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Experimental studies of brain tumour development during exposure to continuous and pulsed 915 MHz radiofrequency radiation
313
Bi.oelectrochemisty and Bioenmgetics, 30 (19931313-318 Elsevier Sequoia S.A., Lausanne
JEC BB 02005
Experimental studies of brain tumour development during exposure to continuous and pulsed 915 MHz radiofrequency radiation Leif G. Salford
l
Division of Eqmimental Neurooncology, Department of Neurosurgery, Lund University, 221 85 Lund (Sweden)
Ame Brun Departmentof Neuropathology, Lund University, 221 85 Lund (Sweden)
Bertil R.R. Persson and Jacob Eberhardt Department of Medical Radiation Physics, Lund University, 221 85 Lund (Sweden)
It has been suggested that electromagnetic fields (EMFs) act as a promoter late in the carcinogenesis process. To date, however, no convincing laboratory evidence has been obtained indicating that EMFs cause tumour promotion at non-thermal exposure levels. The effects of EMF exposure iu a rat brain glioma model were investigated. The exposure consisted of 915 MI-Ix microwaves, both as continuous waves (1 W), and modulated with 4, 8, 16 and 200 Hx in 0.5 ms pulses and 50 I-Ix in 6 ms pulses (2 W per pulse). Fischer 344 rats of both sexes, weighing 150-250 g, were used in the experiments. 5000 RG2 cells in 5 ~1 nutrient solution were injected by the stereotaxic technique into the head of the right caudate nucleus in 37 experimental rats and 37 matched controls. The exposed animals were kept unanaesthetixed in well ventilated transverse electromagnetic (TEM) cells producing 915 MHx continuous or modulated microwaves. Exposure was started on day five after inoculation. The animals were exposed for 7 h d-l for 5 d per week during two to three weeks. The controls were kept in an identical TEM cell without EMF exposure. All brains were examined histopathologicahy and the tumour size was determined. Our study does not show a significant difference in tumour size between animals exposed to 915 MHz microwaves, and those not exposed. Our preliminary results do not support that even an extensive daily exposure to EMF promotes tumour growth when given from the fifth day after the start of tumour growth in the rat brain until the death of the animal which by then has a large brain tumour. Further studies with higher specific absorption rate levels are in progress.
l
To whom correspondence
Elsevier Sequoia S.A.
should be addressed.
314 INTRODUCTION
In 1979 Wertheimer and Leeper [l] reported that high-current primary and secondary wiring close to the home of children, aged less than 19 years, significantly increased the risk of death in leukemia and central nervous system tumours. In a similar study, Tomenius [2] analysed residential 50 Hz magnetic fields for children with diagnosed leukemia. He not only found a significant correlation between magnetic fields exceeding 0.3 PT and the incidence of leukemia, but also frequently observed central nervous system tumours. Lin et al. [3] performed an occupational history study of nearly 1000 patients who died of brain tumours in Maryland during 1969-1982. Occupations where electromagnetic field (EMF) exposures were likely to occur were more frequent among patients with brain tumours, such as astrocytomas, when compared with controls. Several other recent publications report an increased risk of brain tumours among people in electrical occupations [4-61. In the search for agents initiating and promoting cancer, the EMFs have also received attention. It has been suggested that EMFs act as a promoter late in the carcinogenesis process [7]. Adey points out that “there is evidence that intercellular communication plays an essential role in regulation of cell growth. Functional isolation of a cell from its neighbours by the separate or joint actions of EM fields and chemical cancer promoters, both acting at cell membranes, may lead to unregulated growth with tumour formation” [8]. To date, no convincing laboratory evidence has been obtained indicating that EMFs cause damage to DNA at non-thermal exposure levels. Therefore, EMF exposure might play only a minor or no role in cancer-induction. In spite of the epidemiological findings, we have not found any previous work done in vivo in the laboratory to study brain tumour development during EMF exposure. For several years we have been using a rat brain glioma model for therapeutic studies in our laboratory. In a collaboration between the Division of Experimental Neuro-oncology and the Department of Medical Radiation Physics we have examined the effect of 915 MHz microwaves, both as continuous waves (CW), and modulated with 4,18, 16 Hz and 217 Hz in 0.57 ms pulses and 50 Hz in 6 ms pulses. MATERIALS AND METHODS
Animal model
The rat glioma cell-line RG2, originally from an ethylnitrosurea-induced rat tumour, has grown very well in infinite cell culture cycles for two decades E91,and gives rise to glioma-like tumours when inoculated in the brain of Fischer 344 rats.
