  This paper characterizes the microdosimetric spectra of a single-energy
carbon-ion pencil beam at MedAustron using a miniature solid-state silicon
microdosimeter to estimate the impact of the lateral distribution of the
different fragments on the microdosimetric spectra. The microdosimeter was
fixed at one depth and then laterally moved away from the central beam axis in
steps of approximately 2 mm. The measurements were taken in both horizontal and
vertical direction in a water phantom at different depths. In a position on the
distal dose fall-off beyond the Bragg peak, the frequency-mean and the
dose-mean lineal energies were derived using either the entire range of
y-values, or a sub-range of y values, presumingly corresponding mainly to
contributions from primary particles. The measured microdosimetric spectra do
not exhibit a significant change up to 4 mm away from the beam central axis.
For lateral positions more than 4 mm away from the central axis, the relative
contribution of the lower lineal-energy part of the spectrum increases with
lateral distance due to the increased partial dose from secondary fragments.
The average values yF and yD are almost constant for each partial contribution.
However, when all particles are considered together, the average value of yF
and yD varies with distance from the axis due to the changing dose fractions of
these two components varying by 30 % and 10 % respectively up to the most off
axis vertical position. Characteristic features in the microdosimetric spectra
providing strong indications of the presence of helium and boron fragments have
been observed downstream of the distal part of the Bragg peak. We were able to
investigate the radiation quality as function of off-axis position. These
measurements emphasize variation of the radiation quality within the beam and
this has implications in terms of relative biological effectiveness.
