Oncogenic transformation of C3H10T1/2 mouse embryo cells by X-rays, hyperthermia, and combined treatments.

The C3H10T1/2 cell system was used to measure oncogenic transformation after heating and after combined treatments of heat and X-rays. Heat alone at 41.0, 42.0, and 45.0 degrees C caused no transformation. Prolonged heating at 42.0 degrees C, or incubation at 37 degrees C after heating at 45.0 degrees C, resulted in thermotolerance in terms of cell killing, but did not cause transformation. Combined treatments of heat and radiation resulted in small increases in the transformation frequency per viable cell when the two treatments were separated by incubation at 37 degrees C. For simultaneous treatments of heat and X-rays, or treatments immediately following each other, the transformation frequency was about the same as, or lower than, with X-rays alone. When transformation was calculated based on the cells at risk, hyperthermia treatment reduced the X-ray transformation frequency in all cases. Thus hyperthermia may not pose a carcinogenic risk in cancer therapy and could decrease the oncogenic potential of X-rays, depending on the treatment sequence.