[Different effectiveness of hyperthermia with regard to fast electrons and X-rays with different energy spectra...

The synergistic enhancement of the irradiation effect by mild (40.5 degrees C) and strong (43 degree C) hyperthermia (TER = temperature enhancement ratio) was examined for different sparsely ionizing radiation qualities (15 meV electrons in two irradiation depths corresponding to a relative depth dose of 100% and 30%, 200-kV and 29-kV-X-rays). Hypotetraploid ("4n") and diploid (2n), asynchronous, exponentially growing CHO fibroblasts were used as experimental subjects. The survival inthe colony test was used as criterion to assess the effect. The experiments carried out with 4n cells showed that, for the two temperatures, the TER decreases with increasing ionization density and, when applying fast electrons producing an extremely sparse ionization in an irradiation depth of 100%, the TER shows a maximum increase as compared to the more densely ionizing radiation qualities. Already when mild hyperthermia is employed, fast electrons show a TER of 1.25 which rises to about 2.25 at 43 degrees C. In case of 29-kV-X-rays, these values amount to 1 and 1.66 respectively. As to the 2n cells, the TER reaches here the double value and seems to be similarly dependent on the ionization density.