315
When at least 1000 RG2 cells are inoculated, well delineated tumours develop in 100% of our animals within three weeks and the untreated tumours have by then reached a diameter of about 3-6 mm. Fischer 344 rats of both sexes, weighing 150-250 g, were used in the experiments. The animals had free access to water and pellets (SAN-bolagen, Malmij, Sweden). 5000 RG2 cells in 5 ~1 nutrient solution were injected by the stereotaxic technique with a Hamilton syringe into the head of the right caudate nucleus in a total of 72 rats. In order to avoid tumour growth extra-cranially, the injection site was cleaned with 70% ethanol after injection and the burr hole was sealed with WaX.
Groups of two to four animals were inoculated at each instance with cells harvested immediately prior to the procedure. At inoculation, every animal to be exposed to EMF was matched to a control animal which was inoculated with identical tumour cells immediately before the animal to be treated. Thus 37 of the 74 animals served as controls. The other 37 animals were exposed to EMF. Electromagnetic fields The exposed animals were kept unanaesthetixed in transverse electromagnetic (TEM) cells producing 915 MHi continuous or modulated microwaves (see Table 1). Exposure was started on the fifth day after inoculation. The animals were exposed for 7 h d-r for 5 d per week. All animals were given a 30-min break after 4 h of exposure for feeding and stretching of legs. It is noteworthy that the animals in this type of experiment, when given a chance, rush back into the TEM cells where they evidently feel at home. The controls were kept in identical TEM cells without EMF exposure. The TEM cells were well ventilated. The rat rectal temperature was recorded before exposure, after 4 h and after 8 h with an optical temperature device (LUXTRON 2000).
TABLE 1 Exposure scheme Modulation frequency /a
Number of animals
Number of treatments
Pulse length /ms
Peak power in pulse /W
Duty cycle
SAR /WK’
4 8.33 16 50 217 CW
4 4 4 11 7 7
10 10 13 9-13 9 10-15
0.57 0.57 0.57 6 0.57
2 2 2 2 2 1
0.002 0.005 0.009 0.3 0.12 1
0.0077 0.016 0.030 1.00 0.4 1.67
316
Histopathological examination When the exposed animal or its matched control started to develop neurological signs of tumour growth, both animals were sacrificed by perfusion-fixation of the brains under chloralhydrate anaesthesia. All brains were examined histopatholigitally by one of the authors CAB). Five coronal slices from each animal were studied microscopically in cresyl violet staining. In this way the entire telencephalon was covered except the frontal and occipital poles. The area of the tumour in the slice where the tumour was most extended was measured by the pathologist (who was uninformed whether the animals had been exposed to r.f. or were controls), from the formula (a x bXa/4) where a and b are the maximum extensions in two dimensions on the slice. Statistical evaluation Student’s test for paired samples was used for the evaluation. RESULTS
No animals showed signs of unrest with the treatment and returned spontaneously into the TEM cells after lunch-break. The rectal temperature of the animals recorded before, at 4 h and directly after exposure did not fall or rise. All treated animals received between 9 and 15 exposures (see Table 1). All animals, exposed ones as well as controls, developed tumours. These were rounded, polycyclic with well defined boundaries. On the histopathological examination the tumours were usually solid with minor necrotic areas without correlation to treatment, tumour size or time from inoculation to death.
TABLE 2 Brain tumour area of EMF exposed rats and controls Modulation frequency /Hz
4
8.33 16 50 217 cw All
Number of pairs
Tumour area controls /mm2
Tumour area exposed /mm2
Number of rat pairs with increased size in exposed
Relative change /%
1
-4 4 4 11 7 7 37
5.8 3.4 6.4 11.7 5.7 12.3 8.6
9.8 4.7 8.9 14.6 5.1 13.8 10.4
3 3 2 6 3 4 21
329 100 467 130 -6 51 144
0.74 0.55 0.52 0.79 - 0.56 0.31 1.20
P
0.24
317
There were no signs of brain damage outside the tumour areas, either necrosis, gliosis or inflammatory changes that could be ascribed to the r.f. exposure. Table 2 shows results for the different modulation frequencies. The relative change in tumour area between each exposed animal (E) and its matched control (C): (E/C-l) is averaged over the different matched pairs for each exposure condition and is shown as “relative change” in Table 2. Our study does not show any significant difference between animals exposed for 9-15 periods of 88h 915 MHz microwaves, and those not exposed. The standard deviation is large, but this is the result of the great individual variation in the model where the status of the inoculated cells as well as that of the recipient animal is highly influential. No obvious differences are seen between the animals exposed to CW and the animals exposed to modulated fields.
