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

MicroPET-compatible, small animal hyperthermia ultrasound system (SAHUS) for sustainable, collimated and controlled hyperthermia of subcutaneously implanted tumours

An external ultrasound system was developed for the heating of subcutaneously implanted tumours in small animals. This small animal hyperthermia ultrasound system (SAHUS) was designed to be compatible with a microPET (small animal positron emission tomography) scanner to facilitate studies of hyperthermia effects on tumour hypoxia. Collimation and localization of energy deposition, a specific goal for the new device to avoid regional and/or systemic heating of small animals, was demonstrated using thermoradiography following high-power short-time heating of a layered gel phantom. The in vivo heating capabilities of the SAHUS were tested using PC3 cell line tumours (2000-2700 mm(3)) grown in the lateral proximal thighs of Nu-/Nu- nuBR nude mice. Intratumour temperatures were recorded during heating trials with deep and superficial interstitial thermocouples. The experimental data showed that the SAHUS could produce hyperthermia in 8 +/- 2 mm diameter tumours in small animals to a target temperature of 41.5 degrees C and maintain it within a narrow temperature range (+/- 0.3 degrees C) for up to 4 h without raising the core temperature of the animals. PET imaging studies, data to be published separately, were conducted before and during SAHUS-induced hyperthermia. Both devices performed as expected and there was no significant decrease in image quality. In this paper, the new SAHUS is described and data from phantom and in vivo experiments presented.

The potential of using hyperthermia to eliminate radioresistant hypoxic cells

It is known that cells in a nutritionally deprived and acidic environment are sensitive to heat. In general these same cells are chronically hypoxic and therefore heat possesses the potential to eliminate (some) of this radioresistant population. A direct radiosensitization is observed when heat is given simultaneously with radiation. This effect occurs to the same extent in both aerobic and hypoxic cells, thus the oxygen enhancement ratio is unchanged. By giving heat several hours after radiation the direct radiosensitization is avoided and the specific heat killing of the acidic, chronically hypoxic, tumor cells may be utilized to improve the therapeutic gain. The current investigation clearly demonstrates this concept in a C3H mammary carcinoma using a local tumor control assay. This effect could be further enhanced by adding hypoxic radiosensitizers (nimorazole, misonidazole) or a blood flow modifier (nicotinamide) which can eliminate acutely hypoxic cells.

The effect of hyperthermia on the early- and late-appearing mouse foot reactions and on radiation carcinogenesis: Part II. Effect on radiation carcinogenesis (thermal enhancement and oxygen enhancement)

The effect of hyperthermia on radiation carcinogenesis was investigated in the C3Hf/Sed mouse foot. The foot was irradiated under hypoxic conditions, in air, or under hyperbaric oxygen conditions to evaluate the oxygen effect. Hyperthermia at 43.5 degrees C for 45 min was given by immersing the animal foot into a constant temperature water bath. A malignant tumor in the irradiated foot was first observed congruent to 250 days after irradiation. Tumors developed in the irradiated area until day 850. RCD50, or 50% radiation carcinogenesis dose was the endpoint and was calculated based on the tumor incidence 650 days after irradiation. RCD50 following radiation given alone under hypoxic conditions was 66.3 (60.0-73.2) Gy, and the oxygen enhancement ratio (hypoxic/hyperbaric oxygen) was 3.0 (2.5-3.5). Radiation carcinogenesis was enhanced by hyperthermia given with a 20 min treatment interval with no significant alteration in the oxygen effect. Thermal enhancement was greatest when hyperthermia was given 20 min prior to irradiation (2.5 [2.2-2.9] under hypoxia). No thermal enhancement was observed when two treatments were given with a treatment interval of 2 days. The median time to develop a malignant tumor decreased with increasing radiation dose. This median time was shorter following combined hyperthermia and irradiation (423 days) than following radiation alone (504 days). Histological studies revealed that more than 80% of tumors were soft tissue sarcomas, and the most common tumor was fibrosarcoma. Squamous cell carcinoma was found in 7% of all tumors.

A long-term effect of prior irradiation on the thermal enhancement of radiation damage in the mouse ear

The responses of the mouse ear to heat alone, X-rays alone or X-rays combined with heat were measured at 10 months after initial X-ray treatments (19 Gy or 10 X 3.8 Gy), which caused similar acute reactions. Fractionating the initial dose had little effect on the response to retreatment. Prior irradiation increased thermal sensitivity so that the heating time at 43.5 degrees C required to cause necrosis was about 65 per cent that in age-matched controls. Prior irradiation also increased the response to X-rays alone, but had different effects on the susceptibilities to develop acute radiodermatitis and late deformity. For acute radiodermatitis, the second X-ray dose required to cause a given response in previously irradiated ears was 80-90 per cent that in age-matched controls and for late deformity it was 60-65 per cent. Prior irradiation had the same effects on the responses to X-rays given 6 min before mild hyperthermia (43.5 degrees C, 12 min) as on those to X-rays alone but had little effect on the responses to X-rays given 6 min after hyperthermia. Consequently, the thermal enhancement ratios for heat given after X-rays did not depend on prior irradiation whereas those for heat given before X-rays were reduced. This reduction may be due to a reduced ability of irradiated blood vessels to elicit an hyperaemic response to heat.

The response of mouse skin to hyperthermia combined with fast neutrons or X-rays

The effects of hyperthermia combined with fast neutrons (mean energy approximately 7.5 MeV) or X-rays (250 kVp) were studied in the skin of the mouse ear and foot. Hyperthermia was achieved by immersion in water at temperatures of 41.5-43.0 degrees C for 1 hour. The heat treatments used caused no observable tissue injury other than transient erythema but they enhanced the response to both neutrons and X-rays. The enhancement of neutron damage increased as the heating temperature was increased, as is well known for X-rays. When heat was given after irradiation the thermal enhancement ratio (t.e.r.) for neutrons was similar to that for X-rays. When heat was given before irradiation the neutron t.e.r. was less than that for X-rays. Consequently, the relative biological effectiveness of fast neutrons compared with X-rays was not altered by giving heat after irradiation but it was reduced by giving heat before irradiation.

A comparison of thermal enhancement ratios for fast neutron and x irradiation of two normal tissues in rodents

The effects of a hyperthermal treatment of one hour on radiation damage to baby rat cartilage and mouse intestine were compared for 250 kVp X irradiation and cyclotron-produced neutron irradiation (mean energy of about 7.5 MeV). Heat, in the range 41.0 degrees C-43.0 degrees C, caused no observable gross tissue injury when given alone. When heat was given immediately before radiation, the radiation damage was enhanced. There was no qualitative differences between response after fast neutrons and after X rays. Thermal enhancement ratios (TER) were similar for the two tissues and were not affected by the type of radiation used. Thus, the relative biological effectiveness (RBE) of fast neutrons compared with X rays was not markedly altered by combining radiation with hyperthermia.