Substructure in the cell survival response at low radiation dose: effect of different subpopulations

Abstract

In order to obtain more accurate measurements of cell survival after low doses of radiation, we have used the cell sorter assay, in which a cell sorter is used to accurately count out the number of cells plated for colony formation. This method, combined with data averaging, permits measurements of survival with superior precision, which have revealed that there is substructure in the radiation response of asynchronously dividing Chinese hamster cells. The substructure, observed at doses of a few Gy, has features of a 2-component response, consistent with the presence of subpopulations of cells of different cell-cycle-related radiosensitivity. The absence of any substructure in the radiation response of homogeneous (tightly synchronized) cell populations lends strong support to this subpopulation explanation of the substructure. This assay has also been used on a variety of human tumour cell lines, most of which exhibited substructure similar to that of Chinese hamster cells. This paper outlines the application of the cell sorter assay to three different problems: (i) radiosensitizer mechanisms-etanidazole and RB 6145 are shown to enhance primarily the beta term and alpha term, respectively, of tumour cell kill, indicating that sensitizer efficacy may be tumour-specific and predictable from tumour response parameters; (ii) accurate measurement of Relative Biological Effectiveness (RBE) in a modulated clinical proton beam shows that the RBE is both dose- and depth-dependent; and (iii) measurements at lower doses clearly demonstrate a second order of substructure, termed the hypersensitive response, at doses < 1 Gy.

Relative biological effectiveness (RBE) values for proton beam therapy. Variations as a function of biological endpoint, dose, and linear energy transfer

“…Its validity at doses below ~1 Gy (e.g. due to hypersensitivity) (Joiner et al 1996, Skarsgard andWouters 1997) and at doses in excess of ~10 Gy (due to transition of the dose-response to an exponential behavior) (Andisheh et al 2013) is unclear and does depend on the biological system (Kirkpatrick et al 2009). It has been argued that the model might be reliable for photon survival curves up to doses of 18 Gy (Brenner 2008).…”

“…It has been argued that the model might be reliable for photon survival curves up to doses of 18 Gy (Brenner 2008). The predictive power of the model also depends on potential sub-populations in the sample as well as the dose range over which the response curve was fitted to the equation (Skarsgard and Wouters 1997). The linear quadratic model with the parameters α and β to describe cell inactivation as a function of dose was applied in this review because the majority of published proton data were reported using this model.…”

Critical Role for the Protons in FRTL-5 Thyroid Cells: Nuclear Sphingomyelinase Induced-Damage

“…The relative biological effectiveness was both dose- and depth-dependent [3]. In HTB63 human melanoma cells, proton beams inhibited cell growth with G2/M and G1/G0 arrest of the cell cycle and appearance of apoptotic nuclei, even 48 h after irradiation [4].…”