Microdosimetry and radiation quality determinations in radiation protection and radiation therapy.

Beams of different radiation qualities may, for equal absorbed dose, lead to important differences in the degree of harm for a specific biological endpoint. In many practical situations absorbed dose is then not a sufficient measure when for instance the same treatment result or risk level is the focus of attention. In radiation protection, the absorbed dose may be different by a factor of 20 between the most and least effective radiation qualities. In radiation therapy the corresponding factor is approximately 3. Two physical quantities related to the charged particle track structure, LET, and lineal energy, y, are used to characterise radiation quality. Their values are dependent on whether focus is on targets in the micrometer range (chromosomes, cell nucleus, etc.) or in the nanometre range (DNA structures). The two quantities, LET, and y, have important differences, which emphasise different characteristics of a track. Applications will be discussed.