became apparent after approximately the 200th post-treatment day and increased with time. The foot reaction was enhanced by hyperthermia given 20 min before or after irradiation. Dose response curves for radiation given 20 min after hyperthermia for acute-appearing reactions lacked shoulders, whereas those following the same treatment schedule for late-appearing reactions showed significant shoulders. The thermal enhancement ratios (TER) for score 2.0 (complete epilation) early- and late-appearing reactions depended on the treatment interval and sequence. The TER values were greater for a short treatment interval (20 min.) than for a long treatment interval (2 days). Thermal enhancement was greater for hyperthermia given before irradiation compared to the reverse sequence. The TER values were always smaller for the late-appearing reactions than for the acute-appearing reactions. The relationships between early reaction scores and late reaction scores showed that the late reactions following combined heat and radiation are less extensive than those following radiation alone if they were compared at radiation doses which induced an equal level of early reactions. This difference was most significant at low early reaction scores and decreased with increasing score level.     

Thermal enhancement of low dose rate irradiation in a murine tumour system

The effects of localized hyperthermia (HT) in combination with low dose rate irradiation (brachytherapy) have been investigated in vivo using a murine mammary adenocarcinoma. Flank tumours were grown to 0.45-0.70 cm3 in volume, at which time their treatment course was initiated. Tumours were locally heated in a water bath for 15 min at either 44 or 45 degrees C. For tumour irradiations a non-invasive cap was devised to permanently house three iodine-125 sealed sources located at 120 degree intervals around the circumference of the hemispherical cap. During treatment, mice were secured in a modified syringe tube allowing mobility while restricting access to the cap which was placed over the tumour. Calculated dose rates ranged from 15 to 40 cGy/h. Brachytherapy (BT) was delivered for 48 or 72 h to obtain a dose range of 830-2378 cGy. Mice were randomized into one of 10 treatment protocols: BT alone, HT-BT, BT-HT, HT-BT-HT, 1/2BT-HT-1/2BT, four control groups of HT alone and a sham treatment group. Normalized tumour doubling volume growth delays (GDDv) were used to calculate the thermal enhancement ratios (TER). In the 44 degrees C experiments, HT before BT (TER = 1.33 +/- 0.071) was more efficacious than HT after BT (TER = 1.07 +/- 0.042). Two HT treatments, one given before and one after BT (TER = 1.38 +/- 0.152), were not different from a single HT treatment given before BT. However, a single HT treatment given in the middle of an interrupted course of BT resulted in the greatest thermal enhancement (TER = 1.64 +/- 0.072) compared to any other treatment sequence. These data suggest that potentiation of low dose rate irradiation by a single heat treatment may be maximized if the HT is given either in the middle of, or simultaneously with, the BT.     

Photosensitizing efficacy of MTHPC-PDT compared to Photofrin-PDT in the RIF1 mouse tumour and normal skin

The new photosensitizer, meso-tetrahydroxyphenylchlorin (mTPHC) was compared with Photofrin in the murine RIF1 tumour and in normal mouse skin. A range of mTHPC or Photofrin doses were given at intervals of 1 hr to 7 days before illumination. mTHPC-PDT resulted in much higher tumour phototoxicity with longer regrowth delays and more cures. The RIFI tumour could be effectively treated with 30 J cm^?1 (interstitial illumination) at 1 day after mTHPC, whereas 4 to 13 times higher light doses were required with Photofrin for an equivalent anti-tumour effect. High doses of mTHPC also caused more skin phototoxicity (superficial illumination) than Photofrin for the 1-day illumination interval. Evaluating both tumour and normal skin photosensitization, the largest therapeutic gain factor (TGF) for mTHPC-PDT was achieved with a low drug dose (0.15 mg kg^?1) at 1 day before illumination (TGF = 5.6, relative to Photofrin PDT). The duration of cutaneous photosensitivity for mTHPC was shorter than for Photofrin. The light dose required to produce a desquamation response in 50% of the animals increased more than 20-fold over the period 1 to 7 days after high doses of mTHPC, whereas this light dose only increased by a factor of 2 from 1 to 7 days after Photofrin. The large therapeutic gains seen for mTHPC-mediated PDT compared to Photofrin, plus the rapid fading of skin photosensitization, suggest that mTHPC is a potent photosensitizer suitable for clinical testing. © 1995 Wiley-Liss, Inc.     

A moderate elevation of blood glucose level increases the effectiveness of thermoradiotherapy in a rat tumor model II. Improved tumor control at clinically achievable temperatures

