Physical quantification of the biological effectiveness of ionizing radiations

Abstract

The role, and limitations, of the fundamental physical quantities used in our current system of dosimetry for the protection of individuals against the hazardous effects of ionizing radiation is discussed briefly. A major limitation is the inability to correlate biological data, in a unified way, as a function of linear energy transfer with the consequent necessity for quality factors.From consideration of the various interaction processes undergone by charged particles in the equilibrium slowing down spectra generated by the incident radiation field, it is shown that good correlation of biological effects, for all radiation types, can be achieved in terms of linear primary ionization. The implication is that delta-ray effects play at most a very minor role, at moderate fluences, and that, consequently, the absorbed dose is an unsuitable parameter for describing radiation effects. Structure in the radiosensitive targets is observed to have a critical dimension of about 2 nrn. It occurs only when double-stranded DNA is present and the magnitude of the inactivation probability is consistent with double-stranded breaks being the significant lesion. Calculation is made of the yield of lesions as a function of the mean free path for primary ionization.Differential spectra of radiation quality are obtained for monoenergetic electrons (0.2 keV to 30 MeV); for characteristic X-rays from carbon, aluminium, neon, potassium, copper, silver, and tungsten; for 50 kV X-rays, 250 kV X-rays,. 24'Am, "'Cs, and %o gamma rays; and for neutrons with energies between 0.1 and 50 MeV. Damage by electrons is shown to be predominant at the ends of their tracks between 50 and 200 eV, whereas this is not so for the proton recoil spectrum generated by neutrons above 1 MeV.Proposals are made for a unified system of dosimetry that is independent of radiation type and that obviates the need for quality factors. The method is thought to be of general applicability to irradiations by nuclides incorporated into mammalian cells, by ingested emitters, and by external sources.

Physical Parameters that Influence the Effectiveness of Ionizing Radiation at Low Doses

“…This study is based on secondary data from published materials [4,5,6,7]. RBE is mathematically defined using equation (1).…”

“…In radiation protection, Relative Biological Effectiveness (RBE) is defined as the ratio of absorbed doses of a reference radiation to the absorbed dose of a test radiation, which produces the same biological end-point. RBE is also given as the ratio of initial slope of survival curve of tested radiation to the initial slope of the reference curve of 250 kV x-rays or 60Co gamma rays, at low doses region [1].…”

“…However, such bell-shaped response is not universal for all biological end-points. Thus although LET is considered as an essential physical parameter of any radiation type, its application in explaining radiosensitivity (i.e RBE) is limited [3].…”