Effects of hyperthermia on the central nervous system: what was learnt from animal studies?

Animal studies show that nervous tissue is sensitive to heat. Although inter-species variations may play a role, the data indicate that the maximum heat dose without obvious complications after localized hyperthermia in regions of the central nervous system (CNS) lies in the range of 40-60 min at 42-42.5 degrees C or 10-30 min at 43 degrees C. Expression of thermotolerance after a 'conditioning' heat dose was clearly observed in the spinal cord of rodents and the thermotolerance ratio's (ratio between heat doses with and without conditioning required to obtain a certain defined effect) were high, approximately 2. The thermotolerant state of CNS is shown to protect also against other types of injury as well: pre-treatment of rats with hyperthermia protected against spinal cord ischemic injury. During the rather long period required for temperature elevation which is inherent to WBH, some degree of thermotolerance may develop. The correlation between thermotolerance and hsp70 induction in CNS is obvious. Heat, at least if applied shortly after X-rays, enhances the response of nervous tissue to radiation. Data on the combined effects of X-ray irradiation and hyperthermia on rodent spinal cord clearly show that the radiation response can be enhanced with a factor of 1.1-1.3. There are no clear experimental data indicating an increase in adverse effects specific to the CNS after localized or whole body hyperthermia as a result of combined treatment with chemotherapy.

Hyperbaric oxygen: does it promote growth or recurrence of malignancy?

It has been a concern that a therapeutic modality recommended as an adjunct to healing and administered to promote proliferation of fibroblasts, epithelial cells and blood vessels in a wound could also lead to proliferation of malignant cells and angiogenesis in a malignant tumor. The first reported concern that hyperbaric oxygen (HBO2) might have cancer growth enhancing effects appeared in a paper by Johnson and Lauchlan in 1966. In a series of patients treated with HBO2 radiosensitization, they reported a more frequent than expected incidence of metastases and an unusual pattern of metastases. The published literature from clinical reports, animal studies and cell culture studies are reviewed. Putative mechanisms whereby HBO2 could have carcinogenic effects are discussed. The processes of angiogenesis in wound healing and in cancer growth are compared and contrasted. In vitro, in vivo and clinical studies strongly suggest no more than a neutral effect of HBO2 on tumor growth. In fact some studies suggest a negative impact of HBO2 on malignant progression or formation. For angiogenesis, similarities in wound healing and cancer are striking but significant differences are found including the relative importance of angiogenic factors and the process of cessation of angiogenesis. Tumors that grow in hypoxic environments are more prone to metastases and more lethal to the patient. They are also more likely to mutate toward resistant genotypes. Discussion of postulated mechanisms of carcinogenesis including free radical and immunosuppressive effects points out why they are not likely to enhance or cause cancer growth or initiation. In conclusion, the published literature on tumor angiogenesis mechanisms and other possible mechanisms of cancer causation or accelerated growth provides little basis for HBO2 to enhance malignant growth or metastases. A history of malignancy should not be considered a contraindication for HBO2 therapy.

Effects of hyperbaric oxygen and normobaric carbogen on the radiation response of the rat rhabdomyosarcoma R1H.

PURPOSE: Hypoxic tumor cells are an important factor of radioresistance. Hyperbaric oxygen (HBO) and normobaric carbogen (95% oxygen, 5% carbon dioxide) increase the oxygen delivery to tumors. This study was performed to explore changes of tumor oxygenation during a course of fractionated irradiation and to determine the effectiveness of normobaric carbogen and HBO during the final phase of the radiation treatment. METHODS AND MATERIALS: Experiments were performed on the rhabdomyosarcoma R1H growing on WAG/Rij rats. After 20 X-ray fractions of 2 Gy within 4 weeks, oxygen partial pressure (pO2) was measured using the Eppendorf oxygen electrode under ambient conditions, with normobaric carbogen or HBO at a pressure of 240 kPa. Following the 4-week radiation course, a top-up dose of 10-50 Gy was applied in 2-10 fractions of 5 Gy with or without hyperoxygenation. RESULTS: HBO but not carbogen significantly increased the median pO2 in irradiated tumors. The radiation doses to control 50% of tumors were 38.0 Gy, 29.5 Gy, and 25.0 Gy for air, carbogen, and HBO, respectively. Both high oxygen content gas inspirations led to significantly improved tumor responses with oxygen enhancement ratios (OERs) of 1.3 for normobaric carbogen and 1.5 for HBO (air vs. carbogen: p = 0.044; air vs. HBO: p = 0.02; carbogen vs. HBO: p = 0.048). CONCLUSION: Both normobaric carbogen and HBO significantly improved the radiation response of R1H tumors. HBO appeared to be more effective than normobaric carbogen, both with regard to tumor oxygenation and response to irradiation.