PURPOSE: To assess the therapeutic gain (at the TCD(50) level) that can be obtained by boosting thermoradiotherapy with intravenous glucose infusion at different temperatures. This completes our series of studies to determine the optimal conditions and the effectiveness of glucose administration at clinically achievable glucose levels and treatment temperatures. METHODS AND MATERIALS: Subcutaneous rat rhabdomyosarcoma BA1112 was irradiated with graded single doses of 300-kV X-rays (dose range 0-60 Gy). Fifteen minutes after irradiation, a 100-min intravenous infusion was started, consisting of either glucose (20% solution, 2.4-3 g/kg/h) or saline as a control. Then heat was applied to the tumors at 42 degrees C or 43 degrees C (water bath) during a subsequent 100-min period of infusion. Tumor control was scored as the absence of palpable growth at 100 days after treatment. RESULTS: Glucose infusion enhanced tumor control independent of temperature in the range 42-43 degrees C. At 42 degrees C, the TCD(50) for X-irradiation decreased by 5.9 Gy (SEM 1.8 Gy), from 41.6 (1.6) to 35.7 (1.5) Gy, and at 43 degrees C from 33.3 (1.6) to 27.3 (1.5) Gy, representing a glucose enhancement ratio of approximately 1.2. At doses corresponding to the TCD(50) at either 42 or 43 degrees C, the addition of glucose increased tumor control from 50% to 70%. An enhancement ratio of 2.1 was found for the combination of irradiation, glucose infusion, and heating at 43 degrees C, with respect to irradiation alone (TCD(50) 56.3 Gy, reanalyzed earlier data). The contribution of combined heat and glucose to tumor control represented an additive effect, probably on the hypoxic cell population. CONCLUSION: Moderate glucose administration (blood concentration 300 mg/100 mL) sizably improves experimental tumor control after combined X-irradiation and hyperthermia under clinically feasible conditions. Clinical treatment should benefit from this additional modality, in particular if unsatisfactory local control rates are due to insufficient heating. The therapeutic gain has to be evaluated further in clinical studies.     

The significance of thermotolerance after 41 degrees C hyperthermia: in vivo and in vitro tumor and normal tissue investigations

We have investigated the development of thermotolerance in both tumors and normal tissues after 41 degrees C for durations as brief as 15 minutes. The murine RIF tumor, treated by both local radiofrequency and systemic methods, was assayed for thermotolerance by both tumor growth and cell survival analyses. The murine leg and ear, treated by conductive methods, were assayed using pre-defined tissue damage scoring systems. All of these treatments quickly induced substantial levels of thermotolerance. In the tumor studies using local heating, RIF mean diameter doubling time decreased from 17.8 days to a minimum of 13.0 days with a 9 hr interval between 41.0 degrees C for 15 minutes and 44.0 degrees C for 30 minutes (9 hr D1-D2); cell survival increased from 1.2 X 10(-2) to 3.4 X 10(-1) (same interval). Both assays showed some degree of tolerance present immediately after 41.0 degrees C for 15, 30 or 60 minutes (0 hr D1-D2). In the tumor studies using systemic heating, the kinetic pattern of the induced tolerance was similar to that observed after local heating. In the leg studies, 41.0 degrees for 30 minutes increased the time at 45 degrees C necessary to induce a specified level of tissue damage (mean score of 7) by a maximum of 1.8 times (24 hr D1-D2). The kinetic pattern was similar to that for the tumors. In the ear studies, 41.0 degrees C for 30 minutes increased the time at 45 degrees C necessary to induce ear necrosis in 50% of animals by a maximum of 3.5 times (48 hr D1-D2). The peak tolerance level occurred later for the ears, which have a lower normal temperature of 28-30 degrees C, than for the tumors or legs. These results indicate that: thermotolerance can begin to appear in tumors during treatment if hyperthermia sessions involve initial low thermal exposures (near 41 degrees C) for 15 minutes or longer; thermotolerance can develop in tumors after systemic heating and occurs with a kinetic pattern similar to that following local heating; and normal tissues also can develop high levels of thermotolerance after similar thermal exposures.     

Sensitization to hyperthermia induced in a normal tissue by step-down heating

The effect of step-down heating was investigated in the skin of the CDF1 mouse foot. Step-down heating was induced with a 44.7 degrees C/10 min pretreatment followed by a test treatment at a lower temperature for variable time. Step-up heating, that is, a test treatment followed by a 44.7 degrees C/10 min treatment, and single heating were used as controls. The normal tissue reaction was scored at five levels of damage (from slight redness and oedema to loss of a toe or greater reaction), and the heating time to induce each level in 50% of the animals, RD50, was used as the endpoint. The effect of step-down heating was quantified by the step-down ratio, calculated as the ratio of test heating times to obtain the endpoint. A significant reduction of the RD50 was seen at all score levels when the 44.7 degrees C/10 min was given in a step-down heating schedule, and the effect increased with decreasing test treatment temperature. In contrast, the heat sensitivity was only marginally influenced by step-up heating. An analysis of the time-temperature relationship demonstrated a log-linear relationship between temperature and RD50 for single heating in the range 42.2-44.7 degrees C and for step-down heating in the range 41.7-44.7 degrees C. The curve for step-down heating showed a lesser slope indicating a decrease of the activation energy. The kinetics of the SDH effect were investigated by inserting an interval between a primary 44.7 degrees C/10 min treatment and a test treatment performed at 42.2 degrees C. The effect of step-down heating was maximal with no interval between the priming treatment and the test treatment. As the interval was increased to 1.5 hr the step-down sensitization disappeared, and with even longer intervals thermotolerance developed. From a clinical point of view, the present data indicate that step-down heating may increase the extent of both reversible and irreversible heat damage in the normal tissue.     

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.