Microdosimetry of a clinical carbon-ion pencil beam at MedAustron - Part 1: experimental characterization.

Objective: This paper characterizes the microdosimetric spectra of a single-energy carbon-ion pencil beam with a nominal energy of 284.7 MeV/u at MedAustron using a miniature solid-state silicon microdosimeter to estimate the impact on the profile of the microdosimetric spectra when moving at different lateral distances from the beam axis.

Approach: 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 directions due to the fact that the pencil beams are not radially symmetric. These measurements were performed in a water phantom at different depths. At a position on the distal dose fall-off beyond the Bragg peak, the frequency-mean and the dose-mean lineal energies,y¯_F and y¯_D, were derived using either the entire range of y-values, or a sub-range of y values, presumably corresponding mainly to contributions from primary particles and secondary fragments.

Main results: The measured microdosimetric spectra did not exhibit a significant change up to 4 mm away from the beam central axis. For lateral positions greater 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 y¯_F and y¯_D were almost constant for each partial contribution. However, when all particles were considered together, the average value of y¯_F and y¯_D varied 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 were observed downstream of the distal part of the Bragg peak.

Significance: The radiation quality was investigated as function of off-axis position. These measurements emphasized variation of the radiation quality within the beam and this has implications in terms of relative biological effectiveness.