DISCUSSION
Our results hitherto do not support that even an extensive daily exposure to EMF causes tumour promotion when given from the fifth day after the start of tumour growth in the rat brain until the death of the animal which by then has a large brain tumour. It should be pointed out that the tumour areas given in Table 2 are very varied both between the pairs of animals and also within the pairs (as is revealed by the standard deviation). This is an inherent problem of the animal model used, which implies that in order to increase the method’s sensitivity, extended groups of animals are needed. Each pair of animals has taken three weeks to complete as we initially had only one TEM cell available. We have now built several new cells and can perform an extended study faster. The control of body temperature during EMF exposures is important for the exclusion of thermal effects. We found no thermal effects at the EMFs used and in separate experiments we have not seen thermal effects until the power given is five times higher than the one used here (unpublished results). It is noteworthy that for some modulation frequencies the average tumour size in the exposed animals largely exceeds the average size in the controls, despite the fact that the number of matched pairs where the exposed animal shows a larger tumour than the controls only insignificantly exceeds the number of pairs showing the reverse. This might indicate that in the few animals that, for some reason, are sensitive to the exposure, tumour growth is stimulated strongly. We think that our findings merit continued studies, possibly in other experimental models. One reason for the absence of significant differences between EMF treated and control animals might be that our rat glioma tumour cell strain (RG2) is very aggressive. The cells are possibly growing at their maximal speed producing a tumour in three weeks, and the addition of external stimuli may therefore not increase the tumour growth rate.
318
The proliferative activity of several of the established experimental animal brain tumours is high, with labelling indices such as Bromodeoxyuridine (BrdU) higher than for human malignant gliomas. In collaboration with the Department of Tumour Immunology, Lund University, we have recently developed a new ethylnitrosurea induced rat glioma cell line which produces gliomas with only half the growing rate of the RG2 cells [lo], and which may be an appropriate model for continued experiments. Another possibility is to use a xenotransplantation model developed earlier by us which is based upon the implantation of a cell suspension of human gliomas into different regions of immunocompetent, Wistar rat brains, which gives tumour takes in 50% of the cases ill]. These transplants maximally grow to diameters of 3 mm during three months. Promoting effects of EMFs upon this slow, restrictive growth may be easier to demonstrate. We are now initiating studies in these new tumour models but the fact that we found some EMF-treated animals with much larger tumours than their controls also intrigues us to continue our search for EMF effects in the RG2 model as well. ACKNOWLEDGEMENTS
The authors wish to thank Kerstin Sturesson who helped with the histopathological prepartions, and Susanne Stromblad and Catharina Blennow who handled the animals with great skill. The financial support of he Wibergs foundation, Gunnar, Arvid and Elisabeth Nilssons foundation, the Crafoord Foundation and the Medical Faculty of the University of Lund is gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
N. Wertheimer and E. Leeper, Am. J. Epidemiol., 109 (1979) 273. L. Tomenius, Bioelectromagnetics, 7 (1986) 191. R.S. Lin, P.C. Dischinger, J. Conde and K.P. Farrell, J. Occup. Med., 27 (1985) 413. S. Milham, Jr., Environ. Health Perspect., 62 (1985) 297. M.A. Speers, J.G. Dobbins and V.S. Miller, Am. J. Ind. Med., 13 (1988) 629. T.L. Thomas, P.D. Stolley, A. Stemhagen, E.T.H. Fontham, M.L. Bleecker, P.A. Stewart and R.N. Hoover, J. Nat. Cancer Inst., 79 (1987) 233. C.V. Byus, S.E. Pieper and W.R. Adey, Carcinogenesis, 8 (1987) 1385. W.R. Adey in B.W. Wilson, R.G. Stevens and L.E. Anderson (Eds.), Extremely Low Frequency Electromagnetic Fields: the Question of Cancer, Batelle Press, Richland, 1990. W. Wechsler, P. Kleihues, S. Matsumoto, K.I. Zulch, S. Ivankovic, R. Preussman and H. Druckrey, Ann. NY Acad. Sci., 159 (1969) 360. P. SiesjB, E. Visser, M. Lindvall, H.O. Sjijgren and L.G:Salford, Proc. Int. Symp. Adv. NeuroOncology, San Remo, Italy, September 26-29, 1990. L.G. Salford, A. Brun and L.-G. StrBmblad, Abstract to the 6th Int. Conf. on Brain Tumor Research and Therapy, Asheville, NC, 1985.
