  Cao et al (2018) published an article on inverse planning based on
dose-averaged linear energy transfer (LET). Their claim that the problem can be
cast as a linear optimization model relies on an incorrect application of the
Charnes-Cooper transformation. In this comment we show that their linear model
is simlar to one where dose-averaged LET is multiplied with dose, explaining
why their model was nonetheless able to improve the LET distribution.
