X rays may be twice as potent as γ rays for malignant transformation at low doses

Abstract

The introduction in the 1950s of ⁶⁰Co teletherapy units and megavoltage X-ray accelerators for radiotherapy prompted several studies which showed that the relative biological effectiveness (RBE) of orthovoltage X rays, compared with ⁶⁰Co γ rays, was ∼1.1–1.2 (refs 1, 2). Subsequently, radiation therapists confirmed that the effect of established treatment protocols using orthovoltage X rays could be duplicated with the higher energy radiation by increasing the dose by ∼10%, when radiation doses are of the order of tens of grays (or thousands of rads). The dependence of biological effectiveness on photon energy led to recommendations in which the quality factor, Q, which is an RBE for radiation protection purposes, was set at unity for X and γ rays as well as for electrons or other directly ionizing particles having a linear energy transfer (LET) of <3.5 keV µm⁻¹. Over the past decade, however, several studies have shown differences in RBE between various low-LET radiations having LET values within the range designated for standard radiation^3–6. Underbrink et al.⁷ found an RBE for orthovoltage X rays relative to ⁶⁰Co γ rays of ∼2 at 0.04 Gy, for induction of pink mutations in the stamen hairs of Tradescantia. Schmidt et al.⁸ scored chromosome aberrations in human lymphocytes, and at 0.25 Gy found the RBE of 200-kVp X rays relative to 3-MeV electrons to be also ∼2. Here we have compared ⁶⁰Co γ rays and orthovoltage X rays over the dose range 0.03–1.5 Gy using malignant transformation in mammalian cells in vitro^9–13 as an end point. Our findings indicate that whereas the transformation incidence seems similar for X and γ rays at high doses, the malignant potential of X rays is about twice that of γ rays at 0.03 Gy.