Br. J. Cancer (1978) 37, Suppl. III, 194
IN VIVO ASSAY OF THE RADIATION SENSITIVITY OF HYPOXIC TUMOUR CELLS; INFLUENCE OF y-RAYS, CYCLOTRON NEUTRONS, MISONIDAZOLE, HYPERTHERMIA AND MIXED MODALITIES W. PORSCHEN, J. GARTZEN, K. GEWEHR, H. MVHLENSIEPEN, H.-J. WEBER AND L. E. FEINENDEGEN From the Institut fur Medizin, Kernforschungsanlage Jiiltich GmbH., D-5170 Jil4ich, Postfach 1913
Summary.-Tumour cell death can be evaluated in the living mouse by externally measuring the rate of loss of tumour-bound DNA tracer. By sequentially labelling the tumour-bearing animals with 125IUdR and 131IUdR 50 h apart, the average tumour cells at the time of the second injection are labelled by 125IUdR and the euoxic tumour cells are specifically labelled with 131IUdR. Tumour treatment at this stage of labelling permits the observation of the reaction of euoxic cells and average tumour cells and finally yields data on hypoxic cells and thus on the oxygen enhancement ratio. This information adds to results from tumour control and growth delay. With this technique effects were analysed of 60-Co y-rays, cyclotron neutrons (E = 6 MeV), misonidazole (500 mg/kg body wt) and hyperthermia (42°C water-bath), or combinations of these. Misonidazole (15 min before irradiation) altered the oxygen enhancement ratio by a factor of 1.5 for y-rays and of 1-1 for neutrons; when evaluated from tumourgrowth delay and TCD-50 misonidazole gave a dose modifying factor of 1-47 for y-rays and of 1-2-1-3 for neutrons. Based on percentage tumour regression 100 days after treatment, the enhancement ratio from hyperthermia (after irradiation) was 2-75 for y-rays (at 10 Gray) and 2-2 for neutrons (at 3 2 Gray). For neutrons combined with misonidazole and hyperthermia the ratio was 2-4. These results demonstrate that effects of neutron irradiation may be modified by electron-affinic substances and/or hyperthermia.
HYPoxic cells tend to render a tumour relatively resistant to low-LET radiation and thus prevent the full effect of conventional radiotherapy. New modalities have been proposed in order to overcome this problem: (a) Fast neutrons with a lower OER; (b) Electron-affinic substances, like misonidazole; (c) Hyperthermia. It is the aim of this presentation to evaluate the merits of the different strategies in a solid experimental tumour in the living mouse without disturbing the host-tumour relationship. MATERIAL AND METHODS
Sarcoma-180 cells were transplanted into one hind leg of NIMR mice. When the tumour
volume had reached about 300 mm3, the mice were injected i.v. with radioactively labelled 5-iodo-2'-deoxyuridine, in short IUdR. This served the purpose of permanently marking the tumour-cell DNA. IUdR is an analogue of thymidine and specifically incorporated into all DNA-synthesizing cells. It remains stably bound to DNA and upon cell death is inefficiently re-utilized. Loss of tracer from a closed cell population signals cell lysis. The precursor can be labelled with 1251 or 131I which are both easily counted because of their gamma emission. Immediately after a single i.v. injection of 125IUdR, the tracer is autoradiographically observed only within cells around capillary vessels in the tumour. About 70 h after injection, labelled cells are found distributed throughout the tumour and are not localized
IN VIVO ASSAY OF HYPOXIC TUMOUR CELLS
exclusively near the capillaries (Bosiljanoff et al., 1977). Tumour-bound tracers may also be externally assayed by a crystal scintillation counter which shields the body except for the tumour (Porschen and Feinendegen, 1969). In order to label in one tumour differentially both the average tumour cells on one hand, and the cells around the capillaries on the other, a double tracer technique was employed (Feinendegen, 1970). When the tumour in its exponential growth phase had a volume of about 300 mm3, 125IUdR was given twice i.v., 7 h apart. Three days later, the same animals received-this time-131-IUdR again in 2 injections, 7 h apart. At the time of the last injection, '31IUdR is localized in the euoxic cells around blood vessels, and 125 IUdR, at that time, is distributed randomly throughout the tumour and largely traces cells in hypoxic regions. If shortly after the last injection the tumour is treated by irradiation, the daily measurements of 1311 and of 125I signal the fate of such cells, which were at the time of treatment either in euoxic regions or were distributed throughout the
tumour and largely in hypoxic areas (Porschen et al., 1976; Bosiljanoff et al., 1977). The tumours were locally irradiated with either y-rays or neutrons. The y-dose rate was 1.00 Gy/min from a clinical 60-cobalt source. Fast neutrons with a mean energy of 6 MeV were produced by the KFA Compact Cyclotron by bombarding a Be-target with 90 ,uA of 14 MeV deuterons. The neutron field was defined with a benelex collimator. The dose rate to the tumour was 0-38 Gy/min, and ycontamination was about 5% in air. The whole body dose was less than 2% of the tumour dose. Amongst 5 hypoxic sensitizers with yirradiation, misonidazole proved to be most effective when injected i.p. 15 min before irradiation, in an amount of 500 mg/kg (Gewehr et al., 1978). Hyperthermia was applied to the tumours by immersing the tumour-bearing leg in a controlled water bath at 42° ± 0-2°C for an optimal period of 60 min after irradiation. The internal tumour temperature was controlled with a small thermistor (Weber et at.,
1977).