Bi.oelectrochemisty and Bioenmgetics, 30 (19931313-318 Elsevier Sequoia S.A., Lausanne
JEC BB 02005
Experimental studies of brain tumour development during exposure to continuous and pulsed 915 MHz radiofrequency radiation Leif G. Salford
l
Division of Eqmimental Neurooncology, Department of Neurosurgery, Lund University, 221 85 Lund (Sweden)
Ame Brun Departmentof Neuropathology, Lund University, 221 85 Lund (Sweden)
Bertil R.R. Persson and Jacob Eberhardt Department of Medical Radiation Physics, Lund University, 221 85 Lund (Sweden)
It has been suggested that electromagnetic fields (EMFs) act as a promoter late in the carcinogenesis process. To date, however, no convincing laboratory evidence has been obtained indicating that EMFs cause tumour promotion at non-thermal exposure levels. The effects of EMF exposure iu a rat brain glioma model were investigated. The exposure consisted of 915 MI-Ix microwaves, both as continuous waves (1 W), and modulated with 4, 8, 16 and 200 Hx in 0.5 ms pulses and 50 I-Ix in 6 ms pulses (2 W per pulse). Fischer 344 rats of both sexes, weighing 150-250 g, were used in the experiments. 5000 RG2 cells in 5 ~1 nutrient solution were injected by the stereotaxic technique into the head of the right caudate nucleus in 37 experimental rats and 37 matched controls. The exposed animals were kept unanaesthetixed in well ventilated transverse electromagnetic (TEM) cells producing 915 MHx continuous or modulated microwaves. Exposure was started on day five after inoculation. The animals were exposed for 7 h d-l for 5 d per week during two to three weeks. The controls were kept in an identical TEM cell without EMF exposure. All brains were examined histopathologicahy and the tumour size was determined. Our study does not show a significant difference in tumour size between animals exposed to 915 MHz microwaves, and those not exposed. Our preliminary results do not support that even an extensive daily exposure to EMF promotes tumour growth when given from the fifth day after the start of tumour growth in the rat brain until the death of the animal which by then has a large brain tumour. Further studies with higher specific absorption rate levels are in progress.
l
To whom correspondence
Elsevier Sequoia S.A.
should be addressed.
314 INTRODUCTION
In 1979 Wertheimer and Leeper [l] reported that high-current primary and secondary wiring close to the home of children, aged less than 19 years, significantly increased the risk of death in leukemia and central nervous system tumours. In a similar study, Tomenius [2] analysed residential 50 Hz magnetic fields for children with diagnosed leukemia. He not only found a significant correlation between magnetic fields exceeding 0.3 PT and the incidence of leukemia, but also frequently observed central nervous system tumours. Lin et al. [3] performed an occupational history study of nearly 1000 patients who died of brain tumours in Maryland during 1969-1982. Occupations where electromagnetic field (EMF) exposures were likely to occur were more frequent among patients with brain tumours, such as astrocytomas, when compared with controls. Several other recent publications report an increased risk of brain tumours among people in electrical occupations [4-61. In the search for agents initiating and promoting cancer, the EMFs have also received attention. It has been suggested that EMFs act as a promoter late in the carcinogenesis process [7]. Adey points out that “there is evidence that intercellular communication plays an essential role in regulation of cell growth. Functional isolation of a cell from its neighbours by the separate or joint actions of EM fields and chemical cancer promoters, both acting at cell membranes, may lead to unregulated growth with tumour formation” [8]. To date, no convincing laboratory evidence has been obtained indicating that EMFs cause damage to DNA at non-thermal exposure levels. Therefore, EMF exposure might play only a minor or no role in cancer-induction. In spite of the epidemiological findings, we have not found any previous work done in vivo in the laboratory to study brain tumour development during EMF exposure. For several years we have been using a rat brain glioma model for therapeutic studies in our laboratory. In a collaboration between the Division of Experimental Neuro-oncology and the Department of Medical Radiation Physics we have examined the effect of 915 MHz microwaves, both as continuous waves (CW), and modulated with 4,18, 16 Hz and 217 Hz in 0.57 ms pulses and 50 Hz in 6 ms pulses. MATERIALS AND METHODS
Animal model
The rat glioma cell-line RG2, originally from an ethylnitrosurea-induced rat tumour, has grown very well in infinite cell culture cycles for two decades E91,and gives rise to glioma-like tumours when inoculated in the brain of Fischer 344 rats.
