Linear Energy Transfer (LET) is a fundamental physical quantity that describes the rate at which an ionizing particle deposits energy into an absorbing medium as it travels. It is defined as the average energy () locally imparted to the medium by a charged particle of a specified energy per unit path length ():
LET is typically expressed in units of kiloelectronvolts per micrometer () of tissue or water. It is highly related to the concept of collision stopping power, though LET's focus is specifically on energy deposited locally along the track, often excluding energetic secondary electrons (delta rays) that escape the primary track.
Ionizing radiation is categorized into two broad classes based on its LET value:
The deposition of energy as a function of depth for heavy charged particles is beautifully described by the Bragg curve. Initially, when a particle enters a medium at high speed, its interaction time with atoms is low, which corresponds to a lower, stable LET.
As the particle loses kinetic energy, its velocity decreases. According to physical principles, stopping power and LET are inversely proportional to the square of the velocity (). Thus, as the particle slows down near the end of its path, its LET rises sharply. This produces the massive ionization spike known as the Bragg peak before dropping to zero once the particle stops.
In radiobiology, LET is a critical determinant of Relative Biological Effectiveness (RBE). High-LET radiation is biologically much more destructive than low-LET radiation for the same absorbed dose. This is because high-LET radiation creates dense, double-stranded breaks in DNA that are extremely difficult for cell repair mechanisms to fix, whereas low-LET radiation typically causes easily repaired single-strand breaks.