y- IRRADIATION
CYCLOTRON NEUTRONS (E=6MeV)
z
0 z
w
I-
w
o -_ In
80
120 160 HOURS 200 TIME AFTER TREATMENT
195
80
120 160 HOURS 200 TIME AFTER TREATMENT
FIG. 1.-Loss rate of cells which were labelled either with 125IUdR or with 131IUdR. The 131I labelling represents mainly cells near to the capillaries where oxygen tension is assumed to be normal. Labelling with 125IUdR represents the average tumour cells, a large proportion of which is hvnoxie-
196
W. PORSCHEN ET AL.
The combined effect of misonidazole, hyperthermia and irradiation was examined for y-rays and neutrons.
Gy enhances the loss rate of euoxic cells significantly more than the loss rate of the average or hypoxic cells. When misonidazole was used, the loss rate of RESULTS hypoxic cells increased, yet the data The data in Fig. 1 refer to the rate of hardly changed for euoxic cells. On the loss of 125I and of 131J: this means upper right, the results of 3-2 Gy of neuloss of hypoxic cells and euoxic cells after trons without and, on the lower right, with y- and neutron-irradiation. As seen in the misonidazole are shown. As expected, the upper left, gamma irradiation with 19-2 neutron-induced loss rate of hypoxic cells is close to that of euoxic cells. With misonidazole there is little change, as seen SARCOMA 180 on the lower right. COMBINED TREATMENT CYCLOTRON NEUTRONS In the next group of experiments the MISONIDAZOLE 0 5 mg/g influence of pure neutron irradiation and HYPERTHEREMIA 42'C,60min combined schemes with misonidazole and/ Z 7 a or hyperthermia was analysed. 5 CONTROL oX ,,xo The following biological endpoints for a 32Gy a 3 D given dose were evaluated: tumour volume 0 2 /°x / ~~Ro+3.2G after treatment, growth delay, TCD 50/100 32Gy+HYP Lu 1 and the loss rate of hypoxic cells (Fig. 2). Ro+3.2 Gy+ HYP In Fig. 2a the relative tumour volume 0.5 El 0 5 10 15HOURSI2Ois plotted with time after treatment with cr 5 15 HOURS 20 o 10 TIME AFTER TREATMENT 3-2 Gy neutrons. Combined hyperthermia 30I is slightly more effective than misonidab zole, yet the combination of neutrons with d misonidazole and hyperthermia is the most efficient strategy. In Fig. 2b growth delay is plotted as function of dose between 1-6 and 5-6 Gy. The results show that the effects increase from neutrons alone to misonidazole plus neutrons, to neutrons plus hyperthermia, and finally to the triple scheme. Also the percentage tumour regression 100 days after treatment as a function of dose (Fig. 2c) confirms the data on growth 0 2 4 6 8 Gy DOSE delay. Thus, the TCD 50 for the different 9aO I~~~~~~~~~~~~~~~~~~ schemes are for: 100 c ,-z Neutrons alone: 7*8 Gy; Ro+IRRAD.+ HY P , 4IA Misonidazole plus neutrons: 7*2 Gy; Neutrons plus hyperthermia: 6-5 Gy; Neutrons plus misonidazole and hypernurr-LL1 50 thermia about 3 Gy. Hr RAD.IRRAD Thus in the latter instance the dose modification factor is 2-4 ± 0-4. nu 2 4 6 Gy R In Fig. 3, the retention of 1251 as DOSE indicator of hypoxic cells is plotted against FiG. 2. Relative tumour volume, growth the time after treatment with the various delay and tumour regression for pure and combined neutron schemnes. modalities at a neutron dose of 1P6 Gy. 20
Lu
I
0
I
I
0
11
Lu
0
Z LU
Lf)
Lu
LL
cr-
Lu
a.