Enhancement of radiosensitivity of rat rhabdomyosarcoma R1H with normobaric carbogen and hyperbaric oxygen (HBO) using conventionally fractionated irradiation.

Hypoxic clonogenic cells are an important contributory factor in tumour radioresistance. The objective of the present study was to evaluate whether hyperbaric oxygen enhances tumour radiosensitivity, using a conventionally fractionated irradiation schedule, and whether the radiosensitizing potential is different from carbogen. Experiments were performed using the rhabdomyosarcoma R1H model transplanted subcutaneously in the flank of WAG/Rij rats. A total of 30 X-ray fractions of 2 Gy were given either in air, normobaric carbogen or high pressure oxygen (HPO) (240 kPa, 2.37 atm) without anaesthesia. The time taken to achieve complete remission was 38.7 +/- 3.6 days, 36.7 +/- 2.7 days and 32.4 +/- 4.1 days for air, normobaric carbogen and HBO, respectively. The differences between air and HBO (p = 0.002) and carbogen and HBO (p = 0.015) were significant. Use of carbogen and HBO produced the same local control probability at 150 days and this was significantly higher than local control under ambient conditions (p < 0.0001). It was concluded that the time to achieve complete remission of the rat rhabdomyosarcoma R1H can be shortened by HBO. Furthermore, both HBO and carbogen give higher local control probabilities than treatment under ambient conditions when used with a conventionally fractionated radiation schedule.

Hyperthermia, radiation carcinogenesis and the protective potential of vitamin A and N-acetylcysteine.

The in vivo carcinogenic risk of hyperthermia, alone or in combination with irradiation, and the anti-carcinogenic potential of vitamin A and N-acetylcysteine (AcCys) were investigated. Starting 1 month before treatment, 160 rats were divided into four diet groups: no additives, vitamin A-enriched diet, AcCys and the combination vitamin A + AcCys. In 10 animals per diet group, the hind leg was treated with either X-irradiation alone (16 Gy), hyperthermia alone (60 min at 43 degrees C), hyperthermia 5 h prior to irradiation or hyperthermia 5 h after irradiation. Animals were observed for 2 years after treatment with regard to the development of tumours either inside or outside the treated volume. After 16 Gy alone 12 +/- 5% of the animals developed a tumour. Tumour incidence increased to 37 +/- 9% (borderline significance P = 0.07 versus treatment with X-rays alone) when hyperthermia was applied prior to X-rays, and to 24 +/- 8% (NS) with hyperthermia after irradiation. The relative risk ratio (RRR) for tumour induction was increased to 2.4 by hyperthermia if combined with X-irradiation. Pathological characterization of induced tumours showed that these were of the fibrosarcoma, osteosarcoma and carcinoma type. Vitamin A alone or in combination with AcCys slightly protected against the induction of tumours by X-rays without or with hyperthermia (RRR of 0.4). However, morphological changes such as lipid accumulation in hepatocytes and damage to the parenchyma were noticed in livers from all animals that were given a vitamin-A-enriched diet (P < 0.0001). Data from the present and past reports show that hyperthermia alone is not carcinogenic, but that it may increase radiation carcinogenesis. Treatment temperature and time of exposure to heat in addition to the radiation dose applied are important factors in the carcinogenic process. The enhancement of radiation carcinogenesis seems to occur independently of the sequence and time interval between irradiation and hyperthermia. However, not all data are consistent with this interpretation.

Neurological observations after local irradiation and hyperthermia of rat lumbosacral spinal cord.

