On the use of effective dose in the treatment planning of high linear‐energy‐transfer radiation

Abstract

Treatment planning for conventional radiations is based on the assumption that the effect of a combination of doses at any location in the treatment field in a multibeam plan will be equivalent to that of a single dose made up of the total sum of the doses delivered to that location. This is obviously valid for conventional low linear-energy-transfer (LET) radiations when the dose contributions from various beam components are associated with the same relative biological effectiveness (RBE) value of unity. However, this is not the case for the new generation of charged particle beams whose RBEs have been shown to vary significantly with depth. A concept of effective dose, defined as the mathematical product of physical dose and RBE value evaluated for an effect level, is developed for the treatment planning of these high-LET particle radiations. Based on radiobiological results in mixed radiation experiments, it is shown that these effective doses are linearly additive like physical doses and hence, can be used directly for general treatment planning using linear algorithms already developed for the use of physical doses. This is illustrated using examples of simplified one-dimensional plans for the TRIUMF pion beam.