  The radiation effects induced by Co60 serve as a reference system for the
consideration of LET and RBE in normal and tumor tissue dose-effect relations
are usually handled by the linear-quadratic model (LQ). This approximation
excellently works up to the shoulder domain. In particle therapy we have
strictly differ between RBE in the initial plateau and environment of the Bragg
peak. Thus for protons LET and RBE of the initial plateau agree with Co60,
whereas in the Bragg peak domain both properties are increased,, but RBE of
SOBP only varies between 1.1 and 1.17. The RBE of carbon ions is increased once
again Their dose-effect curves are much steeper with a rather small shoulder
domain due to dense ionizing radiation effect. Thus protons are also dense
ionizing in the Bragg peak region, but with rather smaller magnitude compared
to carbon ions. A generalization of the LQ-model based on the nonlinear
reaction-diffusion model is proposed to describe LET and RBE of dense ionizing
particles, which accounts for properties of micro- and nano-dosimetry. A linear
term of a reaction diffusion formula describes the destroy of cells, the
nonlinear term is related to repair and the diffusion term accounts for the
density of the radiation damages. Based on dose-effect properties of Co60 the
parameters of dense ionizing particles can be determined and compared with
measurement data. The local dense of radiation effects and their consequences
in RBE and dose effect curves provide a key of understanding modern therapy
planning with different modalities and properties of nano-dosimetry are
interpreted by mathematical descriptions. The irradiation of spheroids is a
feature of micro-dosimetry, whereas intracellular exposure refers to
nano-dosimetry.