I
IN VIVO ASSAY OF HYPOXIC TUMOUR CELLS SARCOMA 180 NEUTRON IRRADIATION (Eo=6 MeV)
100
80 60z
0
z w 40-
'~--CONTROL R+1.6 Gy
w
6Gy+ Hyp.
20
Ro+ 1.6 Gy+ Hyp.
10c
0
50
100 150 200 HOURS TIME AFTER TREATMENT
N III/1.6 FIG. 3.-125IUdR retention for pure and combined neutron schemes.
Again, hyperthermia is more effective than misonidazole, and the triple scheme is slightly more effective than neutrons plus hyperthermia.
TABLE.-Dose-modifying Factors for Sarcoma- 180 Misonidazole* 500 mg/kg 1Eyperthermiat 60 min 42°C Misonidazole andt
hyperthermia
Co-60 y-Rays
Cycl. Neutrons (E = 6 MeV)
1*47 (1*40-1*54)
1 27 (1*12-1*42)
2*75
2*2
(1*9-2*73) 2*4 (1 *9-2*94)
Based on: * TCD 50/100, growth delay, cell loss rate.
t Percentage tumour regression (100 d).
In the Table, the DMFs from various methods for 60-Co gamma rays are compared with those for neutrons. The gamma
197
values were reported previously. Thus, the DMF values for combinations with neutrons were always smaller than those for gamma irradiation. For neutrons, misonidazole gave a DMF value of 1P27, hyperthermia a value of 2.2 and finally misonidazole plus hyperthermia a value of 2-4. Similar results were found for the system Adenocarcinoma EO-771 in C57 Bi mice, so that the observed neutron enhancement ratios appear not to be specific for the system Sarcoma-180 and NIMRmice. In conclusion, the effect of cyclotron neutrons on hypoxic cells in Sarcoma-180 in the living mouse is augmented by Misonidazole and hyperthermia. The triple scheme of neutrons, misonidazole and hyperthermia was most effective, yielding a DMF of 2*4. REFERENCES BOSILJANOFF, P., PORSCHEN, W., PIEPENBRING, W., MtJIILENSIEPEN, H. & FEINENDEGEN,,L. E. (1977) In-vivo-LYntersuchungen uber die relative Strahlenempfindlichkeit hypoxischer Tumorzellen. Strahlentherapie, 153, 178. FEINENDEGEN, L. E. (1970) Autoradiographische und biochemische Untersuchungen der Zellproliferation in vivo. In: Praoperative Tumorbe8trahlung, Vortrdge vom Deutschen Rdntgenkongress, Ed.: 0. Hug, Munchen. GEWEHR, K.. PORSCHEN, W., MUHLENSIEPEN, H. & FEINENDEGEN, L. E. (1978) Measurements of in vivo Effects of Electron Affinic Radiosensitizers using a Double Tracer Technique. (In press.) PORSCHEN, W. & FEINENDEGEN, L. E. (1969) Invivo-Bestimmung der Zellverlustrate bei Experimentaltumoren mit markiertem Joddesoxyuridin. Strahlentherapie, 137, 718. PORSCHEN, W., BOSILJANOFF, P., GEWEHR, K., MiYHLENSIEPEN, H., WEBER, H. -J., DIETZEL, F. & FEINENDEGEN, L. E. (1976) In vivo Assay of the Radiation-Sensitivity of Hypoxic Tumour Cells; Influence of Radiation-Quality and Hypoxic Sensitization. Int. Symp. Radiobiol. Re8. for the Improvement of Radiother., November, Vienna, IAEA-SM-212/16. WEBER, H.-J., DIETZEL, F., PORSCHEN, W. & FEINENDEGEN, L. E. (1977) In vivo Assay of the Radiation-Sensitivity of Hypoxic Tumour Cells; Influence of Combined Hyperthermia and Irradiation. 2nd Intern. Symp. Hyperthermia and Radiat. in Cancer Therapy, Essen, June, 1977.