315
When at least 1000 RG2 cells are inoculated, well delineated tumours develop in 100% of our animals within three weeks and the untreated tumours have by then reached a diameter of about 3-6 mm. Fischer 344 rats of both sexes, weighing 150-250 g, were used in the experiments. The animals had free access to water and pellets (SAN-bolagen, Malmij, Sweden). 5000 RG2 cells in 5 ~1 nutrient solution were injected by the stereotaxic technique with a Hamilton syringe into the head of the right caudate nucleus in a total of 72 rats. In order to avoid tumour growth extra-cranially, the injection site was cleaned with 70% ethanol after injection and the burr hole was sealed with WaX.
Groups of two to four animals were inoculated at each instance with cells harvested immediately prior to the procedure. At inoculation, every animal to be exposed to EMF was matched to a control animal which was inoculated with identical tumour cells immediately before the animal to be treated. Thus 37 of the 74 animals served as controls. The other 37 animals were exposed to EMF. Electromagnetic fields The exposed animals were kept unanaesthetixed in transverse electromagnetic (TEM) cells producing 915 MHi continuous or modulated microwaves (see Table 1). Exposure was started on the fifth day after inoculation. The animals were exposed for 7 h d-r for 5 d per week. All animals were given a 30-min break after 4 h of exposure for feeding and stretching of legs. It is noteworthy that the animals in this type of experiment, when given a chance, rush back into the TEM cells where they evidently feel at home. The controls were kept in identical TEM cells without EMF exposure. The TEM cells were well ventilated. The rat rectal temperature was recorded before exposure, after 4 h and after 8 h with an optical temperature device (LUXTRON 2000).
TABLE 1 Exposure scheme Modulation frequency /a
Number of animals
Number of treatments
Pulse length /ms
Peak power in pulse /W
Duty cycle
SAR /WK’
4 8.33 16 50 217 CW
4 4 4 11 7 7
10 10 13 9-13 9 10-15
0.57 0.57 0.57 6 0.57
2 2 2 2 2 1
0.002 0.005 0.009 0.3 0.12 1
0.0077 0.016 0.030 1.00 0.4 1.67
316
Histopathological examination When the exposed animal or its matched control started to develop neurological signs of tumour growth, both animals were sacrificed by perfusion-fixation of the brains under chloralhydrate anaesthesia. All brains were examined histopatholigitally by one of the authors CAB). Five coronal slices from each animal were studied microscopically in cresyl violet staining. In this way the entire telencephalon was covered except the frontal and occipital poles. The area of the tumour in the slice where the tumour was most extended was measured by the pathologist (who was uninformed whether the animals had been exposed to r.f. or were controls), from the formula (a x bXa/4) where a and b are the maximum extensions in two dimensions on the slice. Statistical evaluation Student’s test for paired samples was used for the evaluation. RESULTS
No animals showed signs of unrest with the treatment and returned spontaneously into the TEM cells after lunch-break. The rectal temperature of the animals recorded before, at 4 h and directly after exposure did not fall or rise. All treated animals received between 9 and 15 exposures (see Table 1). All animals, exposed ones as well as controls, developed tumours. These were rounded, polycyclic with well defined boundaries. On the histopathological examination the tumours were usually solid with minor necrotic areas without correlation to treatment, tumour size or time from inoculation to death.
TABLE 2 Brain tumour area of EMF exposed rats and controls Modulation frequency /Hz
4
8.33 16 50 217 cw All
Number of pairs
Tumour area controls /mm2
Tumour area exposed /mm2
Number of rat pairs with increased size in exposed
Relative change /%
1
-4 4 4 11 7 7 37
5.8 3.4 6.4 11.7 5.7 12.3 8.6
9.8 4.7 8.9 14.6 5.1 13.8 10.4
3 3 2 6 3 4 21
329 100 467 130 -6 51 144
0.74 0.55 0.52 0.79 - 0.56 0.31 1.20
P
0.24
317
There were no signs of brain damage outside the tumour areas, either necrosis, gliosis or inflammatory changes that could be ascribed to the r.f. exposure. Table 2 shows results for the different modulation frequencies. The relative change in tumour area between each exposed animal (E) and its matched control (C): (E/C-l) is averaged over the different matched pairs for each exposure condition and is shown as “relative change” in Table 2. Our study does not show any significant difference between animals exposed for 9-15 periods of 88h 915 MHz microwaves, and those not exposed. The standard deviation is large, but this is the result of the great individual variation in the model where the status of the inoculated cells as well as that of the recipient animal is highly influential. No obvious differences are seen between the animals exposed to CW and the animals exposed to modulated fields.
