0.3 KEV CARBON-K ULTRASOFT X-RAYS ARE 4 TIMES MORE EFFECTIVE THAN GAMMA-RAYS WHEN INDUCING ONCOGENIC CELL-TRANSFORMATION AT LOW-DOSES

Oncogenic transformation and inactivation were investigated in C3H10T1 /2 mouse embryo fibroblasts exposed to roton-induced 0.28 keV carbon K (C-K)-characteristic X-rays and Co-60 gamma-rays as reference radiati on at high dose-rate (2-3 and 0.7 Gy/min respectively). Both oncogenic cell transformation and cell inactivation followed a linear-quadratic relationship with dose. At low doses where the linear component domin ates C-K ultrasoft X-rays were more effective, by a Factor of 4, at in ducing oncogenic cell transformation and cell inactivation compared wi th Co-60 gamma-rays. For both endpoints the RBE of C-K ultrasoft X-ray s gradually decreased with increasing dose mainly due to the greater q uadratic component for Co-60 gamma-rays compared with C-K ultrasoft X- rays. Our experimental data are in agreement with the hypothesis that single DNA double-strand breaks (dsbs), which are induced by 0.28-keV ultrasoft C-K X-rays, may lead to oncogenic cell transformation. With increasing absorbed dose, i.e. with decreasing mean distance between d sbs induced by 0.28-keV ultrasoft X-rays, oncogenic cell transformatio n and cell inactivation may also be induced by interaction between tho se dsbs.