PURPOSE: Investigation of the effects of hyperthermia on the radiation response of rat lumbosacral spinal cord with respect to: (a) incidence of paralysis, (b) latency, (c) histopathology, and (d) tumor induction. METHODS AND MATERIALS: Rat lumbosacral spinal cord with the cauda equina was single-dose irradiated with 15 to 32 Gy of x-rays. Hyperthermia for 30 min at a spinal cord temperature of 41.1, 42.3, and 42.6 +/- 0.4 degrees C was applied 5 to 10 min after irradiation by means of a 434 MHz microwave applicator. Animals were observed for 21 months while recording myelopathy and development of tumors. RESULTS: The latent period for hind leg paralysis decreased with increasing radiation dose from 359 +/- 31 days (n = 9) after 20 Gy to 200 +/- 4 days (n = 5) after 32 Gy. Hyperthermia enhanced the radiation response of the lumbosacral spinal cord as evidenced by shortening of the latent period for paralysis and a decrease in the biological effective dose. After 20 Gy followed by 30 min 41.1 degrees C, latency was diminished to 214 +/- 16 days (n = 7, p < 0.001 vs. 20 Gy alone). The ED50 was 21.1 Gy, which was diminished to values between 16 and 17 Gy if radiation was followed by hyperthermia, giving a thermal enhancement ratio between 1.24 and 1.32. Histopathological examination of the spinal cord after combined treatment of x-rays and hyperthermia showed necrosis of nerve roots. Irradiation with 16, 20, 24, and 28 Gy (n = 77) alone led to tumor induction in 17 +/- 8% of the animals (pooled data). If followed by hyperthermia (n = 96), it was increased to 33 +/- 12% (p < 0.01). Most tumors induced by radiation and hyperthermia were sarcomas. CONCLUSION: First, the radiation response of rat lumbosacral spinal cord was enhanced by heat. Second, latency for paralysis was shortened in the lower dose range. Third, no difference in pathology between x-rays alone or in combination with hyperthermia. Fourth, hyperthermia did increase radiation carcinogenesis.

Effect of hyperthermia on the central nervous system: a review.

Experimental data show that nervous tissue is sensitive to heat. Animal data indicate that the maximum tolerated heat dose after local hyperthermia of the central nervous system (CNS) lies in the range of 40-60 min at 42-42 x 5 degrees C or 10-30 min at 43 degrees C. No conclusions concerning the heat sensitivity of nervous tissue can be derived from clinical studies using localized hyperthermia. The choice whether or not to exceed the critical heat dose, as derived from laboratory studies, in clinical practice is very much dependent on the clinical situation such as the anatomical site and volume of the tissue involved, and prior therapy. Data on clinical application of whole body hyperthermia (WBH) show that nervous tissue can withstand a slightly higher heat dose than after localized heating, which might be the result of developing thermal resistance during treatment. Expression of thermotolerance was observed in the spinal cord of laboratory animals. After WBH in man at a maximum between 40 and 43 degrees C for 6 h-30 min CNS complications were reported, but other complications seemed to be more life-threatening. Most studies indicate that impairment of the CNS after WBH was not due to direct heat injury to the brain or spinal cord, but was secondary as a result of physiological changes. Heat, at least if applied shortly after X-rays, enhances the response of nervous tissue to radiation. Neurotoxicity of chemotherapeutic drugs does not seem to be a limiting complication in hyperthermia if combined with chemotherapy, but only few data are available. The limited clinical experience shows that safe hyperthermic treatment of CNS malignancies or tumours located close to the CNS seems feasible under appropriate technical conditions with adequate thermometry and taking the sensitivity of the surrounding normal nervous tissue into account.

The influence of hyperthermia on the uptake of cisplatin in the rat cervical spinal cord.