DISCUSSION
Our results hitherto do not support that even an extensive daily exposure to EMF causes tumour promotion when given from the fifth day after the start of tumour growth in the rat brain until the death of the animal which by then has a large brain tumour. It should be pointed out that the tumour areas given in Table 2 are very varied both between the pairs of animals and also within the pairs (as is revealed by the standard deviation). This is an inherent problem of the animal model used, which implies that in order to increase the method’s sensitivity, extended groups of animals are needed. Each pair of animals has taken three weeks to complete as we initially had only one TEM cell available. We have now built several new cells and can perform an extended study faster. The control of body temperature during EMF exposures is important for the exclusion of thermal effects. We found no thermal effects at the EMFs used and in separate experiments we have not seen thermal effects until the power given is five times higher than the one used here (unpublished results). It is noteworthy that for some modulation frequencies the average tumour size in the exposed animals largely exceeds the average size in the controls, despite the fact that the number of matched pairs where the exposed animal shows a larger tumour than the controls only insignificantly exceeds the number of pairs showing the reverse. This might indicate that in the few animals that, for some reason, are sensitive to the exposure, tumour growth is stimulated strongly. We think that our findings merit continued studies, possibly in other experimental models. One reason for the absence of significant differences between EMF treated and control animals might be that our rat glioma tumour cell strain (RG2) is very aggressive. The cells are possibly growing at their maximal speed producing a tumour in three weeks, and the addition of external stimuli may therefore not increase the tumour growth rate.
318
The proliferative activity of several of the established experimental animal brain tumours is high, with labelling indices such as Bromodeoxyuridine (BrdU) higher than for human malignant gliomas. In collaboration with the Department of Tumour Immunology, Lund University, we have recently developed a new ethylnitrosurea induced rat glioma cell line which produces gliomas with only half the growing rate of the RG2 cells [lo], and which may be an appropriate model for continued experiments. Another possibility is to use a xenotransplantation model developed earlier by us which is based upon the implantation of a cell suspension of human gliomas into different regions of immunocompetent, Wistar rat brains, which gives tumour takes in 50% of the cases ill]. These transplants maximally grow to diameters of 3 mm during three months. Promoting effects of EMFs upon this slow, restrictive growth may be easier to demonstrate. We are now initiating studies in these new tumour models but the fact that we found some EMF-treated animals with much larger tumours than their controls also intrigues us to continue our search for EMF effects in the RG2 model as well. ACKNOWLEDGEMENTS
The authors wish to thank Kerstin Sturesson who helped with the histopathological prepartions, and Susanne Stromblad and Catharina Blennow who handled the animals with great skill. The financial support of he Wibergs foundation, Gunnar, Arvid and Elisabeth Nilssons foundation, the Crafoord Foundation and the Medical Faculty of the University of Lund is gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
N. Wertheimer and E. Leeper, Am. J. Epidemiol., 109 (1979) 273. L. Tomenius, Bioelectromagnetics, 7 (1986) 191. R.S. Lin, P.C. Dischinger, J. Conde and K.P. Farrell, J. Occup. Med., 27 (1985) 413. S. Milham, Jr., Environ. Health Perspect., 62 (1985) 297. M.A. Speers, J.G. Dobbins and V.S. Miller, Am. J. Ind. Med., 13 (1988) 629. T.L. Thomas, P.D. Stolley, A. Stemhagen, E.T.H. Fontham, M.L. Bleecker, P.A. Stewart and R.N. Hoover, J. Nat. Cancer Inst., 79 (1987) 233. C.V. Byus, S.E. Pieper and W.R. Adey, Carcinogenesis, 8 (1987) 1385. W.R. Adey in B.W. Wilson, R.G. Stevens and L.E. Anderson (Eds.), Extremely Low Frequency Electromagnetic Fields: the Question of Cancer, Batelle Press, Richland, 1990. W. Wechsler, P. Kleihues, S. Matsumoto, K.I. Zulch, S. Ivankovic, R. Preussman and H. Druckrey, Ann. NY Acad. Sci., 159 (1969) 360. P. SiesjB, E. Visser, M. Lindvall, H.O. Sjijgren and L.G:Salford, Proc. Int. Symp. Adv. NeuroOncology, San Remo, Italy, September 26-29, 1990. L.G. Salford, A. Brun and L.-G. StrBmblad, Abstract to the 6th Int. Conf. on Brain Tumor Research and Therapy, Asheville, NC, 1985.