E6 Biological damage from radiation at the cellular level

Topic

When ionizing radiation passes through a living cell, it ionizes the molecules in its path and can directly break the DNA molecule or generate free radicals that cause indirect damage shortly thereafter. The body possesses cellular repair mechanisms that correct much of this damage as long as the received dose remains below a certain threshold, meaning most everyday damage occurs without visible clinical consequences.

Above that threshold, repair capacity is overwhelmed, and deterministic effects appear: the higher the dose received, the more severe the resulting clinical condition—a mechanism described in detail by the next model in this series. Below the threshold, however, the risk does not disappear but changes in nature: it becomes stochastic—that is, an increased probability of developing cancer years later—with no clear threshold below which the risk is exactly zero.

The same four types of radiation identified in the previous model do not damage a cell equally per unit of absorbed dose; linear energy transfer—the amount of energy a particle deposits per unit of distance traveled within tissue—explains why alpha particles and neutrons are biologically more harmful than gamma radiation, even when the physical absorbed dose is identical. This necessitates a distinction between absorbed dose (the physical energy deposited) and biological equivalent dose (the actual damage that energy causes to the tissue).

For this reason, this model—which begins the moment radiation ionizes a molecule within a cell—serves as the fundamental basis for both radiological medicine and major long-term epidemiological studies: everything known about radiation risk to an individual stems from this very chain of events—from molecular damage to repair, and from repair to the threshold separating deterministic effects from stochastic ones.

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