The influence of local hyperthermia on the uptake of cisplatin in the rat cervical spinal cord was investigated. After single intraperitoneal or intravenous injection of cisplatin (5 mg/kg body weight), the spinal cord region cervical 5-thoracic 2 was heated for 60 min at mean (S.D.) 41.2 (0.4) degrees C or 40 min 42.4 (0.3) degrees C using a 434 MHz microwave heating device. One day after treatment with either hyperthermia alone, cisplatin alone or the combination, none of the animals expressed neurological symptoms. The spinal cord was dissected and platinum levels were measured by flameless atomic absorption spectroscopy. No difference was found in uptake of platinum in the spinal cord between control- and heat treated animals. In a second series of experiments, the spinal cord was heated for 30-60 min. during a 2 h infusion of cisplatin. One day after treatment at 42.3 degrees C for 60 min, neither motor nor sensory functions were affected and platinum levels did not differ significantly between control and treated animals. Also, platinum levels measured in the spinal cord immediately after cisplatin infusion were not influenced by heat treatment at 42.1 or 43.0 degrees C for 30 min. However, after a heat dose of 60 min 43 degrees C, cisplatin uptake was significantly increased (P less than 0.001) by a factor of 2.8 (1.3). The data demonstrate that mild hyperthermia has no effect on the uptake of cisplatin in the spinal cord, while an injurious heat dose leads to a significant increase in cisplatin uptake. The present findings indicate that, in case of treatment of tumours of the central nervous system with hyperthermia and cisplatin, a treatment which might be toxic for the tumour is well tolerated by the normal nervous tissue.

Hyperthermia promotes the incidence of tumours following X-irradiation of the rat cervical cord region.

The cervical region of the rat, including the spinal cord (cervical 5-thoracic 2) was irradiated with single doses of 15-32 Gy 250 kV X-rays. Hyperthermia, at temperatures of 42-, 43- and 44 +/- 0.1 degrees C for 30 min was applied to the cervical vertebral column and immediate adjacent tissues for 5-10 min or 7 h after X-irradiation. Over a period of 18-21 months, animals were followed up to monitor neurological complications occurring as a result of damage to the spinal cord (Sminia et al. 1991). We also noted the development of neoplasms either inside or outside the cervical region. The data on tumour incidence were analysed retrospectively using the actuarial method. Although hyperthermia alone was not carcinogenic, it led to a significant increase of radiation-induced tumours. This increase of radiation carcinogenesis was observed both with hyperthermia applied 5-10 min after X-rays and with an interval of 7 h between X-rays and heat. Cancer induction was highest after the lower radiation doses (16 Gy) combined with high heat doses (30 min 44 degrees C). The latent period for induction of tumours by X-rays was 472 +/- 19 days (mean +/- SEM; n = 24). Latency was significantly shortened by hyperthermia to 404 +/- 34 days (n = 22) if applied 5-10 min after X-rays and to 348 +/- 6 days (n = 33) with an interval of 7 h. Histology revealed that 86% (38/44) of the examined tumours found inside the volume treated with hyperthermia and irradiation were sarcomas. The percentage of animals with a tumour outside the treated volume was almost the same for all treatment groups. Most of these tumours were of the mammary gland type.

Effects of hyperthermia applied to previously irradiated cervical spinal cord in the rat.

Rat cervical spinal cord was X-ray irradiated at doses of 15, 18, 20 and 26 Gy. Ninety days later, approximately the same part of the spinal cord was heated at 42.3 +/- 0.4 degrees C for 50, 60, 75 or 90 min by means of a 434 MHz microwave applicator. After treatment, animals were observed over a period of 18 months for expression of neurological complications. These complications could either be the result of the heat or of the radiation treatment. The time course showed three distinct peaks in the incidence of neurological symptoms. The first peak was due to the acute response to hyperthermia. The ED50 value for neurological complications one day after treatment at 42.3 +/- 0.4 degrees C was 74 +/- 2 min. Previous X-ray irradiation of the spinal cord with 18, 20 and 26 Gy reduced the ED50 to 57 +/- 7, 65 +/- 4 and 55 +/- 5 min (12-26% of control), respectively. Recovery from heat-induced neurological complications was diminished in previously irradiated animals. The second peak (150-300 days after X-rays) concerned the expression of "early delayed" radiation damage. Hyperthermia given 90 days after irradiation did not influence either the percentage of animals with paralysis or the latent period. Neurological symptoms developing after day 300 were due to the "late delayed" radiation response. No significant difference was observed in the data on paralysis induced by radiation alone or radiation followed by heat. The late radiation-induced minor neurological symptoms were, however, influenced by retreatment with heat.