IN VIVO ASSAY OF THE RADIATION SENSITIVITY OF HYPOXIC TUMOUR CELLS; INFLUENCE OF y-RAYS, CYCLOTRON NEUTRONS, MISONIDAZOLE, HYPERTHERMIA AND MIXED MODALITIES W. PORSCHEN, J. GARTZEN, K. GEWEHR, H. MVHLENSIEPEN, H.-J. WEBER AND L. E. FEINENDEGEN From the Institut fur Medizin, Kernforschungsanlage Jiiltich GmbH., D-5170 Jil4ich, Postfach 1913
Summary.-Tumour cell death can be evaluated in the living mouse by externally measuring the rate of loss of tumour-bound DNA tracer. By sequentially labelling the tumour-bearing animals with 125IUdR and 131IUdR 50 h apart, the average tumour cells at the time of the second injection are labelled by 125IUdR and the euoxic tumour cells are specifically labelled with 131IUdR. Tumour treatment at this stage of labelling permits the observation of the reaction of euoxic cells and average tumour cells and finally yields data on hypoxic cells and thus on the oxygen enhancement ratio. This information adds to results from tumour control and growth delay. With this technique effects were analysed of 60-Co y-rays, cyclotron neutrons (E = 6 MeV), misonidazole (500 mg/kg body wt) and hyperthermia (42°C water-bath), or combinations of these. Misonidazole (15 min before irradiation) altered the oxygen enhancement ratio by a factor of 1.5 for y-rays and of 1-1 for neutrons; when evaluated from tumourgrowth delay and TCD-50 misonidazole gave a dose modifying factor of 1-47 for y-rays and of 1-2-1-3 for neutrons. Based on percentage tumour regression 100 days after treatment, the enhancement ratio from hyperthermia (after irradiation) was 2-75 for y-rays (at 10 Gray) and 2-2 for neutrons (at 3 2 Gray). For neutrons combined with misonidazole and hyperthermia the ratio was 2-4. These results demonstrate that effects of neutron irradiation may be modified by electron-affinic substances and/or hyperthermia.
HYPoxic cells tend to render a tumour relatively resistant to low-LET radiation and thus prevent the full effect of conventional radiotherapy. New modalities have been proposed in order to overcome this problem: (a) Fast neutrons with a lower OER; (b) Electron-affinic substances, like misonidazole; (c) Hyperthermia. It is the aim of this presentation to evaluate the merits of the different strategies in a solid experimental tumour in the living mouse without disturbing the host-tumour relationship. MATERIAL AND METHODS
Sarcoma-180 cells were transplanted into one hind leg of NIMR mice. When the tumour
volume had reached about 300 mm3, the mice were injected i.v. with radioactively labelled 5-iodo-2'-deoxyuridine, in short IUdR. This served the purpose of permanently marking the tumour-cell DNA. IUdR is an analogue of thymidine and specifically incorporated into all DNA-synthesizing cells. It remains stably bound to DNA and upon cell death is inefficiently re-utilized. Loss of tracer from a closed cell population signals cell lysis. The precursor can be labelled with 1251 or 131I which are both easily counted because of their gamma emission. Immediately after a single i.v. injection of 125IUdR, the tracer is autoradiographically observed only within cells around capillary vessels in the tumour. About 70 h after injection, labelled cells are found distributed throughout the tumour and are not localized
IN VIVO ASSAY OF HYPOXIC TUMOUR CELLS
exclusively near the capillaries (Bosiljanoff et al., 1977). Tumour-bound tracers may also be externally assayed by a crystal scintillation counter which shields the body except for the tumour (Porschen and Feinendegen, 1969). In order to label in one tumour differentially both the average tumour cells on one hand, and the cells around the capillaries on the other, a double tracer technique was employed (Feinendegen, 1970). When the tumour in its exponential growth phase had a volume of about 300 mm3, 125IUdR was given twice i.v., 7 h apart. Three days later, the same animals received-this time-131-IUdR again in 2 injections, 7 h apart. At the time of the last injection, '31IUdR is localized in the euoxic cells around blood vessels, and 125 IUdR, at that time, is distributed randomly throughout the tumour and largely traces cells in hypoxic regions. If shortly after the last injection the tumour is treated by irradiation, the daily measurements of 1311 and of 125I signal the fate of such cells, which were at the time of treatment either in euoxic regions or were distributed throughout the
tumour and largely in hypoxic areas (Porschen et al., 1976; Bosiljanoff et al., 1977). The tumours were locally irradiated with either y-rays or neutrons. The y-dose rate was 1.00 Gy/min from a clinical 60-cobalt source. Fast neutrons with a mean energy of 6 MeV were produced by the KFA Compact Cyclotron by bombarding a Be-target with 90 ,uA of 14 MeV deuterons. The neutron field was defined with a benelex collimator. The dose rate to the tumour was 0-38 Gy/min, and ycontamination was about 5% in air. The whole body dose was less than 2% of the tumour dose. Amongst 5 hypoxic sensitizers with yirradiation, misonidazole proved to be most effective when injected i.p. 15 min before irradiation, in an amount of 500 mg/kg (Gewehr et al., 1978). Hyperthermia was applied to the tumours by immersing the tumour-bearing leg in a controlled water bath at 42° ± 0-2°C for an optimal period of 60 min after irradiation. The internal tumour temperature was controlled with a small thermistor (Weber et at.,
1977).
