Deposition of Radiation Energy

Energy transfer by radiation to biological systems is both of fundamental and practical importance. The central quantity with ionizing radiation is the dose (absorbed energy per unit mass); However, there is a difference between energy transferred and energy deposited, as will be explained in detail. The spatial distribution of absorption events also plays a significant role which is macroscopically described by the “linear energy transfer” (LET). The applicability of this concept is critically discussed. In microscopical dimensions the stochastic nature of energy absorption events becomes important, an issue which is dealt with under the heading “microdosimetry”. This will be followed by a description of the energy deposition in the close vicinity of the track of an ionizing particle. The concept of dose as defined above is generally not meaningful with optical radiation; thus, other relevant quantities such as photon or energy fluence are introduced.