Optimisation of intraperitoneal cisplatin therapy with regional hyperthermia in rats.

The purpose of this study was to optimise intraperitoneal chemotherapy by combining this modality with regional hyperthermia. In vitro data demonstrated that both the uptake of cisplatin into CC531 tumour cells and cytotoxicity were increased at temperatures of 40 degrees C (factor 4) and 43 degrees C (factor 6) compared to 37 degrees C. The increase of intracellular platinum concentration correlated well with the decrease in survival of these cells. In vivo, rats were treated intraperitoneally with cisplatin (5 mg/kg) in combination with regional hyperthermia of the abdomen (41.5 degrees C, 1 h). The mean (S.D.) temperature in the peritoneal cavity was 41.5 (0.3) degrees C and outside the peritoneal cavity 40.5 (0.3) degrees C. Enhanced platinum concentrations were found in peritoneal tumours (factor 4.1) and kidney, liver, spleen and lung (all around a factor 2.0), after combined cisplatin-hyperthermia treatment. The platinum distribution in peritoneal tumours was more homogeneous after the combined treatment than after cisplatin alone, possibly due to increased penetration of cisplatin into peritoneal tumours. Pharmacokinetic data demonstrated an increased tumour exposure for unfiltered platinum in the peritoneal cavity (area under the curve [AUC] increased from 339 mumol/l/min to 486 mumol/l/min at 37 degrees C and 41.5 degrees C, respectively), and for total and ultrafiltered platinum in the blood. The AUC for total platinum increased from 97.9 to 325.8 mumol/min and for ultrafiltered platinum from 22.2 to 107 mumol/l/min at 37 degrees C and 41.5 degrees C respectively. The latter might be due to a slower elimination of platinum from the blood. The combined treatment, intraperitoneal cisplatin and regional hyperthermia, also increased toxicity. The thermal enhancement ratio (TER) using lethality as endpoint was 1.8.

Effects of 434 MHz microwave hyperthermia applied to the rat in the region of the cervical spinal cord.

Hyperthermia was applied in the region of the vertebral column between the cervical vertebrae 5 and thoracic 2, using a ring-shaped applicator operating at a microwave frequency of 434 MHz. This region was focally heated, including spinal cord, vertebrae, intervertebral discs and nerve roots. In all experiments temperature was measured at a 'reference' thermocouple probe which was placed against one of the cervical vertebrae 6, 7 or thoracic 1. Temperatures inside the vertebral canal were measured separately and proved to be below the 'reference' temperature: at 42 degrees, 43 degrees, 44 degrees and 45 degrees C (+/- 0.1 degree C) respectively the temperature in the canal was 41.2 degrees, 42.3 degrees, 42.9 degrees and 43.2 degrees C (+/- 0.4 degree C). Temperatures in tissues close to the vertebrae (e.g. within 2 mm lateral to the vertebrae, in the region of the brachial plexus) did not differ significantly from the temperature inside the canal. The temperature inside the intervertebral disc was as high as the 'reference' temperature. Temperatures measured at other sites, e.g. in the oesophagus, rectally and in the cervical muscles 5 or 10 mm lateral from the vertebral column showed that these sites were only slightly heated. The effects of hyperthermia at temperatures inside the spinal canal ranging from 41.2-43.2 degrees C for 30-120 min were investigated. One day after treatment at 41.2 degrees C for 120 min or 42.3 degrees C for 60 min neither neurological symptoms nor deaths were observed. Minor neurological symptoms were observed one day after 75 min at 42.3 degrees C. The incidence and severity of the neurological symptoms (ranging from unco-ordinated use of the forelegs to paralysis) increased with increasing temperature and duration of the hyperthermic treatment. Thermal damage even resulted in lethality: 74 per cent of the rats that died did so between 2 and 42 h after treatment. The LD50 value at 60 days at 43.2 degrees C was 30 min, at 42.9 degrees C, 41 min, and at 42.3 degrees C, 92 min. In most rats with neurological symptoms after treatment, recovery from motor dysfunctions took place within about two weeks. Even severe neurological symptoms which did not lead to lethality recovered completely. At day 60 no neurological symptoms were observed.