y- IRRADIATION
CYCLOTRON NEUTRONS (E=6MeV)
z
0 z
w
I-
w
o -_ In
80
120 160 HOURS 200 TIME AFTER TREATMENT
195
80
120 160 HOURS 200 TIME AFTER TREATMENT
FIG. 1.-Loss rate of cells which were labelled either with 125IUdR or with 131IUdR. The 131I labelling represents mainly cells near to the capillaries where oxygen tension is assumed to be normal. Labelling with 125IUdR represents the average tumour cells, a large proportion of which is hvnoxie-
196
W. PORSCHEN ET AL.
The combined effect of misonidazole, hyperthermia and irradiation was examined for y-rays and neutrons.
Gy enhances the loss rate of euoxic cells significantly more than the loss rate of the average or hypoxic cells. When misonidazole was used, the loss rate of RESULTS hypoxic cells increased, yet the data The data in Fig. 1 refer to the rate of hardly changed for euoxic cells. On the loss of 125I and of 131J: this means upper right, the results of 3-2 Gy of neuloss of hypoxic cells and euoxic cells after trons without and, on the lower right, with y- and neutron-irradiation. As seen in the misonidazole are shown. As expected, the upper left, gamma irradiation with 19-2 neutron-induced loss rate of hypoxic cells is close to that of euoxic cells. With misonidazole there is little change, as seen SARCOMA 180 on the lower right. COMBINED TREATMENT CYCLOTRON NEUTRONS In the next group of experiments the MISONIDAZOLE 0 5 mg/g influence of pure neutron irradiation and HYPERTHEREMIA 42'C,60min combined schemes with misonidazole and/ Z 7 a or hyperthermia was analysed. 5 CONTROL oX ,,xo The following biological endpoints for a 32Gy a 3 D given dose were evaluated: tumour volume 0 2 /°x / ~~Ro+3.2G after treatment, growth delay, TCD 50/100 32Gy+HYP Lu 1 and the loss rate of hypoxic cells (Fig. 2). Ro+3.2 Gy+ HYP In Fig. 2a the relative tumour volume 0.5 El 0 5 10 15HOURSI2Ois plotted with time after treatment with cr 5 15 HOURS 20 o 10 TIME AFTER TREATMENT 3-2 Gy neutrons. Combined hyperthermia 30I is slightly more effective than misonidab zole, yet the combination of neutrons with d misonidazole and hyperthermia is the most efficient strategy. In Fig. 2b growth delay is plotted as function of dose between 1-6 and 5-6 Gy. The results show that the effects increase from neutrons alone to misonidazole plus neutrons, to neutrons plus hyperthermia, and finally to the triple scheme. Also the percentage tumour regression 100 days after treatment as a function of dose (Fig. 2c) confirms the data on growth 0 2 4 6 8 Gy DOSE delay. Thus, the TCD 50 for the different 9aO I~~~~~~~~~~~~~~~~~~ schemes are for: 100 c ,-z Neutrons alone: 7*8 Gy; Ro+IRRAD.+ HY P , 4IA Misonidazole plus neutrons: 7*2 Gy; Neutrons plus hyperthermia: 6-5 Gy; Neutrons plus misonidazole and hypernurr-LL1 50 thermia about 3 Gy. Hr RAD.IRRAD Thus in the latter instance the dose modification factor is 2-4 ± 0-4. nu 2 4 6 Gy R In Fig. 3, the retention of 1251 as DOSE indicator of hypoxic cells is plotted against FiG. 2. Relative tumour volume, growth the time after treatment with the various delay and tumour regression for pure and combined neutron schemnes. modalities at a neutron dose of 1P6 Gy. 20
Lu
I
0
I
I
0
11
Lu
0
Z LU
Lf)
Lu
LL
cr-
Lu
a.
