Track structure, radiation quality and initial radiobiological events: Considerations based on the PARTRAC code experience

Purpose: The role of track structures for understanding the
biological effects of radiation has been the subject of research
activities for decades. The physics that describes such processes
is the core Monte Carlo codes, such as the biophysical PARTRAC
(PARticle TRACks) code described in this review, which follow the
mechanisms of radiation-matter interaction from the early stage.
In this paper a review of the track structure theory (and of its
possible extension concerning non-DNA targets) is presented.
Materials and methods: The role of radiation quality and track
structure is analyzed starting from the heavy ions results obtained
with the biophysical Monte Carlo code PARTRAC (PARticles
TRACks). PARTRAC calculates DNA damage in human cells based
on the superposition of simulated track structures in liquid water
to an ‘atom-by-atom’ model of human DNA. Results: Calculations
for DNA fragmentation compared with experimental data for
different radiation qualities are illustrated. As an example,
the strong dependence of the complexity of DNA damage on
radiation track structure, and the very large production of
very small DNA fragments (lower than 1 kbp (kilo base pairs)
usually not detected experimentally) after high LET (high-Linear
Energy Transfer) irradiation is shown. Furthermore the possible
importance of non-nuclear/non-DNA targets is discussed in the
particular case of cellular membrane and mitochondria.
Conclusions: The importance of the track structure is underlined,
in particular the dependence of a given late cellular effect on
the spatial distribution of DNA double-strand breaks (DSB)
along the radiation track. These results show that the relative
biological effectiveness (RBE) for DSB production can be
significantly larger than 1. Moreover the cluster properties of
high LET radiation may determine specific initial targets and
damage evolution.