Practical Proof of the Validity of the Target Theory by Simulating Cellular Targets

Abstract

Objective – To practically prove the validity of the target theory as a mathematical model to describe and understand themechanisms involved in cell killing by ionising radiation. Method – Experimental validation of the target theory was attempted using Bacillus megaterium spores. Sets of 100 vials containing averages of 1, 2, 5, 50 and 500 spores per vial was exposed to varying gamma radiation doses in presence (oxic) and absence (anoxic) of oxygen (O2). The percentage of the vials that exhibited bacterial growth after 6 days of incubation was taken as containing one spore or more, which survived a given dose. For the purpose of simulation each vial was consideredto represent one living cell, as a unit, containing a given number of targets (spores) each of which needed a single hit to be inactivated. The need for single hit was assumed depending on the shape of the dose ln-survival curve of B. megaterium spores, which has a nonzero slope at zero dose. Result – The dose ln-survival curves derived from these radiation experiments are characterized by a shoulder followed by an exponential part. The size of the shoulder increases with increasing number of spores per vial. However, the slope of the exponential parts stays the same. Conclusion – Despite some assumptions imposed to easily manipulate the simulation process, the data obtained from the present study correlate well with those calculated using the multitarget single hit (MTSH) equation: P = 1 – (1 – e–KD)N where P is the surviving fraction, K is the radiation inactivation constant, D is the radiation dose and N is the number of target. This proves the validity of the target theory model as a tool to provide a better understanding of the observed notorious effects of radiation.

Keywords

• target theory, bacterial spore, gamma radiation, simulation