  The "track event theory" (TET) has been developed in recent years as an
alternative to the phenomenological linear-quadratic model for cell survival
under exposure to ionizing radiation, particularly for heavy charged particles.
The TET is based on a few simple model assumptions including the possibility to
derive some of the model parameters from nanodosimetry. This work intends to
carve out more clearly the basic assumptions behind the TET and to critically
review the resulting mathematical model equations. It is demonstrated that the
model assumptions of Poisson distribution and statistical independence of the
frequency distributions of so-called one-track and two-track events follow from
the Poisson distribution of the number of tracks affecting the considered
target volume. It is also shown that the modified TET model equation used in
the literature for consideration of repair is inconsistent with the model
assumptions and requires an additional model parameter. Furthermore, the
derivation of the model parameters from nanodosimetric properties of particle
track structure is revealed to lead to a pure exponential dose dependence when
the potentially large number of relevant nanometric target volumes inside a
cell nucleus is accounted for.
