Combined effect of hyperthermia at 42 degrees C and irradiation dose of 2 Gy on two rat yolk sac tumor cell lines with different radio-thermosensitivity in vitro.
BACKGROUND
Two rat yolk sac tumor cell lines, NMT-1 and NMT-1R, are of the same origin and of different sensitivity to irradiation and to heat. The aim of this study was to investigate the sensitivities of these two cell lines to combined treatments of low-dose irradiation at 2 Gy and hyperthermia at 42 degrees C.


METHODS
The cell survival was assayed by soft agar clonogenic assay. After the survival curves of radiation alone and of heat alone at various temperatures were estimated, not only the effect of irradiation on heat, but the effect of heat on irradiation were evaluated with sequential treatments in both cell lines. These effects on survival curves were evaluated by the enhancement ratios at isosurvival levels of 37%, 10% and 1%, respectively.


RESULTS
NMT-1 was more sensitive to radiation but more resistant to heat than NMT-1R. For 1% survival level, radiosensitivity in NMT-1 was 1.32 times that in NMT-1R, while thermal sensitivity at 42 degrees C in NMT-1R was 2.73 times that in NMT-1. For sequential treatment, thermosensitization by a radiation dose of 2 Gy in radiosensitive NMT-1 was greater than that in radioresistant NMT-1R. Following heat at 42 degrees C for 1 hour, increased radiosensitivity in NMT-1R was significant, whereas the same heat treatment produced an increase in the radiation sensitivity of NMT-1 with a reduction of the survival curve shoulder but with less slope modification. There was no difference in the surviving fraction in the time-course of a combination of heat and irradiation at various intervals within 6 hours for NMT-1 except for heating immediately after irradiation. However a significant increase in survival was observed when heat was applied more than 3 hours after 2 Gy irradiation for NMT1-R.


CONCLUSIONS
These results from our cell lines with the same origin were useful for investigation into the interaction of irradiation with heat.
Tamaki
Yoshio
Y
Department of Radiology and Radiation Oncology, Gunma University School of Medicine, 3-39-22 Showa-machi, Maebashi, Gunma 371-8511. ytamaki@gunma-cc.jp
Mitsuhashi
Norio
N
Sakurai
Hideyuki
H
Islam
Mohammad Shahidul
MS
Takahashi
Takeo
T
Akimoto
Tetsuo
T
Ishikawa
Hitoshi
H
Saitoh
Jun-Ichi
J
Muramatsu
Hiroyuki
H
Niibe
Hideo
H
D000818:Animals; D002470:Cell Survival / Q000502:physiology / Q000528:radiation effects; D003131:Combined Modality Therapy; D004307:Dose-Response Relationship, Radiation; D018240:Endodermal Sinus Tumor / Q000473:pathology / Q000532:radiotherapy / Q000628:therapy*; D006979:Hyperthermia, Induced / Q000379:methods*; D011836:Radiation Tolerance / Q000502:physiology*; D051381:Rats; D013997:Time Factors; D014407:Tumor Cells, Cultured