A Novel Method for Fragmentation Studies in Particle Therapy: Principles of Ion Identification

In carbon ion beam radiation therapy, fragmentation processes within the patient lead to changes in the composition of the particle field with increasing depth. Consequences are alterations of the resulting dose distribution and its biological effectiveness. To enable accurate treatment planning, the characteristics of the ion spectra resulting from fragmentation processes need to be known for various ion energies and target materials. In this work, we present a novel method for ion type identification using a small and highly flexible setup based on a single detector and designed to simplify measurements and overcome current shortages in available fragmentation data.

The presented approach is based on the pixelated, semiconductor detector Timepix. The large number of pixels with small pitch, all individually calibrated for energy deposition, enables detection and visualization of single particle tracks. For discrimination among different ion species, the pattern recognition analysis of the detector signal is used. Fragmentation spectra resulting from a primary carbon ion beam at various depths of tissue-equivalent material were studied to identify different ion species in mixed particle fields. The performance of the method was evaluated quantitatively using reference data from an established technique.

All ion species resulting from carbon ion fragmentation in tissue-equivalent material could be separated. For measurements behind a 158-mm-thick water tank, the relative fractions of H, He, Be, and B ions detected agreed with corresponding reference data within the limits of uncertainty. For the relatively rare lithium ions, the agreement was within 2.3 Δ_ref (uncertainty of reference).

For designated configurations, the presented ion type identification method enables studies of therapeutic carbon ion beams with a simple, small, and configurable detection setup. The technique is promising to enable online fragmentation studies over a wide range of beam and target parameters in the future.