I
IN VIVO ASSAY OF HYPOXIC TUMOUR CELLS SARCOMA 180 NEUTRON IRRADIATION (Eo=6 MeV)
100
80 60z
0
z w 40-
'~--CONTROL R+1.6 Gy
w
6Gy+ Hyp.
20
Ro+ 1.6 Gy+ Hyp.
10c
0
50
100 150 200 HOURS TIME AFTER TREATMENT
N III/1.6 FIG. 3.-125IUdR retention for pure and combined neutron schemes.
Again, hyperthermia is more effective than misonidazole, and the triple scheme is slightly more effective than neutrons plus hyperthermia.
TABLE.-Dose-modifying Factors for Sarcoma- 180 Misonidazole* 500 mg/kg 1Eyperthermiat 60 min 42°C Misonidazole andt
hyperthermia
Co-60 y-Rays
Cycl. Neutrons (E = 6 MeV)
1*47 (1*40-1*54)
1 27 (1*12-1*42)
2*75
2*2
(1*9-2*73) 2*4 (1 *9-2*94)
Based on: * TCD 50/100, growth delay, cell loss rate.
t Percentage tumour regression (100 d).
In the Table, the DMFs from various methods for 60-Co gamma rays are compared with those for neutrons. The gamma
197
values were reported previously. Thus, the DMF values for combinations with neutrons were always smaller than those for gamma irradiation. For neutrons, misonidazole gave a DMF value of 1P27, hyperthermia a value of 2.2 and finally misonidazole plus hyperthermia a value of 2-4. Similar results were found for the system Adenocarcinoma EO-771 in C57 Bi mice, so that the observed neutron enhancement ratios appear not to be specific for the system Sarcoma-180 and NIMRmice. In conclusion, the effect of cyclotron neutrons on hypoxic cells in Sarcoma-180 in the living mouse is augmented by Misonidazole and hyperthermia. The triple scheme of neutrons, misonidazole and hyperthermia was most effective, yielding a DMF of 2*4. REFERENCES BOSILJANOFF, P., PORSCHEN, W., PIEPENBRING, W., MtJIILENSIEPEN, H. & FEINENDEGEN,,L. E. (1977) In-vivo-LYntersuchungen uber die relative Strahlenempfindlichkeit hypoxischer Tumorzellen. Strahlentherapie, 153, 178. FEINENDEGEN, L. E. (1970) Autoradiographische und biochemische Untersuchungen der Zellproliferation in vivo. In: Praoperative Tumorbe8trahlung, Vortrdge vom Deutschen Rdntgenkongress, Ed.: 0. Hug, Munchen. GEWEHR, K.. PORSCHEN, W., MUHLENSIEPEN, H. & FEINENDEGEN, L. E. (1978) Measurements of in vivo Effects of Electron Affinic Radiosensitizers using a Double Tracer Technique. (In press.) PORSCHEN, W. & FEINENDEGEN, L. E. (1969) Invivo-Bestimmung der Zellverlustrate bei Experimentaltumoren mit markiertem Joddesoxyuridin. Strahlentherapie, 137, 718. PORSCHEN, W., BOSILJANOFF, P., GEWEHR, K., MiYHLENSIEPEN, H., WEBER, H. -J., DIETZEL, F. & FEINENDEGEN, L. E. (1976) In vivo Assay of the Radiation-Sensitivity of Hypoxic Tumour Cells; Influence of Radiation-Quality and Hypoxic Sensitization. Int. Symp. Radiobiol. Re8. for the Improvement of Radiother., November, Vienna, IAEA-SM-212/16. WEBER, H.-J., DIETZEL, F., PORSCHEN, W. & FEINENDEGEN, L. E. (1977) In vivo Assay of the Radiation-Sensitivity of Hypoxic Tumour Cells; Influence of Combined Hyperthermia and Irradiation. 2nd Intern. Symp. Hyperthermia and Radiat. in Cancer Therapy, Essen, June, 1977.
