0·3 KeV Carbon K Ultrasoft X-rays Are Four Times More Effective than γ-rays When Inducing Oncogenic Cell Transformation at Low Doses

Abstract

Oncogenic transformation and inactivation were investigated in C3H10T1/2 mouse embryo fibroblasts exposed to proton-induced 0·28 keV carbon K (C_K)-characteristic X-rays and ⁶⁰Co γ-rays as reference radiation 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 dominates C_K ultrasoft X-rays were more effective, by a factor of 4, at inducing oncogenic cell transformation and cell inactivation compared with ⁶⁰Co γ-rays. For both endpoints the RBE of C_K ultrasoft X-rays gradually decreased with increasing dose mainly due to the greater quadratic component for ⁶⁰Co γ-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 dsbs induced by 0·28-keV ultrasoft X-rays, oncogenic cell transformation and cell inactivation may also be induced by interaction between those dsbs.